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U–Pb geochronology of Seychelles granitoids: a Neoproterozoic continental arc fragment
Abstract
New high-precision U–Pb zircon ages for 14 granitoid rocks of the Seychelles, including samples from Mahé, Praslin, La Digue, Ste. Anne, Marianne, Fregate and Recifs Islands, yield dates between 748.4±1.2 Ma and 808.8±1.9 Ma, interpreted as representing magmatic crystallization ages. U–Pb zircon ages as young as 703 Ma have been reported by other workers, and the time span of Seychelles magmatic activity, therefore, is ∼100 Myr. The vast majority of ages, however, fall in the period 748–755 Ma, suggesting a major period of granite plutonism at this time. At least some Seychelles dolerite dikes have equivalent ages, indicating the contemporaneity of granitic and basaltic magmas, and supporting field and chemical evidence for the production of minor quartz dioritic rocks by hybridization and magma mingling. Possible correlatives of the late Neoproterozoic (∼700–800 Ma) granitoids and dolerites of the Seychelles include volcanic and plutonic rocks in Madagascar and northwestern India (Malani Igneous Suite, Rajasthan), which may have formed in a continuous continental (Andean-type) arc located at the western margin of the Rodinia supercontinent. This idea is more consistent with the time span of magmatism, petrologic character of the igneous rocks, and paleomagnetically determined reconstructions, than the commonly held view of an intra-plate extensional setting for the Seychelles and Malani Igneous Suite.
... The coeval magmatic activities in the Seychelles (Tucker et al., 2001;Torsvik et al., 2001) and northern Madagascar have been ascribed to be part of Malani magmatism (Handke et al., 1999;Kröner et al., 1999;. The Seychelles and NW Indian block were located at 30º and 32º north, respectively, jointly forming the western margin of Rodinia Supercontinent (Torsvik et. ...
... The coeval magmatic activities in the Seychelles (Tucker et al., 2001;Torsvik et al., 2001) and northern Madagascar have been ascribed to be part of Malani magmatism (Handke et al., 1999;Kröner et al., 1999;. The Seychelles and NW Indian block were located at 30º and 32º north, respectively, jointly forming the western margin of Rodinia Supercontinent (Torsvik et. ...
... coeval magmatic activities in the Seychelles (Tucker et al., 2001;Torsvik et al., 2001) and northern Madagascar have been ascribed to be part of Malani magmatism (Handke et al., 1999;Kröner et al., 1999;. The Seychelles and NW Indian block were located at 30º and 32º north, respectively, jointly forming the western margin of Rodinia Supercontinent (Torsvik et. al, 2001;Li et al., 2008). Kröner et al., (2000) argued for an extensional environment for the magmatism. Devey et al. (1992) and Stephens et al. (1997) The Malani magmatism of northwestern Indian shield has its bearing on the breakup events of Rodinia. The splitting of Rodonia Supercontinent led to the development of new ocean oors and cratonic ...
Abstract The Neoproterozoic Era witnessed several dynamic events in Earth’s history, the most prominent being amalgamation and subsequent dispersal of the Supercontinent Rodinia. Most of the paleogeographic reconstructions of Rodinia supercontinent assign a key role to the Northwestern Indian block in its accretion and break-up history, and subsequent dispersal. Excellent exposures of well-preserved rocks, suitable for such studies, render this terrain a suitable candidate for undertaking such studies. The present eld trip in NW Indian plate covers the Neoproterozoic magmatic events in this region, for which robust geochronological data are now available, that in concert with the paleomagnetic results, have helped in rening paleoposition of this region in the Rodinia framework. Neoproterozoic magmatism in southwestern Rajasthan is the focus of the eld trip wherein rock exposures allow geochemical and geochronological constraints and relevant data on interpreting tectonic setting and geodynamic process(s) operative at the time of intrusion/effusion, as well as information on the deeper crust/upper mantle. The southern sector of the Delhi Fold Belt (DFB) records a Grenvillian age (~1 Ga) deformation event that corresponds with the timing of amalgamation of the Rodinia supercontinent. The 1024 Ma Ranakpur diorites from the Phulad Ophiolite Suite (Volpe and Macdougall 1990) and the 987 and 968 Ma rhyolites from this sector (Deb et al 2001; 968.3±1 Ma) are coeval with the arc magmatism in this terrain. The 968 ±1 Ma calc-alkaline Sendra granitoids (Pandit et al., 2003), 990 ± 6 and 987 ± 6.4 Ma rhyolites (Deb et al., 2001) also fall within this time bracket. The Sendra Granites invariably show a metamorphic fabric and also dene intrusive relationship with the Delhi metasediments that has been interpreted by Pandit et al. (2003) as syn-tectonic emplacement and effects of later deformation event. Younger and more widespread granitic intrusions in the southern domain of the Delhi Fold Belt and western foreland region have been collectively described as Erinpura Granite and regarded late- to post- kinematic with respect to the Delhi Orogeny (Heron, 1953). In contradiction to the earlier notion of a single event at ~830 Ma (whole rock Rb – Sr ages; Choudhary et al., 1984) several recent and more reliable geochronologic studies (zircon U – Pb; monazite U-Th-Pb EPMA) have identied a much wider time span (870 to 800 Ma) and episodic nature of Erinpura Granite intrusions (Deb et al., 2001; van Lente et al., 2009; Singh et al., 2010; Pradhan et al., 2010; Just et al., 2011, Zhao et al., 2018). Erinpura granites have been noted both in the Delhi Fold Belt and in the Marwar Block, indicating amalgamation of Marwar Block and Aravalli – Bundelkhand Craton prior to this intrusive event. Occupying a geographic domain between NE trending corridor between Delhi Fold Belt and Malani Igneous Suite exposures (undeformed MIS - sensu Bhushan, 1984) the Erinpura granites show anatectic features and localized shear zones. This tectonically active corridor (the Abu-Sirohi terrain) is characterized by late-Neoproterozoic synkinematic granite intrusions, shearing and anatexis (Just et al. 2011; de Wall et al. 2012; Ashwal et al., 2013). De Wall et al. (2014) have identied the deformed eastern sector as imbricate basement/cover sequence, thrust over a pre-Malani basement of unknown age. The age of the reverse thrusting (top-to-thewest) is constrained by formation of new monazites as well as recrystallization of the older ones (775 ± 26 Ma, Just et al., 2011).
... These granitoids were previously considered as the plutonic remnants of an Andean-type magmatic arc resulting from subduction of the Mozambique oceanic plate beneath western Rodinia [3-5], for which the key evidence is the purported long lifespan of~100 m.y. (810-700 Ma) dated by the ID-TIMS technique [4]. As a consequence, commonly ''rift-related" low-d 18 O signatures of the Seychelles granitoids were interpreted to reflect inheritance from a magma source, based on a dataset obtained from wholerock samples and quartz and feldspar grains [6]. ...
... The Seychelles, which occupied a western marginal position in Rodinia (Fig. 1a), consists dominantly of Neoproterozoic granitoids that are some of the first identified low-d 18 O silicic rocks. These granitoids were previously considered as the plutonic remnants of an Andean-type magmatic arc resulting from subduction of the Mozambique oceanic plate beneath western Rodinia [3][4][5], for which the key evidence is the purported long lifespan of~100 m.y. (810-700 Ma) dated by the ID-TIMS technique [4]. ...
... These granitoids were previously considered as the plutonic remnants of an Andean-type magmatic arc resulting from subduction of the Mozambique oceanic plate beneath western Rodinia [3][4][5], for which the key evidence is the purported long lifespan of~100 m.y. (810-700 Ma) dated by the ID-TIMS technique [4]. As a consequence, commonly ''rift-related" low-d 18 O signatures of the Seychelles granitoids were interpreted to reflect inheritance from a magma source, based on a dataset obtained from wholerock samples and quartz and feldspar grains [6]. ...
... 2700 Ma inherited component. Tucker et al. (2001) and Torsvik et al. (2001a) reported high-precision zircon TIMS 206 Pb/ 238 U ages of the granites and a dolerite dyke from the Main Islands with nearly all ages from 748.4 ± 1.2 Ma to 764.2 ± 1.4 Ma. The single exception is Île aux Récifs that yielded an age of 808.8 ± 1.9 Ma. ...
... The single exception is Île aux Récifs that yielded an age of 808.8 ± 1.9 Ma. The weighted-mean zircon U-Pb ages presented in this study are within error of the high precision dates reported by Tucker et al. (2001) for the Mahé and Praslin granites. However, it is the inherited zircon ages that offer insight into the regional temporal correlation with rocks from NW India and Madagascar during the Neoproterozoic. ...
... In other words, thẽ 25 Ma to~60 Ma age gap between the emplacement ages and the inherited zircons indicates there were at least two tectonomagmatic events preserved in the Main Islands during the middle to late Tonian. The~810 Ma age of the inherited zircons overlaps with the emplacement age of 808.8 ± 1.9 Ma reported for Île aux Récifs (Tucker et al., 2001) and a single inherited zircon of 807 ± 40 Ma (2σ) from the Early Paleogene syenite from Silhouette (Tucker et al., 2001;Shellnutt et al., 2015). Thus, based on our findings, it is possible that the reported age of Île aux Récifs is either an emplacement age and represents an earlier period of magmatism within the Praslin granites or the age is indicative of inheritance. ...
The Seychelles microcontinent is located in the western Indian Ocean and is exposed as a series of granitic islands. The Main Islands of the Seychelles consist of the Mahé and Praslin granite groups. They are characterized as ferroan, metaluminous to peraluminous, alkali-calcic to calc-alkalic, and resemble post-collisional A-type granite but have distinct Sr-Nd-Pb-O isotopic compositions. Previous studies showed that the granites and syn-plutonic dolerites were emplacement at 760-750 Ma, although rocks from Île aux Récifs yielded an older age (808.8 ± 1.9 Ma). New in situ zircon U-Pb geochronology of granites from northern Mahé (751.9 ± 4.6 Ma, 755.5 ± 5.8 Ma, 756.2 ± 7.7 Ma) and western Praslin (757 ± 11 Ma, 753.0 ± 7.5 Ma, 751.3 ± 5.8 Ma) yielded weighted-mean ²⁰⁶Pb/²³⁸U ages similar to previous studies but a significant amount of inherited zircon was identified with ages ranging from ∼880 Ma to ∼770 Ma. The weighted-mean ²⁰⁶Pb/²³⁸U ages of the principal inherited zircon populations from the Mahé (777.5 ± 5.7 Ma, 807.7 ± 7.7 Ma) granites are within uncertainty of those identified from the Praslin (777.0 ± 6.1 Ma, 814 ± 11 Ma) granites indicating there was an older period of magmatism within the Seychelles microcontinent. The Hf isotopic data of the magmatic and inherited zircons from the Mahé granite are generally more radiogenic (εHf(t) = +6.7 to +12.0) and have younger model ages (TDM2 = 902 Ma to 1219 Ma) than those from the Praslin granite (εHf(t) = +2.2 to +10.9, TDM2 = 1039 to 1524 Ma). The differences between the zircon Hf isotopes from the Mahé and Praslin granites is consistent with other isotopic systems and indicates that the isotopic dichotomy is an intrinsic feature of the Seychelles microcontinent that extends back to at least ∼840 Ma and possibly to the Mesoproterozoic (1000 Ma to 1400 Ma). It is likely that the Mahé and Praslin granites were emplaced during a period of post-collisional extension/crustal relaxation and that the older inherited zircons are derived from rocks that record an earlier period of magmatism that was likely subduction-related.
... These arcs were accreted to the margin of a continent on the eastern side of the Mozambique Ocean (Whitehouse et al., 2016;Blades et al., 2020), proposed to be the GIS (Alessio et al., 2017). They were located alongside other areas of eastern Gondwana, such as NW India/Pakistan, the Seychelles, and northern Madagascar, all of which comprise basement terranes of the same age and nature (e.g., Tucker et al., 2001;Torsvik et al., 2001;Ashwal et al., 2002;Khan et al., 2012;Meert et al., 2013;Tucker et al., 2014). ...
... This peak points to provenance from the Tonian igneous and metamorphic basement of Oman (Mercolli et al., 2006;Rantakokko et al., 2014;Whitehouse et al., 2016;Alessio et al., 2017;Blades et al., 2020). Basement of similar age and tectonic setting is also found in NW India/Pakistan (Khan et al., 2012;Meert et al., 2013;De Wall et al., 2014), Socotra (Denèle et al., 2012), the Seychelles Tucker et al., 2001;Ashwal et al., 2002), and Madagascar (Tucker et al., 2014). Older Proterozoic and Archean zircons at ca. 1800 Ma and ca. ...
Comprehensive U-Pb detrital zircon geochronology of Ediacaran sandstones from Oman indicates that they originated from Neoproterozoic basement with a peak magmatic age of ca. 850−780 Ma (Tonian), with lesser Paleoproterozoic and Ediacaran sources and renewed magmatic input starting at ca. 550 Ma. Comparison with detrital zircons from the Arabian-Nubian and NW Indian Shields supports an eastern Gondwana affinity for the Ediacaran succession of Oman. Tonian basement sediment sources are present in Oman. Sources for older Paleoproterozoic zircons (ca. 2500 and 1800 Ma) are not found in Oman but are known from the cratonic Indian Shield. The signal of the main magmatic events of the juvenile Arabian-Nubian Shield, peaking at ca. 640−620 Ma, is rare or absent in the Ediacaran rock succession of Oman. However, deformed Ediacaran clastic units with an Arabian-Nubian Shield affinity occur in western Oman. Influx of latest Ediacaran−early Cambrian zircons (550−525 Ma) is interpreted as due to final Cambrian Angudan/Malagasy orogeny-related magmatism.
These results, together with new ages for volcano-sedimentary terranes in the subsurface of SW Oman and seismic observations, support the interpretation that the Ediacaran succession of Oman was deposited along the western, passive margin of the Greater Indian Shield, on the eastern (east and west in this paper refer to present-day coordinates) side of the Mozambique Ocean. Oman then collided with the Arabian-Nubian Shield in the early Cambrian, when the Angudan/Malagasy orogeny led to final consolidation of Gondwana. Our data support the inference that the youngest and final suture between the Indian and Arabian Shields lies in the subsurface of Oman along the early Cambrian western deformation front.
... 75% всього видового складу фауни Мадейри становлять комахи, причому 20% з них -ендеміки, наприклад зникаючий вид -мадерійська капустянка і тарантул, розміром з чоловічу долоню, що мешкає в пустельних районах островів Ілляш-Дезерташ 160 . ...
... Граніти, знайдені на Сейшельських островах, такі самі як і на Мадагаскарі та в північнозахідній Індії. Встановлено, що всі вони утворилися близько 750 млн років тому (Tucker et al. 2001). Проте граніти Мае дещо старші, ніж граніти Праслена. ...
Навчальний посібник розроблено для фізико-географічних курсів, які читаються для спеціальностей 106 «Географія», 014.07 «Середня освіта (Географія)», 103 «Науки про Землю» – «Загальне землезнавство», «Фізична географія материків й океанів», «Геологія загальна та історична», «Геоморфологія», «Метеорологія та кліматологія», «Регіональна кліматологія», «Географія ґрунтів з основами ґрунтознавства» та «Біогеографія».
Фізико-географічна номенклатура виступає каркасом для розуміння регіонального різноманіття та природних закономірностей територіальної організації материків й океанів. Саме тому вивчення номенклатури крізь призму історичних і природних особливостей сприяє географічному мисленню здобувачів і побудові ланцюжка причинно-наслідкових зв’язків природних компонентів і процесів. Окрім того, проаналізовані природні компоненти описаних об’єктів можуть застосовуватися як репрезентативні точки для аналізу геологічних, кліматологічних, ґрунтознавчих і біогеографічних регіональних відмінностей.
Відповідно у виданні висвітлено історію та природу (тектоніку, геологію, клімат, органічний світ) заток: Сідри, Туніської, Хаммамет, Габес, Гвінейської, Біафри, Бенін, Суецької, Акабської й Аденської; Червоного моря; проток: Баб-ель-Мандебської та Мозамбіцької; Сомалійського півострова; островів: Сокотра, Мадейра, Канарських, Зеленого Мису, Біжагош, Біоко (Фернандо-По), Прінсіпі, Сан-Томе, Аннабон, Мадагаскар, Коморських, Сейшельських, Мафії, Занзібару, Пемби, Дахлак, Фарасан і Ханіш.
У книзі не розглянуті острови Маврикій, Реюньйон, Крозе, Принц Едуард, Буве, Трістан-да-Кунья, Святої Єлени, Вознесіння й Азорські, яким буде присвячене окреме видання, оскільки всі вони сформовані в межах серединно-океанічних хребтів Індіського й Атлантичного океанів.
