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Geological evolution of the Antongil Craton, NE Madagascar
Abstract
The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U–Pb zircon analyses to provide an event history for its evolution.The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320 ± 14 Ma to 3231 ± 6 Ma and 3187 ± 2 Ma to 3154 ± 5 Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178 ± 2 Ma and subject to migmatisation at around 2597 ± 49 Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570 ± 18 Ma and 2542 ± 5 Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531 ± 13 Ma and 2513 ± 0.4 Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147 ± 6 Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton.The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792 ± 130 Ma to 763 ± 13 Ma and 553 ± 68 Ma. These events are interpreted as being the only manifestation of the Pan-African orogeny seen in the craton, which led to the assembly of the tectonic blocks that comprise the island.
... The oldest rocks are located along Madagascar's east coast in the Paleoarchean to Paleoproterozoic Antongil and Masora domains (Fig. 1). The c. 3200 Ma Nosy Boraha Suite in the Antongil Domain is an assemblage of polydeformed tonalite-trondhjemite-granodiorite orthogneiss with juvenile isotopic compositions (Schofield et al. 2010;Tucker et al. 2014). These units are considered remnants of the Dharwar Craton of southern India that remained in Madagascar following the Mesozoic break-up of Gondwana (Schofield et al. 2010;Armistead et al. 2018). ...
... The c. 3200 Ma Nosy Boraha Suite in the Antongil Domain is an assemblage of polydeformed tonalite-trondhjemite-granodiorite orthogneiss with juvenile isotopic compositions (Schofield et al. 2010;Tucker et al. 2014). These units are considered remnants of the Dharwar Craton of southern India that remained in Madagascar following the Mesozoic break-up of Gondwana (Schofield et al. 2010;Armistead et al. 2018). ...
... The Tonian Imorona-Itsindro Suite (c. 850-750 Ma) consists of granitoid, syenite and gabbro plutons that intruded all Precambrian lithotectonic domains in central Madagascar Archibald et al. 2016Archibald et al. , 2017 except the Antongil Domain (Schofield et al. 2010). Lithogeochemical and isotopic data indicate emplacement in a suprasubduction zone tectonic environment Boger et al. , 2015Archibald et al. 2016Archibald et al. , 2017Rakotondravaly and Randrianja 2022a, b), although the tectonic setting is controversial (Tucker et al. 2011a;Zhou et al. 2015bZhou et al. , 2018Costa et al. 2021). ...
The Neoarchaean to Palaeoproterozoic Betsiboka Suite of Madagascar mainly consists of granitoid orthogneiss and migmatite variably metamorphosed to amphibolite and granulite facies during the late Neoproterozoic. New U-Pb zircon data yielded emplacement ages of 2512 ± 17 Ma, 2507 ± 17 Ma, 2493 ± 14 Ma, and 2485 ± 16 Ma. Zircon d ¹⁸ O data indicate involvement of the continental crust during magma genesis. Mostly positive ε Hf (t) values plot slightly below the depleted-mantle curve. The isotopic data favour mixing between an older Archaean crustal source and a juvenile depleted-mantle source. Possible local crustal candidates for the older component in the exposed Malagasy basement are Neoarchaean mafic gneisses in the Tsaratanana Complex and the ca. 3200 Ma Nosy Boraha orthogneiss in the Antongil Domain. The new isotopic data support interpretations from previous studies investigating the genesis of Tonian plutonic rocks advocating crustal melting of a Mesoarchaean crustal source in the Antananarivo Domain. In addition, detrital zircon in the Proterozoic Ambatolampy Group and Itremo Group likely had a local source with the same isotopic composition as the Betsiboka Suite. Our study demonstrates the significance of multi-isotopic data from zircon for constraining the sources of detrital zircon grains in sedimentary provenance studies.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6282672
... The Precambrian basement of Madagascar is a complex amalgam of geologic terranes comprised of rocks with middle Archaean to late Cryogenian protolith ages. Although it is widely accepted that these terranes did not achieve their present configuration before the formation of Gondwana (Fig. 1A), an extensive record of older metamorphic and magmatic activity points to a long and presently disputed pre-Gondwana history Tucker et al., 1999Tucker et al., , 2007Tucker et al., , 2011b; Kröner et al., 2000;Jöns and Schenk, 2008;Schofield et al., 2010;Key et al., 2011;Yang et al., 2015;Archibald et al., 2016Archibald et al., , 2018. The crust exposed in the south and west of Madagascar is particularly heterogeneous (Fig. 1B) and, due to the complex nature of this region, little consensus has been reached as to how best subdivide the geology, or how and when this region was constructed. ...
... 3330-3140 Ma) migmatitic orthogneisses of the Nosy Boraha and Befody suites (BGS-USGS-GLW, 2008). These rocks are relatively unevolved (initial εNd = −1.25 to +1.3) and comprised of a suite of tonalities, trondhjemites and granodiorites 2011b;Schofield et al., 2010;Key et al., 2011;Ichiki et al., 2015). Their inferred host rocks are paragneisses of the Ambodiriana and Fenoarivo Groups in the Antongil Domain, and the Vohilava and Nosivolo Groups in the Masora Domain (BGS-USGS-GLW, 2008;Schofield et al., 2010;Tucker et al., 2014). ...
... These rocks are relatively unevolved (initial εNd = −1.25 to +1.3) and comprised of a suite of tonalities, trondhjemites and granodiorites 2011b;Schofield et al., 2010;Key et al., 2011;Ichiki et al., 2015). Their inferred host rocks are paragneisses of the Ambodiriana and Fenoarivo Groups in the Antongil Domain, and the Vohilava and Nosivolo Groups in the Masora Domain (BGS-USGS-GLW, 2008;Schofield et al., 2010;Tucker et al., 2014). ...
Southern and western Madagascar is comprised of five tectonic provinces that, from northeast to southwest, are defined by the: (i) Ikalamavony, (ii) Anosyen, (iii) Androyen, (iv) Graphite and (v) Vohibory Domains. The Ikalamavony, Graphite and Vohibory Domains all have intermediate and felsic igneous protoliths of tonalite-trondhjemite-granodiorite-granite composition, with positive εNd, and low Sr and Pb isotopic ratios. All three domains are interpreted to be the products of intra-oceanic island arc magmatism. The protoliths of the Ikalamavony and Graphite Domains formed repectively between c. 1080–980 Ma and 1000–920 Ma, whereas those of the Vohibory Domain are younger and date to between c. 670–630 Ma. Different post-formation geologic histories tie the Vohibory-Graphite and Ikalamavony Domains to opposite sides of the pre-Gondwana Mozambique Ocean.
By contrast, the Androyen and Anosyen Domains record long crustal histories. Intermediate to felsic igneous protoliths in the Androyen Domain are of Palaeoproterozoic age (c. 2200–1800 Ma), of tonalite-trondhjemite-granodiorite-granite composition, and show negative εNd, moderate to high ⁸⁷Sr/⁸⁶Sr and variable Pb isotopic compositions. The felsic igneous protoliths of the Anosyen Domain are of granitic composition and, when compared to felsic gneisses of the Androyen Domain, show consistently lower Sr/Y and markedly higher Sr and Pb isotope ratios. Like the Vohibory and Graphite Domains, the Androyen Domain can be linked to the western side of the Mozambique Ocean, while the Anosyen Domain shares magmatic and detrital zircon commonalities with the Ikalamavony Domain. It is consequently linked to the opposing eastern side of this ocean. The first common event observed in all domains dates to c. 580–520 Ma and marks the closure of the Mozambique Ocean. The trace of this suture lies along the boundary between the Androyen and Anosyen Domains and is defined by the Beraketa high-strain zone.
... The oldest rocks are located along Madagascar's east coast in the Palaeoarchaean to Palaeoproterozoic Antongil and Masora Domains (Fig. 1). These units are considered remnants of the Dharwar Craton of southern India left behind following the Mesozoic break-up of Gondwana (Schofield et al. 2010). ...
... The Imorona-Itsindro Suite (c. 850 -750 Ma) consists of granitoid, syenite and gabbro that intrude all central Malagasy Precambrian domains except the Antongil Domain (Schofield et al. 2010) and these intrusive rocks are not found in southern Madagascar Tucker et al. 2012). Geochemical and petrographic data indicate that the magmatic suite was emplaced in a suprasubduction-zone tectonic environment Archibald et al. 2016Archibald et al. , 2017Boger et al. , 2015. ...
... The Ikalamavony Domain must then have amalgamated with the Antananarivo Domain prior to c. 850 Ma when subduction of Mozambique Ocean crust beneath central Madagascar resulted in emplacement of the Imorona-Itsindro Suite across all lithotectonic domains except the Antongil Domain and southern Madagascar (Schofield et al. 2010;Tucker et al. 2014;Archibald et al. 2016Archibald et al. , 2017. The lack of evidence for intense pre-850 Ma metamorphism suggests that the collision between the Ikalamavony and Antananarivo (Itremo) Domains was probably a 'soft' collision (GAF-BGR 2008d;CGS 2009a), that any metamorphic fabrics developed during a pre-850 Ma 'harder' collision were overprinted by Ediacaran to Cambrian high-grade metamorphism and/or that the Ikalamavony Domain was the upper plate in the collision and, being at higher crustal levels, did not experience intense metamorphism. ...
Madagascar is a complex, composite geological terrane that occupies an important location in Late Proterozoic plate reconstructions. The recognition in one of the Madagascan terranes of a Stenian to Tonian-aged arc magmatic suite (the Dabolava Suite) demonstrates that its host terrane resided at a plate boundary from c. 1080 to 980 Ma. Gabbroic and granitoid intrusions of the Dabolava Suite are recognized only in the Ikalamavony Domain in west–central Madagascar. The oxygen isotopic compositions of zircon indicate that the parental magmas involved crustal contributions that were fractionated by a hydrological cycle, whereas hafnium isotopic compositions reflect near depleted-mantle signatures with only minor deflection to more crustal values. Together, these trends suggest mantle derivation of parental magmas coupled with upper-crustal assimilation of Stenian-aged pre-existing plutonic and volcanic rocks. These magmatic rocks, together with the coeval sedimentary rocks of the Ikalamavony Group, are taken to represent a subduction-related magmatic arc that formed in an oceanic-arc tectonic setting in the Mozambique Ocean outboard of the Archaean to early Palaeoproterozoic shield of Madagascar. The arc accreted to the older craton before the initiation of the Imorona–Itsindro Suite magmatism at c. 850 Ma that intruded both domains.
Supplementary material: A complete description of the analytical methods and supplementary data are available at https://doi.org/10.6084/m9.figshare.c.3823717
... The W Indian passive margin also follows the NNE foliation trend of the Western Dharwar gneisses (Fig. 4.9). But, this margin did not develop along any mobile belt and separated the *3300-2500 Ma Antongil-Masora cratons of Madagascar (Fig. 4.10) in west from the Dharwar craton of India (Veeraswamy and Raval 2004;Schofield et al. 2010). The Paleo-proterozoic to Neo-proterozoic Bemarivo mobile belt lies immediately N of the Antongil craton with the Andaparaty thrust in between (Schofield et al. 2010;Key et al. 2011) (Fig. 4.10). ...
... But, this margin did not develop along any mobile belt and separated the *3300-2500 Ma Antongil-Masora cratons of Madagascar (Fig. 4.10) in west from the Dharwar craton of India (Veeraswamy and Raval 2004;Schofield et al. 2010). The Paleo-proterozoic to Neo-proterozoic Bemarivo mobile belt lies immediately N of the Antongil craton with the Andaparaty thrust in between (Schofield et al. 2010;Key et al. 2011) (Fig. 4.10). Note that there is a conspicuous *NNW trending crustal-scale lithological boundary between the Dharwar craton and the Closepet Granite ( Fig. 4.9), which also parallels the passive margin. ...
... The west Indian passive margin is unique since it did not break along any mobile belt but separated the Archean craton into the Western Dharwar Craton, in India, and the Antongil and the Masora cratons in E Madagascar (Schofield et al. 2010) (Fig. 4.10). The Anaboriana-Manampotsy (AM) mobile belt (Fig. 4.10) immediate W to the Antongil-Masora cratons was avoided i.e. the passive margin did not form along it. ...
Extensional geodynamics is controlled strongly by lithosphere rheology. Lithospheric strength inversely relates to its thickness and temperature and also to its composition. Reorientation of stress axes can uplift rift shoulders. A narrow-, wide- and core complex mode of rifting happens in progressively hot and weak lithosphere. Combined thermal state of the lithosphere and strain rates manifest differences in rift architecture.
... In central-eastern Madagascar, the Masora and Antongil domains are separated from the Antananarivo domain by the north-south trending Betsimisaraka suture zone, which lies almost parallel to the Angavo-Ifanadiana shear zones Kröner et al., 2000;Collins and Windley, 2002;Collins, 2006;Raharimahefa and Kusky, 2006. It is generally accepted that the Masora and Antongil domains belong to an isolated part of the Western Dharwar Craton of India (e.g., Veeraswamy, 2003, 2011;Collins, 2006;Schofield et al., 2010;Tucker et al., 2011aTucker et al., , 2014Ishwar-Kumar et al., 2013;Rekha et al., 2013Rekha et al., , 2014Collins et al., 2014). However, the correlation of the Antananarivo domain with the surrounding Archean domains that are now located in India remains debated. ...
... Both the Masora and Antananarivo domains contain felsic and mafic intrusive rocks that have been metamor- (Kröner et al., 1999(Kröner et al., , 2000Tucker et al., 1999Tucker et al., , 2007Tucker et al., , 2011aPaquette et al., 2003;Kabete et al., 2006;Smith et al., 2008aSmith et al., , 2008bSmith et al., , 2008cRaharimahefa and Kusky, 2010;Schofield et al., 2010;. Abbreviations are same as in Figure phosed to produce felsic gneisses and mafic gneisses, respectively (e.g., Handke et al., 1999;Randriamananjara, 2008;Boger et al., 2009). ...
... COLG, collision zone granite; ORG, ocean ridge granite; VAG, volcanic arc granite; WPG, within-plate granite. MH73 shows the composition of the tonalitic gneiss sample from the Antongil domain (Schofield et al., 2010). study (barring those from sample MG08101232) are long and prismatic, euhedral to subhedral, and they exhibit oscillatory and sector zoning under CL images. ...
In reconstructions of the Gondwana supercontinent, correlations of Archean domains between Madagascar and India remain debated. In this paper, we aim to establish correlations among these Archean domains using whole-rock geochemistry and U-Pb zircon geochronology of meta-granitoids from the Masora and the Antananarivo domains, central-eastern Madagascar. A meta-granitoid from the central part of Masora domain is dated at 3277 Ma and shows a typical Archean tonalite-trondhjemite-granodiorite composition, whereas a tonalitic gneiss from the southeastern part of the Antananarivo domain gives an age of 2744 Ma. The geochemical signature of this tonalitic gneiss differs from that of the-2500 Ma granitoids of the northwestern part of Antananarivo domain. In addition, the geochemical composition of the-760 Ma granitic gneisses is consistent with a volcanic-arc origin for the protolith. Based on the geochemical and geochronological results, along with existing data, we identified three episodes of granitic magmatism in central-eastern Madagascar at-3300, 2700, and 2500 Ma. These three magmatic events are consistent with those reported for the Dharwar Craton in India, suggesting that the Archean Masora and Antananarivo domains in Madagascar were part of the Greater Dharwar Craton during the period of 3300-2500 Ma. The 700-800 Ma volcanic arc granites identified in eastern Madagascar have not been reported in India. Therefore, the subduction of the oceanic plate that led to the formation of these granites likely took place at the western margin of the Greater Dharwar Craton, which included part of eastern Madagascar.
... The oldest of these are the Palaeoarchaean to Palaeoproterozoic Antongil and Masora blocks (Fig. 1b). These units crop out on the island's east coast and are considered to be parts of the Dharwar Craton of southern India (Collins, 2006;Collins and Windley, 2002;Collins et al., 2003b;Schofield et al., 2010;Tucker et al., 1999Tucker et al., , 2011aTucker et al., , 2011bTucker et al., , 2014. ...
... The data were used to demonstrate an~710 Ma maximum depositional age, and as such, the sequence may be considered an equivalent to the Manampotsy Group as discussed above (Fig. 8e). Recently, Schofield et al. (2010) argued that the Fenoarivo and Soanierano Ivongo metasedimentary rocks are not Neoproterozoic, but instead were deposited in the Mesoarchaean or Neoarchaean with a minimum depositional age being constrained by the upper intercept age of an array of mainly discordant zircon cores and rims from a foliation parallel granitoid. This age was quoted despite the discordant array yielding a Neoproterozoic lower intercept allowing a permissive interpretation that the granite crystallised in the Neoproterozoic, with the upper intercept representing partially reset zircon inherited from the surrounding metasedimentary rocks. ...
