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The origin of Madagascar chromitites
Abstract
Precambrian rocks of Madagascar host numerous chromitite occurrences, ranging from centimeter-thick lenses and seams to orebodies containing millions of tons. Production of chromite concentrates and lumpy, coming from Bemanevika mine that was estimated to have a remaining life of 15 years (Rahaga, 2009), establishes Madagascar as the world 15th chromite producer. The five most important chromitite localities, investigated for this work, are all characterized by outcropping chromitite bodies hosted within mafic/ultramafic intrusions of poorly understood age. They may range from Archean to Cambrian in age although they probably date back to Neoproterozoic to Cambrian. Metamorphism and alteration have variously affected all of the chromitites, but never completely obliterated their primary characteristics. Chromitite host rocks are peridotite, orthopyroxenite or orthoamphibolite, and primary gangue phases are orthopyroxene, olivine, rare plagioclase, ilmenite, rutile, pyrrhotite and pentlandite. Secondary assemblages comprise serpentine, talc, Cr-chlorite, tremolitic to actinolitic amphibole and magnetite. Geologic, textural, mineralogical and mineral chemistry data best fit a layered intrusion origin for North Toamasina, North Belobaka, Antanimbary and Andriamena chromitites, while Befandriana chromitites, even in a general layered intrusion scenario, show some contrasting features more similar to ophiolite chromitites. Differences between the studied chromitites can be ascribed to the position of the chromitite bodies within the stratigraphic sequence of a layered intrusion. The most striking chromitites are those from Antanimbary that show features assimilating them to the Cr-bearing Ti-magnetite layers of the Upper Zone of Bushveld complex. Chromitite alteration mostly affected gangue silicates whose primary assemblage was partially to almost totally obliterated, while chromites underwent at North Belobaka and North Toamasina partial and at Antanimbary complete ferritchromitization.
... In NE, Madagascar the Ranomena Complex has been investigated by numerous authors in the last few decades ago on various aspects like lithology, length, position, age i-e 700 m long and 300 m wide lens and composed of harzburgite, clinopyroxenite and orthopyroxenite and layered-Chromitite harzburgite and two pyroxene-hornblende gabbro (Kröner et al., 2000;Collins & Windley, 2002;Hottin, 1969). The Chromitite associated in layered of harzburgite and pyroxenite as alternating layers with each other (Grieco et al., 2012;Grieco et al., 2014). The metamorphism in Ranomena Complex has been investigated as garnet-sillimanite grade with amphibolite facies and 3100 Ma igneous and metamorphic migmatitic gneiss studied by (Bauer & Key, 2005). ...
... The paragneisses and mica-schist dip shallowing to the west major lithology of Betsimisaraka Suture Zone. The Ranomena Complex comprised of harzburgite, orthopyroxenite, clinopyroxenite, garnet-websterite, chromite layered harzburgite and layered gabbro (Hottin, 1969;Bauer & Key, 2005;Grieco et al., 2012;Grieco et al., 2014). ...
... There was an ocean known as Manampotsy basin which has been consumed during Neoarchean time (Tucker et al. 2011). The Platinum Group Mineral (PGM) studies indicated that the Ranomena Complexes is layered intrusion or stratiform complex and comprised of many relict mafic-ultramafic complexes (Grieco et al., 2012;Grieco et al., 2014;Hottin, 1969). Tahirkheli et al., 1979;Searle & Khan, 1996;Coward et al., 1986). ...