... Just et al. (2011) (Li et al. 2003(Li et al. , 2008. Similar aged magmatic activities are reported in Seychelles (Tucker et al. 2001;Torsvik et al. 2001) and in Madagascar (Handke et al. 1999;Kr€ oner et al. 1999Kr€ oner et al. , 2000Tucker et al. 1999Tucker et al. , 2001. ...
... Just et al. (2011) (Li et al. 2003(Li et al. , 2008. Similar aged magmatic activities are reported in Seychelles (Tucker et al. 2001;Torsvik et al. 2001) and in Madagascar (Handke et al. 1999;Kr€ oner et al. 1999Kr€ oner et al. , 2000Tucker et al. 1999Tucker et al. , 2001. ...
The study incorporates geochemistry and geochronology of a gabbro, located near Daba, from the Neoproterozoic Sirohi Group along the northwestern Canks of the Indian Shield. The Daba gabbro is massive, cumulate, undeformed and forms a linear body bounded by shear zones at margins. The Neoproterozoic Sirohi Group marks the end of the compressional tectonic regime and the emplacement of gabbro marks amalgamation processes of the Rodinia Supercontinent in the northwestern Indian Shield. The gabbro is characterized by enriched TiO2,P2O5, LILE, LREE, and HREE depletion; and high La/ SmN, Gd/YbN, Zr/Nb ratios with no positive Eu anomalies and no Nb, Ta and Ti negative anomalies suggesting an Oceanic Floor Basalt (OFB) aDnity. The Ti–V plot of the Daba gabbro aDrms OFB aDnity, whereas this parameter for proximal maBcs from the Delhi Supergroup shows Oceanic Island Basalts (OIB) aDnity. Primitive mantle normalized multi-element pattern and chondrite-normalized pattern for trace element and REE show OFB aDnity. Depletion of Nb–Ta, P and Ti with a Cat distribution of LILE and HFSE reCects limited degrees of crustal contamination (exception Zr positive anomaly). SHRIMP geochronology of magmatic zircons from the two gabbro samples has a well-deBned 206Pb/238U Concordia at 825 ± 4 and 828 ± 4 Ma. This data corresponds to the gabbroic emplacement was almost synkinematic with felsic magmatism, coeval with the Sirohi orogeny (*835 Ma), denoting the end of compression tectonic regime. This marks the closure of the Rodinia amalgamation process in the northwestern Indian shield.
... Just et al. (2011) (Li et al. 2003(Li et al. , 2008. Similar aged magmatic activities are reported in Seychelles (Tucker et al. 2001;Torsvik et al. 2001) and in Madagascar (Handke et al. 1999;Kr€ oner et al. 1999Kr€ oner et al. , 2000Tucker et al. 1999Tucker et al. , 2001. ...
... Just et al. (2011) (Li et al. 2003(Li et al. , 2008. Similar aged magmatic activities are reported in Seychelles (Tucker et al. 2001;Torsvik et al. 2001) and in Madagascar (Handke et al. 1999;Kr€ oner et al. 1999Kr€ oner et al. , 2000Tucker et al. 1999Tucker et al. , 2001. ...
The study incorporates geochemistry and geochronology of a gabbro, located near Daba, from the Neoproterozoic Sirohi Group along the northwestern flanks of the Indian Shield. The Daba gabbro is massive, cumulate, undeformed and forms a linear body bounded by shear zones at margins. The Neoproterozoic Sirohi Group marks the end of the compressional tectonic regime and the emplacement of gabbro marks amalgamation processes of the Rodinia Supercontinent in the northwestern Indian Shield. The gabbro is characterized by enriched TiO2, P2O5, LILE, LREE, and HREE depletion; and high La/SmN, Gd/YbN, Zr/Nb ratios with no positive Eu anomalies and no Nb, Ta and Ti negative anomalies suggesting an Oceanic Floor Basalt (OFB) affinity. The Ti–V plot of the Daba gabbro affirms OFB affinity, whereas this parameter for proximal mafics from the Delhi Supergroup shows Oceanic Island Basalts (OIB) affinity. Primitive mantle normalized multi-element pattern and chondrite-normalized pattern for trace element and REE show OFB affinity. Depletion of Nb–Ta, P and Ti with a flat distribution of LILE and HFSE reflects limited degrees of crustal contamination (exception Zr positive anomaly). SHRIMP geochronology of magmatic zircons from the two gabbro samples has a well-defined 206Pb/238U Concordia at 825 ± 4 and 828 ± 4 Ma. This data corresponds to the gabbroic emplacement was almost synkinematic with felsic magmatism, coeval with the Sirohi orogeny (~835 Ma), denoting the end of compression tectonic regime. This marks the closure of the Rodinia amalgamation process in the northwestern Indian shield.
... Voluminous 870-750 Ma arc-related rocks are well developed at the northern and western margins of the Yangtze craton Dong et al., 2012;Luo et al., 2018;Zhao et al., 2018b;Ao et al., 2019). Similar Neoproterozoic arc magmatism was also identified in Madagascar, the Seychelles islands and the Malani Igneous Suite in India (Tucker et al., 2001;Torsvik et al., 2001;Ashwal et al., 2002;Zhou et al., 2006). These rock assemblages are thought to represent an extensive arc system formed at eastward-dipping subduction zone on the western to northern margins of Rodinia (Tucker et al., 2001;Torsvik et al., 2001;Ashwal et al., 2002;Zhou et al., 2006;Zhao et al., 2017Zhao et al., , 2019. ...
... Similar Neoproterozoic arc magmatism was also identified in Madagascar, the Seychelles islands and the Malani Igneous Suite in India (Tucker et al., 2001;Torsvik et al., 2001;Ashwal et al., 2002;Zhou et al., 2006). These rock assemblages are thought to represent an extensive arc system formed at eastward-dipping subduction zone on the western to northern margins of Rodinia (Tucker et al., 2001;Torsvik et al., 2001;Ashwal et al., 2002;Zhou et al., 2006;Zhao et al., 2017Zhao et al., , 2019. As discussed above, the Nesoproterozoic magmatism is also widespread in the Qinling block. ...
The Meso-Neoproterozoic Rodinia supercontinent formed a coherent large landmass, which was later dispersed over all current major continents and a number of microcontinents. The Qinling block is a Precambrian continental mass, located in the Qinling orogenic belt, which marks the junction of the North China and South China cratons. In this paper, we present a systematic study of the petrology, whole-rock geochemistry and geochronology of metasedimentary rocks from the Xiahe Group, which is a key unit of the Precambrian basement in the Qinling block, to constrain the paleogeographic reconstruction of Rodinia. The studied metasedimentary rocks have detrital zircon ages of 3054–1082 Ma that peak at 1172 Ma and 1582 Ma. The protoliths of the metasedimentary rocks are mainly shales and wackes with maximum depositional ages from 1147 to 1082 Ma, representing a part of a continental arc-related sedimentary sequence. Combining our results with existing data, we propose that their protoliths constituted a sedimentary series including an older sequence of basement and a younger sequence with depositional ages between 1262 and 840 Ma. The detritus that is older than 1266 Ma was sourced from the continental margin of the Indian craton, which was part of the Columbia supercontinent. There was a Paleo-Mesoproterozoic continental nucleus in the Qinling block that split from the margin of the Indian craton during the breakup of Columbia. The source rocks for the younger detritus were arc magmatic rocks in the Qinling block, which formed in a continental arc by oceanic lithosphere subduction during the assembly of Rodinia. A sequence of oceanic subduction beneath the continental nucleus of the Qinling block (1262–981 Ma), arc-continent collision between the continental nucleus and the Indian craton (981–911 Ma), continuous subduction of oceanic crust beneath the block with formation of a mature volcanic arc (929–833 Ma), and continental rifting (833–774 Ma) during the formation of the Qinling block was identified. The block faced the Neoproterozoic ocean during the assembly of Rodinia, and finally rifted off the supercontinent during its breakup to form an isolated arc terrane
... The global Neoproterozoic magmatism is commonly related to the assembly, configuration, and breakup of the Rodinia supercontinent and hasevoked geoscientific attention regardingits petrogenesis and tectonic settings (Heaman et al., 1992;Li et al., 1999Li et al., , 2002Li et al., , 2006Li et al., , 2008Li et al., , 2013Wang et al., 2009Wang et al., , 2018Bybee et al., 2010;Spencer et al., 2013;Cawood et al., 2018). The evidences of Neoproterozoic magmatism have been reported from several continental fragments, such as South China (e.g., Li et al., 1999Li et al., , 2002Li et al., , 2003Li et al., , 2008Li et al., , 2010Cawood et al., 2013;Zhao et al., 2013;Liu et al., 2020), India (e.g., Bhushan, 2000;Gyani et al., 2001;Gregory et al., 2009;Santosh et al., 2012Santosh et al., , 2014Santosh et al., , 2017Ashwal et al., 2013;He et al., 2016;Wang et al., 2018;Zhao et al., 2018;Yang et al., 2020), Seychelles (e.g., Torsvik et al., 2001;Tucker et al., 2001;Ashwal et al., 2002;Hammond et al., 2013), Madagascar (e.g., Nédélec et al., 1995Nédélec et al., , 2016Handke et al., 1999;Bybee et al., 2010;Boger et al., 2014;Zhou et al., 2015), Lhasa (e.g., Zhang et al., 2012aZhang et al., , 2012bHu et al., 2018aHu et al., , 2018bHu et al., , 2018cDong et al., 2020), Tarim (e.g., Xu et al., 2005Xu et al., , 2013He et al., 2012;Zhang et al., 2012cZhang et al., , 2016Wu et al., 2017;Wen et al., 2019), Australia (e.g., Michael et al., 1998;Li et al., 2006;Huang et al., 2015), Pakistan (e.g., Khan et al., 2012;Ahmad et al., 2013;Qasim et al., 2018), and Laurentia (e.g., Goodge and Vervoort, 2006;Goodge et al., 2017;Cawood and Pisarevsky, 2017). However, the paleogeographic settings of diffenent terranes are still controversial. ...
... As has been stated in previous studies, most of the Neoproterozoic magmatic events are attributed to mantle plumes or a mantle superplume that caused the rifting and fragmentation of the Neoproterozoic Rodinia supercontinent (e.g., Heaman et al., 1992;Park et al., 1995;Shellnutt et al., 2004;Maruyama et al., 2007;Li et al., 1999Li et al., , 2002Li et al., , 2006Li et al., , 2008Li et al., , 2013Cui et al., 2015;Lyu et al., 2017;Zhang et al., 2017a;Zhu et al., 2018). However, a few other studies have suggested that the Neoproterozoic magmas from northwestern India, Madagascar, Seychelles, and Tarim were formed in a continental arc setting, pointing to active Andean-type orogeny on the northwestern margin of the Rodinia supercontinent (e.g., Torsvik et al., 1996;Tucker et al., 2001;Rino et al., 2008;Gregory et al., 2009;Bybee et al., 2010;He et al., 2012;Tang et al., 2016;Liao et al., 2017;Wu et al., 2017;Wang et al., 2018;Zhao et al., 2018). Although this debate has not been well resolved so far, there is no doubt that studies on the origin and geodynamics of Neoproterozoic magmas will contribute to our understanding of the Rodinia supercontinent evolution. ...
Neoproterozoic magmatism associated with the assembly and configuration of the Rodinia supercontinent is widely distributed in the India-Himalayan terrane. However, its petrogenesis and tectonic settings remain controversial. This study provides new geochronological and geochemical data on the Neoproterozoic bimodal magmatism from the eastern Himalayan orogen. In situ zircon UPb dating revealed that the protoliths of amphibolites were emplaced at ca. 826 Ma and the granitic gneisses have crystallization ages of 825–820 Ma. The granitic gneisses exhibit geochemical features of A-type granites, with high initial (⁸⁷Sr/⁸⁶Sr)i ratio (0.7182–0.7394), low whole-rock εNd(t) (−8.4 to −6.6), and variable zircon εHf(t) (−7.4 to +1.0) values. They were probably generated by partial melting of the ancient lower crust with minor input of mantle components. The amphibolite samples are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), suggesting an arc affinity. They have relatively high initial (⁸⁷Sr/⁸⁶Sr)i ratios (0.7113–0.7136), low whole-rock εNd(t) (−1.1 to 1.4) and a wide range of zircon εHf(t) (−4.1 to 8.3) values, indicating that the protoliths of amphibolites were likely generated by partial melting of an enriched subduction-modified continental lithospheric mantle. Their geochemical signatures are similar to typical back-arc basin basalts. The presence of coeval A-type granites and arc-related mafic rocks is probably due to the existence of a back-arc system. We argue that the Neoproterozoic bimodal magmatism is a product of back-arc extension initiated at an early stage, resulting from the rollback of the Mozambique Oceanic slab. Combined with previous studies on Neoproterozoic magmas from India and the Himalayas, we suggest that an extensive Neoproterozoic back-arc system may have existed along the northwestern margin of the Rodinia supercontinent. This theory supports a scenario of an Andean-type continental margin for the India-Himalayan terrane during the middle Neoproterozoic.
... The range in ages of the felsic volcanic rocks is 751 to 771 Ma (n = 9) (Torsvik et al., 2001b;Gregory et al., 2009;Van Lente et al., 2009;Dharma Rao et al., 2012), which overlaps with those of the granitoids (764 to 768 Ma, n = 3) . A dolerite dyke 5 m thick yielded an age of 752 ± 18 Ma (Meert et al., 2013), demonstrating that the dolerites are coeval with the granitic rocks that they Tucker et al. (2001) and Stephens et al. (1999). Modified from Ashwal et al. (2002) and Tucker et al. (2001). ...
... A dolerite dyke 5 m thick yielded an age of 752 ± 18 Ma (Meert et al., 2013), demonstrating that the dolerites are coeval with the granitic rocks that they Tucker et al. (2001) and Stephens et al. (1999). Modified from Ashwal et al. (2002) and Tucker et al. (2001). ...
On the last page of his 1937 book “Our Wandering Continents” Alex Du Toit advised the geological community to develop the field of “comparative geology”, which he defined as “the study of continental fragments”. This is precisely the theme of this paper, which outlines my research activities for the past 28 years, on the continental fragments of the Indian Ocean.
In the early 1990s, my colleagues and I were working in Madagascar, and we recognized the need to appreciate the excellent geological mapping (pioneered in the 1950s by Henri Besairie) in a more modern geodynamic context, by applying new ideas and analytical techniques, to a large and understudied piece of continental crust. One result of this work was the identification of a 700 to 800 Ma belt of plutons and volcanic equivalents, about 450 km long, which we suggested might represent an Andean-type arc, produced by Neoproterozoic subduction. We wondered if similar examples of this magmatic belt might be present elsewhere, and we began working in the Seychelles, where late Precambrian granites are exposed on about 40 of the >100 islands in the archipelago. Based on our new petrological, geochemical and geochronological measurements, we built a case that these ~750 Ma rocks also represent an Andean-type arc, coeval with and equivalent to the one present in Madagascar. By using similar types of approaches, we tracked this arc even further, into the Malani Igneous Province of Rajasthan, in northwest India. Our paleomagnetic data place these three entities adjacent to each other at ~750 Ma, and were positioned at the margins, rather than in the central parts of the Rodinia supercontinent, further supporting their formation in a subduction-related continental arc.
A widespread view is that in the Neoproterozoic, Rodinia began to break apart, and the more familiar Gondwana supercontinent was assembled by Pan-African (~500 to 600 Ma) continental collisions, marked by the highly deformed and metamorphosed rocks of the East African Orogen. It was my mentor, Kevin Burke, who suggested that the present-day locations of Alkaline Rocks and Carbonatites (called “ARCs”) and their Deformed equivalents (called “DARCs”), might mark the outlines of two well-defined parts of the Wilson cycle. We can be confident that ARCs formed originally in intracontinental rift settings, and we postulated that DARCs represent suture zones, where vanished oceans have closed. We also found that the isotopic record of these events can be preserved in DARC minerals. In a nepheline syenite gneiss from Malawi, the U-Pb age of zircons is 730 Ma (marking the rifting of Rodinia), and that of monazites is 522 Ma (marking the collisional construction of Gondwana).
A general outline of how and when Gondwana broke apart into the current configuration of continental entities, starting at about 165 Ma, has been known for some time, because this record is preserved in the magnetic properties of ocean-floor basalts, which can be precisely dated. A current topic of active research is the role that deep mantle plumes may have played in initiating, or assisting, continental fragmentation. I am part of a group of colleagues and students who are applying complementary datasets to understand how the Karoo (182 Ma), Etendeka (132 Ma), Marion (90 Ma) and Réunion (65 Ma) plumes influenced the break-up of Gondwana and the development of the Indian Ocean. Shortly after the impingement of the Karoo plume at 182 Ma, Gondwana fragmentation began as Madagascar + India + Antarctica separated from Africa, and drifted southward. Only after 90 Ma, when Madagascar was blanketed by lavas of the Marion plume, did India begin to rift, and rapidly drifted northward, assisted by the Marion and Deccan (65 Ma) plumes, eventually colliding with Asia to produce the Himalayas. It is interesting that a record of these plate kinematics is preserved in the large Permian – Eocene sedimentary basins of western Madagascar: transtensional pull-apart structures are dextral in Jurassic rocks (recording initial southward drift with respect to Africa), but change to sinistral in the Eocene, recording India’s northward drift.