... Interpreting the protolith of rocks of this type is notoriously difficult and unimodal age populations are certainly not unique to metavolcanic rocks. A crosscutting leucosome contains discordant zircons with Neoarchaean upper intercept ages and Neoproterozoic lower intercept ages (similar to the data presented in Schofield et al., 2010) and a late crosscutting dyke contains Neoarchaean zircons. Both samples from Tucker et al. (2011a) and Schofield et al. (2010) contain near-concordant Tonian/Cryogenian zircon that are dismissed as due to an undefined hydrothermal event (for which no other evidence is provided). ...
... In recent years, in areas neighboring the NE coast of Madagascar, new geological (Pili et al., 1997;Kusky, 2006, 2010;Tucker et al., 2011a,b), petrological (Buchwaldt et al., 2003;Jöns et al., 2009) and chronological (Paquette et al., 2003;Schofield et al., 2010;Thomas et al., 2009) data have helped to identify different lithodemic units and shear/thrust zones separating these units (Fig. 1). The continuity of "equivalent" rocks eastwards into western India is unknown due to the lack of geological, petrological and chronological data along the western coast of India. ...
... The age range (2.4-2.6 Ga) is attributed to tectonism related to E-W amalgamation of the East and the West Dharwar Cratons and the emplacement of syn-orogenic granitoids typified by the N-trending Fig. 15. A comparison between monazite ages from the northern Konkan coast (this study) and available age data from the Antongil Block (after Schofield et al., 2010). The extent of the Fenerivo Group is adopted from Tucker et al. (2011a). ...
... The northern Antongil Block, the southern Masora Block and the central Antananarivo Block constitute the Archean crustal domains along the northeastern coast of Madagascar (Schofield et al., 2010;Tucker et al., 2011a). The rock association, lithology and metamorphic grade vary greatly across the domain. ...
Synthesis of mesoscopic structures and microscopic analyses of fabric superposition and deformation microstructures are combined with Th–U–Pb (total) ages in monazites to constrain the Meso/Neoarchean crustal domains in the Western Dharwar Craton (WDC) along the western coast of India; the domains are correlated with those in NE Madagascar to configure the assembly of crustal domains in the East Gondwanaland prior to the Mesozoic break up. In the WDC, para-schists and phyllites of the Shimoga schist belt, SSB (unmixed age components, 3067 ± 26 and 3158 ± 110 Ma) and the Peninsular gneisses (3138 ± 35 Ma) are intruded by granitoid plutons (2924 ± 50 Ma) deformed at low-T. Five generations of deformation events are identified in the SSB and Peninsular gneisses. But the two early tectonic fabrics in Peninsular gneisses formed at anatectic amphibolite facies conditions are in sharp contrast to fabrics in SSB formed at greenschist facies. The two lithodemic units are inferred to share a pre-3.2 Ga tectonic contact.
... To the east are the Antongil and Masora cratons, both of which contain rocks that are c. 3100 Ma and are likely a continuation of the Dharwar Craton of India (Armistead et al., 2017;Schofield et al., 2010;Tucker, Ashwal, Handke, et al., 1999). To the southwest of the Antananarivo Craton is the Itremo Group, composed of quartzites, schists, and marbles with a maximum depositional age of c. 1600 Ma (Cox et al., 1998;Fernandez et al., 2003). ...
... Northern Madagascar comprises the c. 3100 Ma Antongil Craton, the c. 2500 Ma Antananarivo Craton, and the c. 750-700 Ma Bemarivo Domain (Figure 1), all of which have debatable geological histories. It is well documented that the Antongil Craton of northern Madagascar shares many characteristics with the Dharwar Craton of India and that these two terranes were probably contiguous until the breakup of Gondwana (Armistead et al., 2017;Bauer et al., 2011;Collins & Windley, 2002;Schofield et al., 2010). The Dharwar Craton and Antongil Cratons both contain abundant c. 3100 and c. 2500 Ma magmatic rocks, and both cratons contain Archean metasedimentary rocks with indistinguishable detrital zircon U-Pb and Hf isotope signatures (Armistead et al., 2017). ...
Madagascar is a key area for unraveling the geodynamic evolution of the transition between the Rodinia and Gondwana supercontinents as it contains several suites of c. 850–700 Ma magmatic rocks that have been postulated to correlate with other Rodinian terranes. The Bemarivo Domain of northern Madagascar contains the youngest of these units that date to c. 750–700 Ma. We present zircon Hf and O isotope data to understand northern Madagascar's place in the Neoproterozoic plate tectonic reconfiguration. We demonstrate that the northern component of the Bemarivo Domain is distinct from the southern part of the Bemarivo Domain and have therefore assigned new names—the Bobakindro Terrane and Marojejy Terrane, respectively. Magmatic rocks of the Marojejy Terrane and Anaboriana Belt are characterized by evolved εHf(t) signatures and a range of δ¹⁸O values, similar to the Imorona‐Itsindro Suite of central Madagascar. These magmatic suites likely formed together in the same long‐lived volcanic arc. In contrast, the Bobakindro Terrane contains juvenile εHf(t) and mantle‐like δ¹⁸O values, with no probable link to the rest of Madagascar. We propose that the Bobakindro Terrane formed in a juvenile arc system that included the Seychelles, the Malani Igneous Suite of northwest India, Oman, and the Yangtze Belt of south China, which at the time were all outboard from continental India and south China. The final assembly of northern Madagascar and amalgamation of the Bobakindro Terrane and Marojejy Terrane occurred along the Antsaba subduction zone, with collision occurring at c. 540 Ma.
... The dyke-like Ankavanana gabbroic intrusion from Antongil craton yields a U-Pb zircon age of 2147 ± 6 Ma (Schofield et al., 2010). The Atongil craton along with the Masora and Antananarivo cratons makes up the Archean basement of present-day Madagascar, and the latter probably juxtaposed the Antongil-Masora craton during the assembly of Gondwana (Schofield et al., 2010). ...
... The dyke-like Ankavanana gabbroic intrusion from Antongil craton yields a U-Pb zircon age of 2147 ± 6 Ma (Schofield et al., 2010). The Atongil craton along with the Masora and Antananarivo cratons makes up the Archean basement of present-day Madagascar, and the latter probably juxtaposed the Antongil-Masora craton during the assembly of Gondwana (Schofield et al., 2010). The Atongil-Masora craton is postulated to be a fragmented extension of the western Dharwar Craton, accreted at ca. 2.5-2.45 ...
Progress with supercontinental reconstructions relies on accurate age determinations for continental geological units such as mafic dyke swarms. Here we present zircon and baddeleyite U-Pb ID-TIMS isotope data of six mafic dykes from the Archean North Atlantic craton in present-day southern West and South-East Greenland. Two dykes in southern West Greenland yield crystallization ages of 2021 ± 4 Ma, for the NE-trending Hamborgersund dyke, and 2125 ± 9 Ma, for the E-trending Angissunguaq dyke. Additional age determinations of mafic dykes from South-East Greenland are 2166 ± 8 Ma and 2158 ± 8 Ma for two E–W trending dykes, herein named the Skjoldungen dykes, and 2137 ± 11 and 2124 ± 11 for two ENE and NE trending dykes, respectively. The name Ruinnæsset dykes is proposed for this slightly younger, ca. 2137–2124 Ma, generation of dykes in SE Greenland, and Nuuk dykes for coeval ca. 2125 Ma dykes in SW Greenland. The Skjoldungen and the Ruinnæsset dykes have primordial mantle geochemical signatures, with only minor LILE-enrichments. These signatures differ from other, more ‘lithospheric’ Proterozoic dykes within the region, and may reflect highly attenuated lithospheric extension during their emplacement. Coeval mafic magmatism in North Atlantic and Dharwar cratons suggests that these two fragments coexisted within a common Paleoproterozoic supercraton. A lack of younger age-matches further argue for ca. 2170–2140 Ma rifting and subsequent break-up of this Dharwar-North Atlantic connection, prior to the 2137–2124 Ma emplacement of the Ruinnæsset dykes. Other global age correlations are discussed and a likely paleogeographic reconstruction of North Atlantic craton together with the Dharwar and Superior cratons within a Paleoproterozoic supercraton is presented.
... Based on the results of PGRM, as well as considering the purpose of this paper, the Precambrian crystalline basement of Madagascar may be divided into seven geological domains (Fig. 2a). The Antongil-Masora Domain is characterized by the presence of Paleo-and Mesoarchean TTG gneisses, which are the oldest rocks known in Madagascar (Schofield et al., 2010;Tucker et al., 2011a;Ichiki et al., 2015). There is a general consensus that this domain represents a fragment split from the Indian Dharwar Craton during Gondwana break-up (Collins, 2006;Tucker et al., 2014). ...
... Particularly, previous workers suggested that the Antongil-Masora Domain escaped mid-Neoproterozoic tectonothermal reworking, a positive line of evidence to interpret the Manampotsy-Anaboriana Domain as a Neoproterozoic suture (Collins, 2006). However, representatives of the Imorona-Itsindro Suite in the Masora Sub-domain and metamorphic zircon growth in mid-Neoproterozoic time in the Antongil Sub-domain have recently been reported (Schofield et al., 2010;Tucker et al., 2011a;Archibald et al., 2016). Our U-Pb dating results of sample RT-12B-12 from the Beheloka area are very interesting in this context, demonstrating that members of the Imorona-Itsindro Suite may be also present in the Antongil Sub-domain. ...
The Neoproterozoic Imorona-Itsindro Suite of central Madagascar represents one of the crucial petrological records of the supercontinent transformation from Rodinia into Gondwana, whereas its tectonic setting remains open to debate. Here newly reported and existing zircon U/Pb-Hf-O isotope data are used to better understand the tempo-spatial features and petrogenetic mechanisms of this enigmatic suite. The following results are at least more compatible with, if not suggestive of, a continental rift setting than the traditionally accepted continental arc setting. (1) This suite is present throughout central Madagascar, including the Manampotsy-Anaboriana Domain and probably also the Antongil Sub-domain. Particularly, the high-flux magmatic pulse at ∼790 Ma has a width of more than 300 km. (2) This suite is in essence an igneous expression of lithospheric reworking with no asthenosphere contribution being clearly detected. (3) The felsic and mafic end-members of this suite have independent sources and were derived from the crustal basement and lithospheric mantle, respectively. (4) Input of mantle materials (via magma mixing process) into the crust-derived felsic end-member increased at the ∼790 Ma high-flux period, as indicated by a discernible positive zircon εHf(t) excursion. (5) Felsic end-member magma systems may have interacted with meteoric water at a high temperature, as implied by a previously reported zircon low-δ¹⁸O signature. Thus, there is perhaps no Neoproterozoic Mozambique Ocean suture across the Archean basement of central Madagascar. The most important implication of this study is that an elongate Andean-type orogenic system assumed in many Neoproterozoic paleogeography maps may not have been developed along the western margin of the supercontinent Rodinia.
... The five metasedimentary samples used for zircon U-Pb and Lu-Hf analysis are described in Table 1. A further eight samples from Madagascar were selected for Lu-Hf analysis from zircon mounts previously analysed for U-Pb geochronology as part of a BGS World Bank project (BGS-USGS-GLW, 2008;Schofield et al., 2010) (Table 1). Four of these samples (those with prefix BT) are from the Antongil Domain, which is considered to be an extension of the Dharwar Craton (Collins et al., 2003;Tucker et al., 1999). ...
... To understand the provenance of our analysed zircon grains, it is essential to compare them to other published detrital zircon samples. We have compiled over 7000 published U-Pb zircon analyses (∼5000 from detrital samples) from Madagascar and India, many of which have complex age distributions (Archibald et al., 2015;BGS-USGS-GLW, 2008;Collins et al., 2012Collins et al., , 2003Collins et al., , 2015Collins et al., , 2007Cox et al., 1998Cox et al., , 2004De Waele et al., 2011;Fitzsimons and Hulscher, 2005;Ishwar-Kumar et al., 2013;Lancaster et al., 2015;Li et al., 2017;Maibam et al., 2016Maibam et al., , 2011Plavsa et al., 2014;Sarma et al., 2012;Schofield et al., 2010;Tucker et al., 2011aTucker et al., , 2011b. Traditional techniques of visually comparing age peaks between detrital samples or regions is no longer sufficient when dealing with such large quantities of complexly distributed data. ...
It has long been recognised that Madagascar was contiguous with India until the Late Cretaceous. However, the timing and nature of the amalgamation of these two regions remain highly contentious as is the location of Madagascar against India in Gondwana. Here we address these issues with new U-Pb and Lu-Hf zircon data from five metasedimentary samples from the Karwar Block of India and new Lu-Hf data from eight previously dated igneous rocks from central Madagascar and the Antongil-Masora domains of eastern Madagascar. New U-Pb data from Karwar-region detrital zircon grains yield two dominant age peaks at c. 3100Ma and c. 2500Ma. The c. 3100Ma population has relatively juvenile εHf(t) values that trend toward an evolved signature at c. 2500Ma. The c. 2500Ma population shows a wide range of εHf(t) values reflecting mixing of an evolved source with a juvenile source at that time. These data, and the new Lu-Hf data from Madagascar, are compared with our new compilation of over 7000 U-Pb and 1000 Lu-Hf analyses from Madagascar and India. We have used multidimensional scaling to assess similarities in these data in a statistically robust way. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. Based on εHf(t) signatures we also suggest that India (and the Antongil-Masora domains of Madagascar) were palaeogeographically isolated from central Madagascar (the Antananarivo Domain) during the Palaeoproterozoic. This supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.
... and 2.8-2.6 Ga greenstone intruded by~2.6 Ga granite Peucat et al., 2013). Equivalent units in Madagascar are the Antongil Domain, with 3.3-3.1 Ga tonalite intruded by 2.6-2.5 Ga granite (Tucker et al., Pan-African orogens (0.7-0.5 Ga) of Gondwana (modified from Gray et al., 2008;Meert and Lieberman, 2008;Harley et al., 2013 1999; Schofield et al., 2010), and the Masora Domain with similar protoliths but a stronger Pan-African overprint discussed further below (Tucker et al., 2011a;Key et al., 2011;Ichiki et al., 2015). Although Indian and African sides of the orogen both record 2.6-2.5 Ga magmatism, they are set apart by major magmatic events at 3.5-3.0 ...
... The age of metamorphism in the Malagasy suture is con-troversial with kyanite-staurolite schist west of the Antongil Domain dated both as Archean (Schofield et al., 2010;Tucker et al., 2011a) and Cambrian (Collins et al., 2003). A Cambrian age is confirmed by 0.52 Ga monazite included in staurolite and kyanite (Maxwell et al., 2014), consistent with high-strain rocks west of the Masora Domain that record metamorphic ages of 0.56-0.52 ...
Granulites of the southern East African Orogen formed by continental collision during Gondwana assembly. The highest metamorphic gradients of 25–50 °C km−1 were attained at 0.58–0.53 Ga in a microcontinental block that was sandwiched between two collisional sutures and is now exposed in Madagascar and southern India. The 50 Myr duration of extreme pressure–temperature (P–T) conditions and lack of coeval mantle magmatism suggest that metamorphism was driven by radiogenic heat accumulation beneath a long–lived orogenic plateau. Bounding sutures most likely record transfer of this microcontinent across the Neoproterozoic Mozambique Ocean, analogous to Gondwanan terranes that crossed Tethys before final India–Asia collision and, like Tibet, these sutures mark the edges of a plateau that formed following terminal ocean closure and collision. Both sutures record moderate metamorphic gradients of 15–25 °C km−1 but with quite different ages. Metamorphism along the western suture at 0.65–0.61 Ga followed the end of magmatism in an adjacent 0.85–0.65 Ga ocean–arc terrane. It has an anti–clockwise P–T path that reflects preferential thickening of the hot arc during early stages of collision, and dates ocean closure at the western suture. Metamorphism along the eastern suture at 0.53–0.51 Ga has a clockwise P–T path and is widely assumed to date terminal collision in the East African Orogen. However, this event was coeval with rapid exhumation of granulites in the adjacent plateau and is more likely to reflect reactivation of a much older eastern suture during plateau collapse. Great care should be taken when using metamorphism to date ocean closure in ancient orogens. Rocks with hot metamorphic gradients give poor age constraints on initial collision because peak T is attained >50 Myr after ocean closure if radioactivity is a major part of the heat budget. Suture zone rocks with moderate metamorphic gradients can provide more reliable estimates for the time of ocean closure but are also prone to later reactivation in orogens with protracted histories.
... 1. The Archaean Antongil and Masora cratons on the eastern coast of Madagascar (Schofield et al., 2010) (Fig. 1) have been correlated with the equivalent Archaean Dharwar craton of western India (Chadwick et al., 2000;Chardon et al., 2008;Jayananda et al., 2013) by e.g., Katz and Premoli (1979), Kröner et al. (1999), Collins and Windley (2002), Collins et al. (2003a), Tucker et al. (2011a.b;2014), Ishwar- Kumar et al. (2013), Rekha et al. (2013, A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT al., 2011;Bauer et al., 2011). ...