The margin of Indian Plate is limited to Suture Zones at North, North-West
and East, those Suture Zones have marked distinctively by Ophiolites and
Mafic-Ultramafic complexes in Pakistan. The order of the Ophiolites and
Mafic-Ultramafic Complexes from South to North as (a) Bela, (b) ZhobMuslimbagh, (c) Waziristan, (d) Dargai, (e) Shangla-Mingora, (f) Jijal
Complex, (f) Sapat Complex and (g) Chilas Complex. These Ophiolites and
Mafic-Ultramafic Complexes are entirely characterized by segregated,
lenticular or disseminated Chromite associations. The present study is a
critical extensive review of previous works about Chromite Chemistry and
their petrogenetic indications. In particular, the Chromite Chemistry played
an important role to interpreted the tectonic setup of particular maficultramafic Complex, Ophiolites in Pakistan and Ranomena Ultramafic
Complex NE, Madagascar. The Ophiolites and Mafic-Ultramafic complexes in
Pakistan classified based on chromium number (Chromium No: >60 Class I,
Class-II 15-60 & <60 III). The Chilas Complex i-e Chromite. No <60 is related to
rifting origin, the Sapat and Jijal Complex Chromite. No >60 formed in Island
Arc tectonic setting while others formed in Complex origin particularly SupraSubduction Zone (SSZ) environment. The Mafic-Ultramafic Complexes of
Pakistan has been correlated with Ranomena Ultramafic Complex North-East
Madagascar and interpreted that the Bela, Muslimbagh-Zhobe, Waziristan,
Dargai and Shangla-Mingora Ophiolites NW, Pakistan and Ranomena
Ultramafic Complex NE, Madagascar originated at Supra Subduction Zone
tectonic setting.
... In recent studies, the existence and exact position, shape and age of the Betsimisaraka suture has become controversial (Tucker et al. 1999(Tucker et al. , 2011(Tucker et al. , 2014Key et al. 2011;Ishwar-Kumar et al. 2013,2016bRekha et al. 2013;Rekha, Bhattacharya & Prabhakar, 2014;Ratheesh-Kumar et al. 2015). The Ranomena complex consists of layered gabbro, harzburgite, orthopyroxenite, clinopyroxenite, garnet websterite and chromitite-layered harzburgite (Hottin, 1969;Bauer & Key, 2005;Grieco, Merlini & Cazzaniga, 2012;Grieco et al. 2014). The chromite in the chromitite may potentially reveal valuable information about its petrogenesis and tectonic setting. ...
... The Ranomena ultramafic complex (hereafter Ranomena complex) is located c. 25 km NW (17°45 S; 48°06 E) of Toamasina (Tamatave) town in northeastern Madagascar (Fig. 1) (Kröner et al. 2000;Collins & Windley, 2002). It is a c. 700 m long, 300 m wide lens that consists of harzburgite, orthopyroxenite, clinopyroxenite, chromitite-layered harzburgite and two pyroxene-hornblende gabbro (Hottin, 1969), the chromitites occurring between alternating layers of harzburgite and pyroxenite (Grieco, Merlini & Cazzaniga, 2012;Grieco et al. 2014). The Ranomena complex occurs in garnet-sillimanite paragneiss, amphibolite and c. 3100 Ma migmatitic gneiss (Bauer & Key, 2005). ...
... The Betsimisaraka suture contains several relict mafic-ultramafic complexes (Hottin, 1969). From a study of platinum-group minerals (PGM), Grieco, Merlini & Cazzaniga (2012) and Grieco et al. (2014) interpreted the Ranomena complex as a continental layered/stratiform intrusion. The Antananarivo block to the west of the suture mainly consists of Neoarchean (c. ...
The Ranomena ultramafic complex in NE Madagascar consists of layered gabbro, harzburgite, orthopyroxenite, clinopyroxenite, garnet websterite and chromitite-layered peridotite. This study of the Ranomena chromite chemistry aims to better understand the petrogenesis and palaeotectonic environment of the complex. The chromite from the Ranomena chromitite is unzoned/weakly zoned and has a Cr# (Cr/(Cr + Al)) of 0.59–0.69, a Mg# (Mg/(Fe + Mg)) of 0.37–0.44, and low Al 2 O 3 (15–23 wt %) suggesting derivation from a supra-subduction zone arc setting. Calculation of parental melt composition suggests that the parental magma composition of the Ranomena chromitite was similar to that of a primitive tholeiitic basalt formed at a high degree of mantle melting, suggesting the parental melt composition was equivalent to that of an island-arc tholeiite (IAT). The parental magma of the Ranomena chromite had a FeO/MgO ratio of 0.9 to 1.8, suggesting arc derivation. The parental magma was Al- and Fe-rich, similar to a tholeiitic basaltic magma. The composition of orthopyroxene from the chromitite indicates a crystallization temperature range of 1250–1300°C at 1.0 GPa. The chemistry of the chromite in the Ranomena chromitite further suggests that the complex formed in a supra-subduction zone arc tectonic setting.