Our latest work has begun to reveal that small continental fragments are present in unexpected places. In the young (max. 9 Ma) plume-related, volcanic island of Mauritius, we found Precambrian zircons with ages between 660 and 3000 Ma, in beach sands and trachytic lavas. This can only mean that a fragment of ancient continent must exist beneath the young volcanoes there, and that the old zircons were picked up by ascending magmas on their way to surface eruption sites. We speculate, based on gravity inversion modelling, that continental fragments may also be present beneath the Nazareth, Saya de Malha and Chagos Banks, as well as the Maldives and Laccadives. These were once joined together in a microcontinent we called “Mauritia”, and became scattered across the Indian Ocean during Gondwana break-up, probably by mid-ocean ridge “jumps”. This work, widely reported in international news media, allows a more refined reconstruction of Gondwana, suggests that continental break-up is far more complex than previously perceived, and has important implications for regional geological correlations and exploration models. Our results, as interesting as they may be, are merely follow-ups that build upon the prescient and pioneering ideas of Alex Du Toit, whose legacy I appreciatively acknowledge.
... Back arc model for the generation of UHT metamorphism in the eastern Ghats is being proposed by Dasgupta et al. (2013). The cratonization age of the EGMB is considered as ~0.90 Ga, though minor activities were still going on around 0.85 Ga, when India and South China were located at the North Pole (Tucker et al. 2001;Santosh et al. 2016). Afterward, the united mass moved south toward Gondwana. ...
... Afterward, the united mass moved south toward Gondwana. Yangtze and northwestern India were fully assembled by this time, and a hiatus between 0.85 and 0.80 Ga in subduction is inferred when magmatism along the southern margin of India (Tucker et al. 2001), in the Seychelles and on the (present-day) northern margin of the Yangtze Craton (Yan et al. 2004;Zhou et al. 2002), started again. This collision orogeny of Grenvillian age welded proto-India against eastern Antarctica, and resultant eastern Ghats belt-Rayner complex terrane, which was not separated during the breakup of Rodinia, remained an entity until the fragmentation of Gondwana (Fitzsimons 2003). ...
The term supercontinent generally implies grouping of formerly dispersed continents and/or their fragments in a close packing accounting for about 75% of earth’s landmass in a given interval of geologic time. The assembly and disruption of supercontinents rely on plate tectonic processes, and therefore, much speculation is involved particularly considering the debates surrounding the applicability of differential plate motion, the key to plate tectonics during the early Precambrian. The presence of Precambrian orogenic belts in all major continents is often considered as the marker of ancient collisional or accretionary sutures, which provide us clues to the history of periodic assembly of ancient supercontinents. Testing of any model assembly/breakup depends on precise age data and paleomagnetic pole reconstruction. The record of dispersal of the continents and release of enormous stress lie in extensional geological features, such as rift valleys, regionally extensive flood basalts, granite-rhyolite terrane, anorthosite complexes, mafic dyke swarms, and remnants of ancient mid-oceanic ridges.
... The north-western margin of Greater India has several extensive subduction-related mid-Neoproterozoic igneous suites (e.g. Hill and Walter 2000;Tucker et al. 2001;Ashwal et al. 2002;Li et al. 2006;Piper 2007;Thomas et al. 2009;Van Lente et al. 2009;Dharma Rao et al. 2013; Qi et al. 2014), and protoliths of mid-Neoproterozoic ages in high-pressure metamorphic suites (e.g. North Lhasa terrane, Zang et al. 2012Zang et al. , 2014 reported in published literature. ...
In this study, we report for the first time an Early Palaeozoic basement diorite from the drilled well Nirona-A in the Banni Half-Graben of the Kutch basin, western India. The 40 Ar-39 Ar dates provided a plateau age of 441.84 ± 2.66 Ma and another pseudo plateau of 441.28 ± 5.82 to 388.08 ± 16.65 Ma for the basement diorite. These ages constrain the basement formation age to the Late Ordovician-Early Silurian period. The obtained basement ages are correlatable with the later part of Cambro-Ordovician alkaline magmatism that has been reported from the Huqf area in Central Oman, whereas their lithological and petrographic correlativity with basement diorites occurring in the Dinsi Body of Nagar Parkar igneous complex in Pakistan can also be envisaged. The geochemical studies characterized the diorite with enrichment of LILE (Rb, Ba, and K) and LREE (La, Ce, Nd), strong depletion of HFSE (Nb, Sr, P, and Ti), along with weakly negative Eu anomalies. The geochemical signatures indicate their petrogenetic affiliation with mantle-derived magmas, as well as their tectonic setting to be arc-related, having post-collisional continental-arc type affinity. The * 440 Ma basement of Kutch, therefore, appears to represent the later thermal event associated with the reworked Neoproterozoic subduction-related suite from Greater India's northwest edge, which has implications for Gond-wana assembly in the northwest Indian subcontinent.
... Flanking these two orogenic belts are ancient continental blocks, like the Indian Shield, the Congo Craton, the Kalahari Craton, the East Antarctic Craton, which all exhibit Neoproterozoic to Cambrian metamorphic overprinting (Fig. 1a). The East African Orogen consists of reworked continental crust (like the East Sahara Ghost Craton and central Madagascar, Karmakar and Schenk, 2015;Archibald et al., 2023;Collins, 2006;Archibald et al., 2023;Tucker et al., 2014), juvenile oceanic components (like the Arabian-Nubian Shield, Fritz et al., 2013;Li et al., 2018;Meert, 2003;Stern et al., 2010), magmatic arc components (like the Seychelles Islands and southern India, Ashwal et al., 2002;Hammond et al., 2013;Meert and Van Der Voo, 1997;Santosh et al., 2012;Tucker et al., 2001) and ancient continental fragments (like the Antongil terrane in Madagascar and southern India, Bhattacharya et al., 2014;Brandt et al., 2014;Fritz et al., 2013;Kröner, 2001;Kröner et al., 2012;Tucker et al., 2014). The spatial occurrence as well as the components of the younger Kuunga Orogenic belt are still unclear, but the different orogenic belts meet each other in the central part of Gondwana ( Fig. 1a and d, Collins and Pisarevsky, 2005;Fritz et al., 2013;Kröner, 2001;Meert, 2003;Meert and Van Der Voo, 1997;Satish-Kumar et al., 2013). ...
The reconstruction of megacontinent Gondwana was arguably the first crowning achievement of paleogeography, nonetheless the kinematics of its assembly remains controversial. The now-outmoded concept that Gondwana assembly was simply the amalgamation of East and West Gondwana has been replaced by models involving multiple terrane accretions and continental collisions along different orogenic belts. Although the affinities of large cratons are well established, figuring out the relationships between the multiple “Pan-African” orogenic belts has been hindered by poorly defined smaller intervening terranes. At the center of Gondwana, the polymetamorphic terranes in Sri Lanka have been correlated to various crustal blocks without broad consensus, mainly because of a paucity of high-quality geochronological results. Terrane correlations and figuring out the spatial distributions of “Pan-African” orogenic belts in the central part of Gondwana are, therefore, hampered by the controversies about Sri Lanka. We present new data on the high-grade metamorphic rocks occurring in the Highland Complex of Sri Lanka, which reveal the widespread Paleoproterozoic magmatism with U-Pb ages of ≥2465 – 1803 Ma and uniform Archean Hf model ages. Our data confirm the pervasive metamorphic ages of ~610 – 510 Ma, and for the first time, we present multiple episodes of metamorphism at Paleoproterozoic (1854 – 1803 Ma) and Neoproterozoic (~637 – 635 Ma) which are comparable with those reported from the East African Orogen. Based on a comparison with neighboring crustal blocks, we suggest that these early Precambrian basement components as well as the multiple metamorphism of the Highland Complex in Sri Lanka can be correlated with micro-continent Azania. These new data not only confirm the widespread early Precambrian basement components in Sri Lanka, but also open up new ways for terrane correlations from East Africa through Sri Lanka to East Antarctic.
... Voluminous Neoproterozoic magmatic rocks were also identified in Madagascar and Seychelles (western Greater India) and NW India (Fig. 14C) (Archibald et al., 2016;Ashwal et al., 2002;Tucker et al., 2001), which show similar emplacement ages (850-700 Ma) and continental arc-type geochemical characters to those from the northern, western and southwestern Yangtze margin. Furthermore, coeval Neoproterozoic low δ 18 O magmatisms have been identified along the northern/western margin of the South China, NW India, Madagascar and Seychelles (Archibald et al., 2016;Armistead et al., 2019;Harris and Ashwal, 2002;Huang et al., 2019;Wang et al., 2017;Wu et al., 2020;Zhang and Zheng, 2013;Zheng et al., 2007). ...
The Neoproterozoic magmatic and tectonic history of the South China Craton (SCC) is critical for the reconstruction of Rodinia break-up and subsequent Gondwana assembly. Nonetheless, the tectonic attribution of the Neoproterozoic igneous rocks in the southwestern Yangtze is contentious, and the mechanism for crustal anatexis remains unclear. Here, we report mineralogical, whole-rock geochemical and Sr-Nd isotope data, and zircon U-Pb-Hf-O isotope data from the newly found Neoproterozoic trondhjemites and granites in the Diancangshan-Ailaoshan (DCS-ALS) fold belt along the southwestern Yangtze. Both the trondhjemites (ca. 771-762 Ma) and granites (ca. 767 Ma) have high SiO 2 and low MgO contents. The trondhjemites have relatively high CaO, Na 2 O and Sr contents, but low K 2 O contents and Rb/Sr ratios, and show positive to slightly negative Eu anomalies and heavy rare earth elements (HREEs) depletions, resulting in high Sr/Y and La/Yb ratios. In contrast, the granites have lower CaO, Na 2 O, and Sr contents, but higher K 2 O and Rb/Sr, and display negative Eu anomalies and flat HREEs patterns. The trondhjemites have negative εNd(t) (-4.12 to − 3.85), positive to negative εHf(t) (-2.50 to + 5.28) and low δ 18 O (4.37 ‰-7.27 ‰) values. In comparison, the granites have positive εNd(t) (0.09 to 0.26), εHf (t) (+1.24 to + 9.86) and higher δ 18 O (6.50 ‰-7.24 ‰) values. The distinct elemental and isotopic compositions between the trondhjemite and granite samples reflect two different types of crustal anataxis within different crustal levels. The trondhjemites were formed by water-fluxed melting of the juvenile mafic lower continental arc crust (with addition of ancient Yangtze basement). The granites were derived from dehydration melting of juvenile felsic middle crust. Our results imply that the trondhjemites and granites were formed in a continental arc setting. The melting processes revealed here should be applicable to decipher the widespread Neoproterozoic crustal melting along the southwestern Yangtze, which was most likely fluid-controlled via subduction-induced asthenosphere upwelling. Combined with available regional geological data, we suggest that a long-lived Neo-proterozoic (~880-730 Ma) subduction zone may have extended from the Panxi-Hanna region in the northern/ western Yangtze Block to the DCS-ALS fold belt in the southwestern Yangtze. Thus, the SCC may have located on the northwestern margin of Rodinia.
... Comparing available ages for the MIS it seems that rifting and earliest intrusions were concentrated in front of the western margin of the DFB (770 to 760 Ma, Mt. Abu batholith, Sindreth rift, Ashwal et al., 2013;Wang et al., 2018) while ages spectra reported for the Seychelles are in the range of 750 to 760 Ma (Tucker et al 2001). ...
Here we introduce a new model for a Tonian active continental margin in the Neoproterozoic western foreland of the Delhi Fold Belt of NW India. This study gives indication for a transition from continental crust into an active arc setting prior to emplacement of the volcano-plutonic Malani Igneous Suite. Erinpura Granites (890 to 860 Ma) are exposed in a large area west of the Delhi Fold Belt in NW India. The Erinpura Granites are S-type (εNd −10 to −3) whereas granites, granodiorites and gabbros exposed in the westernmost basement outcrops (Bhinmal, Harsani) are I-type granitoids and show positive Nd ratios (εNd + 4 to + 6). These intrusions are derived from crustal melting with a lithospheric component and affinity to a subduction setting, as can also be inferred from trace element ratios. Our study places the Bhinmal granite as youngest intrusion in this setting (U-Pb analyses of zircon 794 ± 17 Ma and titanite 793 ± 19 Ma). High temperature linear solid-state fabrics in the granites indicate intrusion and deformation during crustal extension. After uplift and exhumation bimodal dykes and granites of the Malani Igneous Suite (770–750 Ma) intruded into this older crust (pre-Malani basement).
An evolutionary model for a Tonian active continent margin is established by integrating data from Nagar Parkar in East Pakistan and Neoproterozoic basement inliers in East Oman. The data indicate 130 Myr of ongoing subduction (900 to 770 Ma) along the continental margin. Magmatic activity is identified as episodic with three major pulses starting at ca. 840 Ma, ca. 825 Ma and ca. 770 Ma. High temperature melting constitutes the last Tonian magmatic pulse (Malani Igneous Suite) attributed to roll-back of the subducting slab. The implications for Neoproterozoic plate tectonic reconstructions are discussed.
... This correlation is based on coeval magmatism in the EAO, i.e., Seychelles granitoids (~0.81-0.75 Ga, Torsvik et al., 2001b;Tucker et al., 2001) and granitoids in the Bemarivo Belt of NE Madagascar (~0.75-0.73 Ga, Collins, 2006;Tucker et al., 2014), and Malani igneous suite in northwestern India (~0.75 Ga, Fig. 15f; de Wall et al., 2018;Zhao et al., 2018). ...
The Aravalli Craton, representing the Precambrian nucleus of northwestern India, consists of the Archean Banded Gneissic Complex (BGC; 3.3–2.5 Ga) overlain by Paleoproterozoic (~2.2–1.7 Ga) and Paleo- to Neoproterozoic (~1.7–0.7 Ga) metasedimentary sequences of the Aravalli and Delhi supergroups, respectively. The extensively reworked Late Paleoproterozoic terrane located between the Aravalli and Delhi supracrustal sequences is known as the Sandmata Complex. The BGC, Sandmata Complex and supracrustal sequences, collectively known as Aravalli Craton, were developed by multiple accretionary-collisional processes from ~3.3 to 0.7 Ga and are regarded as classical terranes for understanding Precambrian crustal evolution. The previous multidisciplinary studies have invariably described the litho-tectonic relationships of the Aravalli Craton. Considering the voluminous literature and arguable interpretations, we present a holistic review addressing the Mesoarchean to Neoproterozoic tectonic evolution of the basement and the polydeformed supracrustal sequences of Aravalli and Delhi supergroups. We suggest that the Aravalli Craton evolved by the accretionary-collisional interactions between three major crustal domains, viz., the Mewar gneissic terrane and intrusive granitoids (~3.3–2.5 Ga), the Aravalli fold belt (~2.2–1.7 Ga) and the Delhi fold belt (~1.7–0.7 Ga). The Mewar gneissic terrane formed between 3.3 Ga and 2.7 Ga by partial melting of hydrated mafic crust, where the terrane evolved continuously and finally stabilized due to the collision between the Bundelkhand and Aravalli cratons, resulting in the emplacement of several granitoids between 2.6 and 2.4 Ga. The subsequent development of the Aravalli fold belt (~2.2–1.7 Ga) to the west of Mewar gneissic terrane was characterized by the ~2.2–2.1 Ga mafic-ultramafic volcanism and ~1.8–1.7 Ga felsic magmatism, marking the opening and closing of the Aravalli Basin, respectively. The final closure of this basin was contemporaneous with the exhumation of the Sandmata granulite terrane along the western margin of Aravalli fold belt. Although the Sandmata Complex was previously interpreted as a reworked equivalent of the basement gneisses, based on contrasting lithology, deformation styles and metamorphic grade, we infer that the Sandmata Complex possibly represents an independent terrane with a distinct tectonothermal history. The tectonic evolution of the Delhi Basin most likely took place in two stages from ~1.7 to 0.7 Ga. The initial stage (~1.7–1.4 Ga) led to the development of the north Delhi fold belt and emplacement of A-type granitoids (~1.5–1.4 Ga), whereas the high-grade metamorphism and I- and S-type granite magmatism in the southern part characterize the later stage (~1.3–0.7 Ga) of the Delhi Basin. Following the Delhi Basin closure, the areas to the west of the Aravalli Craton witnessed the emplacement of the Malani Igneous Suite and the development of the Sirohi and Marwar basins. Altogether, the available key information on structural patterns, magmatic-metamorphic histories and geochronology allows more detailed correlations with possible contiguous orogens of the Great Indian Proterozoic Fold Belt. Our synthesis and tectonic interpretations help us discuss and provide alternate explanations for some of the controversial issues from existing tectonic models. Further, we summarize important unresolved issues, which require special attention to improve our knowledge of the Archean to Proterozoic crustal evolution in northwestern India.