... This deep continental crustal cross-section with its distinctive sequence of units and its tectonic polarity is similar to that across many upper crustal suture zones in Phanerozoic orogenic belts (Dewey, 1977;Windley, 1995). Because of inaccessibility, there are less modern detailed studies of the mineralogy and petrology of the metamorphosed mafic-ultramafic complexes in the suture zone, which A C C E P T E D M A N U S C R I P T (Hottin, 1969;Goodenough et al., 2010;Grieco et al., 2012Grieco et al., , 2014 . 2). ...
The occurrence of high-pressure mafic–ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet–clinopyroxene–quartz coronas that formed after the breakdown of orthopyroxene–plagioclase–ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 system indicate a pressure–temperature (P–T) evolution from an isothermal (780 °C) pressure up to c. 24 kbar to decompression and cooling. Such a P–T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U–Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625 Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818 Ma and Th/U ratios vary from 1.08 to 2.09. K–Ar dating of muscovite and biotite from biotite–kyanite–sillimanite gneiss and garnet–biotite gneiss yields age of 486 ± 9 Ma and 459 ± 9 Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43 km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar.
... The present-day crystalline basement of Madagascar is made up of Archean to Neoproterozoic crustal units with equivalents in India and East Africa (Stern 1994; Unrug 1996; Blasband et al. 2000; Kröner et al. 2000; Veevers 2004; Collins and Pisarevsky 2005). The Antongil-Masora Block of Madagascar can be correlated with the Western Dharwar Craton (WDC) of India on the basis of the coherent Paleo/Mesoarchean and Neoarchean evolutionary histories of the two crustal units (e.g., Tucker et al. 1999b, 2011a, 2011b, 2014; Schofield et al. 2010; Rekha et al. 2013; 2014). Together, the two terranes constituted the Greater Dharwar Craton (GDC). ...
... Juxtaposition of India and Madagascar at 120 Ma and suggested correlation of shear zones and tectonic units, as proposed by Ishwar Kumar et al. (2013). The published ages of the crustal units in western India and northcentral Madagascar are shown (age data compiled from Balasubramanian et al. 1978; Swami Nath and Ramakrishnan 1981; Naqvi and Rogers 1987; Gupta et al. 1988; Agarwal et al. 1992; Jayananda et al. 1995, 2000; Peaucat et al. 1995; Russel et al. 1996; Tucker et al. 1999b, 2011b, 2014; Chawwick et al. 2000; Paquette et al. 2003; Ghosh et al. 2004; French and Heaman 2010; Schofield et al. 2010; Ishwar Kumar et al. 2013; Rekha et al. 2013). A major part of the Peninsular Indian basement along the Konkan coast is covered and hidden by volcanic rocks of the Deccan Traps, making reliable correlations with Madagascar difficult. ...
Paleogeographic reconstructions of India and Madagascar before their late Cretaceous rifting juxtapose the Antongil Block of Madagascar against the Deccan Traps of India, indicating that the Western Dharwar Craton extends below the Deccan lavas. Some recent studies have suggested that the South Maharashtra Shear Zone along the northern Konkan coast of India limits the northern extent of the Western Dharwar Craton, implying that the craton does not extend below the Deccan Traps, raising a question mark on paleogeographic reconstructions of India and Madagascar. The continuity of the Western Dharwar Craton north of the South Maharashtra Shear Zone below the Deccan Traps—or its lack thereof—is critical for validating tectonic models correlating Madagascar with India. In this study, zircons in tonalitic basement xenoliths hosted in Deccan Trap dykes were dated in situ, using the U-Pb isotope system. The data furnish U-Pb ages that define three populations at 2527 ± 6, 2456 ± 6, and 2379 ± 9 Ma. The 2527 ± 6 Ma ages correspond to the igneous crystallization of the tonalites, whereas the 2456 ± 6 and 2379 ± 9 Ma ages date metamorphic overprints. The results help to establish for the first time that the basement is a part of the Neoarchean granitoid suite of the Western Dharwar Craton, which extends northward up to at least Talvade in central and Kihim beach in the western Deccan. By implication, the South Maharashtra Shear Zone cannot be the northern limit of the Western Dharwar Craton. The granitoids are correlated with the Neoarchean felsic intrusions (2.57–2.49) of the Masaola suite in the Antongil Block of Madagascar, supporting the existence of a Neoarchean Greater Dharwar Craton comprising the Western Dharwar Craton and the Antongil-Masora Block.
... The first critical process, regional metamorphism and magmatism, has occurred throughout much of the island's Precambrian basement, excluding the Antongil domain (BGS-USGS-GLW, 2008; Schofield et al., 2010;Fritz et al., 2013). In order to map the other two critical factors, we used the Geological Map of Madagascar at the 1: 1,000,000 scale (Roig et al., 2012) to identify: (a) major shear zones, and (b) geological units with prospective lithologies (marble, mafic-ultramafic rocks, aluminous metasedimentary rocks, skarns, alkaline volcanic rocks) based on the classifications of Giuliani et al. (2020 ; Table S1). ...
... 3100 Ma and ca. 2500 Ma; Paquette et al., 2003;Schofield et al., 2010), and the Paleoproterozoic Itremo subdomain (Group) in central Madagascar (Tucker et al., 2007). The Antananarivo and Antongil-Masora Domains contain meta-igneous and meta-sedimentary rocks with a history of Proterozoic reworking. ...
Southeast Madagascar hosts several major deposits of beach placers, the provenance of which is little understood. To redress this imbalance, we present new laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages, and trace elements of zircons and monazites, as well as Hf isotopes of zircons from heavy mineral beach sands at Taolagnaro (Fort Dauphin) in SE Madagascar. The ages of detrital zircon grains range from 650 Ma to 450 Ma and from 1900 Ma to 1700 Ma (one grain has an age of 2633 Ma), and detrital monazite ages range from 574 Ma to 484 Ma. The Ediacaran−Cambrian zircons (650−450 Ma) and monazites (574−484 Ma) record a major Pan-African orogenic event in SE Madagascar. Geologically, the 650−450 Ma ages correlate well with coeval granitic gneisses in southern Madagascar, and a few 1900−1700 Ma ages can be linked to putative Paleoproterozoic protoliths of metasedimentary rocks from the Anosyen Domain and from the Itremo Group of central Madagascar. The zircon mineral chemistry indicates that some of the grains (P>25 μmol g−1) were sourced from S-type granitic rocks. The monazite chemistry points toward a metamorphic provenance from garnet-bearing amphibolite facies rocks, such as migmatitic gneisses in southern Madagascar. The Hf isotopic compositions of detrital zircons indicate reworking of their probable Paleoproterozoic−Archean source rocks. We conclude that the beach sand zircons and monazites were largely sourced from the uplifted and eroded Anosyen and Androyan Domains in SE Madagascar, and with minor contributions from central Madagascar, which were all transported by rivers to the ambient ocean to the southeast.
... In the west of Betsimisaraka Suture the Antananarivo block dominantly comprised of Neoarchean (c 2500 Ma) granulite to amphibolite facies orthogneisses intruded by granitic and gabbroic rocks of arc tectonic setting at about 820-740 Ma (Kröner et al., 2000;Tucker et al., 2011). The Archean Antongil craton on the east side of the suture consists of thin quartzite remnant of shelf setting has been imbricated with gneisses in distinction to thrusted Archean Antongil craton (Windley et al., 1994;Collins & Windley, 2002;Schofield et al., 2010). The Ranomena Complex is lies in a small slice of gneisses which has been thrusted eastward in distinction to west dipping Betsimisaraka Suture over Antongil Complex (Kumar et al., 2016). ...
The margin of Indian Plate is limited to Suture Zones at North, North-West
and East, those Suture Zones have marked distinctively by Ophiolites and
Mafic-Ultramafic complexes in Pakistan. The order of the Ophiolites and
Mafic-Ultramafic Complexes from South to North as (a) Bela, (b) ZhobMuslimbagh, (c) Waziristan, (d) Dargai, (e) Shangla-Mingora, (f) Jijal
Complex, (f) Sapat Complex and (g) Chilas Complex. These Ophiolites and
Mafic-Ultramafic Complexes are entirely characterized by segregated,
lenticular or disseminated Chromite associations. The present study is a
critical extensive review of previous works about Chromite Chemistry and
their petrogenetic indications. In particular, the Chromite Chemistry played
an important role to interpreted the tectonic setup of particular maficultramafic Complex, Ophiolites in Pakistan and Ranomena Ultramafic
Complex NE, Madagascar. The Ophiolites and Mafic-Ultramafic complexes in
Pakistan classified based on chromium number (Chromium No: >60 Class I,
Class-II 15-60 & <60 III). The Chilas Complex i-e Chromite. No <60 is related to
rifting origin, the Sapat and Jijal Complex Chromite. No >60 formed in Island
Arc tectonic setting while others formed in Complex origin particularly SupraSubduction Zone (SSZ) environment. The Mafic-Ultramafic Complexes of
Pakistan has been correlated with Ranomena Ultramafic Complex North-East
Madagascar and interpreted that the Bela, Muslimbagh-Zhobe, Waziristan,
Dargai and Shangla-Mingora Ophiolites NW, Pakistan and Ranomena
Ultramafic Complex NE, Madagascar originated at Supra Subduction Zone
tectonic setting.
... Central Madagascar consists of the Antananarivo Domain, which is composed of c. 2500 Ma magmatic gneisses of the Betsiboka Suite (Collins and Windley, 2002;Kröner et al., 2000) and amphibolite-granulite facies metasedimentary rocks of the Ambatolampy Group (Archibald et al., 2015). To the east are the Antongil and Masora Domains, which contain c. 3100 Ma rocks and are likely a continuation of the Dharwar Craton of India Schofield et al., 2010;Tucker et al., 1999;Tucker et al., 2011a). To the southwest of the Antananarivo Domain, and locally unconformable on it (Cox et al., 1998), is the Itremo Group, composed of quartzites, schists and marbles with a maximum depositional age of c. 1700 Ma (Cox et al., 1998;Cox et al., 2004;Fernandez et al., 2003). ...
... Two-third of the geology of Madagascar is made up of rocks of Precambrian age, occurring in the central and eastern part of the country, from north to south (Fig. 2). The crystalline basement is composed of a collage of accreted terranes ranging in age from Archaean to Neoproterozoic (Schofield et al. 2010). The Archaean and Proterozoic rocks age range from ~ 550 to 2500 Ma (Kroner et al. 2000). ...
Nitrate contamination of groundwater was assessed in the crowded area of Ambohidrapeto, Antananarivo city-Madagascar using hydrochemistry and multivariate analysis. Seventeen dug well waters and three spring waters were collected and measured in the field for physical parameters and in the laboratory for major ions. The results showed that all water samples have nitrate concentration above the standard value of 50 mg/L-NO3 set by the World Health Organization, with a minimum value of 79.3 mg/L-NO3 and a maximum value of 394.5 mg/L-NO3. Anthropogenic activities, from the surrounding pit latrines are the main sources of the groundwater nitrate contamination and are dominant over natural processes, which contribute to a lower degree to the groundwater mineralization. Ionic ratios indicated the occurrence of mineral dissolution and silicate weathering as the natural sources of magnesium and sodium, respectively. Furthermore, the correlation between calcium and sodium as well as the Piper diagram revealed the occurrence of ion exchange, resulting in sodium being the dominant cation over calcium. Forty percent (40%) of the water samples have the hydrochemical facies Na–Mg–Cl, followed by Na–Mg–Ca–Cl facies for 25% and Na–Mg–Cl–HCO3 facies for 20%. By applying principal component analysis, the influence of anthropogenic activities is confirmed. The first principal component extracted, which explained 52.88% of the total variance showed strong positive loadings on TDS, NO3−, Cl−, Mg2+, Na+, K+ and Ca2+, indicating that these ions contribute as anthropogenic sources, although Mg2+, Na+, K+ and Ca2+ contribute to a lesser extent than NO3−, and Cl−.
... The centre of Madagascar is made up of the Antananarivo Domain, which has a basement of c. 2500 Ma magmatic gneisses (Tucker et al. 1999;Kröner et al. 2000;Collins and Windley 2002), known as the Betsiboka Suite (Roig et al. 2012), interleaved with the Ambatolampy Group granulite-and amphibolite-facies metasedimentary rocks (Archibald et al. 2015). To the east of the Antananarivo Domain is the Antongil-Masora Domain, which contains gneisses that have ages of c. 3100 and c. 2500 Ma and are interpreted as a continuation of the Dharwar Craton of India (Tucker et al. 1999;Schofield et al. 2010;Armistead et al. 2017). ...
Madagascar occupied an important place in the amalgamation of Gondwana and preserves a record of several Neoproterozoic events that are linked to orogenesis of the East African Orogen. In this study, we integrate remote sensing, field data and thermochronology to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S 1 ), followed by south- to SW-directed, tight to isoclinal, recumbent folding (D 2 ). These are overprinted by north-trending upright folds that formed during an approximately east–west shortening event (D 3 ). Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. We show that the Itremo and Ikalamavony domains were deformed together in the same orogenic system, which we interpret as the c . 630 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, and probably formed in response to final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India. Apatite U–Pb and novel laser ablation triple quadrupole inductively coupled plasma mass spectrometry (LA-QQQ-ICP-MS) muscovite and biotite Rb–Sr thermochronology indicates that much of central Madagascar cooled through c . 500°C at c . 500 Ma.
Supplementary material: A detailed geological map of central Madagascar (Supplementary A), detailed methods and geo- and thermochronology results (Supplementary B), isotopic data for geo- and thermochronology (Supplementary C) and Landsat images and structural interpretation examples (Supplementary D) are available at https://doi.org/10.6084/m9.figshare.c.4840575
... 3.32-3.18 Ga; Tucker et al., 1999Tucker et al., , 2011aSchofield et al., 2010;Key et al., 2011;De Waele et al., 2011) and Antananarivo cratons (granulite facies metamorphism 2.5 Ga; Kroner et al., 2000) were considered part of the greater Dharwar craton (e.g., Tucker et al., 2011a, b). In this model, the Salem-Namakkal blocks could have been contiguous with Antananarivo block. ...
Structural mapping, strain analysis, and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the Salem-Attur shear zone in the Southern Granulite terrane of South India. The Salem-Namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss, charnockite and mafic
granulite, and had undergone multiple phases of magmatism spanning over a period of 3.2–0.5 Ga. The rocks were deformed by four phases of deformation D1–D4. The D1 deformation was characterized by isoclinal and recumbent NE-SW trending F1 fold with a pervasive subhorizontal axial planar granulitic fabric, S1, and associated quartzofeldspathic leucosomes. Granulite metamorphism was dated at ca. 2.5–2.3 Ga. The F1 fold and S1 fabric were coaxially refolded by tight to isoclinal, upright to steeply inclined NE-SW trending F2 folds during D2 deformation. The D2 deformation was associated with F2 axial planar shear zones, crenulations and leucosomes, S2 fabric. Large-scale D2 shear zones characterized by high-temperature ductile shear fabric with a vertical flow
host syntectonic syenite pluton which was dated at ca. 2.5–2.4 Ga. A P-T condition of 7 kb/600 °C was inferred for the D2 deformation. The D3 deformation was characterized by NW-SE to E-W trending F3 folds and the Salem-Attur shear zone. The shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down-dip stretching lineation defined by quartz, biotite and hornblende minerals and dated at ca. 2.0 Ga. It indicated N-NNE vergence of thrusting with the mean kinematic vorticity number, Wm, as 0.7 suggesting general simple shear strain with 50% pure shear component. The D4 deformation was manifested as NNE-SSW striking strike-slip faults and NW-SE striking extensional normal faults. Pseudotachylite veins having an age of 1.9 Ga injected during strike-slip faulting and granite-pegmatite veins showing age of 0.8–0.5 Ga intruded during normal faulting. The Salem-Namakkal blocks thus recorded a longlived shearing history. We suggest that the Salem-Attur shear zone and other shear zones such as Palghat-Cauvery, Moyar, Bhavani, Karur-Kambam-Painavu-Trichur and Achankovil shear zones, were Paleoproterozoic intraterrane shear zones which were overprinted by Meso-Neoproterozoic-Cambrian ductile and brittle deformations.
... Madagascar contains several Precambrian to earliest Palaeozoic 'basement' units overlain by Phanerozoic sedimentary and volcanic rocks (Collins, 2006;Roig et al., 2012). The oldest rocks are located along Madagascar's east coast in the Palaeoarchaean to Palaeoproterozoic Antongil and Masora Domains (Fig. 1b) that are considered remnants of the Dharwar Craton of southern India (Tucker et al., 1999;Schofield et al., 2010;Armistead et al., 2018). The Antananarivo Domain underlies most of central Madagascar. ...