... %) [30], typical of nonmetasomatised ophiolite mantle, to elevated TiO2 contents in Cr-spinel from dunites (>0.25 wt. %) [30], indicative of metasomatism by rock/melt reaction ( [30][31][32][33][34] and references therein). Figure 5 shows the distribution pattern of TiO2 content versus Cr# and Mg# for Cr-spinel from harzburgites and dunites (TDC and NDL). ...
... All spinels within the dunite have TiO2 values above the conventional 0.25 wt. %, defined as the limit between not metasomatised Cr-spinels, which have not reacted with a melt (Time 3), and metasomatised Crspinels ( [30,31] and references therein). ...
Holes BA1B and BA3A were drilled into the Wadi Tayin Massif, southern ophiolite complex of Oman, a fragment of the Tethyan oceanic lithosphere obducted onto the Arabian continent. Within the sequence, we have studied a portion of the shallow mantle, composed mainly of strongly serpentinised harzburgite that embeds dunitic levels, the biggest being over 150 m thick. The formation of thick dunitic channels, already approached via published structural and mathematical models, is here investigated with a mineral chemistry approach. We focused on Crspinel, the only widespread phase preserved during serpentinization, whose TiO2 content displays a wide variability from low in harzburgite, (TiO2 < 0.25 wt. %), typical of non‐metasomatised ophiolite mantle, to moderately high in dunite (TiO2 < 1.10 wt. %) characterizing a rock/melt interactions. The high variability of TiO2, accompanied by similar patterns of Cr# and Mg# is observed, in a fractal pattern, at all scales of investigation, from the whole channel scale to the single
thin section, where it affects even single grain zonings. Our results suggest that the over 150 m thick dunite channel here investigated was formed by coalescence of different scale melt channels and reaction zones with different sizes, confirming the published structural model.
... Chromite chemistry within ophiolitic suites has long been used as a petrogenetic indicator for establishing their geotectonic environment, the composition of the parental magma(s) and the genesis of chromitites of various types (e.g., Irvine, 1967Irvine, , 1977Pearce et al., 1984;Roeder and Reynolds, 1991;Arai, 1992;Arai, 1997;Malpas et al., 1997; Zhou and Robinson, 1997;Arai and Matsukage, 1998;Barnes and Roeder, 2001;Kamenetsky et al., 2001;Ahmed and Economou-Eliopoulos, 2008;Pag? and Barnes, 2009;Kapsiotis et al., 2011;Kapsiotis, 2013;Grieco et al., 2014;Rollinson and Adetunji, 2015;Uysal et al., 2015;Zhou et al., 2015). Chromite chemistry can be affected during subsolidus reactions by re-equilibration processes with silicate mineral phases and fluids (Irvine, 1967;Arai, 1980;Ozawa, 1985;Roeder and Campbell, 1985;Auge, 1987;Christodoulou and Michailidis, 1990;Rassios and Kostopoulos, 1990;Van der Veen and Maaskant, 1995;Peltonen, 1995;Filippidis, 1997). ...
... It should be noted that our interpretations cannot be compatible with the identification of Alaskan-type complexes in the Lavatrafo area based mainly on PGE data (Bybee et al., 2010). However, precise dating results for their studied rocks are not available, and as pointed out by Grieco et al. (2014), chromites in these intrusions have composition distinct from the Alaskan-type field. Thus, to classify these intrusions as Alaskantype complexes is debatable. ...