... The peak ages of 910-1080 Ma corresponds to the merging of Indian Gondwana with Antarctic Gondwana and is responsible for the large-scale magmatic activity in Indian Gondwana and the closure of the Purana basin (Gregory et al. 2006;Malone et al. 2008;Basu and Bickford 2014;Turner et al. 2014). Similarly, the peak ages of 710-830 Ma corresponds to the development of continental arcs in the northern part of Indian Gondwana, Madagascar and African Seychelles (Torsvik et al. 1996;Tucker et al. 2001;Guangwei et al. 2010). Also, the peak ages of 470-620 Ma corresponds to Pan-African orogenic event at 600-500 Ma (Meert 2003;Veevers and Saeed 2007;Torsvik and Cocks 2013) and Andean-type orogenic event along the northern margin of Indian Gondwana at 530-470 Ma (Cawood and Buchan 2007;Hu et al. 2013). ...
Using the integrated approach of sedimentology, petrography and geochemistry, we tried to understand the provenance, palaeo-weathering and depositional pattern of Carboniferous Fenestella Shale Formation (consisting of shales and sandstones) exposed in the Kashmir Valley, northwest Tethys Himalaya. Five lithofacies are identiBed from base to top of the studied succession, viz., quartz-arenite facies, pebbly sandstone facies, sandstone-siltstone facies, silty-shale facies and shale facies collectively suggest a shallow and deep-water depositional environment. The dominance of small-scale bi-directional cross-stratiBcation relative to large-scale planar cross-bedding in the quartz-arenite facies intercalated with shale facies reCects deposition in a shelf to shore setting during regressive to transgressive phases. The discrimination diagrams also suggest the passive margin depositional setting of these rocks. The chemical index of alteration (ranging from 53 to 77) and weathering index of Parker (ranging from 37 to 57) reCect that the source rocks underwent a low-to-moderate degree of chemical weathering. The presence of plutonic (granitic) rock fragments and dominance of heavy mineral grains, e.g., zircon, rutile and tourmaline in the Fenestella sandstones suggest plutonic rocks as pristine source rocks. Based on coupled sedimentological and geochemical characteristics and available geochronological data sets, it is suggested that the Neo-proterozoic Pan-African granites situated south of the Tethys Himalaya could be the possible source rocks for Fenestella Shale Formation.
... The peak ages of 910-1080 Ma corresponds to the merging of Indian Gondwana with Antarctic Gondwana and is responsible for the large-scale magmatic activity in Indian Gondwana and the closure of the Purana basin (Gregory et al. 2006;Malone et al. 2008;Basu and Bickford 2014;Turner et al. 2014). Similarly, the peak ages of 710-830 Ma corresponds to the development of continental arcs in the northern part of Indian Gondwana, Madagascar and African Seychelles (Torsvik et al. 1996;Tucker et al. 2001;Guangwei et al. 2010). Also, the peak ages of 470-620 Ma corresponds to Pan-African orogenic event at 600-500 Ma (Meert 2003;Veevers and Saeed 2007;Torsvik and Cocks 2013) and Andean-type orogenic event along the northern margin of Indian Gondwana at 530-470 Ma (Cawood and Buchan 2007;Hu et al. 2013). ...
... The peak ages of 910-1080 Ma corresponds to the merging of Indian Gondwana with Antarctic Gondwana and is responsible for the large-scale magmatic activity in Indian Gondwana and the closure of the Purana basin (Gregory et al. 2006;Malone et al. 2008;Basu and Bickford 2014;Turner et al. 2014). Similarly, the peak ages of 710-830 Ma corresponds to the development of continental arcs in the northern part of Indian Gondwana, Madagascar and African Seychelles (Torsvik et al. 1996;Tucker et al. 2001;Guangwei et al. 2010). Also, the peak ages of 470-620 Ma corresponds to Pan-African orogenic event at 600-500 Ma (Meert 2003;Veevers and Saeed 2007;Torsvik and Cocks 2013) and Andean-type orogenic event along the northern margin of Indian Gondwana at 530-470 Ma (Cawood and Buchan 2007;Hu et al. 2013). ...
Using the integrated approach of sedimentology, petrography and geochemistry, we tried to understand the provenance, palaeo-weathering and depositional pattern of Carboniferous Fenestella Shale Formation (consisting of shales and sandstones) exposed in the Kashmir Valley, north-west Tethys Himalaya. Five lithofacies are identified from base to top of the studied succession, viz., quartz-arenite facies, pebbly sandstone facies, sandstone-siltstone facies, silty-shale facies and shale facies collectively suggest a shallow and deep-water depositional environment. The dominance of small-scale bi-directional cross-stratification relative to large-scale planar cross-bedding in the quartz-arenite facies intercalated with shale facies reflects deposition in a shelf to shore setting during regressive to transgressive phases. The discrimination diagrams also suggest the passive margin depositional setting of these rocks. The chemical index of alteration (ranging from 53 to 77) and weathering index of Parker (ranging from 37 to 57) reflect that the source rocks underwent a low-to-moderate degree of chemical weathering. The presence of plutonic (granitic) rock fragments and dominance of heavy mineral grains, e.g., zircon, rutile and tourmaline in the Fenestella sandstones suggest plutonic rocks as pristine source rocks. Based on coupled sedimentological and geochemical characteristics and available geochronological data sets, it is suggested that the Neoproterozoic Pan-African granites situated south of the Tethys Himalaya could be the possible source rocks for Fenestella Shale Formation.
... However, the India and South China Cratons had probably rifted away from the northwestern Australia Craton by ca. 755 Ma (Li et al., 2008b), owing to the breakup Rodinia supercontinent linked dynamically to subduction retreat along its northern margin (e.g., Torsvik et al., 2001a, b;Tucker et al., 2001;Ashwal et al., 2002;Keppie, 2015). Therefore, the Tarim-Qilian-Quanji continent and the South China, India and Australia Cratons once had a valid connection during early Neoproterozoic. ...
The occurrence of pre-existing crust in orogenic belts has important implications for constraining and testing models of orogenic processes and for making paleogeographic reconstructions. Asynchronous magmatic–metamorphic rocks provide an opportunity to reconstruct the positions and constituent blocks of certain Precambrian fragments. Here we report Neoarchean to Neoproterozoic crustal rocks preserved in the North Wulan Terrane (NWT), northwestern China. Our new zircon geochronological data reveal the ca. 2.57 Ga magmatic and 2.0–1.72 Ga metamorphic events in the NWT, implying its involvement in the formation of Columbia supercontinent. Meanwhile, the NWT also recorded the 1.5–1.3, 1.1, 0.9–0.8 and 0.51–0.48 Ga magmatic-metamorphic activities, corresponding to the involvement of regional Precambrian fragments in cyclic supercontinent-related processes from Columbia to Greater Gondwana. Most of the ca. 2.57 and 0.9–0.8 Ga granites are characterized by enriched zircon εHf(t) values of –5.7 to +0.9 and –5.2 to +7.1. Moreover, some of these granites show fractionated characteristics, with high silica contents (80.3–82.3 wt% and 73.9–75.2 wt%), indicating that these rocks were derived mainly from remelting of 3.6–2.7 Ga mature crustal materials and 1.6–2.0 Ga crustal materials without or with minor contribution of juvenile materials, respectively. On the basis of these new results, regional stratigraphy, and previously determined geochemical data and tectonic signatures, it is concluded that the Quanji Massif and the Central Qilian, Hualong and North Wulan terranes probably formed a coherent Qilian–Quanji block as the southeastern extension of the Tarim Craton initiated from the Neoarchean. We propose that from the Neoarchean to early Paleoproterozoic, the primal Qilian–Quanji block was formed by long-term arc-related magma emplacement and reworking of ancient mature crustal materials in southeastern Tarim Craton. Subsequently, the juvenile Tarim–Qilian–Quanji continent was involved in tectonic cycles during its involvement in the supercontinents of Columbia and Rodinia.
... Several authors (Torsvik et al., 2001a(Torsvik et al., , 2001bAshwal et al., 2002;Gregory et al., 2009;Meert et al., 2013) use the paleomagnetic data of the MIS to propose the co-existence of India with Seychelles-Mauritia, Madagascar, Sri Lanka and the Enderby Land-Prydz Bay region of East Antarctica in east Gondwana configuration. The MIS volcanics originated as part of a long-lived subduction and continental arc system that existed along the margins of Rodinia where subduction of the Mozambique Ocean occurred beneath Seychelles-Mauritia, northern Madagascar and northwestern India during Gondwanaland formation (Tucker et al., 2001;Torsvik et al., 2001aTorsvik et al., , 2001bAshwal et al., 2002;Meert and Lieberman, 2008;Meert et al., 2013). Some previous studies (Bhushan, 2000;Eby and Kochhar, 1990;Sharma, 2004;Li et al., 1999) however considered the Malani volcanism to be a product of Neoproterozoic crustal extension leading to Rodinia breakup. ...
The Greater India Proterozoic Fold Belt (GIPFOB) is a curved highly tectonized zone dominated by Early Paleoproterozoic to Early Neoproterozoic magmatic and metamorphic rocks extending from NW India (Aravalli Delhi Fold Belt, ADFB) through central India (the Satpura Mobile Belt, SMB) to eastern India (the Chottanagpur Gneiss Complex, CGC). The continuity of the crustal domains within GIPFOB is obscured by the Gondwana Formation, the Mesozoic Deccan basalts, the intertrappean Bagh Beds and the infratrappean Lameta Formations, and younger alluvium. In supercontinent reconstructions the GIPFOB is speculated to be continuous with Proterozoic mobile belts in Western Australia and the Trans North China Orogen. The NNE-striking western arm (ADFB) of the GIPFOB is flanked by the North India Block (NIB) in the east and the Marwar Craton in the west, whereas the E-striking southern arm (CGC-SMB) is sandwiched between the South India Block (SIB) and the NIB. In the Godhra-Chhota Udepur sector (west-central India) the two arms converge.
We synthesize and compare existing data on mesoscale structures, U-Pb (zircon) and monazite chemical ages, and magmatic and metamorphic histories in the Precambrian crystalline rocks in the three crustal blocks to constrain the accretion dynamics in the GIPFOB with an emphasis on the origin of the curvature in the Godhra-Chhota Udepur sector. The CGC and the central and southern domains of the SMB share considerable homogeneity in chronology and regional structures. In the CGC, the early, N-striking steep-dipping tectonic fabrics in ~1.6 Ga anatectic gneisses with ~1.45 Ga and 1.05–0.9 Ga granitoids are modified into a carapace of shallowly-dipping tectonic mélange of recumbently folded basement gneisses and granitoid mylonites thurst over by allochthonous supracrustal rocks at ~0.95 Ga. The central and southern domains within SMB also exhibit the shallowly-dipping foliations and the emplacement of 1.05–0.9 Ga granitoids. The unmodified basement and the overlying tectonic mélange in the CGC-SMB are traversed by 1.0–0.9 Ga E-striking steep-dipping sinistral (dominant) and dextral (uncommon) shear zones that accommodated the Early Neoproterozoic transpressive deformation induced by the NIB-SIB oblique collision. The mesoscale structures in the rocks of the Godhra-Chhota Udepur sector intruded by Early Neoproterozoic granitoids and followed by oblique collision are similar to CGC-SMB. In the ADFB, by contrast, the basement rocks are older (Archean to Early Paleoproterozoic), high-grade metamorphism and felsic magmatism are older (1.8–1.7 Ga), the expansive ~1.45 Ga granitoids are absent, nappe structures are locally present, and expansive domains of the shallowly-dipping foliations are lacking. Overall, the tectonic evolution of the ADFB is incoherent with those in CGC-SMB and the GC sector.
We suggest that the structures in the N/NNE-striking western accretion arm terminate against the E-striking southern arm. The Early Neoproterozoic (1.0−0.9 Ga) integration of the crustal domains within the GIPFOB resulted due to broadly contemporaneous convergence of the NIB, the SIB and the Marwar Craton (?) during the Rodinia supercontinent assembly, but the accretion along the southern arm post-dated the accretion in the western arm. [482 words]
... The periphery of the South China block might have evolved into Andean-type active continental margin belt (either an arc or a collisional orogen) during the middle Neoproterozoic, as supported by the widespread 850-700 Ma igneous belts along the western and northern margins of the Yangtze Block Kröner and Cordani, 2003;Tucker et al., 2001). A supra-subduction zone likely formed along the western and northern margins of Yangtze Block during the period of middle Neoproterozoic. ...
The Neoproterozoic NE-striking Jiangnan orogen in the South China Block (SCB) separates the Cathaysia microcontinent on the southeast from the Yangtze craton in the northwest. The origin and evolution of the Jiangnan orogen is unresolved, with individual models explaining part of the history, such as various magmatic, deformational, metamorphic, and sedimentary events including extensional, contractional, and transpressional modes. Here, we present detailed geochronological and Nd-Hf isotopic data for Neoproterozoic mafic and felsic intrusions in the Jiangnan orogen and identify three episodes of Neoproterozoic magmatism. The first stage ranging from ~1013–942 Ma is preserved predominantly in the eastern segment of the orogen, including a series of mafic rocks with ages of ~1010–952 Ma, contrasting with weak magmatic activity in the western segment with a peak at ~997 Ma. The second stage ranges from ~906 Ma to ~820 Ma, with intensive and extensive bimodal magmatism occurring in the eastern and western parts. The last stage mainly occurred in the western Jiangnan orogen in the period from ~800 to 780 Ma, associated with drastic mafic magmatism and weakly bimodal volcanic activities. The bimodal volcanism in the eastern segment exhibits a striking weak trend after ~823 Ma, distinctly different from the western segment where the magmatism lasted until ~785 Ma, indicating its syn- to post-collisional in nature. The pre-800 Ma magmatic rocks in western Jiangnan orogen have a dominant negative Nd isotopic signature but exhibit a significant positive trend after 800 Ma, different from those in east, which have a decreasing trend. Both parts display a roughly decoupled Nd-Hf isotopic character presented by a dominant positive pattern in Hf isotopes of the Neoproterozoic intrusive rocks, indicating a significant addition of previous slab-derived components or accretionary wedges in the pre-800 Ma magma evolution, and more depleted materials involved in the source in the later stages. Oceanic subduction under the northwestern Cathaysia Block occurred around ~1.0 Ga, slightly earlier than the subduction to the southeastern Yangtze. A gradual tectonic transition from post-orogenic extension to intracontinental rifting dominated the tectonics in Jiangnan orogen in the middle-late Neoproterozoic, followed by slab failure after ~790 Ma. Arc magmatism was the dominant source of crustal growth before 820 Ma in the SCB, but was exceeded in volume by later slab failure magmatism after ~790 Ma.
... The Kuluketage Group is also correlated with the well-known glacial deposits in South China, Australia, and North America (Xiao et al., 2004;Xu et al., 2005). Thus, the northern arc of the Tarim Block was probably part of the Neoproterozoic giant Andean-type arc system which has been defined by volcanic and plutonic rocks in Madagascar, Seychelles, and northwestern India (Tucker et al., 2001;Cawood et al., 2013;Tang et al., 2016;Chen et al., 2017) and the western margin of the Yangtze Block in South China Zhao et al., 2011. These lines of evidence suggest that the Xingdier intrusion was formed in an active continental margin during the Neoproterozoic. ...
The magmatic processes beneath the active continental margins are very complicated and affect structures and compositions of the arc roots. Neoproterozoic igneous rocks are widely distributed around the margins of the Tarim Block in NW China. The Xingdier mafic-ultramafic intrusion is a composite body, located at the northern margin of the Tarim Block, and consists of gabbro, pyroxenite, and peridotite units. The gabbro unit has a secondary ion mass spectrometry zircon U-Pb age of 727 ± 5 Ma. Rocks from the Xingdier intrusion have a large range of MgO (12.9−32.8 wt%) and SiO2 (43.0−57.9 wt%), and low K2O+Na2O (0.11−2.25 wt%) contents. They have right inclined chondrite-normalized rare earth element patterns with (La/Yb)N ratios of 2.2−8.6. Their primitive mantle normalized trace element patterns show arc-affinity geochemical features characterized by enrichment in Rb, Ba, Th, U, and Pb and depletion in Nb, Ta, and Ti. They have variable initial 87Sr/86Sr ratios (0.7063−0.7093), εNd(t) values (−2.9 to −7.8), 206Pb/204Pb (17.08−17.80), 207Pb/204Pb (15.42−15.49), and 208Pb/204Pb ratios (37.48−38.05), forming an evolution trend from the peridotite unit to the gabbro and pyroxenite units. Clinopyroxene in the three units is chemically similar to those formed in hydrous magmas. The spinel inclusions in olivine from the peridotite unit show unmixing texture and have high Al contents and oxygen fugacity of ∼FMQ+1. Therefore, the parental magma was probably derived from a lithospheric mantle enriched by slab-derived fluids. Rocks from the gabbro and peridotite units are proposed to have been derived from olivine-normative melts, whereas rocks from the pyroxenite unit are cumulates from the quartz-normative melts. Such contrasting parental magmas resulted from variable degrees of crustal contamination and fractional crystallization in the arc root.