The East African Orogen involves a collage of Proterozoic microcontinents and arc terranes that became wedged between older cratonic blocks during the assembly of Gondwana. The Ediacaran–Cambrian Ambalavao and Maevarano Suites in Madagascar were emplaced during the waning orogenic stages and consist of weakly deformed to undeformed plutonic rocks and dykes of mainly porphyritic granite but also gabbro, diorite and charnockite. U-Pb geochronological data date emplacement of the Ambalavao Suite to between ca. 580 and 540 Ma and the Maevarano Suite to between ca. 537 and 522 Ma. Major and trace element concentrations are consistent with emplacement in a syn- to post-collisional tectonic setting as A-type (anorogenic) suites. Oxygen (δ18O 5.27 to 7.45 ‰) and hafnium (ɛHf(t) -27.8 to -12.3) isotopic data from plutons in the Itremo and Antananarivo Domains are consistent with incorporation of an ancient crustal source. More primitive δ18O (5.27 to 5.32‰) and ɛHf(t) (0.0 to +0.2) isotopic values recorded in samples collected from the Ikalamavony Domain demonstrate the isotopic variation of basement sources present in the Malagasy crust. The Hf isotopic composition of Malagasy zircon are unlike more juvenile Ediacaran–Cambrian zircon sources elsewhere in the East African Orogen and, as such, Madagascar represents a distinct and identifiable detrital zircon source region in Phanerozoic sedimentary provenance studies. Taken together, these data indicate that high-T crustal anatexis, crustal assimilation and interaction of crustal material with mantle-derived melts were the processes operating during magma emplacement. This magmatism was coeval with polyphase deformation throughout Madagascar during the amalgamation of Gondwana and magmatism is interpreted to reflect lithospheric delamination of an extensive orogenic plateau.
... 3.8, 3.0, 2.8, 2.5 Ga; Harley and Black, 1997), and the Antongil Block of eastern Madagascar (ca. 3.3e3.1 and 2.7e2.5 Ga; Schofield et al., 2010). Similar Neoarchean to Paleoproterozoic basements are also distributed in the CongoeTanzaniaeBangweulu Block of east and central Africa, such as the Tanzania Craton (ca. ...
The Lützow-Holm Complex (LHC) of East Antarctica has been regarded as a collage of Neoarchean (ca. 2.5 Ga), Paleoproterozoic (ca. 1.8 Ga), and Neoproterozoic (ca. 1.0 Ga) magmatic arcs which were amalgamated through the latest Neoproterozoic collisional events during the assembly of Gondwana supercontinent. Here, we report new geochronological data on detrital zircons in metasediments associated with the magmatic rocks from the LHC, and compare the age spectra with those in the adjacent terranes for evaluating the tectonic correlation of East Antarctica and Sri Lanka. Cores of detrital zircon grains with high Th/U ratio in eight metasediment samples can be subdivided into two dominant groups: (1) late Meso- to Neoproterozoic (1.1-0.63 Ga) zircons from the northeastern part of the LHC in Prince Olav Coast and northern Sôya Coast areas, and (2) dominantly Neoarchean to Paleoproterozoic (2.8-2.4 Ga) zircons from the southwestern part of the LHC in southern Lützow-Holm Bay area. The ca. 1.0 Ga and ca. 2.5 Ga magmatic suites in the LHC could be proximal provenances of the detrital zircons in the northeastern and southwestern LHC, respectively. Subordinate middle to late Mesoproterozoic (1.3-1.2 Ga) detrital zircons obtained from Akarui Point and Langhovde could have been derived from adjacent Gondwana fragments (e.g., Rayner Complex, Eastern Ghats Belt). Meso- to Neoproterozoic domains such as Vijayan and Wanni Complexes of Sri Lanka, the southern Madurai Block of southern India, and the central-western Madagascar could be alternative distal sources of the late Meso- to Neoproterozoic zircons. Paleo- to Mesoarchean domains in India, Africa, and Antarctica might also be distal sources for the minor ∼2.8 Ga detrital zircons from Skallevikshalsen. The detrital zircons from the Highland Complex of Sri Lanka show similar Neoarchean to Paleoproterozoic (ca. 2.5 Ga) and Neoproterozoic (ca. 1.0 Ga) ages, which are comparable with those of the LHC, suggesting that the two complexes might have formed under similar tectonic regimes. We consider that the Highland Complex and metasedimentary unit of the LHC formed a unified latest Neoproterozoic suture zone with a large block of northern LH-Vijayan Complex caught up as remnant of the ca. 1.0 Ga magmatic arc. © 2017 China University of Geosciences (Beijing) and Peking University.
... The oldest rocks are located along Madagascar's east coast in the Paleoarchean to Paleoproterozoic Antongil and Masora Domains ( fig. 1b). These units are considered remnants of the Dharwar Craton of southern India that were separated from the craton during the Mesozoic breakup of Gondwana (Collins et al. 2003b;Collins 2006;Schofield et al. 2010;Tucker et al. 2011aTucker et al. , 2011bTucker et al. , 2014, although exact correlation with terranes in India is controversial (Rekha et al. 2014;Ratheesh-Kumar et al. 2015;Ishwar-Kumar et al. 2016). ...
The East African Orogen is one of the largest orogens that formed during the Ediacaran to Cambrian amalgamation of Gondwana. In the Mozambique Belt, the East African Orogen represents the amalgamation of Neoproterozoic India with the Congo-Tanzania-Bangweulu Block. In the Arabian-Nubian Shield, the orogen consists of a pre-Neoproterozoic continental terrane surrounded by Neoproterozoic juvenile oceanic arc–like terranes. A large ocean named the Mozambique Ocean divided these Neoproterozoic landmasses. Many oceanic sutures tie together the various terranes in the northern orogen, but the location of potential sutures become less obvious moving south. The ca. 100 My duration of subduction of the Mozambique Ocean resulted in voluminous magmatism in Madagascar, named the Imorona-Itsindro Suite, but the location of possible suture zones in Madagascar is controversial. Understanding the petrogenesis of this suite is critical for developing Neoproterozoic paleogeographic plate reconstructions. The Tonian age of the Imorona- Itsindro Suite is well documented, but the geochemistry of the suite provides ambiguous evidence for the tectonic setting. Geochemically, these rocks are predominantly calc-alkaline, with characteristics consistent with emplacement within a volcanic or continental arc. The suite has variable but mostly high⁸⁷Sr/⁸⁶Sr and low¹⁴³Nd/¹⁴⁴Nd signatures, indicating significant crustal involvement. Changes in subduction-zone dynamics, accompanied by variable crustal anatexis and assimilation, contributed to geochemical ambiguities that have prompted some authors to suggest an active rift as an alternative tectonic model. However, the geochemical signatures are more compatible with an Andean-like arc tectonic setting. We suggest that a prolonged history of subduction (1100 My) provided sufficient time for the continental arc to mature and for the development of shallow mantle source metasomatism.
... 2500 Ma) granulite to amphibolite facies orthogneisses intruded by arc-generated 820-740 Ma aged granitic rocks and gabbros (Kröner et al. 1999(Kröner et al. , 2000Tucker et al. 1999Tucker et al. , 2011Tucker et al. , 2014. On the eastern side of the suture is a remnant, thin quartzite-dominated shelf that has been imbricated with gneisses from the under-thrusted Archean Antongil craton (Windley et al. 1994;Collins & Windley, 2002;Schofield et al. 2010). Figure 1 shows that the Ranomena complex is situated in a small slice of gneisses that has been thrust eastwards from the westdipping Betsimisaraka suture over the Antongil block. ...
The Ranomena ultramafic complex in NE Madagascar consists of layered gabbro, harzburgite, orthopyroxenite, clinopyroxenite, garnet websterite and chromitite-layered peridotite. This study of the Ranomena chromite chemistry aims to better understand the petrogenesis and palaeotectonic environment of the complex. The chromite from the Ranomena chromitite is unzoned/weakly zoned and has a Cr# (Cr/(Cr + Al)) of 0.59–0.69, a Mg# (Mg/(Fe + Mg)) of 0.37–0.44, and low Al 2 O 3 (15–23 wt %) suggesting derivation from a supra-subduction zone arc setting. Calculation of parental melt composition suggests that the parental magma composition of the Ranomena chromitite was similar to that of a primitive tholeiitic basalt formed at a high degree of mantle melting, suggesting the parental melt composition was equivalent to that of an island-arc tholeiite (IAT). The parental magma of the Ranomena chromite had a FeO/MgO ratio of 0.9 to 1.8, suggesting arc derivation. The parental magma was Al- and Fe-rich, similar to a tholeiitic basaltic magma. The composition of orthopyroxene from the chromitite indicates a crystallization temperature range of 1250–1300°C at 1.0 GPa. The chemistry of the chromite in the Ranomena chromitite further suggests that the complex formed in a supra-subduction zone arc tectonic setting.
... 3800, 3000, 2800, 2500 Ma: Harley and Black, 1997) and the Antongil Block of East Madagascar (ca. 3300-3100 and 2700-2500 Ma: Schofield et al., 2010). Although the southern Indian terranes could be one of the major source regions, they might not be the sole provenance of the HC sediments because of the lack of dominant Paleoproterozoic (ca. ...
... Tucker et al. (1999) proposed, based on U-Pb zircon ages, that the Antongil Block in NE Madagascar and the anatectic Peninsular gneisses and granitoids of the Western Dharwar Craton (WDC) in India (Beckinsale et al., 1980;Bhaskar Rao et al., 1991;Meen et al., 1992;Peucat et al., 1995;Taylor et al., 1984) constituted a coherently evolved crustal domain prior to the Cenozoic rifting of the Gondwana supercontinent. This suggestion is endorsed by later workers (Paquette et al., 2003;Rekha et al., 2013b;Schofield et al., 2010;Thomas et al., 2009;Tucker et al., 2011a). ...
New structural–mineralogical data and U–Th-total Pb monazite chemical ages in 27 samples in a 430 km long corridor along the west coast of India are combined with existing data to reconstruct the tectonic set up of the Meso/Neoarchean crystalline rocks in the Western Dharwar Craton (WDC). The data helps to delineate two NW-trending Paleoproterozoic ductile shear zones that limit the southern and the northern margins of the WDC. The southern shear zone (metamorphic age: 2.3–2.4 Ga) separates the greenschist facies supracrustal belts (2.5 and 3.3 Ga), foliated granitoids (2.5 and 2.9 Ga) and amphibolite facies anatectic gneisses (>3.0 Ga) of the WDC from the >2.9 Ga granulite facies ortho/para-gneisses of the Coorg Block. This shear zone is correlated with the ∼2.4 Ga Betsimisaraka suture zone in east-central Madagascar that demarcates the accretion zone between the Antongil Block (≈WDC) and the granulite facies lithologies of the Antananarivo domain (≈Coorg Block). The northern shear zone system (metamorphic age: 2.2–1.8 Ga) extending NW into Madagascar possibly exists as a hitherto undiscovered tectonic zone forming the basement of the Mesoproterozoic Sahantaha Formation underlying the Neoproterozoic Bemarivo Belt supracrustals in NE Madagascar.
Within the WDC, the Meso/Neoproterozoic ages retrieved from poorly-defined margins in monazite are uncommon, dispersed within the craton, and do not define localized zones within the craton. The chemical ages of metamorphic monazites formed at greenschist/amphibolite facies conditions preclude metamorphism–deformation associated with accretion of crustal blocks within the WDC during the Rodinia assembly.
... Such evidence could be concealed in the recently (Cenozoic) rifted away Seychelles Block and/or the Antongil Terrane of Madagascar. However, these blocks were strongly tectonically overprinted in the middle and late Neoproterozoic-Cambrian East African orogen (Tucker et al., 2001;Schofield et al., 2010). Similarly, the south-western margin of Sarmatia is mostly covered and probably strongly overprinted by the Cadomian orogeny. ...
A set of global paleogeographic reconstructions for the 1770–1270 Ma time interval is presented here through a compilation of reliable paleomagnetic data (at the 2009 Nordic Paleomagnetic Workshop in Luleå, Sweden) and geological constraints. Although currently available paleomagnetic results do not rule out the possibility of the formation of a supercontinent as early as ca. 1750 Ma, our synthesis suggests that the supercontinent Nuna/Columbia was assembled by at least ca. 1650–1580 Ma through joining at least two stable continental landmasses formed by ca. 1.7 Ga: West Nuna (Laurentia, Baltica and possibly India) and East Nuna (North, West and South Australia, Mawson craton of Antarctica and North China). It is possible, but not convincingly proven, that Siberia and Congo/São Francisco were combined as a third rigid continental entity and collided with Nuna at ca.1500 Ma. Nuna is suggested to have broken up at ca. 1450–1380 Ma. West Nuna, Siberia and possibly Congo/São Francisco were rigidly connected until after 1270 Ma. East Nuna was deformed during the breakup, and North China separated from it. There is currently no strong evidence indicating that Amazonia, West Africa and Kalahari were parts of Nuna.
... In the Antongil subdomain, Neoarchean granitoids are represented by monzogranites, granodiorites and syenogranites (Masoala Suite, 2.55-2.50 Ga) that form a ubiquitous suite of weakly deformed batholiths, stocks, and small intrusive massifs (Paquette et al., 2003;Schofield et al., 2010). Both suites of Neoarchean granitoids yield a range of Sm-Nd model ages (T DM ) between 3.2 and 2.5 Ga (Kröner et al., 2000;Tucker et al., 1999a), which are interpreted to reflect variable amounts of mixing between Neoarchean depleted mantle and Mesoarchean crust (e.g. ...
Available U–Pb geochronology of the Precambrian shield of Madagascar is summarized and integrated into a synthesis of the region’s geological history. The shield is described in terms of six geodynamic domains, from northeast to southwest, the Bemarivo, Antongil–Masora, Antananarivo, Ikalamavony, Androyan–Anosyan, and Vohibory domains. Each domain is defined by distinctive suites of metaigneous rocks and metasedimentary groups, and a unique history of Archean (∼2.5 Ga) and Proterozoic (∼1.0 Ga, ∼0.80 Ga, and ∼0.55 Ga) reworking. Superimposed within and across these domains are scores of Neoproterozoic granitic stocks and batholiths as well as kilometer long zones of steeply dipping, highly strained rocks that record the effects of Gondwana’s amalgamation and shortening in latest Neoproterozoic time (0.560–0.520 Ga).
... The latter, firstly described by Besairie (1970Besairie ( , 1971 and previously known as the Betsimisaraka belt (e.g. Collins, 2000;Collins and Windley, 2002;Raharimahefa and Kusky, 2009;Schofield et al., 2010), is a strongly debated structure, whose interpretation is pivotal in understanding the geodynamic evolution of Madagascar (e.g. Collins, 2006;Collins and Pisarevsky, 2005;Collins and Windley, 2002;Collins et al., 2003;Kroner et al., 2000;Raharimahefa and Kusky, 2009;Tucker et al., 2010Tucker et al., , 2011. ...
Precambrian rocks of Madagascar host numerous chromitite occurrences, ranging from centimeter-thick lenses and seams to orebodies containing millions of tons. Production of chromite concentrates and lumpy, coming from Bemanevika mine that was estimated to have a remaining life of 15 years (Rahaga, 2009), establishes Madagascar as the world 15th chromite producer. The five most important chromitite localities, investigated for this work, are all characterized by outcropping chromitite bodies hosted within mafic/ultramafic intrusions of poorly understood age. They may range from Archean to Cambrian in age although they probably date back to Neoproterozoic to Cambrian. Metamorphism and alteration have variously affected all of the chromitites, but never completely obliterated their primary characteristics. Chromitite host rocks are peridotite, orthopyroxenite or orthoamphibolite, and primary gangue phases are orthopyroxene, olivine, rare plagioclase, ilmenite, rutile, pyrrhotite and pentlandite. Secondary assemblages comprise serpentine, talc, Cr-chlorite, tremolitic to actinolitic amphibole and magnetite. Geologic, textural, mineralogical and mineral chemistry data best fit a layered intrusion origin for North Toamasina, North Belobaka, Antanimbary and Andriamena chromitites, while Befandriana chromitites, even in a general layered intrusion scenario, show some contrasting features more similar to ophiolite chromitites. Differences between the studied chromitites can be ascribed to the position of the chromitite bodies within the stratigraphic sequence of a layered intrusion. The most striking chromitites are those from Antanimbary that show features assimilating them to the Cr-bearing Ti-magnetite layers of the Upper Zone of Bushveld complex. Chromitite alteration mostly affected gangue silicates whose primary assemblage was partially to almost totally obliterated, while chromites underwent at North Belobaka and North Toamasina partial and at Antanimbary complete ferritchromitization.