The occurrence of high-pressure mafic–ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet–clinopyroxene–quartz coronas that formed after the breakdown of orthopyroxene–plagioclase–ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 system indicate a pressure–temperature (P–T) evolution from an isothermal (780 °C) pressure up to c. 24 kbar to decompression and cooling. Such a P–T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U–Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625 Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818 Ma and Th/U ratios vary from 1.08 to 2.09. K–Ar dating of muscovite and biotite from biotite–kyanite–sillimanite gneiss and garnet–biotite gneiss yields age of 486 ± 9 Ma and 459 ± 9 Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43 km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar.
The Fifield suite of ultramafic-mafic intrusions occur as an N-S linear belt within the Lachlan Orogen extending from north of Condobolin to south of Bourke, NSW and are associated with Australia’s only significant platinum production from nearby placer deposits. These Alaskan-type or ankaramitic plutons intrude the highly deformed quartz-rich turbidites of the Middle to Late Ordovician Girilambone Group, which were deformed during the Late Ordovician-Early Silurian Benambran Orogeny. The lack of any internal fabric or significant alteration indicates the Fifield intrusive suite is a post-Benambran phase of extension related magmatism. U-Pb SHRIMP dating of zircons extracted from a hornblende gabbro of the Tout intrusion give an age of 439.6 ± 8.5 Ma (Early Silurian), providing the first accurate date for the intrusions and by association an age constraint to the termination of the Benambran Orogeny. Thirty-six samples were collected from outcrop and drill core from the Owendale, Tout, Murga and Murrumbogie intrusions for use in petrographic and whole-rock geochemical analysis. Petrographic analysis yielded a crystallisation sequence typical of Alaskan-type intrusions of olivine-clinopyroxene-orthopyroxene-amphibole-plagioclase-K-feldspar-quartz with minor biotite, magnetite, chromite and sulfide phases. This was supported by geochemical results, with the ultramafic association having >40 wt. % MgO and the cumulate association having an average of 12.66 wt. % MgO (pyroxenite – 18.83 wt. %, gabbro – 12.68 wt. %, monzonite/diorite – 6.49 wt. %). Harker diagrams reveal that two distinct suites of igneous rocks are present within the Fifield igneous suite; 1) an ultramafic association characterised by serpentinised peridotite (hazburgite) with an Mg number of 80 and REE patterns that indicate a depleted chondrite-normalised signature; and 2) the cumulate association characterised by progressive evolution from pyroxenite-hornblendite to gabbro, diorite and monzonite compositions that show clear geochemical fractionation trends from a single alkali basaltic melt source. Chromite compositions from the ultramafic rocks are consistent with Alaskan-type or island arc origins. The ultramafic rocks are interpreted to represent the residual depleted hazburgitic material from which the alkali basaltic melt was extracted via partial melting then undergo fractional crystallisation to produce the mafic felsic cumulate rocks. The Fifield intrusive suite is concluded to have been derived from a fertile mantle source which was emplaced as a solid but ductile diapir into the wet sediments of the Girilambone Group, where the addition of water into the diapir initiated partial melting. The cumulate association was then formed by fractional crystallisation. This is concluded to have occurred in an extensional setting similar to that associated with post-collisional, extension
iv
related alkali volcanism in northern Taiwan. Emplacement of the Fifield intrusive suite is interpreted to have occurred during a brief period of regional extension that occurred immediately after the accretion of the Macquarie Arc and is coeval with ‘Phase 4’ magmatism associated with the alkali ‘shoshonitic’ porphyry mineralisation at Cadia and Northparkes.