... It is therefore, reafBrmed that proximity and similarity in petrography, geochemistry and radiometric dates lend strong support in considering the Nagar Parkar rocks as part of the Malani igneous suite (Jan et al. 2017). Neoproterozoic anorogenic magmatism in the northwestern Indian shield, Madagascar and Seychelles is associated with the Rodinia splitting and assembly with continued heat accumulation in the silicic crust (Tucker et al. 2001;Torsvik et al. 2001;Sharma 2004). Kochhar (1984), Bhushan (1999Bhushan ( , 2000, Raval (2000) and Roy (2001) attributed Malani magmatism to hotspot activity and that there was no movement of the Indian plate relative to the hotspot between 1,500 and 750 Ma. ...
An extremely small but significant exposure of possibly Precambrian or Pre-Deccan Mesozoic plutonic rocks spanning 200 × 90 m in size with 15 m elevation, remotely located amidst vast salt flats of the Great Rann of Kachchh received attention when it was first reported by Biswas in 1968; however, systematic petrological and petrographical studies remained undone. Therefore, in the present study, we addressed complete geological accounts to establish its possible origin. Our detailed geological study indicates that the complex is made up of nepheline and aegirine bearing alkali feldspar syenite with numerous fine-to-medium grained felsic dykes. It lays two viewpoints: (1) it has affinity with Trans Aravalli Belt anorogenic felsic magmatism, i.e., Precambrian and (2) it is genetically related to the pre-Deccan Mesozoic plutonic intrusives like Nir Wandh in Pachham Island and Mundwara and Sarnu–Dandali complexes in Rajasthan. The nearest felsic magmatic suite is an exposure of gray and pink granites of Nagar Parkar of Neoproterozoic age. Presence of undersaturated minerals and xenoliths of diorites within syenite parent body may suggest partial melting of lower crust probably during Rodinia supercontinent rifting. However, presence of alkali-rich minerals, alkali feldspars, kaersutite, aegirine, apatite and rutile needles and the complex being part of the Kachchh rift basin suggests its genetic resemblance with Pre-Deccan Mesozoic plutonic events. The radiometric age will confirm the age of this plutonic massif.
... 775-760 Ma (Ashwal et al., 2013;Gregory et al., 2009;Meert et al., 2013;Van Lente et al., 2009). The latter magmatic and extrusive phase forms one of the largest felsic igneous provinces on the planet-the Malani Igneous Suite-which is also traced to the Seychelles (Fig. 15c-e) Tucker et al., 2001). Arc accretion continued outboard to Oman where two main phases of subduction and arc magmatism occur, at ca. 850 Ma and ca. ...
Recent progress in plate tectonic reconstructions has seen models move beyond the classical idea of continental drift by attempting to reconstruct the full evolving configuration of tectonic plates and plate boundaries. A particular problem for the Neoproterozoic and Cambrian is that many existing interpretations of geological and palaeomagnetic data have remained disconnected from younger, better-constrained periods in Earth history. An important test of deep time reconstructions is therefore to demonstrate the continuous kinematic viability of tectonic motions across multiple supercontinent cycles. We present, for the first time, a continuous full-plate model spanning 1 Ga to the present-day, that includes a revised and improved model for the Neoproterozoic–Cambrian (1000–520 Ma) that connects with models of the Phanerozoic, thereby opening up pre-Gondwana times for quantitative analysis and further regional refinements. In this contribution, we first summarise methodological approaches to full-plate modelling and review the existing full-plate models in order to select appropriate models that produce a single continuous model. Our model is presented in a palaeomagnetic reference frame, with a newly-derived apparent polar wander path for Gondwana from 540 to 320 Ma, and a global apparent polar wander path from 320 to 0 Ma. We stress, though while we have used palaeomagnetic data when available, the model is also geologically constrained, based on preserved data from past-plate boundaries. This study is intended as a first step in the direction of a detailed and self-consistent tectonic reconstruction for the last billion years of Earth history, and our model files are released to facilitate community development.
... Considering how little was known then about the sea floor, and about the ages of rocks, what du Toit said in 19375 presages the insights that were to be gained by the geological mapping of the Seychelles by Baker in 1963 10 , and the more recent geochronological and palaeomagnetic studies of Tucker et al.11 and Torsvik et al.12 , summarised most recently in the 35th Alex du Toit Memorial Lecture by Professor Lewis D. Ashwal13 . Du Toit's remarkable conjecture, made in 1937, in visualising the crystalline rocks of the Seychelles, and 'perhaps the nucleus of Mauritius also' as being fragments left behind by the rifting away of India from Madagascar, was finally proven to be correct 80 years later. ...
Alexander Logie du Toit (1878–1948) was South Africa’s most famous geologist during his lifetime, having authored five books which brought him world renown. In December 1937 to January 1938, accompanied by his wife Evelyn, he visited India in order to attend the Jubilee Indian Science Congress in Calcutta and to do field work in coal and diamond mines. On the return journey to Africa by ship, they stopped for a few hours in Port Victoria on Mahé Island in the Seychelles archipelago. They also passed by Silhouette Island. Du Toit recorded his activities in a diary, and his geological observations in a notebook, where he also drew a sketch of Mahé, and recorded steep structures on the east coast of Silhouette. Although he had not visited the Seychelles before, his deep understanding of the problems of Seychelles geology resulted from his comprehensive research on Indian Ocean geology for his 1937 book Our Wandering Continents. He made remarkably accurate observations on the geomorphology and structure, some of which were only confirmed decades later when the Seychelles were mapped in the 1960s to 1990s. His bold and prescient ideas on the breakup of the Gondwana continent, and on the formation of the Indian Ocean, have been amply confirmed by modern studies, especially by those of Lewis D. Ashwal and his collaborators.
Significance:
• South African geologist Alexander Logie du Toit’s impressions of the Seychelles in 1938 are recorded for the first time, based on entries in his diaries. His observations of structures on Mahé and Silhouette Islands were prescient. His deep understanding of Seychelles geology was the result of his research for his 1937 book Our Wandering Continents. His bold conjecture that the Mascerene Ridge, made of continental crust, was the nucleus of Mauritius, was finally proved in 2017.
... Considering the ages of magmatic zircons with low δ 18 O values only, they show similar age peaks at 780-750 Ma (Fig. 13b). The contemporaneous development of low δ 18 O magmatism at the similar ages in different continents strongly suggests that these continents would be close to each other during the Tucker et al., 2001;Harris and Ashwal, 2002;Zheng et al., 2004Zheng et al., , 2006Zheng et al., , 2007aZheng et al., , 2008Tang et al., 2008;Chen et al., 2011;Fu et al., 2013;Liu and Zhang, 2013;He et al., 2016;Yang et al., 2016;Wang et al., 2017;Archibald et al., 2016;Armistead et al., 2019). (b) Cumulative histograms of U-Pb ages for igneous zircons with low δ 18 O values. ...
Low δ¹⁸O felsic igneous rocks are reported for the first time in the northwestern margin of the South China Block. Zircon U-Pb dating indicates that intermediate to felsic plutons were emplaced at 780 Ma and 750 Ma, respectively, and rhyolite was erupted at about 750 Ma. Compared with other rock-forming minerals that have variable δ¹⁸O values (-2.3‰ to 8.7‰ for quartz;, -5.4‰ to 6.3‰ for plagioclase, -7.6‰ to 2.5‰ for K-feldspar), zircon δ¹⁸O values mainly range from 1.0‰ to 4.7‰ with a few negative values of -7.7‰ to -2.9‰. A two-stage ¹⁸O-depletion scenario was proposed, 1) high-temperature meteoric water-rock reaction in a continental rift setting, generating low δ¹⁸O melt in which precipitated zircons with significant lower δ¹⁸O values than normal mantle; 2) post-crystallization sub-solidus high-T meteoric-hydrothermal alteration, resulting in varying δ¹⁸O values for rock-forming minerals that leads to δ¹⁸O- disequilibrium between zircon and other minerals. In addition, zircons from these rocks show positive εHf(t) values of 4.6 to 11.2 with Hf model ages of ca. 1.0-1.4 Ga, suggesting a magma source of the juvenile crust due to mantle melting during the Grenvillian subduction of oceanic lithosphere in response to accretion of the South China Block to Rodinia in the Late Mesoproterozoic. The occurrence of low δ¹⁸O felsic rocks in the northwestern margin of the South China Block indicates the development of a low δ¹⁸O magmatic belt during the rifting of the South China Block away from Rodinia at about 750 Ma. The coeval occurrence of Neoproterozoic low δ¹⁸O magmatism in the NW India, South China, Madagascar and Seychelles suggests a link between these continental blocks in the Rodinia configuration.
... This is the most favourable tectonic setting for high-temperature reactions between water and rocks during the genesis of a low-δ 18 O magma (e.g. Bindeman et al., 1998Bindeman et al., , 2008Harris and Ashwal, 2002;Tucker et al., 2001;Wang et al., 2011;Watts et al., 2010;Yang et al., 2008;Zhang and Zheng, 2011). Typical examples are the Pleistocene hightemperature rhyolite beds at the Yellowstone National Park, USA, and Paleocene granites in western Scotland, which formed in the extensional setting related to mantle plumes (Bindeman et al., 2008;Monani and Valley, 2001;Watts et al., 2010). ...
A better understanding of the origin and evolution of widespread Neoproterozoic granites along the western margin of the Yangtze Block is important in locating the position of South China within the Rodinia Supercontinent, and the mechanism by which the supercontinent was fragmented. Published and our new detailed geochronological and geochemical data from the Dengganping Complex show that it formed during an extended magmatic event during the Neoproterozoic. This was during the emplacement of a fractionated ca. 815 Ma syenogranite, which was followed by partial melting during ca. 740 Ma and the emplacement of a biotite monzogranite. The geochemistry of the syenogranite indicates it is: (1) a peraluminous high-K type of granite; (2) calc-alkaline; (3) fractionated; (4) enriched in Th, U, Zr, Hf and LREE (LREE/HREE values of 5.3–9); (5) depleted in Nb and Sr; (6) has a negative Eu anomaly (δEu = 0.3–0.5); (7) has whole rock εNd(t) value of −0.7 to +0.5; (8) has zircon εHf(t) value of −1.7 to +5.2; and (9) has zircon ¹⁸O values ranging from 5.13 to 7.38‰. These characteristics are indicative of a source derived from the partial melting of crustal material. The biotite monzogranite is: (i) peraluminous calc-alkaline to high-K calc-alkaline; (ii) enriched in LREE (LREE/HREE = 9.9–14.6), Rb, Ba, Th and U; (iii) depleted in Nb and Ta; (iv) has weak negative δEu and δCe anomalies; (v) has whole rock εNd(t) value of between −3.2 and − 1.8; has relatively high positive zircon εHf(t) values of +4.1 to +13; and has zircon ¹⁸O values ranging from 3.44 to 6.75‰. These features are indicative of partial melting of a juvenile mafic crust that might have experienced a high-temperature water-rock interaction. By comparing the geochronology of Neoproterozoic magmatic rocks along the faulted western margin of the Yangtze Block, it is suggested that the region was part of the Rodinia Supercontinent when it started to breakup during widespread rifting between ca. 825 and 710 Ma. We believe the fault zone formed in response to an opening ocean was induced by the presence of a mantle plume during this period.
... Based on the results discussed above and a synthesis of data from global tectono-thermal events, we propose that the Neoproterozoic magmatic rocks in the Bureya Block were formed during an Andeantype orogeny on the northwestern edge of the Rodinia supercontinent (e.g., Meert and Torsvik, 2003;Li et al., 2008;Ding and Zhang, 2016). First of all, an increasing number of studies argued that the Neoproterozoic granitoids and associated basaltic magmas from the Seychelles, Madagascar, India, Lhasa, South China, Tarim, and Australia were formed in a contiguous continental arc, indicating the existence of an active Andean-type orogeny on the northwestern edge of the Rodinia supercontinent (e.g., Torsvik et al., 1996;Tucker et al., 2001;Meert and Torsvik, 2003;Zhou et al., 2006;Li et al., 2008;Rino et al., 2008;Gregory et al., 2009;Bybee et al., 2010;Dong et al., 2011;Guynn et al., 2012;Zhang et al., 2012;Ding and Zhang, 2016;Zhao et al., 2019). Secondly, the main age populations of detrital zircons from the Cambrian siltstones in the Bureya Block are similar to those of detrital zircons from the Neoproterozoic and early Paleozoic deposits within the Jiamusi Block (Ovchinnikov et al., 2019). ...
The Bureya Block, located in the easternmost segment of the Central Asian Orogenic Belt (CAOB), is one of the least understood blocks in Russian Far East. Here we describe newly discovered Neoproterozoic magmatic rocks along the Bureya River in the northern Bureya Block. LA–ICP–MS zircon U–Pb dating demonstrates that these rocks were emplaced in two stages during the early Neoproterozoic: (1) 937–933 Ma, forming an intrusive rock association of gabbros, granodiorites, and monzogranites; and (2) 896–891 Ma, representing by a suite of syenogranites. The good correlation on the Neoproterozoic to Late Triassic geological events between the Jiamusi and Bureya blocks suggests the two blocks formed a contiguous crustal unit. Field observations, together with petrographic and geochemical features, and zircon Hf isotopic data, indicate a magma mixing/mingling origin for the 937–933 Ma gneissic granitoids and amphibolites, with the felsic melts generated by partial melting of a dominantly Mesoproterozoic lower crustal source with possible little addition of ancient crustal materials, whereas the mafic melts were derived from partial melting of a depleted lithospheric mantle wedge metasomatized by fluids dehydrated from the subducted oceanic slab. In addition, the 896–891 Ma syenogranites share a common crust source with the 937–933 Ma granitoids but experienced extensive assimilation and fractional crystallization during magma ascent and evolution. In the context of assembly, growth and breakup of the Rodinia supercontinent, together with the recognition of Neoproterozoic continental arc-type magmatic rocks in the Bureya–Jiamusi Block as well as the nearly coeval subduction-related magmatism recorded elsewhere on the globe, such as Seychelles, Madagascar, India, Lhasa, South China, and Australia, we conclude that the 937–891 Ma magmatism in the Bureya Block resulted from the Andean-type orogeny that formed along the northwestern margin of the Rodinia supercontinent.
... A detailed comparison of the magmatic characteristics, geochemistry, and timing reveals that the Neoproterozoic magmatic rocks of the southwestern Yangtze block in Neoproterozoic Panxi-Hannan Belt were close to those of northern India in Malani Igneous suite ( Figure 13; Zhou et al. 2006aQi et al. 2014, Qi et al. 2016; Cawood et al. 2018). The emplacement ages (800-700 Ma) and geochemical characteristics of volcanic-intrusive rocks in Seychelles, Madagascar and northwestern India (Malani Igneous Suite) suggested that these magmatic rocks can be part of the continental magmatic arc along the Panxi-Hannan belt ( Archibald et al. 2016;Tucker et al. 2001;Ashwal et al. 2002Ashwal et al. , 2013Zhou et al. 2006aZhou et al. , 2006bThomas et al. 2009;Archibald et al. 2017;Cawood et al. 2018). Subduction process continued until the end of 700 Ma in the western Yangtze, which was also found in Seychelles, northern Madagascar, and northwestern India. ...
Neoproterozoic rocks in the Phan Si Pan Zone, NW Vietnam are considered to be the remnant of the ancient continental crust. They recorded the evolution of the Gondwana supercontinent. A combined study of U-Pb geochronology and Sr-Nd-Hf isotopic geochemistry has been carried out for the Neoproterozoic granites in the Phan Si Pan Zone. Zircon LA-ICP-MS data yield crystallization ages ranging from 758.0 ± 5.0 to 762.0 ± 8.1 Ma. The Neoproterozoic granites have variable (87Sr/86Sr)i ratios of 0.7029 to 0.708 and a wide range of εNd(t) values (−2.02 to 0.04) and of zircon εHf(t) values (−5.1 to 0.0). All these characteristics suggest an origin through partial melting of ancient crustal source rocks with small amount input from the mantle material. Whole-rock Nd and zircon Hf isotopic model ages mostly vary from 1.67 Ga to 1.99 Ga, indicating the existence of the Paleoproterozoic source material in the study area. Neoproterozoic granites in the Phan Si Pan Zone, NW Vietnam have a similar tectonic evolution to those in SW Yangtze, South China.