... The basement of Madagascar was classified into five main tectonic blocks (Collins, 2006;Collins and Windley, 2002;Collins et al., 2000;Kröner et al., 2000): (1) the Bemarivo block ( Fig. 1), forming the youngest Proterozoic orogenic belt of Madagascar, composed of granite batholiths, metasediments, metavolcanic rocks, gneisses and migmatites (Bésairie, 1964;Buchwaldt et al., 2002;Collins et al., 2001;Peters et al., 2003;Tucker et al., 2001); (2) the Antongil (Fig. 1) and Masora block (Collins et al., 2003b,c;Hottin, 1976;Paquette et al., 2003;Schofield et al., 2010) composed mainly of granodiorite, ortho-and paragneisses, migmatite and granite suites (Paquette et al., 2003). This block includes a 3187 Ma tonalitic gneiss and is intruded by~2500 Ma granitic bodies as well as mafic suites; these Archean blocks of Madagascar experienced Late Archean greenschist-facies metamorphism; it is equivalent to Fig. 1). ...
In this study, we applied classification and tectonic setting diagrams, N-MORB-normalised trace element, and incompatible element temporal variation diagrams, Nb/Nb*, Pb/Pb*, La/Smn, La/Nbn, Th/Nbn and Pb/Cen ratios and field-structural relationships to Archean tonalites, trondhjemites and granodiorites (TTGs) and Phanerozoic arc-generated TTGs. Geochemical analyses were compiled from the literature to elucidate how Archean continental crust formed, whether plate tectonics operated in the Archean, if this Archean form of plate tectonics resembled modern-style plate tectonics, and when plate tectonics commenced in the early Earth. Archean TTGs used for this study were not noticeably influenced by alteration or crustal contamination; therefore, their geochemistry is indicative of their original juvenile sources and the tectonic environments in which they were generated. These rocks are predominantly calcic to calc-alkalic and magnesian granitoids derived from low-K mafic/tholeiitic sources. Most Archean TTGs have high La/Yb(cn) and Sr/Y ratios and low Yb(n) values and Y contents characteristics of adakites and formed by partial melting of subducting oceanic crust or the lower arc crust. However, some Archean TTGs resemble modern arc andesites, dacites and rhyolites that have low La/Yb(cn) and Sr/Y ratios and high Yb(cn) and Y contents and formed by fractional crystallisation of basaltic magmas derived from partial melting of the sub-arc mantle wedge. Archean TTGs overlap with their Phanerozoic counterparts and mainly plot in the arc fields of tectonic setting discrimination diagrams. The N-MORB-normalised trace element patterns of Archean TTGs from well-studied cratons bear striking resemblance to those of TTGs from modern active arcs (e.g., Izu-Bonin-Mariana, Andean) and from older Phanerozoic arcs (e.g., Sierra Nevada, Gangdese), consistent with their formation in arcs. The temporal variations in the trace element geochemistry of Archean TTGs underwent a noticeable change on a global scale at ca. 3500–3200 Ma. The vast majority (99%) of Archean TTGs have Nb/Nb* and Pb/Pb* anomaly ratios of <1 and > 1, respectively, and are interpreted to have formed in arc settings; the remainder had non-arc origins. The La/Smn, La/Nbn, Th/Nbn and Pb/Cen ratios of these TTGs suggest that 99% formed in supra-subduction zone settings, namely arcs, forearcs and back-arcs, the remainder being derived from mantle plumes and mid-ocean ridges. This study suggests that throughout the Archean, TTGs were generated in subduction zones by modern-style plate tectonic processes, beginning in the Hadean (>4000 Ma). TTGs began to form in intra-oceanic arcs at ca. 4020 Ma prior to a global-scale switch to Andean-style continental arc magmatism at ca. 3500–3200 Ma, which led to the genesis of TTGs in continental arcs worldwide in the Paleoarchean. Modern-style plate tectonic processes predominantly contributed to the formation of Archean continental crust. As such, global-scale vertical tectonic processes did not play a significant role in the formation of Archean continental crust. Most plutons are emplaced vertically into the crust, as exemplified by modern arcs. Vertical tectonic processes are locally observed in Archean cratons, leading many researchers to erroneously conclude that vertical tectonic processes were predominant in the early Earth.
Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic-ultramafic intrusions to the south of the BHC.
Central and southern Madagascar comprise a number of distinctive Archaean crustal blocks (the Antongil-Masora and Antananarivo domains) overlain by Proterozoic supracrustal sequences, preserved in the East African Orogen. Here, we present U–Pb and Lu–Hf isotopic data for two supracrustal units from detrital and metamorphic zircon grains. The lower sequence is comprised of quartzite and calc-silicate units with a major Palaeoproterozoic detrital zircon source and a minor Archaean contribution with a maximum depositional age of ca. 1780 Ma. This sequence reflects a stable shelf sedimentation within the Antananarivo Domain and is correlated with the Itremo Group. U–Pb and Hf data are equivocal in determining the direct sources for the Archaean and early Palaeoproterozoic detrital zircon grains. However, the abundant ca. 2.3-1.8 Ga detrital grains are correlative with the Congo-Tanzania-Bangweulu Block and as these are close to the inferred age of the Itremo Basin, these are interpreted to be single cycle detritus. This implies that the Congo-Tanzania-Bangweulu craton was close to central Madagascar at ca. 1.8-1.6 Ga and the lower sequence would correspond to an originally contiguous late Palaeoproterozoic to early Mesoproterozoic sedimentary basin across central Madagascar. The upper metasedimentary unit has contrasting detrital sources and is represented mostly by biotite-plagioclase paragneiss, with an inferred psammitic protolith interleaved with volcanic/subvolcanic andesitic/rhyolitic dikes. The predominant Tonian-aged population (ca. 860-710 Ma) are igneous zircon grains with εHf(t) values varying from -15.1 to -29.2 and TDM Hf model ages between ca. 3.4 and 2.6 Ga. These grains were derived from the ca. 850-750 Ma Imorona-Itsindro magmatic suite. Their Neoarchaean-Palaeoproterozoic cores are interpreted as xenocrysts, reinforcing that the Imorona-Itsindro magmatism has a prominent continental reworking component. The probable tectonic setting for this Early Cryogenian sedimentary basin would represent a transition from an intra-arc to an intracontinental setting related to an outboard subduction, partially jammed at ca. 710 Ma due to the subduction of a ridge-transform system. The analogue would be the western US, where the Basin and Range region corresponds to a wide rift associated with a major mantle thermal anomaly. The absence of geological units and structures between ca. 720 and 635 Ma in central Madagascar corroborate with this model for a transition to a transform continental setting. The pre-Gondwana amalgamation convergence in the Ediacaran-Cambrian, that deformed and metamorphosed all units in central Madagascar units, is accounted for by ca. 550 Ma metamorphic rims on zircon grains from the quartzites in the Itremo Group.
Madagascar hosts several Paleoproterozoic sedimentary sequences that are key to unraveling the geodynamic evolution of past supercontinents on Earth. New detrital zircon U–Pb and Hf data, and a substantial new database of ∼15,000 analyses are used here to compare and contrast sedimentary sequences in Madagascar, Africa, and India. The Itremo Group in central Madagascar, the Sahantaha Group in northern Madagascar, the Maha Group in eastern Madagascar, and the Ambatolampy Group in central Madagascar have indistinguishable age and isotopic characteristics. These samples have maximum depositional ages >1700 Ma, with major zircon age peaks at c. 2500 Ma, c. 2000 Ma, and c. 1850 Ma. We name this the Greater Itremo Basin, which covered a vast area of Madagascar in the late Paleoproterozoic. These samples are also compared with those from the Tanzania and the Congo cratons of Africa, and the Dharwar Craton and Southern Granulite Terrane of India. We show that the Greater Itremo Basin and sedimentary sequences in the Tanzania Craton of Africa are correlatives. These also tentatively correlate with sedimentary protoliths in the Southern Granulite Terrane of India, which together formed a major intra‐Nuna/Columbia sedimentary basin that we name the Itremo‐Muva‐Pandyan Basin. A new Paleoproterozoic plate tectonic configuration is proposed where central Madagascar is contiguous with the Tanzania Craton to the west and the Southern Granulite Terrane to the east. This model strongly supports an ancient Proterozoic origin for central Madagascar and a position adjacent to the Tanzania Craton of East Africa.
Two distinct NW- to WNW-trending mafic dike swarms of the previously dated ca. 2.21 Ga Anantapur-Kunigal and ca. 2.18 Ga Mahbubnagar-Dandeli swarms, emplaced in the Dharwar craton, are studied in greater petrological detail. Petrography classifies these as dolerite, olivine-dolerite, and meta-dolerite. The bulk rock geochemistry of 39 new and 11 published samples of both dike swarms are compared to those of three ca. 2.21 Ga and two ca. 2.18 Ga samples previously dated through baddeleyite U-Pb geochronology. All 50 samples are sub-alkaline tholeiite basalt to basaltic andesite in compositions and both swarms have similar differentiation/crystallization trends. Nonetheless, based on the slightly enriched REE patterns of ca. 2.21 Ga samples, as opposed to practically flat ca. 2.18 Ga REE patterns; we propose that 41 samples constitute the ca. 2.21 Ga Anantapur-Kunigal dike swarm and 9 samples belong to the ca. 2.18 Ga Mahbubnagar-Dandeli swarm. Petrogenetic modelling suggests that each swarm was derived through different degrees and proportions of partial melting of both an ambient asthenospheric and metasomatized sub-continental lithospheric mantle (SCLM) and further differentiated through similar fractional crystallization processes. Both mantle melts segregated within the garnet to garnet-spinel transition zone. Geodynamic conditions suggest that dikes of both swarms were emplaced in a similar tectonic system within a single LIP, which was likely triggered through the heating by the same (pulsating?) mantle plume. Such an indirect influence from a mantle plume, not recorded in geochemical signatures, is supported by ~1576°C potential mantle temperature (Tp) estimates for more magnesian parents within both swarms. Available geochronological data on mafic dikes and Palaeopositions of the Dharwar and North American cratons during ca. 2.21–2.18 Ga are well supportive of their link to the Superia supercraton reconstruction.
The Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the ∼ 2.5 Ga orogeny and underwent high temperature (up to 1050ºC) and high pressure (up to 11.5 kbar) granulite facies metamorphism. Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed. The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks, as thin layers, or as lenses at different scales in the host rocks. Here, we report new petrographic data, zircon LA–ICP–MS U–Pb ages, and Lu–Hf isotopic data for felsic leucosomes within this complex. Anatexis, as identified by the petrological study of felsic leucosomes in the field and in thin sections, involved initial ternary feldspar exsolving to produce antiperthite and a quartz + plagioclase ± K‐feldspar + sericite mineral assemblage around feldspar grain boundaries. Dissolution is apparent along muscovite grain boundaries, and residual sericite is present around the margins of feldspar and quartz, all suggesting that anatexis was driven by reactions involving muscovite. Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma. The majority of samples have positive ∊Hf(t) values, although a few have negative values, suggesting their formation from magmas predominantly sourced from the depleted mantle, possibly with the involvement of minor amounts of crustal materials. Two‐stage Hf model ages and ∊Hf(t) values for these samples are consistent with those for gneisses of the basement, indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso‐Neoarchean (ca. 3.1–2.7 Ga). As such, the crystallization age of the felsic leucosome (∼2.4 Ga) represents the timing of regional anatexis and a change to post‐orogenic tectonism. And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar, providing an important constraints on the correlation of the two continental fragments.
Madagascar occupied an important place in the amalgamation of Gondwana, and preserves a record of several Neoproterozoic events that can be linked to orogenesis of the East African Orogen. We integrate remote sensing and field data to unravel complex deformation in the Ikalamavony and Itremo domains of central Madagascar. The deformation sequence comprises a gneissic foliation (S1), followed by south to south-west directed, tight to isoclinal, recumbent folding (D2). These are overprinted by north-trending upright folds that formed during a ~E–W shortening event. Together these produced type 1 and type 2 fold interference patterns throughout the Itremo and Ikalamavony domains. Apatite U–Pb and muscovite and biotite Rb–Sr thermochronometers indicate that much of central Madagascar was thermally reset to at least ~500oC at c. 500 Ma. Deformation in west-central Madagascar occurred between c. 750 Ma and c. 550 Ma, and we suggest this deformation formed in response to the c. 650 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, which formed in response to the final closure of the Mozambique Ocean along the Betsimisaraka Suture that amalgamated Madagascar with the Dharwar Craton of India.
Madagascar occupies an important location within the East African Orogen (EAO). The EAO comprises an assemblage of Neoproterozoic microcontinents and arc terranes lodged between older cratonic blocks during the final assembly of the supercontinent Gondwana. The Imorona-Itsindro Suite of central Madagascar represents voluminous Tonian-aged (850-750 Ma) magmatism with controversial petrogenesis. Early work proposed arc magma generation coinciding with oceanic plate subduction during closure of the Mozambique Ocean along the ‘Betsimisaraka Suture’ in eastern Madagascar. Recently, others have questioned the existence of such a suture in Madagascar and rather suggest extension related emplacement into the middle and upper crust through a system of pre-existing structures. New U-Pb (zircon) geochronological data coupled with in-situ oxygen and hafnium isotopic analyses demonstrate that the Imorona-Itsindro Suite had several source components. Most of the Tonian-aged magmatic rocks were derived by mixing between ancient crust and mantle derived melts. δ18O values show variation that indicates significant involvement of crustal material and hydrothermal fluids. Predominantly low negative εHf(t) values are also variable and indicate significant crustal involvement in the genesis of the Tonian magmas. A compilation of all available geochronological data shows magmatism was essentially continuous for ∼100 Myr but with periods of increased activity at ∼800 Ma, ∼791 Ma and ∼784 Ma. Temporal analysis shows magmatic cycles of enrichment and depletion on the scale of 15-40 Ma. Spatial variations in isotope compositions reflect the heterogeneity of probable crustal source rocks present in the Ikalamavony, Itremo, Antananarivo and Masora Domains. A tectonic model is proposed for the Imorona-Itsindro Suite as a long-lived Andean-like arc on the margin of the Mozambique Ocean. The longevity and temporal isotopic trends are interpreted as reflecting cycles of arc advance and retreat.
Geochemical mapping of North and Central Madagascar was carried out using stream sediments at an average density of one sample per 11 km2. Over 50 elements were determined from some 13 300 stream sediments using a sample size fraction of <150 μm following hot aqua regia digestion. Partially extractable concentrations of six major elements and seven base metals reveal that the Andriamena 'greenstone' Belt of the Tsaratanana Complex is geochemically distinct from the other geological divisions within the Precambrian basement of central and northern Madagascar. In particular, this study has shown the detailed spatial distribution of anomalous concentrations of base metals which confirm known areas of chromite mineralisation within the Andriamena Belt. Base metal anomalies also occur in relation to Cretaceous and Neogene volcanic rocks, for example at Nosy Be, and mafic-ultramafic intrusions such as along the Maroala deformation zone. The distribution of anomalous Au in stream sediments showed good correspondence with known gold districts of central and northern Madagascar. Highest concentrations were measured in stream sediments from within the Ampasary-Mananjary (southern Androna-Mandritsara) and Andavakoera (northern margin of North Bemarivo) gold districts. The results of the stream sediment geochemistry show that these new data provide valuable information for current and future mineral exploration and environmental studies in Madagascar, at both regional and local scale. © 2012 Institute of Materials, Minerals and Mining and The AusIMM Published by Maney on behalf of the Institute and The AusIMM.
U–Pb age data from southwest Madagascar provide a compelling case that the pre-Gondwana Indian plate was stitched with the arc terranes of the Arabian Nubian Shield along a suture that closed between 580 Ma and 520 Ma. The key observations supportive of this interpretation are: (1) metamorphism dated to 630–600 Ma is manifested only on the west side of the suture in rocks that have affinities with the oceanic and island arc terranes of the Arabian Nubian Shield, or which represent continental rocks welded to these terranes prior to the amalgamation of Gondwana, and (2) orogenesis at 580–520 Ma is manifest in rocks on both sides of the suture, an observation taken to mark the timing of collision and to reflect spatial continuity across the suture. In southwest Madagascar the distribution of metamorphic ages places the suture along the Beraketa high-strain zone, the tectonic boundary between the Androyen and Anosyen domains. Similar age relationships allow for the extrapolation of this tectonic boundary into both East Antarctica and Africa.
The end of the Archean eon (3.0–2.5 Ga) was a period of fundamental change in many aspects of the geological record. In Archean cratons, this timespan is marked by a considerable diversification in both the nature and petrogenesis of granitoid rocks. In this article, we review the nature, petrogenesis and global evolution of late-Archean granitoids and discuss their geodynamic significance.