Small (3(Sb,As)] phase - in the shallowest of the three zones. Electron microprobe analysis of these PGM indicates the presence of oxygen and that at least two species exist. The resulting stoichiometries suggest that at least one species could be a hydrated form of palladinite [PdO.(H2O)n]. The other phase could be a hydroxide [Pd(OH)2] or a less strongly hydrated form of palladinite. Textural evidence suggests that the Pd-O species formed via the replacement of a precursor Pd-rich PGM, with only a limited removal of Pd, rather than via precipitation of a Pd-O PGM from a fluid that was Pd-bearing. The limited thermal stability of hydrated Pd oxides and the apparent restriction of the Pd-O phases to a shallow zone which is affected by the seasonal movement of groundwater, suggests that they may have formed at low temperatures via the leaching and replacement of other elements from a precursor Pd-rich PGM by oxygen and water during alternating water-saturated and dry conditions. If this is the case, the water table interface might be another environment, in addition to the surface, in which to look for the development of PGE oxides.
Chromitite layers are common in large mafic layered intrusions. A widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma. This forces supersaturation of the mixture in chromite, which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer. To evaluate this and other genetic hypotheses to explain the chromitite layers of the Bushveld Complex, we have conducted a detailed study of the silicate-rich layers immediately above and below the UG2 chromitite and another chromitite layer lower in the stratigraphic section, at the top of the Lower Critical Zone. The UG2 chromitite is well known because it is enriched in the platinum-group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the Bushveld Complex. Where we have studied the sequence in the central sector of the eastern Bushveld, the UG2 chromitite is embedded in a massive, 25 m thick plagioclase pyroxenite consisting of 60–70 vol. % granular (cumulus) orthopyroxene with interstitial plagioclase, clinopyroxene, and accessory phases. Throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements (Mg-number = 79·3–81·1). However, the 6 m of pyroxenite below the chromitite (footwall pyroxenite) is petrographically distinct from the 17 m of hanging wall pyroxenite. Among the differences are (1) phlogopite, K-feldspar, and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite, and (2) plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite (An 45–60 vs An 70–75 ). The Lower Critical Zone chromitite is also hosted by orthopyroxenite, but in this case the rocks above and below the chromitite are texturally and compositionally identical. For the UG2, we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below. In either case, the melt was trapped in the footwall pyroxenite because the overlying UG2 chromitite was less permeable. If this interpretation is correct, the footwall and hanging wall pyroxenites were essentially identical when they initially formed. However, all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different. This leads us to propose that the Bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals. The model is supported by the common observation of phenocrysts, including those of chromite, in lavas and hypabyssal rocks, and by chromite abundances in lavas and peridotite sills associated with the Bushveld Complex indicating that geologically reasonable amounts of magma can account for even the massive, 70 cm thick UG2 chromitite. The model requires some crystallization to have occurred in a deeper chamber, for which there is ample geochemical evidence.
Very large crystals of magnetite, up to 2 cm, are found in pure magnetitite layers in the upper zone of the Bushveld Complex. Detailed electron microprobe analysis of one of these indicates a vertical compositional zoning of Cr (a highly compatible element in magnetite) quite different from the concentric zonation often found, for example, in feldspars in intrusive rocks. It is shown that these crystals could not have grown to their present size in suspension in the magma chamber. Annealing of many small crystals into a single grain could occur either by the Ostwald ripening process at temperatures close to the liquidus or by subsolidus recrystallization. Alternatively, these grains could have formed directly from the magma by upward growth from the floor of the magma chamber, by a process analogous to crescumulate or heteradcumulate growth but in the absence of other crystallizing phases. All of these models require the observed Cr gradient to be produced during the initial crystallization event and not as a secondary effect.