... In this connection the tectonics of northwestern India has been extensively discussed and debated (Buick et al., 2010;Bhowmik et al., 2010;Chatterjee et al., 2017;Deb et al., 2001;Pandit et al., 2003;Singh et al., 2010;Tiwari & Biswal, 2019). Palaeomagnetic studies in the~750 Ma Malani Igneous Suite (MIS) (Eby & Kochhar, 1990;Pandit et al., 1999;Pareek, 1984;Torsvik, Carter, et al., 2001) in the Marwar craton suggest that MIS-Seychelles-North Madagascar was a coherent terrane separated from the remaining parts of the present-day Peninsular India at~750 Ma Torsvik, Carter, et al., 2001;Tucker et al., 2001). However, the timing and suturing of MIS hosting Marwar craton with the remaining India remains speculative. ...
In northwestern India, Phulad Shear Zone (PSZ, ~810 Ma) demarcates the boundary between South Delhi Fold Belt to the east and Marwar craton to the west. This shear zone has regional NE‐SW trend with small bends of N‐S orientations. PSZ is characterized by steep southeasterly dipping mylonitic foliation with steep oblique stretching lineation. The PSZ has developed in a ductile transpression with top‐to‐the‐NNW reverse sense of movement associated with a component of sinistral‐slip movement on horizontal section. The 200 by 6 km porphyritic Phulad granite occurs along and across the PSZ, and it is variably deformed. Phulad granite shows evidence of magmatic foliation with preservation of parallel alignment of euhedral feldspars phenocrysts and microgranitoid enclaves. The feldspar phenocrysts show simple twin interfaces parallel to the direction of elongation. This granite also shows development of solid‐state foliation parallel to this magmatic foliation. Detailed study of structural elements suggests that Phulad granite has formed during the regional deformation in the country rock shear zone prior to its complete crystallization. Our data indicate that the releasing bends of N‐S orientation within the PSZ have provided the space required for emplacement of Phulad granite in a transpressional regime. EPMA U‐Pb‐Th monazite and U‐Pb LA‐ICP‐MS zircon ages in Phulad granite indicate a magmatic age of 819.1 ± 4 and 818 ± 18 Ma, respectively. PSZ represents the suture along which Marwar craton accreted with remaining India to form the Greater India landmass. The present study suggests that Phulad granite acted as a stitching pluton during this suturing along PSZ at 810–820 Ma.
The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO2 ≈ 57 wt%), syenite (SiO2 = 61–65 wt%), and microsyenite (SiO2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A1-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (87Sr/86Sri = 0.704095–0.707533; Nd(t) = +1.2–+1.9) and zircon Hf (Hf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/NbPM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions (FMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.
The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO 2 ≈ 57 wt%), syenite (SiO 2 = 61–65 wt%), and microsyenite (SiO 2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A 1 -type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd ( ⁸⁷ Sr/ ⁸⁶ Sr i = 0.704095–0.707533; ε Nd (t) = +1.2–+1.9) and zircon Hf ( ε Hf (t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/Nb PM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions ( Δ FMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.
The serpentine orogenic belts that formed during the Neoproterozoic assembly of Gondwana resulted in geodynamic changes on the planet in advance of the Cambrian radiation. The details of Gondwana assembly associated with the closure of the Mozambique Ocean are enigmatic. We compile published geological and paleomagnetic data to argue that the Tarim block was associated with the Azania and Afif-Abas-Lhasa terranes and they were the locus of long-lived Andean-type subduction during the~900-650 Ma interval. Our model suggests a subduction system reorganization between 750-720 Ma, which resulted in two distinct phases of Mozambique ocean evolution. Between 870-750 Ma, a N-S oriented subduction system marks the locus of ocean crust consumption driven by the extension of the Mozambique Ocean. Beginning~720 Ma, a newly developed~E-W oriented subduction system began to consume the Mozambique Ocean and led to the assembly of eastern Gondwana. Our new reconstruction uses true polar wander to constrain the relative paleo-longitude of Tarim, South China and West Africa. In this scenario, the closure of the Mozambique Ocean and formation of Gondwana was orthogonal to the preceding super-continent Rodinia.
What drives the breakup of a supercontinent remains contentious. Previously proposed mechanisms include mantle plumes, subduction retreat, and basal traction from mantle convection. Here we review the geological record of plumes, orogens, and subduction zones during the Pangea breakup and investigate the potential roles played by these factors through 4D spherical geodynamic modelling. We found that mantle plumes provided the dominant force that drove Pangea's breakup, particularly in triggering the initial breakup. Young orogens as continental lithospheric weak zones generally guided the development of continental rifts, consistent with the geological record that rifting within Pangea commonly developed along pre-existing orogens. However, the marginal drag force produced by subduction retreat, and basal traction associated with subduction-related mantle flow, likely also played a role in Pangea's breakup. In addition, the weakening effect of plume-induced melts can sometimes help to break the continental lithosphere away from orogens, as exemplified by the breakup between Antarctica and Australia. Furthermore, geodynamic modeling suggests that subduction is responsible for generating mantle plumes. A particular such example is the formation of the Kerguelen plume, triggered by subduction along the northern margin of Australia, which facilitated the breakup between East Antarctica and Australia.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6670136
The age of emplacement and geochemistry of the lamprophyres are of tectonic significance as they have potential to unravel global scale geodynamic processes. We present petrology, U-Pb SHRIMP apatite and titanite ages and bulk-rock Sr-Nd isotope data for an unmetamorphosed and undeformed lamprophyre dyke from the Simdega area of the Chhotanagpur Gneissic Complex (CGC) which is a component of the E-W trending Central Indian Tectonic Zone (CITZ), India. The CITZ is a major intercontinental suture which separates the northern Indian and the southern Indian blocks whose polarity of their subduction is a contentious issue. The lamprophyre exhibits a strong porphyritic-panidiomorphic texture imparted by the megacrysts/phenocrysts of mica and amphibole with feldspar, apatite, titanite, zircon and opaques confined to the groundmass. Based on combined mineralogy and geochemistry, the lamprophyre is classified to be of calc-alkaline variety (minette) with shoshonitic affinities. Mg# (70.7–78.2) contents highlight the primitive melt character whereas incompatible trace element ratios exclude crustal contamination and are indistinguishable from those of the subduction-related global as well as Eastern Dharwar craton (southern India) calc-alkaline lamprophyres. U-Pb dating of apatite gave an emplacement age of 944 ± 82 Ma which is indistinguishable, within the error limits, from the U-Pb titanite age of 942.1 ± 5Ma demonstrating a Neoproterozoic magmatic emplacement age of the lamprophyre synchronous with the Rodinia assembly. Bulk-rock 87Sr/86Srinitial (0.70899 and 0.71004) and ɛNdinitial (-16.9 to -15.8) highlights the involvement of an enriched mantle source. Calculated Nd TDM (depleted mantle) Paleoproterozoic model ages of 2.1 Ga of the lamprophyre are indistinguishable from those of the co-spatial amphibolite dykes.Petrogenetic modeling involving rare earth elements reveals that the derivation of the lamprophyre magma from 2–3 % partial melting of a mixed garnet (70%) and spinel (30%) lherzolitic mantle source with minor phlogopite. Our study highlights that the western part of the CGC was less affected, relative to the eastern part, by the M3 regional amphibolite grade metamorphic event (ca. 920-880 Ma) and also supports the geodynamic models involving northward-directed subduction of the Southern Indian block under the Northern Indian block.
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Neoproterozoic felsic magmatic suites are important proxies to investigate the breakup history of the Rodinia supercontinent which likely coincided with the emplacement of voluminous Silicic Large Igneous Provinces (SLIPs). Here we report new zircon U-Pb ages with comprehensive whole-rock and mineral chemistry data on the felsic volcano-plutonic rocks from the Tusham Ring Complex (TRC) that forms part of the anorogenic Malani Igneous Suite (MIS) (~780-750 Ma) in NW India. The plutonic rocks (granites) and contemporaneous volcanic rocks (rhyolites) show affinity to A-type granitoids, hypersolvus to subsolvus, and metaluminous to peraluminous. The granitoids investigated in this study are enriched in SiO2, Na2O+K2O, Fe/Mg, Ga/Al, U, Th, REE (except Eu), and depleted in MgO, CaO, Cr, Ni, P, Ti, Sr, and Eu contents. The zircon U-Pb dating for felsic volcanic rocks (four rhyolite samples) yielded Neoproterozoic ages ranging from 827 to 764 Ma whereas zircon in five felsic plutonic rocks (granite) shows ages ranging from 830 to 787 Ma, indicating coeval nature of the intrusive and extrusive rocks. The zircon grains have mostly negative εHf(t) values up to -3.62 and yield crustal Hf model ages from 1.6 Ga to 1.9 Ga, suggesting that the magmatic event involved melting of a Paleoproterozoic source. Based on geochemical features, we propose that the partial melting of crustal protoliths, fractional crystallization, and crustal contamination played a significant role in the magmatic evolution of these rocks. We also infer that the felsic anorogenic magmatism in the TRC occurred in an extensional tectonic regime, possibly associated with a mantle plume event. Our results, in conjunction with previous studies, support the model of anorogenic magmatism linked with the disruption of the Neoproterozoic Rodinia supercontinent.
The present study deals with the petrogenesis and age implication of anorogenic peralkaline granitoid rocks exposed along the North Puruliya Shear Zone (NPSZ) in Jhalda area of Puruliya district, West Bengal. Alkali granite consists of quartz, alkali feldspar, aegirine, riebeckite, arfvedsonite and biotite. These granitoid rocks have a high range of silica, very high total alkali content and are poor in CaO, Al2O3, FeO and MgO content. Geochemically, they are ferroan, alkalic, reduced and peralkaline granitoid rocks and have many similarities with A-type granites. Crystallisation temperatures of these granitoid rocks are greater than 900°C. U–Pb isotopic ages of zircon indicate a major age cluster ~966.7 ± 7.0 Ma. The oldest lower crustal rocks in and around Jhalda area are charnockite, khondalite, garnetiferous granite gneiss, which might have acted as source rocks. Trace element model indicates that a moderate degree partial melting (5–20%) of charnockite + khondalite source rock followed by ~30% fractional crystallisation of plagioclase feldspar is responsible to generate parent magma of alkali granite. Similar and overlapping crystallisation ages of 966.7 ± 7.0 Ma of the per-alkaline anorogenic/post-orogenic granites of present study with already reported orogenic I-type granites from Jhalda and S-type granites from nearby Raghunathpur area (age 1000 Ma) may indicate origin and emplacement of post-orogenic granites of Jhalda during orogeny–anorogeny transition at the time of waning stage of orogenic activity. Mantle upwelling in late to post-orogenic stage provides additional heat to initiate partial melting of lower crustal source rocks.
Molecular phylogenetic studies have shown that the characters of the reduced shell of the false limpets of the genus Siphonaria Sowerby I, 1823 are highly variable and often insufficient for species delimitation. The taxonomy and distribution of Siphonaria in the Indian Ocean are poorly known. We sampled Siphonaria in the Seychelles Bank to check the occurrence of recorded species using DNA sequences and to study the paths through which Siphonaria species have colonised the Seychelles Bank by reconstructing their phylogenetic relationships. Analyses of a dataset comprising 16 S rRNA gene sequences of 33 specimens from the Seychelles Bank and 300 additional Siphonaria sequences from other regions from GenBank with various methods for species delimitation resulted in 19–102 primary species hypotheses. Assemble Species by Automatic Partitioning provided a conservative estimate of the species number (42) in which several indisputable species were lumped. The results of Automatic Barcode Gap Discovery depended strongly on the assumed prior maximum intraspecific divergence, whereas the tree‐based methods Generalised Mixed Yule Coalescent and Poisson Tree Processes resulted in high overestimates. The specimens from the Seychelles Bank represent three clades, belonging to the Siphonaria ‘atra’ group, the Siphonaria ‘normalis’ group and a possibly undescribed species recorded previously only from Hainan. At least two of the three species recorded from the Seychelles Bank came from the east, i.e., from the Coral Triangle in the Indo‐Australian Archipelago, the region with the highest marine biodiversity worldwide. A major transport mechanism across the Indian Ocean was probably the South Equatorial Current.
Recent progress in plate tectonic reconstructions has seen models move beyond the classical idea of continental drift by attempting to reconstruct the full evolving configuration of tectonic plates and plate boundaries. A particular problem for the Neoproterozoic and Cambrian is that many existing interpretations of geological and palaeomagnetic data have remained disconnected from younger, better-constrained periods in Earth history. An important test of deep time reconstructions is therefore to demonstrate the continuous kinematic viability of tectonic motions across multiple supercontinent cycles. We present, for the first time, a continuous full-plate model spanning 1 Ga to the present-day, that includes a revised and improved model for the Neoproterozoic–Cambrian (1000–520 Ma) that connects with models of the Phanerozoic, thereby opening up pre-Gondwana times for quantitative analysis and further regional refinements. In this contribution, we first summarise methodological approaches to full-plate modelling and review the existing full-plate models in order to select appropriate models that produce a single continuous model. Our model is presented in a palaeomagnetic reference frame, with a newly-derived apparent polar wander path for Gondwana from 540 to 320 Ma, and a global apparent polar wander path from 320 to 0 Ma. We stress, though while we have used palaeomagnetic data when available, the model is also geologically constrained, based on preserved data from past-plate boundaries. This study is intended as a first step in the direction of a detailed and self-consistent tectonic reconstruction for the last billion years of Earth history, and our model files are released to facilitate community development.
Precambrian and Early Paleozoic magmatic activities in the Lhasa terrane provides important context for distinguishing between the North and South Lhasa terranes, and deciphering the nature and geotectonic history of the northern margin of the Rodinia and Gondwana supercontinents. Here, we conducted a detailed petrological, geochemical, and geochronological study of mafic to granitic meta-intrusive rocks of the Bomi complex in the eastern segment of the Lhasa terrane. Zircon U–Pb dating indicates that these rocks record Neoproterozoic and Cambrian magmatic events, with protolith crystallization ages of ca. 805 Ma, ca. 505 Ma, and ca. 503 Ma for dioritic gneisses, amphibolites (meta-gabbros), and granitic gneisses, respectively. These rocks were later metamorphosed during the Late Jurassic, with zircon U–Pb ages of ca. 158 Ma obtained from the amphibolites. All rocks are enriched in Rb, U and LREEs, and depleted in Nb, Ta, Sr and HREEs, showing affinity with arc-related magmas. With positive zircon εHf(t) values of +1.4 to +6.9, the Neoproterozoic dioritic gneisses are interpreted to have formed by fractional crystallization of mantle-derived magmas. Cambrian amphibolites have εHf(t) values of –3.0 to +6.0, differing from those of coeval granitic gneisses (+0.3 to +3.2). These amphibolites are interpreted to have been derived from partial melting of enriched continental lithospheric mantle with contamination of subducted sediments, whereas the granitic gneisses probably originated from anatexis of lower crust that was induced by coeval emplacement of mantle-derived magma. These rocks are interpreted to be formed in a continental arc setting. When comparing these data with those of other Precambrian and Cambrian magmatic rocks in the Lhasa terrane and other blocks, we propose that the North Lhasa terrane and South Lhasa terrane have distinctive Precambrian basements. The North Lhasa terrane diachronically witnessed an Andean-type orogeny along the northwestern margin of Rodinia during the Neoproterozoic and the northeastern margin of Gondwana during the Early Paleozoic.
Mid-Neoproterozoic low-δ¹⁸O metamafic rocks from the Leeuwin Complex, southwestern Australia, are reported for the first time. Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating of these upper amphibolite- to granulite-facies mafic rocks yields igneous protolith ages of 674–660 Ma. The metamafic rocks are generally classified as subalkaline tholeiitic rocks with an ocean island basalt (OIB) affinity. They have low Mg# values (22–50) and Cr (0.19–105 ppm) and Ni (0.62–115 ppm) contents, with whole-rock εNd(t) values of − 1.4 to + 1.5 and zircon εHf(t) values of − 0.3 to + 3.5. Using these data in combination with the incompatible trace element characteristics, it is inferred that the protoliths of the rocks were derived from low-degree partial melting of relatively depleted asthenospheric mantle in a continental rift environment, and the magmas underwent some crustal contamination and fractional crystallization of mafic minerals. Zircon cores from the metamafic rocks yield δ¹⁸O values of 0.89 to 4.10‰, which are lower than normal mantle values (5.3 ± 0.3‰). These cores preserve oscillatory zoning or banding in cathodoluminescence images, and individual samples have concordant ages and preserve a narrow range of δ¹⁸O values, suggesting that the low-δ¹⁸O signatures are of primary magmatic origin. It is inferred that these low-δ¹⁸O metamafic rocks were generated by contamination by low-δ¹⁸O felsic crustal wall rocks and interaction of the magma with surface water at shallow depths in an extensional regime during the mid-Neoproterozoic.