Late-Archean granitoids can be classified into four groups: (1) volumetrically-dominant and juvenile tonalites, trondhjemites and granodiorites (TTGs), whose geochemistry is consistent with an origin through partial melting of meta-igneous mafic rocks at various pressures; (2) Mg-, Fe- and K-rich, metaluminous (monzo)diorites and granodiorites, referred to as sanukitoids s.l., which derive primarily from hybridization between mantle peridotite and a component rich in incompatible elements; (3) peraluminous and K-rich biotite- and two-mica granites, formed through melting of older crustal lithologies (TTGs and meta-sediments, respectively); and (4) hybrid high-K granites with mixed characteristics from the first three groups, showing great diversity from one craton to another.
The chronology of granitoid emplacement in late-Archean times is different from one craton to another but, in general, follows a very specific two-stage sequence: (1) a long period (0.2–0.5 Ga) of TTG emplacement; (2) a shorter period (0.02–0.15 Ga) during which all other granitoid types were generated. We propose that this sequence represents the first global subduction-collision cycle in the Earth's history. Although possibly present in the geological record prior to 3.0 Ga, such mechanisms became progressively prevalent on a planetary scale only between 3.0 and 2.5 Ga, indicating that the late-Archean geodynamic changes resulted from the global initiation of "modern-style" plate tectonics. The Archean-Proterozoic transition thus represents a major change in the mechanisms of the Earth's heat loss: before 3.0–2.5 Ga, it took place by large-scale magmatic differentiation characterized by generation of proto-continents that underwent crustal maturation locally, but without obvious cyclic activity on a planetary scale. After this, heat loss became accommodated by plate tectonics and global Wilson subduction-collision cycles. These changes were the consequence of the Earth's cooling on a global scale, which in turn controlled a number of different parameters locally (thickness, temperature, volume and rheology of the crust). This explains why the changes took place over a short timespan (~ 0.5 Ga) relative to the Earth's history, but at different times and with different characteristics from one craton to another.
The Anosyen Domain of southeastern Madagascar represents a part of a vast sequence of predominantly siliciclastic and carbonaceous sedimentary and felsic volcanic rocks that can be traced from Madagascar through southern India, Sri Lanka, and into East Antarctica. We present new age and geochemical data from the Iakora and Horombe groups: units that represent the sedimentary and volcanogenic components of the Anosyen Domain. Our data demonstrate that the siliciclastic sedimentary rocks of the Iakora Group contain predominantly 2400–1600 Ma aged detrital zircon with a modal age peak at approximately 1850 Ma. This age spectrum is comparable to those reported from the majority of the other sedimentary rocks exposed in Madagascar. The felsic volcanic rocks of the Horombe Group are of both metaluminous and peraluminous composition and have a similar chemistry and the same age as the Imorona–Itsindro Suite (840–700 Ma), a voluminous suite of continental arc-related granitoids that intrude central and northern Madagascar.
Coupled U-Pb and Hf isotopic analysis of detrital zircons from metasedimentary rocks of the Southern Granulite terrane (India) provides provenance information that helps unravel their paleotectonic position before Gondwana amalgamated. The metasedimentary packages of the Salem block (southernmost extension of Dharwar craton) record a restricted juvenile late Archean to early Paleoproterozoic (2.7-2.45 Ga) source provenance and epsilon Hf values between +0.3 and +8.8. Similar late Archean juvenile crust is found throughout the Dharwar craton and represents a likely source for the Salem block metasedimentary rocks. By contrast, the metasedimentary rocks of the Madurai block (south of the Salem block) show a predominantly Archean to Paleoproterozoic provenance (3.2-1.7 Ga) in the northern part of the Madurai block and a largely late Mesoproterozoic to Neoproterozoic provenance (1.1-0.65 Ga) in the southern part of the Madurai block. Collectively, the Madurai block metasedimentary rocks record a mixture of reworked Archean and Paleoproterozoic sources, as well as juvenile Paleoproterozoic, late Mesoproterozoic, and evolved Neoproterozoic sources. These detrital signatures best fit the combined basement ages of the Congo-Tanzania-Bangweulu block and central Madagascar (Azania), thus linking the tectonic evolution of the southernmost tip of India to these domains throughout much of the Proterozoic. The diachroneity of metamorphic ages obtained from the rims of Madurai block detrital zircons attests to their poly-metamorphic history that is different from that of the Salem block. The contrasting metamorphic and depositional histories between the Salem and Madurai blocks place them on opposite sides of the Mozambique Ocean until the latest Neoproterozoic when they came together to form Gondwana.
The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and
Madagascar in the south, is the world´ s largest Neoproterozoic to Cambrian orogenic complex. It comprises a
collage of individual oceanic domains and continental fragments between the Archean Sahara–Congo–Kalahari
Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct
phases of orogeny between �850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield,
is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The
ocean closed during a protracted period of island-arc and microcontinent accretion between �850 and
620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of
southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique
Ocean and experienced a �650–620Ma granulite-facies metamorphism. Completion of the nappe assembly
around 620Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean.
Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu
Cratons and the Zimbabwe–Kalahari Craton. They closed during the �600–500Ma Kuungan or Malagasy
Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique,
Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of
post-orogenic extension. Early �600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern
Granulite–Cabo Delgado Nappe Complex. Later �550–480 Ma extension affected Mozambique and southern
Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric
delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution
of distinctly different orogen styles. The Arabian–Nubian Shield is an accretion-type orogen comprising a
stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite–
Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly
extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen
developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a
Himalayan-type orogen composed of partly doubly thickened crust.
We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsite-haematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18 kbar at c. 550 degrees C, followed by near-isothermal decompression. The detrital SHRIMP U-Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993 Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K-Ar metamorphic ages of ca. 1326 and ca. 1385 Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750 Ma and ca. 1697 Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K-Ar biotite ages from gabbro in the range 1644-1536 Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571 Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733 Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U-Pb zircon magmatic ages of ca. 3200 Ma with a rare inherited core age of ca. 3601 Ma. The K-Ar biotite cooling age from the TTGs (1746 Ma and 1796 Ma) and amphibolite (ca. 1697 Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380 Ma, and a progression toward the south until ca. 750 Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000 km long Rodinian suture.
A region of metamorphosed supracrustal rocks (pelite, quartzite, marble, and graywacke) and coeval intrusive igneous rocks crop out in a 250 km long orogenic belt in northern Madagascar. The NW-SE trending belt is situated between a juvenile Neoproterozoic magmatic arc terrane (to the north) and an Archean craton, strongly reworked in early Neoproterozoic times (∼800-670 Ma), to the south. Pelitic schist and granulite exposed along a ∼70 km long transect from Andapa to Sambava contain assemblages ranging from sillimanite-garnet-biotite-orthoclase-cordierite to sillimanite-garnet-biotite-orthoclase and sillimanite-garnet-biotite-muscovite. These assemblages crop out over much of the area in which migmatites and hornblende + augite ± hypersthene ± biotite + perthite granites are common. Partial melting, biotite dehydration reactions, and granite emplacement are interpreted to have been nearly synchronous on the basis of field, structural, and petrographic observations. Pressure and temperature estimates from garnetiferious metapelitic granulite and pelitic migmatite are generally in the range of 6.5-8.5 kbar and 800-900°C using conventional thermobarometric methods. The occurrence of cordierite moats and discordant, but synmetamorphic, leucosomes in the granulites suggest an isothermal decompression-type path. Similarly, a core-rim P-T trajectory indicates ca. 2-3 kbar decompression at high temperature in the metapelitic granulite. U-Pb geochronology of sphene, monazite, and zircon in various metamorphic and igneous rocks from the same region yields a Cambrian age of 510-520 Ma for the time of gneiss formation, granulite metamorphism, and igneous activity. Calculated minimum cooling rates range from 6 to 18°C/Ma. Such cooling rates are more rapid than those associated with normal isostatic processes, and suggest that the terrane was tectonically exhumed at high temperature.
New U-Pb zircon ages and Sm-Nd and Rls-Sr isotopic data are presented for orthogneisses from north-central Madagascar, including Ile Sainte Marie, Alaotra-Beforona, Maevatanana, and Ambatolampy-Ambatomarina. A migmatite tonalite gneiss from Ile Sainte Marie is dated precisely at 3187 +/- 2 Ma and has a Sm-Nd model age (T-DM) of 3204 Ma, thereby establishing a Middle Archean age for the oldest, juvenile gneisses in northeast Madagascar. Dated orthogneisses, intrusive into the schist/paragneiss sequences, range in age between 2522 and 2494 Ma and have Sm-Nd model ages (T-DM) between 3207 Ma and 2541 Ma. These data establish a Late Archean or older age for two of the schist/paragneiss sequences of Madagascar and suggest that the cratonal regions of north-central Madagascar and south India were once contiguous. Strontium and neodymium isotopic data from the Late Archean rocks are interpreted to reflect mixing between depleted mantle magmas and evolving Middle Archean crust. U-Pb geochronology of other plutonic igneous rocks demonstrates that the Middle Neoproterozoic (800-640 Ma) represents an important period of igneous activity throughout north-central Madagascar. In addition, a latest Neoproterozoic-Early Cambrian (580-520 Ma) period of high-grade metamorphism and intrusive igneous activity is recorded in western and central parts of north Madagascar. We attribute this later activity to the effects of continental collision between East and West Gondwana.
Proterozoic metasediments of the Itremo Group in central Madagascar probably represent a passive margin sequence predating Gondwana assembly. The quartzites are well-sorted quartz arenites that contain flat lamination, wave ripples, current ripple cross-lamination, and dune cross bedding. The carbonate rocks preserve abundant stromatolites and algal laminates. A continental source is indicated by mudrock major and trace element chemistry. The combination of lithologic association, sediment architecture and mudrock chemistry indicates that the sequence was deposited on a continental shelf or platform.
SHRIMP data from detrital zircons indicate that the source area included early Proterozoic and late Archean rocks with ages between 1.85 and 2.69 Ga, and that the depositional age of the Itremo Group must be less than 1855 ± 11 Ma. The sequence has been deformed into a series of large-scale folds separated by ductile shear zones. SHRIMP data indicate both massive lead loss from detrital zircons and new zircon growth in the metasediments at 833 ± 112 Ma, which we interpret as the age of metamorphism of the sequence. Comparison of detrital grain ages with basement ages in East Africa and in India indicates that the source area for the Itremo Group probably lay on the present African mainland.
We present combined field, structural data and spot image analysis, petrographic data, U–Pb zircon ages, Nd isotopes and whole-rock geochemical data for the late Archaean granite plutons from Arsikere–Banavara (AB suite) and Chitradurga–Jampalnaikankote–Hosdurga (CJH suite), in the western Dharwar craton (WDC). AB plutons are syn-kinematic with respect to the development of dome-and-basin patterns resulting from partial crustal diapiric overturn (D1), whilst CJH suite plutons show deformation patterns resulting from the superimposition of large-scale regional strike-slip shearing (D2) onto D1 syn-emplacement fabrics. SIMS U–Pb zircon ages of 2614 ± 10 Ma for the Chitradurga pluton (CJH) and of 2617 ± 3 Ma for the Arsikere pluton (AB) define the minimum age of the D1 strain pattern.The CJH and AB suites are mainly high-K granites whose origin is to be sought in partial melting of an old (>3.0 Ga) depleted lower crust of intermediate composition. The CJH and AB suites correspond mainly to the CA1- and CA2-types of Archaean calc-alkaline granites and some facies to the transitional TTG. Their heterogeneous chemical compositions indicate involvement of various sources and also significantly different depths of partial melting to produce these granites.These results allow documenting regional partial reworking of the WDC 2615 Ma ago, requiring an HT event to have taken place before 2.61 Ga and thus different from the 2.55 to 2.51 Ga granulite facies metamorphic episode associated to regional-scale strike-slip shearing (D2) that affected the entire craton. Partial reworking of the older lower crust at 2.61 Ga and associated deformation would result from heat advection and/or crustal thickening that may relate to voluminous mafic greenstone volcanism and moderate shortening at that time.
We report single zircon 207Pb/206 Pb evaporation and SHRIMP ages, combined with whole-rock Nd isotopic systematics for granitoid rocks from the Antananarivo Block (terrane), one of five tectono-metamorphic units making up the Precambrian basement of central and northern Madagascar. Our data reveal three distinct age groups at 560 to 530, 820 to 720, and 2520 to 2500 Ma respectively that reflect major magmatic events and correlate with similar events in various parts of East Africa and Sri Lanka but not in southwestern India. A widespread high-grade metamorphic event at 550 Ma transformed many of the earlier granitoid gneisses into enderbite-charnockite assemblages. This granulite-facies event is common to Madagascar, East Africa, and southernmost India/Sri Lanka and reflects the final amalgamation of East and West Gondwana. Contrary to previous interpretations, there is a distinct lack of Kibaran-Grenvillian magmatism or metamorphism in Madagascar, making it unlikely that the island played a major role in the accretionary history and amalgamation of the supercontinent Rodinia. The widespread and voluminous granitoid magmatism at 824 to 720 Ma remains enigmatic, and the tectonic scenario with which it is associated is difficult to reconstruct due to severe tectonic transposition of most gneisses. The Nd isotopic systematics as well as abundant zircon xenocrysts attest to extensive remelting of Archean and Paleoproterozoic crust. On presently available data the 740 to 820 Ma granitoids are either related to magmatic underplating following plume generation, subcrustal mantle delamination during break-up and dispersal of Rodinia, or to continental arc magmatism related to subduction of the Mozambique ocean. They were emplaced into the ancient crust of central Madagascar as it lay either attached to East Africa or formed a microcontinent within the Mozambique ocean.
Decades of field and microscope studies, and more recent quantitative
geochemical analyses have resulted in a vast, and sometimes
overwhelming, array of nomenclature and terminology associated with
igneous rocks. This book presents a complete classification of igneous
rocks based on all the recommendations of the International Union of
Geological Sciences (IUGS) Subcommission on the Systematics of Igneous
Rocks. The glossary of igneous terms has been fully updated since the
first edition and now includes 1637 entries, of which 316 are
recommended by the Subcommission. Incorporating a comprehensive
bibliography of source references for all the terms included in the
glossary, this book is an indispensable reference guide for all
geologists studying igneous rocks, either in the field or the
laboratory. It presents a standardised and widely accepted naming scheme
that will allow geologists to interpret terminology in the primary
literature and provide formal names for rock samples based on
petrographic analyses. It is also supported by a website with
downloadable code for chemical classifications.
The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U-Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil-Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo-Mesoarchean (3.32-3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55-2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the "Betsimisaraka suture" is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro-Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560-520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo-Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.
The first zircon U–Pb SHRIMP dating on high-grade meta-igneous units in the northernmost parts of the Fraser Belt along the southern margin of the Western Australian Yilgarn Craton, reveal crystallisation ages between 1299 + 10 and 1250 + 23 Ma. A small number of older xenocrystic zircons, incorporated in some samples, indicate the presence of Late Paleoproterozoic crust in the region. Zircon that crystallised within a melt accumulated in the neck of a boudinaged mafic unit was dated at 1296 + 4 Ma, indicating that the emplacement of the igneous protoliths took place syntectonically. The anisotropy of magnetic susceptibility of the granulites indicates minimum axes with a mean inclination of 48 towards 1308, corresponding to a nearly vertical southwest–northeast (50–2308) magnetic foliation. This is very close to the structural trend of the Fraser Belt suggesting that the magnetic fabric was acquired syntectonically, during the collision between the Yilgarn and Gawler Cratons. The paleomagnetic data on the granulites overlap with published poles for various 1.2 Ga units in the Albany Belt and the 1.2 Ga Fraser dykes, possibly suggesting that the remanence was acquired during the second stage of the Fraser tectonism. A younger magnetisation component resembles a pole of uncertain age published for Bremer Bay in the Albany Belt.
Recent work in central and northern Madagascar has identified five tectonic units of the East African Orogen (EAO), a large collisional zone fundamental to the amalgamation of Gondwana. These five units are the Antongil block, the Antananarivo block, the Tsaratanana sheet, the Itremo sheet, and the Bemarivo belt. Geochronological, lithological, metamorphic, and geochemical characteristics of these units and their relationships to each other are used as a type area to compare and contrast with surrounding regions of Gondwana. The Antananarivo block of central Madagascar, part of a broad band of pre-1000-Ma continental crust that stretches from Yemen through Somalia and eastern Ethiopia into Madagascar, is sandwiched between two suture zones we interpret as marking strands of the Neoproterozoic Mozambique Ocean. The eastern suture connects the Al-Mukalla terrane (Yemen), the Maydh greenstone belt (northern Somalia), the Betsimisaraka suture (east Madagascar), and the Palghat-Cauvery shear zone system (south India). The western suture projects the Al-Bayda terrane (Yemen) through a change in crustal age in Ethiopia to the region west of Madagascar. Our new framework for the central EAO links the Mozambique belt with the Arabian/Nubian Shield and highlights the power of tectonic analysis in unraveling the complex tectonic collage of the EAO.
Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an FeNi core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at ∼ 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an FeNi alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence (∼ 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk of ∼ 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth (∼ 14%) and the core (∼ 86%). Chondritic meteorites display a range of ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.