The similarities in age (c. 2·06 Ga) and isotopic composition of the Bushveld Igneous Complex and the overlying Rooiberg Group felsic volcanic rocks
suggest the possibility that the two are cogenetic. To investigate this possibility we derive a robust new estimate of the
major and trace element bulk composition of the Bushveld Upper Zone and Upper Main Zone (UUMZ). Using MELTS thermodynamic
modeling, we show that this composition fails to reproduce fundamental features of the cumulate sequence, such as the presence
of primary orthopyroxene at the base of the magma column. We investigate the possibility that some amount of evolved magma
escaped from the magma chamber and derive a new estimate of the UUMZ parent magma composition that reproduces generally the
observed cumulate sequence. Our results constrain the amount and composition of the escaped magma and provide evidence of
a genetic link between the UUMZ of the Bushveld layered series and the felsic volcanic rocks immediately overlying it. We
conclude that between 15 and 25% of the original magma volume was expelled and is manifest either as part of the upper Rooiberg
Group lava sequence or the Rashoop Granophyre (distinction is hindered by their almost identical major and trace element compositions).
A parent magma in which 15–25% Rooiberg or Rashoop material is added back to the extant cumulate sequence is initially saturated
in orthopyroxene and is in major and trace element equilibrium with the phases at the base of the UUMZ.
The Betsimisaraka Suture (B.S.) of Madagascar is an important structural zone defining the collision between Eastern and Western Gondwana. It is represented by highly deformed high-grade metamorphic rocks with mineral assemblages typical of ophiolitic material, including chromitite and nickel bearing rocks consistent with a suture zone setting. Analysis of satellite imagery coupled with field investigations has helped to elucidate the structure and evolution of the B.S. Digital image processing of Landsat ETM+ data was integrated with Synthetic Aperture Radar (SAR) to reduce the effects of the dense vegetation cover. Enhanced false color composite (7-4-2 and 4-2-3), single Landsat bands and band ratio composites including 2/7-1/7-2/5, 5/3-5/1-7/3, 5/1-7/1-4/1 and 5/7-5/1-5/4×3/4 (in RGB) improve the lithologic contrast, reduce the effects of topography and enhance the structural lineaments. Ductile deformation deduced from structural features mapped on Landsat enhanced images indicates three generations of folding (F1, F2, and F3) coupled with shearing: (1) F1 folds with NE striking axial surfaces; (2) F2 related with N–S striking axial surfaces, (3) and F3 associated with ENE–WSW axial surfaces, indicating NNW and SSE contractional strain similar to the deformation in the southern B.S. Mapping these structures enables three types of shearing to be delineated: (1) NW–SE dextral shearing as seen in the Befandriana region; (2) NW–SE sinistral shearing defined by sigmoidal bodies in Mandritsara and Ankijanilava-Marotandrano regions, (3) and NE–SW striking dextral shears recorded in the Lake Alaotra region. Several faults, joints and fractures represent brittle deformation events. Lineaments analyzed within the B.S. are divisible into two groups of brittle structures: (1) N–S trending lineaments correlated with the Gondwanan collision events and (2) much younger NE and NW trending lineaments that are mainly found in the Antananarivo block. The latter may represent an active tectonic event in the central plateau that bounds the western part of the B.S.