Orogens that form at convergent plate boundaries typically consist of accreted rock units that form a highly incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed crust of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the paleogeographic distribution of oceans and continents and bathymetric and topographic features that existed thereon, such as igneous plateaus or seamounts, microcontinents or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction (continent-continent collision, continent-ocean collision (obduction)) and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications heavily hinge on dynamic interpretations linking downgoing plate paleogeography, particularly continental collision, to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave like accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or melanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrusted lower crustal and mantle lithospheric underpinnings. Upper plate paleogeography and deformation responds to the competition between absolute motion of the upper plate and the subducting slab. Our navigation approach through orogenic architecture contains no a priori dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate and thus provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis’ that link the rules of rigid plate tectonics with the reality of plate deformation. We illustrate the use of these rules with a thought experiment, in which we predict orogenic architecture that results from subducting the present-day Indian ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture comparable to elements of modern orogens, unlocking the well-known modern geography for interpreting paleogeography from orogens that formed since the birth of plate tectonics.
Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed crust of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction (continent-continent collision, continent-ocean collision (obduction)) and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave like accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motion of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid a priori dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis’ that link the rules of rigid plate tectonics with the reality of plate deformation. We use these rules for a thought experiment, in which we predict orogenic architecture that will result from subducting the present-day Indian ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of plate tectonics.
The Aravalli Craton is situated on the northwestern edge of India and has a geological history ranging from the Paleoarchean to the Quaternary. The basement of the Aravalli Craton (Banded Gneissic Complex or BGC) and its supracrustal sequences (Aravalli and Delhi Supergroups) serve as an excellent archive to understand the crustal and geodynamic evolution of continental crusts during the Precambrian period of the Earth's history. Here, we present a brief overview along with the recent developments on the status of its Precambrian geology.
The palaeogeographic locations of ancient continental blocks in the Tibetan Plateau, especially of micro-continental blocks during the Neoproterozoic supercontinent cycle, remain enigmatic. To clarify the petrogenesis of Neoproterozoic granitic gneisses and the early evolution of the Amdo micro-continent, we conducted an integrated study of the petrography, geochronology, and geochemistry of biotite monzonitic gneisses from the Precambrian metamorphic basement. The biotite monzonitic gneiss sample yielded a weighted mean zircon U–Pb age of 820 ± 11 Ma, indicating an early Neoproterozoic magmatic event. All gneiss samples show enrichment in light rare-earth elements and depletion in high-field-strength elements in N-MORB-normalized spider diagrams, which suggests that their precursor magma was generated in a subduction-related active continental margin. In addition, these rocks have mostly positive εHf(t) values (−1.0 to 7.0) and corresponding crustal model ages of 1.3 − 1.8 Ga. Zircon Hf isotopes and whole-rock geochemistry indicate that the gneisses were derived from partial melting of juvenile crust material. Recently published age spectra of detrital zircons from different blocks suggest that the Amdo micro-continent had a closer affinity to the Lhasa terrane than to the Qiangtang terrane prior to and during the break-up of the Rodinian supercontinent. However, the gneisses are geochemically distinct from back-arc magmatic rocks in the Lhasa terrane and similar to coeval Andean-type magmatic rocks in blocks that constituted the northwestern margin of Rodinia (including Madagascar, Seychelles, and Malani). This study therefore provides new constraints on the Neoproterozoic palaeogeographic location and evolution of the Amdo micro-continent.
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.
Published geologic maps, regional geological relations, and new U-Pb
ages for intrusive igneous rocks in west-central Madagascar define a
450-km-long belt of rocks emplaced in middle Neoproterozoic time. We
report precise U-Pb zircon and baddeleyite ages for 11 coeval gabbro and
granitoid plutons from the Itremo region, located in the southern part
of this belt. The ages for all gabbroic and granitic plutons and
deformed equivalents define an ˜25 m.y. period of magmatic
activity between 804 and 779 Ma (at maximum uncertainty). Granitoids
intrusive into the Quartzo-Schisto-Calcaire series provide a minimum
depositional age of 791 Ma for this Mesoproterozoic platformal
sedimentary sequence. Our results, combined with other recent U-Pb age
determinations, define a belt of plutonic rocks in west-central
Madagascar emplaced between 804 and 776 Ma. We propose that these middle
Neoproterozoic rocks constitute the root of a continental magmatic arc
emplaced at the time of, or slightly preceding, the breakup of the
Rodinian supercontinent. Neoproterozoic plate reconstructions place
Madagascar on the putative margin of Rodinia, and therefore the plutonic
belt in west-central Madagascar provides important constraints on the
timing and extent of middle Neoproterozoic tectonic events in Rodinia
and the critical period of Rodinia's transformation into Gondwana.
Isotopic age dating of igneous rocks from the Seychelles and its related Gondwana fragments reveals seven major igneous events related to four phases of rifting during the fragmentation of Gondwana. Two phases of rift-related Late Precambrian granitic magmatism occurred on Mahé between ∼710-680 Ma, followed by the Praslin/La Digue/Félicité granites at ∼665 Ma, then by basic dykes, which intruded these granites at ∼620 Ma. The Early Jurassic (∼190 Ma) layers of devitrified ash can be correlated with the Karoo flood-basalt volcanism and its equivalents.Five important igneous events are recorded in the Cretaceous/Tertiary:- 1.i ∼135 Ma: basic dykes temporally equivalent to the Marion hotspot-related Lebombo and Movene volcanics of South Africa and Mozambique.2.ii) ∼124-113 Ma: igneous interbeds possibly correlative of the postulated volcanic development of the Saya de Mahla from the Marion hotspot3.iii) ∼82–?65 Ma: tholeiitic basalts representing the formation of the Amirante ridge complex4.iv) ∼70-60 Ma: representing the Deccan event and divisible into pre-Deccan tholeiitic (∼70-68 Ma), main Deccan basic (∼68-63 Ma) and post-Deccan acidic magmatism(∼63-60 Ma)5.v) ∼47 Ma: basalts extruded when the drift of Seychelles/Mascarene coincided with the Deccan-Reunion hotspot at Saya de Mahla.Acid volcanics from the northeast coast of Madagascar, and offshore the Indian west coast are related to the beginning (∼96 Ma) and end (∼84 Ma) of rifting between Seychelles/ India and Madagascar.
The Seychelles islands consist of undeformed and unmetamorphosed, metaluminous monzogranites and granodiorites of Neoproterozoic age (∼750 Ma). Subsolvus, and lesser hypersolvus granitoids are crosscut by coeval dolerite dykes, dominantly of olivine tholeiite composition. Field relations suggest that mixing between granitoid and doleritic magmas generated a variety of minor intermediate rocks that occur as irregular masses and enclaves; their compositions plot as linear arrays between those of dolerites and granitoids. Two groups of granitoids can be distinguished based on colour, chemistry and isotopic signature. Mah e ´ Group granitoids are grey, with relatively low incompatible element concentrations and primitive isotopic signatures that cluster at &egr; Nd 750 = +2·85 ± 0·17 and I Sr 750 = 0·7031 ± 0·0008 (some samples with impossibly low I Sr <0·700 were affected by open-system processes). Praslin Group granitoids are characteristically red to pink, with Rb >180 ppm, U >4·2 ppm, Th >20 ppm, Pb > 30 ppm, and correspondingly evolved and variable isotopic signatures (&egr; Nd, T = +0·80 to −3·71 and I Sr, T = 0·7032–0·7263). Both groups of granitoids appear to have been derived from a mixed source dominated by a juvenile, mantle-derived component, with variable amounts of an ancient, possibly Archaean, silicic source constituent or contaminant. Potential source materials are unexposed in the Seychelles, but the juvenile component may resemble ∼750 Ma intermediate to mafic metavolcanic rocks of NW Madagascar or NE India; candidates for the ancient silicic component, whose signature is slightly enhanced in granitoids of the Praslin Group, may be represented by 2·5–3·3 Ga tonalitic gneisses of the Banded Gneiss Complex in Rajasthan (NW India), or by similar materials present in central–northern Madagascar. Seychelles dolerites show variable Nd and Sr isotopic compositions (&egr; Nd 750 = +5·46 to −0·87; I Sr 750 = 0·7021–0·7061) that can be modelled as basaltic magmas derived from depleted mantle, variably contaminated (0–15%) by Archaean silicic crust. All petrologic, petrographic, geochemical, isotopic and chronologic data for Neoproterozoic magmatic rocks of the Seychelles, coupled with palaeomagnetic data indicating its position at the margin of the Rodinia supercontinent at ∼750 Ma, are at least consistent with, if not suggestive of, a continental or Andean-type arc setting. We argue, therefore, that the conventionally accepted notion of an extensional (i.e. rift or plume) setting for Seychelles magmatism is vulnerable.
Potassium-argon and 40Ar/39Ar measurements on samples from six mafic dikes that intrude Precambrian granites in the Seychelles Islands (Indian Ocean) indicate that they crystallized about 620 Ma and were uralitized penecontemporaneously or soon thereafter. Stable paleomagnetic directions from two of the six dikes sampled determine a pole position that, in a Gondwana reconstruction, agrees well with the one reported from the late Precambrian Nama Group in southwestern Africa. The paleomagnetic data thus support previous plate reconstructions of the Seychelles Bank between northern Madagascar and western India, adjacent to the Somali coast of eastern Africa. -Authors
Results of U-Pb zircon, titanite and monazite dating from 29 Proterozoic gneissic rocks from an area of ca. 9800 km² in the central portion of the Western Gneiss Region (WGR), Norway are presented. The major period of crust formation is identified, and the age and approximate domains of Middle Proterozoic (Sveconorwegian) and Phanerozoic (Caledonian) isotopic resetting in the region are determined. Results of detailed dating of carefully studied outcrops south of latitude 62°30′N suggest that Caledonian resetting is much less penetrative than farther north. However, Sveconorwegian effects are widespread in this region, and are manifest as crosscutting granodiorite and pegmatite dykes that have been dated at ca. 951 to 942 +5/-4 Ma. It is proposed, therefore, that the north-central portion of the WGR best correlates with the Transscandinavian Granite-Porphyry Belt (southern Scandinavia) and part of the Trans-Labrador Batholith (eastern Labrador), and that the south-central portion of the WGR is correlative with the Southwest Scandinavian Domain (southern Scandinavia) and the Groswater Bay or Lake Melville terranes (eastern Labrador). -from Authors
The rocks of the Seychelles can be divided into two age groups, namely Precambrian granites and younger (Cretaceous/Tertiary) intrusive rocks. The latter can be further subdivided into alkaline ring complexes (as found on the islands of Silhouette and North Island) and basic dykes (on Praslin, Felicité and Mahé islands). Evidence from offshore seismic work and drill holes suggests that Cretaceous/Tertiary magmatism occurred over the whole Seychelles Bank, producing both flood basalts and central volcanic complexes. The flood basalts extend at least as far south as 10°S/60°E.
The younger igneous rocks of the Seychelles show close similarities to the Deccan igneous rocks of India. Tholeiitic dykes from Praslin have previously been shown to resemble Bushe Formation tholeiites from the Deccan, and here we show that the Felicité Island dykes also resemble Bushe. We show also that the alkaline dykes of Mahé and North Island are chemically similar to the dykes at Murud on the west coast of India. Isotopically the Seychelles undersaturated rocks fall within the fields of the Deccan tholeiites.
In India, alkaline magmatism post-dates the tholeiitic magmatism; the age difference is of the order of 3 Ma. This is similar to the age difference between shield-building and rejuvenated-stage magmatism on Hawaiian volcanoes, which has been related to reactivation of the volcanoes by the passage of the Hawaiian Arch. We propose that the Deccan alkaline magmatism is a continental equivalent of oceanic rejuvenated-stage volcanism.
D\H and ¹⁸ O\ ¹⁶ O data have now been obtained on a wide variety of granitic batholiths of various ages. The primary δD values of the biotites and hornblendes are remarkably constant at about –50 to –85, identical to the values in regional metamorphic rocks, marine sediments and greeenstones, and most weathering products in temperate climates. Therefore the primary H 2 O in these igneous rocks is probably not ‘juvenile’, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducated lithosphere. Most granitic rocks have δ ¹⁸ O = + 7.0 to +10.0, probably indicating significant involvement of high - ¹⁸ O metasedimentary or altered volcanic rocks in the melting process; such an origin is required for many other granodiorites and tonaloites that have δ ¹⁸ O = + 10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ ¹⁸ O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low- ¹⁸ O magmas probably formed by melting or assimilation of hydrothermally altered roofrocks. However, the water/rockratios were typically low enough that over wide areas the only evidence for meteoric H 2 O exchange in the batholiths is given by low D/H ratios (δD as low as –180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves from through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks haves δD values lower than –120. The lowering of δD values commonly correlates
A sector of the Peruvian Coastal Batholith 120 km long has been mapped in detail. Little altered volcanics of Cretaceous and Lower Tertiary age make up the bulk of the country rocks and these and their structures are flagrantly cross-cut by a great composite intrusion in which some 16 distinct petrographic units are distributed in many time separated plutons. Though tonalites bulk large, substantial volumes of basic and acid rocks are involved in a clearly marked basic to acid sequence, a primary rhythm, in the distribution of which there is often a marked symmetry: gabbros on the flanks, tonalites and quartz diorites internally and in a medial position there are centred complexes in which adamellites are particularly important. Each of the main petrographic units involved a secondary rhythm of in situ, high level, differentiation in their emplacement. The adamellites have followed earlier arcuate basic intrusions and the resulting associations of multiple plutons and ring dykes are thought to represent the basal wrecks of volcanoes which were active over the entire intrusion history of the batholith. The space problem has not been fully solved and the resolution of this problem will depend on structural analysis of the envelope over a wide area. It has however been demonstrated that stoping and cauldron subsidence were important and because the attitude of many of the main contacts was controlled by contemporaneous fracturing it seems that a general uplift on faults may have provided some of the space for intrusion.
In 1976 the IUGS Subcomission of Geochronology recommended that a new set of decay and abundance constants be adopted for the calculation of nearly all K-Ar laboratories. Two tables reproduced here are an aid for the rapid and easy conversion of K-Ar ages calculated with either set of old constants. This is necessary because the conversion is non-linear.-J.Clayton
This paper reports Rb-Sr isotope ages of the Neoproterozoic volcanics, and associated granitoids of the trans-Aravalli belt of northwestern India. All these rocks along with the earlier reported 779±10 Ma old felsic volcanics from Diri, and Gurapratap Singh of Pali district, Rajasthan, constitute the Malani Group. The study indicates that different rock suites belonging to the Malani Group represent a polyphase igneous activity which spanned for about 100 Ma ranging from 780 to 680 Ma. The granitoids of the Malani Group, i.e. peraluminous Jalore type, and peralkaline Siwana type, were emplaced around 730, and 700 Ma ago, respectively. These plutonic suites represent two different magmatic episodes within a short time interval. The initial Sr ratios of these granitoids suggest lower crustal derivation of the magma. The peralkaline granitoids, and the associated peralkaline rhyolites (pantellerites) are coeval, and cogenetic. The ultrapotassic rhyolite exposed at Manihari of Pali district represents the youngest magmatic activity at 681±20 Ma, having a very high initial Sr ratio of 0.7135±0.0033. The high initial Sr ratio of these rocks may be due to incorporation of radiogenic 87Sr from the country rock, by assimilation or fusion, into the residual fraction of the magma in the crust which gave rise to other differentiated rocks of the Group.40Ar39Ar studies of two Jalore granite samples indicate presence of post crystallisation thermal disturbance between 500550 Ma ago. The timing of this thermal overprinting on the Malani rocks is related to the widespread Pan-African thermo-tectonic event which is witnessed, and magmatically manifested in different part of the Indian shield.
Due to different initial Pb incorporation, the PbPb whole-rock data of the Seychelles anorogenic complex of Late Precambrian age form linear arrays (on 24 samples) with no time significance.The existence of two groups of rocks in this complex with different initial Pb and Nd isotopic characteristics has been evidenced: 1.(1) The Mahé group, comprising the biotite—hornblende granite, the weakly alkaline granite and their melanocratic (dioritic to gabbroic) xenoliths, yields ϵNdBE (705 Ma) = +3.1; their Pb isotopic compositions in K-feldspar are only slightly more radiogenic than those of mantle-derived rocks of the same age.2.(2) The Praslin-La Digue-Félicité group, comprising the biotite granite and its leucocratic xenoliths, yields ϵNdBE (705 Ma) = −1.9; K-feldspar Pb ratios are significantly more radiogenic than those of Mahé rocks.Thus, the parental basic magma of this alkaline anorogenic complex appears to originate from a slightly-depleted mantle source underlying the African continental crust. This magma has been contaminated by the high-level part of this crust to different degrees in the two groups, the Praslin one having been more affected, particularly by late-magmatic fluids.