The results of field, geochronologic, geochemical and isotopic studies are presented for the granitoids that occur east of the Closepet batholith up to the Kolar schist belt (KSB). Field data, such as common foliation, strong shear deformation occasionally leading to mylonitization, together with petrographic data, including reduction in grain size with corroded borders, show characteristics of the syn-kinematic emplacement of the granitoids. Single zircon evaporation ages define a minimum age of 3127 Ma for the tonalitic–trondhjemitic–granodioritic (TTG) basement and 2552–2534 Ma plateau ages for the emplacement of the granitoids, which slightly predate (20–30 Ma) the emplacement of the 2518 Ma Closepet batholith.Major and trace element data, together with isotopic data, suggest at least four magmatic suites from Closepet batholith to the east, which have independent magmatic evolution histories. The observed data are compatible with magma mixing for the Closepet batholith, melting of TTG and assimilation–fractional crystallization processes for Bangalore granites, either melting of heterogeneous source or different degree of melting of the same source for the granitoids of Hoskote–Kolar and fractional crystallization for the western margin of the KSB. Isotopic (Nd–Sr) and geochemical data (LREE and LIL elements) suggest highly enriched mantle and ancient TTG crust for the Closepet batholith, enriched mantle and TTG crust for the Bangalore granites, c.a. chondritic mantle source for the granitoids of Hoskote–Kolar and the quartz monzonites of the western margin of the KSB and slightly depleted mantle for granodiorites of the eastern margin of the KSB.We interpret all these geochronologic, geochemical and isotopic characteristics of granitoids from the Closepet batholith to the east up to the KSB in terms of a plume model. The centre of the plume would be an enriched ‘hot spot’ in the mantle that lies below the present exposure level of the Closepet batholith. Melting of such an enriched mantle hot spot produces high temperature magmas (Closepet) that penetrate overlying ancient crust, where they strongly interact and induce partial melting of the surrounding crust. These magmas cool very slowly, as the hot spot maintains high temperatures for a long time; thus they appear younger (2518 Ma). On the contrary, to the east the plume induces melting of c.a. chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas, which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older (2552–2534 Ma). This plume model can also account for late Archaean geodynamic evolution, including juvenile magmatism, heat source for reworking, inverse diapirism and granulite metamorphism in the Dharwar craton.
Fifty whole rocks Rb/Sr isochron data (lambda = 1.42 . 10-11/year) and the geological synthesis published by G.Hottin in 1973 allowed the following conclusions: the two thirds of the island are ancient archean terrains, with some katarchean cores (more than 3200 m.y.) which were metamorphosed ina large extent during the Kibaran and Pan-African orogenies. This is pointed out by important granitisations and migmatisations, dated 1150 m.y. and 675 m.y. Middle or lower Proterozoic formations exist in the South-Center and in the North-East. The Pan-African metamorphism is superimposed to the Kibaran metamorphism. The interpretation of the 'Androyan complex' in the South of Madagascar is not yet clear, the ages obtained belonging to the Katarchean up to the upper Proterozoic time. -English summary
The Rhodesian craton has been a coherent tectonic unit with little internal deformation for at least 2.6 Ga. Evidence from within the craton suggests that a continental nucleus has existed since early Archaean time, and the main (Bulawayan) greenstone belt sequences were erupted onto preexisting continental crust. The Limpopo belt to the south of the craton has had a similarly long history. Various tectonic models are discussed to explain the geological development of the craton: since the craton has behaved as an integral unit with little internal deformation it has been a plate (or part of a plate) since the Archaean, but the ancient tectonic regime may have been very different from that of today.
New limits have been set on the age of the provenance and the depositional period of the oldest known Archaean supracrustal rocks (Sargur Group) in southern India. Detrital zircon grains from a pelitic schist and a quartzite within major tracts of supracrustal rocks older than their host regional grey orthogneisses (Peninsular Gneiss, c.3000-2900 Ma) have yielded U-Pb ages in the range 3580-2960 Ma. The data indicate that granitoid rocks in the age range 3580-3130 Ma were a significant component of the provenance of the sedimentary protoliths. Exhumation of the granitoid provenance, deposition of the sedimentary protoliths, intrusion of major gabbroic and peridotitic complexes and possible basaltic volcanism took place in the period 3130-2960 Ma. This age range is at variance with previous suggestions that the Sargur Group represents early Archaean or primitive crust. -from Authors
We identify 304 mafic magmatic events ranging in age from ca. 3500 Ma to the present and assess their potential for linkage to the arrival of mantle-plume heads. For each, we catalogue components (flood basalts or their erosional remnants, giant mafic dike swarms, mafic sill provinces, and mafic-ultramafic layered intrusions), age constraints, and references to published literature. On the basis of criteria involving large amounts of magma emplaced in a short time, and/or giant radiating dike swarms, 34 events have been confidently linked to the arrival of a mantle-plume head. By using other criteria including geochemistry, an additional 194 events are considered probably related to plume arrival. The remaining 76 events, the majority of which are rift related, require further study to assess the plume-head link. Our analysis of the events that are confidently or probably linked to plume arrival yields several preliminary conclusions. Plume-head events occur throughout the geologic record since at least 3500 Ma and probably since 3800 Ma, with no plume-free intervals greater than ∼200 m.y. Plume arrival does not follow any obvious periodicity. Plumehead locations are known for only a handful of events. However, in such cases, associated sills and flow packages can be emplaced (presumably via lateral flow in dikes) as far as 2500 km away from the plume center. There are numerous precise age correlations between mafic units on different continents. However, further work is required to determine which coeval events can be reconstructed into a single plume event and which represent separate events occurring at the same time.
Reliably dated continental crust older than 3,000 Myr is only known from restricted segments of cratons in southern Africa, Greenland-Labrador, and Minnesota (summarised in ref. 1). These rare Archaean nucleii are very important in constraining the timing and mechanism of the differentiation of continental material from the mantle. We report here preliminary Rb-Sr whole rock data for Archaean gneisses from the Gorur-Hassan area, South Indian Craton which yield the oldest age yet reported from the Indian sub continent (3,358±66 Myr) with a very unradiogenic initial 87Sr/86Sr ratio (0.7000±0.0004) which implies that the igneous precursors of these gneisses were derived from a relatively Rb-depleted source within the mantle.
The Indian Precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes. The bulk of this crust was formed prior to 2600 Ma and remobilized at least twice between 2600-2000 Ma (early proterozoic Mobile Belt, EPMB) and 2000-1500 Ma (middle Proterozoic Mobile Belt MPMB). Three early Precambrian nucleii Karnataka (KN), Jeypore-Bastar (JBN), and Singhbhum (SN) appear to have survived in the craton and are characterized by low-grade supracrustals and tonalitic trondhjemite gneisses, formed 3800-2600 Ma. The available data provide constraints for a model which suggests that older schist belts were developed in shallow water basins on a simatic crust. On the other hand, the platformal components of the younger greenstone belts were laid down in rifted basins on a sialic basement. Crustal deformation and thickening gave rise to the EPMB. At 2000-1500 Ma, another intensive mobile belt event occurred in which subduction and flexure at the E and N margins of the Dharwar-Singhbhum Protocontinent gave rise to Proterozoic sedimentary basins, rift valleys, and igneous and metamorphic suites. Plate tectonic regimes had clearly set in by 2000 Ma; the middle Proterozoic orogeny shows clear evidence of modern-style collision tectonics. -from Authors
In the course of the year 1891 it fell to my lot to undertake a long and interesting journey of about 1200 miles in the island of Madagascar, during which I was able to make a few geological notes that may not be unworthy of record. Leaving the capital, I proceeded in a north-easterly direction as far as Imerimandroso, in the province of Antsihanaka; then struck due east, reaching the sea at Fenoarivo, a town situate about lat. 17° 25′ S. From here I followed the eastern coast, mostly along the water's edge, as far as the Bay of Diego Suarez, in the extreme north of the island; and thence I travelled down the north-western coast as far as the River Mevarano, in lat. 14° 37′ S., finally proceeding by boat to the island of Nosibe (long. 48° 15′ E., lat. 13° 20′ S.), visiting several of the smaller islands on the way. It is, of course, impossible to give a detailed account of the geological features of the country traversed during such a journey; but, as so little is known of the geology of Madagascar, a general sketch may not be without value. I may say that specimens of all the different rock-types met with on the route were collected,
For the sake of convenience, the part of the island travelled over may be divided into four sections : (1) that between the capital and the east coast, (2) the east coast (that is, the northern half of it),
SHRIMP U/Pb dating of zircon grains from a schistose acid volcanic rock from the Daginkatte Formation in the Chitradurga Group of the Late Archaean Dharwar Supergroup in western Karnataka has yielded a precise concordant age of 2614 ± 8 Ma which indicates the time of melt crystallisation. In contrast, zircons from similar schistose acid volcanic rocks in the Vibhuti Gudda Formation of the Sandur Group in the Sandur schist belt of eastern Karnataka yielded less precise concordia intercept ages of 2658 ± 14 and 2691 ± 18 Ma. Zircons from high-strain grey gneisses which appear to be the oldest recognisable component of the Late Archaean granite complex adjacent to the Sandur schist belt have an imprecise age of 2719 ± 40 Ma. Zircons from the youngest recognisable granite adjacent to the belt have also yielded an imprecise, but younger, age of 2570 ± 62 Ma. The ages show that granite emplacement and deformation adjacent to the. Sandur schist belt took place in a period of c.150 Ma. The imprecise ages of zircon in the acid volcanic rocks in the Sandur Group and the adjacent granites are related to Neoproterozoic loss of lead which may have been an effect of either weathering or a regional thermal event. We favour the latter in the light of the record of Pan-African thermal effects in the east and south of southern Peninsular India.
This contribution summarizes and brings up to date the recommendations made by the IUGS Subcommission on the Systematics of
Igneous Rocks for the classification of volcanic rocks when modal analyses are lacking. The classification is on a non-genetic
basis using the total alkali-silica (TAS) diagram, and is as nearly consistent as possible with the QAPF modal classification.
The diagram is divided into 15 fields, two of which contain two root names which are separated according to other chemical
criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, trachybasalt, basaltic
trachyandesite, trachyandesite, trachyte, trachydacite, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite
and foidite. Using Na-K criteria, trachybasalt may be further divided into the sub-root names hawaiite and potassic trachybasalt,
basaltic trachyandesite into the sub-root names mugearite and shoshonite, and trachyandesite into the sub-root names benmoreite
and latite.
The first zircon U–Pb SHRIMP dating on high-grade meta-igneous units in the northernmost parts of the Fraser Belt along the southern margin of the Western Australian Yilgarn Craton, reveal crystallisation ages between 1299 + 10 and 1250 + 23 Ma. A small number of older xenocrystic zircons, incorporated in some samples, indicate the presence of Late Paleoproterozoic crust in the region. Zircon that crystallised within a melt accumulated in the neck of a boudinaged mafic unit was dated at 1296 + 4 Ma, indicating that the emplacement of the igneous protoliths took place syntectonically. The anisotropy of magnetic susceptibility of the granulites indicates minimum axes with a mean inclination of 48 towards 1308, corresponding to a nearly vertical southwest–northeast (50–2308) magnetic foliation. This is very close to the structural trend of the Fraser Belt suggesting that the magnetic fabric was acquired syntectonically, during the collision between the Yilgarn and Gawler Cratons. The paleomagnetic data on the granulites overlap with published poles for various 1.2 Ga units in the Albany Belt and the 1.2 Ga Fraser dykes, possibly suggesting that the remanence was acquired during the second stage of the Fraser tectonism. A younger magnetisation component resembles a pole of uncertain age published for Bremer Bay in the Albany Belt.
A geochronological study using the step-wise dissolution technique and in situ ion microprobe analysis is applied to late Archean metamict zircons of granitoids from northeastern Madagascar basement. Both techniques provide concordant U–Pb ages. Additionally, the ion microprobe study provided in situ measurements on the Pb isotopic composition of the metamict domains. Finally, Pb–Pb evaporation ages were found to be slightly but systematically younger than the dating obtained by the two other methods. Based on these data, granites from the Antongil belt were emplaced during at least three episodes separated by periods of magmatic inactivity of about 10 Ma at 2513, 2523 and 2532 Ma, respectively. These results are consistent with previous whole-rock Rb–Sr and U–Pb zircon dating from this part of Madagascar. The occurrence of this late Archean basement in northeastern Madagascar can be related to southern India where large volumes of contemporaneous granitoids were emplaced.
A U–Pb baddeleyite age of 2367 ± 1 Ma from a diabase dyke together with previously published age data, suggests that a major early Proterozoic dyke swarm cuts across the structural grain of the Archean Dharwar craton in India. Paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 • with convergence to the west. It was originally emplaced at high latitudes, and together with the Widgiemooltha dykes of the Yilgarn block of Australia, may have been a segment of a larger radiating swarm related to a long-lived plume event that was active for about 50 My from 2418 to 2367 Ma. A regional change in the intensity of brown feldspar clouding in the dykes suggests that the Dharwar craton was tilted northwards, in harmony with previous observations on the structure and metamorphism of the Archean rocks. Towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker, more "sooty" appearance. The black clouding, whose precise origins remain unknown, is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at ∼800 Ma, and with a shear zone and change in structural trend related to Pan-African deformation at ∼550 Ma. Paleomagnetic studies suggest that (i) the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and (ii) as a more general observation, diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black.
Available lithostratigraphic, tectonothermal, geochronological and paleomagnetic data from 2.1–1.8 Ga collisional orogens and related cratonic blocks around the world have established connections between South America and West Africa; Western Australia and South Africa; Laurentia and Baltica; Siberia and Laurentia; Laurentia and Central Australia; East Antarctica and Laurentia, and North China and India. These links are interpreted to indicate the presence of a supercontinent existing before Rodinia, referred to herein as Columbia, a name recently proposed by Rogers and Santosh [Gondwana Res. 5 (2002) 5] for a Paleo-Mesoproterozoic supercontinent. In this supercontinent, the Archean to Paleoproterozoic cratonic blocks were welded by the global 2.1–1.8 Ga collisional belts. The cratonic blocks in South America and West Africa were welded by the 2.1–2.0 Ga Transamazonian and Eburnean Orogens; the Kaapvaal and Zimbabwe Cratons in southern Africa were collided along the ∼2.0 Ga Limpopo Belt; the cratonic blocks of Laurentia were sutured along the 1.9–1.8 Ga Trans-Hudson, Penokean, Taltson–Thelon, Wopmay, Ungava, Torngat and Nagssugtoqidian Orogens; the Kola, Karelia, Volgo–Uralia and Sarmatia (Ukrainian) Cratons in Baltica (Eastern Europe) were joined by the 1.9–1.8 Ga Kola–Karelia, Svecofennian, Volhyn–Central Russian and Pachelma Orogens; the Anabar and Aldan Cratons in Siberia were connected by the 1.9–1.8 Ga Akitkan and Central Aldan Orogens; the East Antarctica and an unknown continental block were joined by the Transantarctic Mountains Orogen; the South and North Indian Blocks were amalgamated along the Central Indian Tectonic Zone; and the Eastern and Western Blocks of the North China Craton were welded together by the ∼1.85 Ga Trans-North China Orogen.
The island of Madagascar straddles the poorly understood eastern margin of the East African Orogen, which is characterised by a high-grade gneissic basement that was structurally and thermally reworked during the Neoproterozoic collision of the Dharwar craton of India with the Congo/Tanzania/Bangweulu craton of Africa. An east-west traverse across the eastern margin of the Antananarivo block of central Madagascar and the overlying Beforona belt of the Tsaratanana sheet has identified four north-south trending structural domains, which represent the easternmost units of the East African Orogen. Dominant structures in these four domains are ascribed to two main deformation events, D2 and D3, whereas a poorly preserved early foliation and late folding event are classified as D1 and D4, respectively. D2 structures developed during east-west shortening comprise relatively low-strain, upright, north-south trending folds and steeply dipping, planar, north-south trending high-strain zones, both of which are associated with a stretching lineation that plunges gently north-south. D3 structures comprise a >20 km wide mylonitic high-strain zone and smaller scale discrete shear zones that dip gently west and exhibit top-to-the-east movement, again interpreted as a result of east-west shortening. The Antananarivo block and Beforona belt were juxtaposed sometime before D2 folding, which deforms the contact between the two units, but some time after an early granulite-facies metamorphic event that is preserved only in low strain zones of the Antananarivo block. D2 coaxial strain and D3 non-coaxial strain both occurred under amphibolite-facies metamorphic conditions and, although D3 shear zones locally cut D2 structures, these two events need not be temporally distinct. Published geochronological data from rocks exposed along this traverse constrain the likely timing of D2 and D3 to 630–515 Ma. The age, style and magnitude of strain along the eastern Antananarivo block indicates that eastern Madagascar was a focus for deformation during the final stages of collision between the Dharwar and Congo/Tanzania/Bangweulu cratons.