THE Seychelles Archipelago is situated in the Indian Ocean some 1,100 miles to the east of Mombasa. It is made up of nineteen islands and a number of islets of which all but two are granitic in composition. The only two coral islands in the group are Bird and Denis, which lie at a distance of about sixty miles north of Victoria, Mahé. Mahé itself is the largest of the islands, being seventeen miles long and up to seven miles wide. Rugged peaks of granite rise to a height of almost 3,000 ft. above sea-level and show marked erosion features. Praslin, Silhouette and Digue are the other main granitic islands of the group.
Concordant $Ar^{40}-K^{40} + Sr^{87}-Rb^{87}$ ages of 189 m.y. were obtained on biotite from a Bounty Islands granite. $Sr^{87}-Rb^{87}$ ages of 638 m.y. and 681 m.y., respectively, were determined on a microcline and a whole rock sample of granite from Mahe Island in the Seychelles Archipelago.
Rb/Sr and K/Ar dating of the younger igneous rocks of the Seychelles Islands reveals a peak of magmatic activity at 63 Ma. This corresponds very well with the timing of continental rifting between the Seychelles microcontinent and the Indian plate deduced from ocean floor magnetic anomaly patterns.18O/16O,87Sr/86Sr,143Nd/144Nd and Pb isotope compositions of Cretaceous/Tertiary dolerite dykes indicate strong contamination by continental basement, but there is evidence for only limited contamination of Tertiary alkaline differentiated plutons. The alkaline magmas have Nd/Sr isotope compositions near the Kerguelen field and Pb isotope compositions intermediate between Indian Ocean ridges and islands. They are attributed to a mantle plume which initiated sea-floor spreading on the Carlsberg Ridge. The data suggest that the distinctive chemistry of the Indian Ocean mantle has persisted for at least the Tertiary period.
New determinations of the half-lives of 235U and 238U have been made. Improved techniques have allowed the half-life values to be measured with greater accuracy than has been heretofore achieved. Samples were prepared by molecular plating and counted in a intermediate-geometry α-proportional counter with an extremely flat pulse-height plateau. The small amount of residual nonplated uranium was counted in a 2π counter. Energy analysis with a silicon-junction detector was used to measure the presence of "foreign" activities. For 235U, the measured specific activity was (4798.1±3.3) (dis/min)/(mg 235U), corresponding to a half-life of (7.0381±0.0048) × 108 yr. For 238U, the specific activity was measured as (746.19±0.41) (dis/min)/(mg 238U), corresponding to a half-life of (4.4683±0.0024) × 109 yr. Errors quoted are statistical (standard error of the mean), based upon the observed scatter of the data. This scatter exceeds that expected from counting statistics alone. We believe that systematic errors, if present, will no more than double the quoted errors.
The geology of Mahé-Island and adjoining islets, Seychelles, consists of three types of late Precambrian granitic rock series: (1) a gneissose granodiorite-gneissose tonalite and amphibolite-leucogranodiorite series, (2) a porphyritic granite-pink granite series, and (3) a grey granite-gneissose grey granite series. All these granitic rocks are cut by dykes of doleritic compositions, mostly with a WNW-ESE trend. The gneissose granodiorite is a calcalkaline I-type granite with normative Ab ⪢ Or, the porphyritic granite is a sub-alkaline A-type granite with normative Ab > Or, and the grey granite is an alkaline A-type granite with normative Ab > Or. RbSr radiometric analyses yielded ages of 713±19 Ma for the gneissose granodiorite, 683±16 Ma for the porphyritic granite and 570±5 Ma for the grey granite.Petrological, geological, geochronological and geochemical studies suggest that the late Precambrian granitic rocks of the Seychelles are very similar to, and are well correlated with those of the Arabian-Nubian shield.408 late Precambrian granite specimens were collected from the Mahé island group, Seychelles. Progressive alternating field demagnetization and thermal demagnetization have been successfully performed to isolate stable remanence with well-grouped direction from natural remanent magnetization of the granites. The stable remanences of the granite fall around a northerly mean direction, of which the declination is 348.5° and the inclination +48.7°. The stable remanences of the Mahé island group granite yield a palaeomagnetic pole at 54.0°N, 38.2°E.From the above data, in conjunction with reported polar wander path and plate rotation poles, it is concluded that the Seychelles were originally located near the eastern end of the Horn of Africa and have drifted to the present position since continental break-up.
The geology of the Seychelles Islands in the Indian Ocean is dominated by granitoid rocks and to a lesser extent by basaltic dykes. A U–Pb zircon age from the Takamaka dolerite dyke (Mahé Island) gives an intrusion age of 750.29 2.5 Ma. The dyke age is considerably older than previous age estimates and suggests that some of the Mahé dolerite dykes are almost coeval with the granitoid rocks. The Mahé dykes show variable degrees of magnetic overprinting, but the proposed oldest magnetization, component A (Decl. = 001.4°, Incl. = +49.7° and h 95 = 11.2; palaeomagnetic pole: Lat. =54.8°N and Long. =057.6°E), is identified as a high unblocking component in most dykes, and compares favorably with palaeomagnetic data from the Mahé granitoids. A new Seychelles– India fit (Euler pole: Lat. =25.8°, Long. =330° and rotation angle = 28°) produces a good match of palaeomagnetic poles from ca. 750 Ma magmatic rocks in the Seychelles and NW India (Malani), and places these regions only 600 km apart. Together with Madagascar, this tectonic trio formed an outboard continental terrane of the Rodinia supercontinent during the Neoproterozoic (ca. 750 Ma). The position of the Seychelles at this time marks the incipient formation of a microcontinent because there is no evidence for older continental crust than the 750– 755 Ma granitoid rocks. The Seychelles formed at 30°N and most likely as part of an Andean-type arc along the western margin of the former Rodinia supercontinent. © 2001 Elsevier Science B.V. All rights reserved.
A new Rb−Sr age of 779±10 Ma has been obtained for a suite of andesite-daciterhyolite from the Malani Igneous Province of
southwestern Rajasthan, dated earlier at 745±10 Ma by Crawford and Compston (1970). The associated basalts may be slightly
younger than the felsic volcanics and have a mantle source. The felsic volcanics on the other hand were most probably derived
by fractional crystallization of a crustal magma (Srivastavaet al 1989a, b).40Ar−39Ar systematics of three samples viz., a basalt, a dacite and a rhyolite show disturbed age spectra indicating a thermal event
around 500–550 Ma ago. This secondary thermal event is quite wide-spread and possibly related to the Pan-African thermo-tectonic
episode observed in the Himalayas and south India.
The U-Pb dating of 18 samples, representing the principal rock types of the 4000 km2 Salmi anorthosite-rapakivi granite complex and its satellite Uljalegi pluton, southeastern Baltic (Fennoscandian) Shield,
reveals that six temporally distinct episodes of igneous activity occurred in a timespan of 17 million years. From oldest
to youngest they are: (1) gabbronorite and monzonite at 1546.7 Ma; (2) syenogranite at 1543.4 Ma; (3) early wiborgite and
pyterlite at 1540.6–1537.9 Ma; (4) biotite granite and more evolved granite at 1538.4–1535 Ma; (5) late pyterlite at 1535.2
Ma; (6) olivine gabbro and biotite-amphibole granite at 1530 Ma. The resolvable intervals between magmatic episodes are 3.5–5.0
million years. Early wiborgite and pyterlite (3, above) and biotite granite (4, above) probably crystallized from multiple
magma intrusions. Age differences of 3.4±1.5 million years between zircon and baddeleyite in olivine gabbro (6, above) are
probably a result of xenocrystic origin of baddeleyite extracted from an earlier mafic phase of the Salmi complex. The ages
and chemical features of early and late zircon populations, together with our modeling of magma crystallization and zircon
growth, show that the duration of magma crystallization and Pb-diffusion in zircon was short lived and insignificant compared
to the precision of dating of about ±1–2 million years. Hence, the range of U-Pb ages for each of the major rock types may
approximate the emplacement intervals of their respective magmas. Average rate of magma emplacement was about 0.01 km3/year for the most voluminous phase of early biotite-amphibole rapakivi granite, and about 0.0024 km3/year for the Salmi complex as a whole. Compositional changes of the Salmi magmas over time are in agreement with the model
of magmatism related to lithospheric extension.
During the Neoproterozoic and Palaeozoic the two continents of Baltica and Laurentia witnessed the break-up of one supercontinent, Rodinia, and the formation of another, but less long-lived, Pangea. Baltica and Laurentia played central roles in a tectonic menage a trois that included major orogenic events, a redistribution of palaeogeography and a brief involvement of both with Gondwana. Many of these plate re-organisations took place over a short time interval and invite a re-evaluation of earlier geodynamic models which limited the speeds at which large continental plates could move to an arbitrarily low value.
A simple procedure for the decomposition of zircon and the extraction of U and Pb for isotopic age determinations has been developed and tested (Krogh, 1971a,b). Samples are decomposed at 220°C with 48 per cent HF in a Teflon® capsule confined by a self-sealing stainless steel jacket. Uranium and lead are isolated on a Teflon® anion exchange column using Dowex 1 resin. Measured lead contamination levels range from 0.5 to 5.0 ng. In contrast, lead blanks for the borax fusion technique used in all previous zircon analyses are typically 0.2 to 1.0 μg.Eight small samples weighing 0.3 to 7 mg contained 30 to 260 ng of Pb206. The average value for the total amount of common lead present in the lead isotopic composition analysis, contributed from both the sample and the chemical procedures, was 1.4 ng. The highest ratio measured to date (126,000) was obtained on a 0.1-g sample that contained 50 ppm of Pb206. An exact determination of the amount of common lead in zircons is now possible. A maximum value of 0.3 ppm was found for fourteen nonmagnetic zircon fractions from granites and rhyolites. Higher values reported in the literature suggest that lead contamination levels are often underestimated in the analysis of zircons by the borax fusion technique.The silica-gel loading technique for lead provides stable emission for small samples as well as limited isotopic fractionation in the mass spectrometer. These features, combined with the low levels of lead contamination and the high precision of mass spectrometric analyses, make possible an average reproducibility (for duplicate decompositions of the same finely ground sample) of 0.3 m.y. for lead ages of 2750 m.y. The new method requires fewer reagents and is much easier than the borax fusion technique.
Well-dated paleomagnetic poles for the interval 1100–800 Ma have been compiled for the Laurentia, Baltica, São Francisco, Congo and Kalahari cratons in order to construct apparent polar wander paths (APWPs) for this interval. Laurentia's APWP consists of a well-determined Keweenawan track for 1100–1000 Ma and a 1000–800 Ma Grenville loop. We use a counterclockwise APW loop for the Grenville poles based on ages for post-metamorphic cooling through ∼500°C for the Grenville Province between 1000 and 950 Ma, and the temporal and spatial similarities with Proterozoic counterclockwise APWP's for other cratons. Baltica's APWP is comprised of seven dated poles that define a similar loop, counterclockwise and hinged at 950 Ma, that can be superimposed on the Laurentian Grenville loop. This loop is also seen in the seven poles of the APWP for the combined São Francisco–Congo craton; superposition of these loops leads to a reconstruction in which the São Francisco–Congo craton is to the south-southeast of Laurentia in present-day coordinates. A long 1090–985 Ma APWP track for the Kalahari is in reasonable agreement with the roughly coeval Keweenawan track, when the Kalahari craton is rotated ∼40° counterclockwise away from the Congo craton while remaining hinged at the Zambezi belt. The resulting Rodinia reconstruction resembles those previously proposed on geological grounds for Laurentia, East Gondwana, Baltica, São Francisco–Congo, and the Kalahari craton.
Abrasion combined with an improved paramagnetic separation technique eliminates 90 to 100 percent of discordance so that ages of unprecedented accuracy (±1 to 3 m.y.) can be achieved for virtually all 2700 m.y. old zircon populations from plutonic or volcanic rocks. The procedures work even better for younger zircons. Besides removing outer layers that may have been leached, high-U parts are preferentially removed by abrasion because they are softened by radiation damage. Altered and cracked zircons also tend to be eliminated.In most cases, the new concordant data move up the line established by previously analysed paramagnetic fractions but a number of anomalies have been found where old data give upper intersections that are in error by as much as 25 m.y. Reducing or eliminating paramagnetically correlated Pb loss greatly enhances our ability to define mixing lines for igneous or metamorphic rocks when two ages of zircon are present.The abrasion technique allows detection of an inherited component if it exists by enhancing the sample in core material. Abrasion in many cases removes about 80 percent of the common lead, thus allowing a direct evaluation of this component.When the outer parts of grains are removed, the correlation between magnetic susceptibility and uranium content is maintained but the usual correlation of uranium level with lead loss is reduced or eliminated. Therefore, only near surface uranium is involved in the classic discordance versus uranium level correlation of Silver (1963).
Equations are derived for the estimation of errors and error correlations for various types of U-Pb isotope data, taking into account ion-beam instabilities, run-to-run variability in mass-discrimination, uncertainties in Pb and U concentrations, and uncertainties in initial-Pb and blank-Pb amount and isotopic composition. Equations are also given for the calculation of concordia intercept errors.
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.
名古屋大学博士学位論文 学位の種類:理学博士 (論文) 学位授与年月日:昭和61年1月14日 A variety of granitic rocks from granodiorite to alkaline granite is developed in Mah'e island, Seychelles. Microprobe analyses were made on amphiboles and coexisting minerals. Amphibole constitutes the most prominent ferromagnesian minerals in the Seychelles granitic rocks. Its chemical composition ranges widely from calcic through sodic-calcic to alkali amphiboles and amphibole composition evolves systematically from Fe-poor to Fe-rich: magnesiohornblende →ferrohornblende →ferroeaenite →silicic ferroedenite →ferrorichterite and ferrowinchite →riebeckite. Riebeckite occurs abundantly in the alkaline rocks as subsolidus minerals. Throughout the evolution two types of isomorphous substitution, Mg Fe2 + and A1+ Ca Si + Na principally took place. Compositions of clinopyroxene and biotite also evolve from Fe-poor variety to Fe-rich variety. All these compositional evolutions of the constituent minerals suggest a comagmatic origin of the Seychelles granitic rocks studied. In the Seychelles alkaline magma, ferrorichterite crystallized at the late-magmatic stage under conditions of 650-700℃ in temperature and of slightly above the QFM-buffer in oxygen fugacity. With falling temperature, oxidizing condition prevailed and riebeckite crystallized. Generally, in alkaline granite and quarts syenite magmas, ferrorichterite evolves continuously to arfvedsonitic compositions when oxygen fugacity is defined by QFM-buffer even during subsolidus stage. On the other hand, ferrorichterite evolves to riebeckite composition when oxidizing condition prevails. But, in this case, continuous solid-solution between ferrorichterite and riebeckite is not found, presumably owing to an existence of a compositional gap between them.
Notes sur la constitution gë des Iles Seychelles
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18O depletion in granites as a signature of extensional tectonics: Seychelles basement granites and implications for Late Proterozoic Gondwana (abstract)
- Stephens
W.E. Stephens, A.E. Fallick, R.M. Ellam, 18 O depletion in granites as a signature of extensional tectonics: Sey-chelles basement granites and implications for Late Pro-terozoic Gondwana (abstract), Terra Nova 7 (1995) 145.
The granitic rocks of the Seychelles Islands and their enclaves: Part I: REE and Sr isotope geochemistry (abstract), Terra Cogn
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Rb-Sr whole rock ages of granitic rocks from the Seychelles Islands, 8th Prelim
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- Y Wakizaka
- K Suwa
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Geology and petrology of the Seychelles Islands, 8th Prelim
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- T Yanagi
- K Tokieda
- H Umemura
- M Asa-Mi
- M Hoshino
K. Suwa, T. Yanagi, K. Tokieda, H. Umemura, M. Asa-mi, M. Hoshino, Geology and petrology of the Seychelles Islands, 8th Prelim. Rept. Afr. Studies, Nagoya Univer-sity, 1983, pp. 3^21.
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M.J. Handke, R.D. Tucker, L.D. Ashwal, Neoproterozoic
continental arc magmatism in west^central Madagascar,
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J. Michot, S. Deutsch, Les Seychelles, un nuclëus sialique,
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de Wit, Seychelles revisited, J. Afr. Earth Sci. 27 (1998)
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An isotopic and geochronological investigation of the Seychelles microcontinent
- Dickin