Madagascar lay in an interesting position in Gondwana, straddling one of the largest orogens that formed as the supercontinent amalgamated. The Malagasy basement preserves a record of the timing and style of this amalgamation, and in addition contains much information as to the palaeogeography of the eastern Mozambique Ocean.Madagascar consists of a number of tectonic units that amalgamated in the Ediacaran–Cambrian. The tectonic units are: The Antongil Block; the Antananarivo Block; the Tsaratanana Sheet and the Bemarivo Belt. In addition to these, there are a number of regions dominated by Neoproterozoic metasedimentary rocks, including the Molo, Betsimisaraka, Vohibory and Androyen regions. In this review I outline these units, discuss their amalgamation history and implications for Neoproterozoic–Cambrian palaeogeography, and highlight a few key questions for future study.
The broadly east-west trending, Late Neoproterozoic Bemarivo Belt in northern Madagascar has been re-surveyed at 1:100 000 scale as part of a large multi-disciplinary World Bank-sponsored project. The work included acquisition of 14 U–Pb zircon dates and whole-rock major and trace element geochemical data of representative rocks. The belt has previously been modelled as a juvenile Neoproterozoic arc and our findings broadly support that model. The integrated datasets indicate that the Bemarivo Belt is separated by a major ductile shear zone into northern and southern “terranes”, each with different lithostratigraphy and ages. However, both formed as Neoproterozoic arc/marginal basin assemblages that were translated southwards over the north-south trending domains of “cratonic” Madagascar, during the main collisional phase of the East African Orogeny at ca. 540 Ma. The older, southern terrane consists of a sequence of high-grade paragneisses (Sahantaha Group), which were derived from a Palaeoproterozoic source and formed a marginal sequence to the Archaean cratons to the south. These rocks are intruded by an extensive suite of arc-generated metamorphosed plutonic rocks, known as the Antsirabe Nord Suite. Four samples from this suite yielded U–Pb SHRIMP ages at ca. 750 Ma. The northern terrane consists of three groups of metamorphosed supracrustal rocks, including a possible Archaean sequence (Betsiaka Group: maximum depositional age approximately 2477 Ma) and two volcano-sedimentary sequences (high-grade Milanoa Group: maximum depositional age approximately 750 Ma; low grade Daraina Group: extrusive age = 720–740 Ma). These supracrustal rocks are intruded by another suite of arc-generated metamorphosed plutonic rocks, known as the Manambato Suite, 4 samples of which gave U–Pb SHRIMP ages between 705 and 718 Ma.
Well constrained SmNd arrays of whole rocks are reported for the first time from three important stratigraphic units of the Dharwar volcano-sedimentary sequence of south India. If interpreted as isochrons, these arrays will correspond to the following ages and initial Nd ratios relative to CHUR: 2.911±0.049 Ga and −0.20 ± 0.40 for the Kalasapura mafic basal metavolcanics of the Bababudan greenstone belt; 2.848±0.070 Ga and −0.88 ± 0.82 for the Santaveri Formation comprising mafic to felsic metavolcanics overlying the Kalasapura rocks; and 2.747±0.015 Ga and +1.72 ± 0.10 for the Ingaldhal mafic-felsic metavolcanic suite of the Chitradurga greenstone belt. The possibility that these SmNd arrays are mixing lines between unrelated components depleted and enriched, respectively, in light rare earth elements has been discussed, leading to the conclusion that the ages closely correspond to their time of eruption: the older suites from the Bababudan belt from a mantle source of CHUR-like composition, the younger suite from the Chitradurga belt from one with a prior history of LREE depletion.
Both geochemical and experimental petrological research indicate that Archaean continental crust was generated by partial melting of an Archaean tholeiite transformed into a garnet-bearing amphibolite or eclogite. The geodynamic context of tholeiite melting is the subject of controversy. It is assumed to be either (1) subduction (melting of a hot subducting slab), or (2) hot spot (melting of underplated basalts). These hypotheses are considered in the light of modern adakite genesis. Adakites are intermediate to felsic volcanic rocks, andesitic to rhyolitic in composition (basaltic members are lacking). They have trondhjemitic affinities (high-Na2O contents and K2O/Na2O∼0.5) and their Mg no. (0.5), Ni (20–40 ppm) and Cr (30–50 ppm) contents are higher than in typical calc-alkaline magmas. Sr contents are high (>300 ppm, until 2000 ppm) and REE show strongly fractionated patterns with very low heavy REE (HREE) contents (Yb≤1.8 ppm, Y≤18 ppm). Consequently, high Sr/Y and La/Yb ratios are typical and discriminating features of adakitic magmas, indicative of melting of a mafic source where garnet and/or hornblende are residual phases. Adakitic magmas are only found in subduction zone environments, exclusively where the subduction and/or the subducted slab are young (<20 Ma). This situation is well-exemplified in Southern Chile where the Chile ridge is subducted and where the adakitic character of the lavas correlates well with the young age of the subducting oceanic lithosphere. In typical subduction zones, the subducted lithosphere is older than 20 Ma, it is cool and the geothermal gradient along the Benioff plane is low such that the oceanic crust dehydrates before it reaches the solidus temperature of hydrated tholeiite. Consequently, the basaltic slab cannot melt. The released large ion lithophile element (LILE)-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting. Afterwards, the residue is made up of olivine+clinopyroxene+orthopyroxene, such that the partial melts are HREE-rich (low La/Yb and Sr/Y). Contrarily, when a young (<20 Ma) and hot oceanic lithosphere is subducted, the geothermal gradient along the Benioff plane is high, so the temperature of hydrated tholeiite solidus is reached before dehydration occurs. Under these conditions, garnet and/or hornblende are the main residual phases giving rise to HREE-depleted magmas (high La/Yb). The lack of residual plagioclase accounts for the Sr enrichment (high Sr/Y) of the magma. Experimental petrologic data show that the liquids produced by melting of tholeiite in subduction-like P–T conditions are adakitic in composition. However, natural adakites systematically have higher Mg no., Ni and Cr contents, which are interpreted as reflecting interactions between the ascending adakitic magma generated in the subducted slab and the overlying mantle wedge. This interpretation has been recently corroborated by studies on ultramafic enclaves in Batan lavas where olivine crystals contain glass inclusions with adakitic compositions [Schiano, P., Clochiatti, R., Shimizu, N., Maury, R., Jochum, K.P., Hofmann, A.W., 1995. Hydrous, silica-rich melts in the sub-arc mantle and their relationships with erupted arc lavas. Nature 377 595–600.]. This is interpreted as demonstrating that adakitic magmas passed through the mantle wedge and interacted with it. Sajona [Sajona, F.G., 1995. Fusion de la croûte océanique en contexte de subduction collision: géochimie, géochronologie et pétrologie du magmatisme plioquaternaire de Mindanao (Philippines). Unpublished thesis, Brest University, France, 223 pp.] also considers that the high-Nb basalts, which are associated with adakites, reflect mantle–adakite interactions. Recent structural studies have demonstrated that plate tectonics operated during the first half of Earth history. The very strong similarities that exist between modern adakites and Archaean tonalite, trondhjemite and granodiorite (TTG) attest that both have the same source and petrogenesis. Consequently, when Archaean-like P–T conditions are exceptionally realised in modern subduction zones, Archaean-like magmas are generated. Contrarily, hot spots never produce TTG-like magmas, thus, strongly supporting the hypothesis of the generation of the Archaean continental crust within a subduction environment. However, Archaean TTG are poorer in Mg, Ni and Cr than adakites, indicating that mantle–magma interactions were less efficient, probably due to the shallower depth of slab melting. In this case, the slab-derived magmas rise through a thinner mantle wedge, thus, reducing the efficiency of the interactions. This is corroborated by the absence of a positive Sr anomaly in TTG, which indicates that plagioclase could have been a residual phase during their genesis.
Late tectonic, post-collisional granite suites are a feature of many parts of the Late Neoproterozoic to Cambrian East African Orogen (EAO), where they are generally attributed to late extensional collapse of the orogen, accompanied by high heat flow and asthenospheric uprise. The Maevarano Suite comprises voluminous plutons which were emplaced in some of the tectonostratigraphic terranes of northern Madagascar, in the central part of the EAO, following collision and assembly during a major orogeny at ca. 550 Ma. The suite comprises three main magmatic phases: a minor early phase of foliated gabbros, quartz diorites, and granodiorites; a main phase of large batholiths of porphyritic granitoids and charnockites; and a late phase of small-scale plutons and sheets of monzonite, syenite, leucogranite and microgranite. The main phase intrusions tend to be massive, but with variably foliated margins. New U–Pb SHRIMP zircon data show that the whole suite was emplaced between ca. 537 and 522 Ma. Geochemically, all the rocks of the suite are enriched in the LILE, especially K, and the LREE, but are relatively depleted in Nb, Ta and the HREE. These characteristics are typical of post-collisional granitoids in the EAO and many other orogenic belts. It is proposed that the Maevarano Suite magmas were derived by melting of sub-continental lithospheric mantle that had been enriched in the LILE during earlier subduction events. The melting occurred during lithospheric delamination, which was associated with extensional collapse of the East African Orogen.
The Closepet granite, in the Dharwar craton of south India, is a large, late Archaean magmatic body. Its composition can be explained as a result of interactions between mantle-derived melts and pre-existing continental crust (TTG gneisses). Using geochemical modelling based upon major and trace element compositions the following petrogenetic model is proposed for the formation of the batholith: (i) an enriched mantle (garnet- and amphibole-bearing lherzolite) melts to produce a basaltic liquid. (ii) The basaltic liquid undergoes limited fractional crystallization of biotite and amphibole (F>0.9). (iii) The differentiated liquid rises into the old continental crust, and induces water-saturated anatexis of the TTG gneisses (K-Feldspar+Qtz+Plagioclase→melt). (iv) Both mantle-derived and anatectic liquid mix and give rise to magma compositions ranging from quartz-monzonite to granite. The origin of the mantle enrichment is discussed. In the context of the regional geological setting the most likely possibility seems to be metasomatism by slab melts the metasomatism of a mantle wedge by slab melts. This suggests a two-stage evolution for the Dharwar craton during the late Archaean: (i) a subduction related event with the formation of TTGs, sanukitoids, and with associated mantle metasomatism; and (ii) re-melting of the metasomatized mantle. This evolutionary history implies that at least some of the K-rich, late-Archaean granites are juvenile, rather than products of intracrustal reworking, as frequently assumed.
Examination of an extensive adakite geochemical database identifies two distinct compositional groups. One consists of high-SiO2 adakites (HSA) which is considered to represent subducted basaltic slab-melts that have reacted with peridotite during ascent through mantle wedge. The second group consists of low-SiO2 adakites (LSA) which we interpret to have formed by melting of a peridotitic mantle wedge whose composition has been modified by reaction with felsic slab-melts.The chemical composition of less differentiated (primitive) Archaean tonalite–trondhjemite–granodiorite (TTG) magmas evolved from 4.0 to 2.5 Ga. Mg# (molecular Mg/(Mg+Fe2+), Ni, and Cr contents increased over this period of time and we interpret these changes in terms of changes in the degree to which the TTG magmas interacted with mantle peridotite. Over the same period, concentrations of (CaO+Na2O) and Sr also increased, as the amount of plagioclase, residual from basalt melting, decreased in response to increased pressures at the site of slab-melting. In the Early Archaean, it appears that these interactions were very rare or absent thus leading to the conclusion that subduction was typically flat and lacked the development of a mantle wedge. In contrast, the relatively lower heat production by ∼2.5 Ga meant that slab-melting occurred at greater depth, where plagioclase was no longer stable, and where the development of a thick mantle wedge ensured interaction between the slab-melts and mantle peridotite.Close compositional similarities between HSA and Late Archaean TTG (T<∼3.0 Ga) strongly suggest a petrogenetic analogy. However, an analogy between the older Archaean TTG and HSA is not complete because evidence for mantle wedge interaction is missing in most Early Archaean TTGs.Late Archaean sanukitoids and the compositionally similar Closepet-type granites have compositions significantly different from TTG of all ages. However, they show some affinity with LSA which could be considered as their possible analogue. These magmas are all thought to result from melting of a mantle peridotite whose composition has been modified by reaction with slab-melts.We propose that all these magmas are directly linked to slab-melting. Archaean TTG and HSA represent slab-melts that have interacted with peridotite to varying extent, whereas sanukitoids, Closepet-type granites, and LSA correspond to melts of peridotite previously metasomatised by slab-melt. The changes observed from Early Archaean TTG to Late Archaean TTG and to sanukitoids reflect change in both the nature and efficiency of interaction between slab-melt and mantle wedge peridotite. Comparisons between all of these rocks suggest that ancient styles of subduction that have operated since at least ∼3.3 Ga persist in a limited way today. The secular changes in the degree and style of these interactions is a direct consequence of the cooling of Earth that modified the thermal and dynamic parameters at the subducted slab–mantle wedge interface.
Extensive field work in the Eastern Dharwar Craton, associated with petrographic and geochemical (major and trace elements) investigations, allows four main types of Late Archaean granitoids to be distinguished. (1) Na-rich granitoids of trondhjemitic, tonalitic and granodioritic composition (TTG) that are characterised by strongly fractionated REE patterns and low HREE contents and generally interpreted as "slab melts" generated by partial melting of metamorphosed hydrated basalt, most likely in a subduction environment. (2) Sanukitoids, which are K- and Mg-rich monzonites and granodiorites with TTG-like REE patterns associated with marked LILE-enrichment, and considered to result from the reaction of slab melts generated in a subduction environment with, and assimilation of, mantle wedge peridotite. (3) Uncommon high-HFSE, Mg and K granites with strongly REE and LILE-enrichment that, probably formed by partial melting of an enriched mantle source; unlike in the genesis of sanukitoids, in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals, resulting in mantle-enrichment. Subsequent melting of this enriched mantle (probably in a post-subduction setting) gives rise to high-HFSE, Mg and K magmas. As demonstrated for the Closepet Granite (Dharwar Craton), the hot mantle-derived magma can induce melting of continental crust and then mix with the anatectic products. (4) K-rich, Mg-poor anatectic biotite-granites with REE Patterns that are less fractionated and show negative Eu anomalies. These granites result from the remelting of old basement or recently accreted plutons, both with TTG compositions. Such anatexis can occur either in a subduction or in a post-subduction environment.
The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen.U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between ∼800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200–3000, ∼2650, ∼2500 and 800–700 Ma.The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.
Understanding of the style and timing of collision between East and West Gondwanaland to form the East African Orogen is incomplete, but the general outline of this event is slowly emerging. A model embodying the present understanding of this evolution is presented. Present reconstructions of the earliest stages in the EAO orogenic cycle may be more speculation than understanding, but initiation by rifting of a continent is consistent with the data. This rifting is identified as part of the break up of Rodinia at about 850-900 Ma. The sequence of events that begins with sea-floor spreading and formation of arcs and back-arc basins and continues with the accretion of these tectonic cells into juvenile crust is coming into sharper focus, at least for the Arabian-Nubian Shield. There is evidence that these processes were underway as early as 870 Ca and continued until at least 690 Ma. Continental collision first led to crustal thickening and uplift, beginning perhaps as early as 750 Ma but certainly by 700 Ma, and continued with orogenic collapse and escape tectonics until the end of the Precambrian. At least as far as the EAO is concerned, this dates the consolidation of East and West Gondwanaland. Tectonic escape led to the development of major rift basins in the northern EAO and environs which led directly to sea-floor spreading and formation of a passive margin on the remnants of Gondwanaland and the formation of an ocean basin to the north, at about 550 Ma ago. -from Author
Parameters on which models for terrestrial lead isotope evolution are based have recently been revised. These parameters are the isotopic composition of troilite lead, the age of the meteorite system and the decay constants of uranium and thorium. As a result, the normal single-stage model in which the age of the earth is taken to be that of the meteorite system is now untenable.A two-stage model has been constructed which permits the age of the earth to be that of the meteorite system and which also yields good model ages for samples of all ages. The new model postulates that lead developed initially from a primordial composition assumed to be that of troilite lead beginning at 4.57 b.y. ago. The average values of 238U/204Pb and 232Th/204Pb for this first stage were 7.19 and 32.21 respectively. At approximately 3.7 b.y. ago differentiation processes brought about the conditions of a second stage, in which 238U/204Pb ≈ 9.74 and 232Th/204Pb ≈ 37.19 in those portions of the earth which took part in mixing events, giving rise to average lead.
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- H Martin
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A chemical classifica-tionofvolcanicrocksbasedonthetotalalkali-silicadiagram
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12 List of mineral abbreviations Recommendations of the IUGS Subcommission on the Systematics of Metamorphic Rocks Approximation of terrestrial lead isotopic evolution by a two-stage model
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