Primary stratiform platinum-group element (PGE) mineralizations associated with chromitites in the Critical zone of the Bushveld Complex, South Africa, are confined to cumulate layers that are regionally persistent over tens of kilometers. Four categories of chromitite are recognized: Chromitites at the bases of cycles, comprising the Lower Group, Middle Group, and Upper Group, are further categorized into types Ia, Ib, IIa, and IIb on the basis of lithostratigraphy, chromite and PGE chemistry, and sulfide content. Type IIb chromitites contain significant quantities of sulfides and are not investigated here. New whole-rock and electron microprobe chemical analyses of the sulfide-poor type Ia, Ib, and IIa chromitites, however, record regular variations with height. These variations are interpreted as a differentiation trend. Repeated replenishment of the magma chamber resulted in mixing with resident liquid that became increasingly differentiated with height. Type Ia chromitites are ascribed to replenishment and mixing with the relatively primitive Lower zone residue, type Ib crystallized as a result of mixing with Lower Critical zone resident liquid, and type IIa, due to mixing with more evolved Upper Critical zone resident liquid. -from Authors
New and recently published platinum-group element data of chromite-rich layers in the Bushveld Complex are evaluated. Chondrite-normalized
platinum-group element plots of these data show that trends for chromitite layers differ from that of the Merensky reef. The
data furthermore indicate remarkable consistencies in compositional trends for many kilometers along strike of individual
layers, suggesting that the conditions of formation of chromitite layers and their associated platinum-group element mineralization
were very uniform over large areas of the magma chamber. Significant differences in the proportions of platinum-group elements
within a particular layer are only evident on a regional scale.The platinum-group minerals of the middle group of chromitite
layers in the Marikana area of the western Bushveld are mainly sulfides and arsenides. Laurite is the most common platinum-group
mineral and is mostly enclosed by chromite. A variety of other platinum-group minerals occur in close association with base
metal sulfides. Extremely low bulk sulfur content in the chromitite layers is ascribed to a postcumulus loss of sulfur. Postcumulus
loss of sulfur and the existence of laurite enclosed within chromite casts doubt on the validity of any petrogenetic models
for platinum mineralization in chromitite-rich layers based on partition coefficients and the R factor.
Compositional fields for spinels from a wide variety of maficultramafic igneous rock types and tectonic environments have been determined from a global database of over 26 000 analyses. These fields are defined using contoured data density plots based on the spinel prism, and plots of TiO2 us ferric iron, for mantle xenoliths, ophiolitic rocks, continental layered instrusions, alkalic and lamprophyric rocks, tholeiitic basalts, Alaskan ultramafic complexes and komatiites. Several trends appear regularly in the various environments: a trend of widely variable Cr/(Cr + Al) at low Fe2+/(Mg + Fe2+) (the Cr-Al trend); increasing Fe3+, Fe2+/ (Mg +Fe2+) and TiO2 at constant Cr/(Cr + Al) (Fe-Ti trend); a trend found primarily in kimberlites, similar to Fe-Ti but at constant Fe2+/ (Mg + Fe2+); and an unusual trend of increasing Al found only in layered intrusions. The Cr-Al and Fe-Ti trends are both found to varying degrees in tholeiitic basalts. The Cr-Al trend is prevalent in rocks that have equilibrated over a range of pressures, whereas the Fe-Ti trend is dominantly due to low-pressure fractionation. The most Cr-rich chromites found in nature occur in boninites, diamond-bearing kimberlites, some komatiites and ophiolitic chromitites. Exceptionally reduced chromites are found in some komatiites and in ophiolitic chromitites. Detrital chromites from the Witwatersrand conglomerates are of komatiitic provenance.
The Precambrian shield of south-central Madagascar, excluding the Vohibory region, consists of three geologic domains, from north to south: Antananarivo, Ikalamavony-Itremo, and Anosyen–Androyen. The northern Antananarivo domain represents the Neoarchean sector of the Greater Dharwar Craton amalgamated at 2.52–2.48Ga. The Greater Dharwar Craton is overlain by several groups of Meso- to Neoproterozoic supracrustal rocks (Ambatolampy, Manampotsy, Ampasary, Sahantaha, and Maha Groups) each with a common and diagnostic signature of Paleoproterozoic detrital zircons (2.2–1.8Ga). The central domain (Ikalamavony-Itremo) consists of two distinct parts. The Itremo Sub-domain, in the east, is a structurally intercalated sequence of Neoarchean gneiss and shallow marine metasedimentary rocks of Paleo-Mesoproterozoic age (Itremo Group), the latter with Paleoproterozoic detrital zircons ranging in age between 2.2 and 1.8Ga. The Ikalamavony Sub-domain, to the west, contains abundant volcano-clastic metasediments and lesser quartzite (Ikalamavony Group), formed between 1.03Ga and 0.98Ga, and intruded by igneous rocks (Dabolava Suite) of Stenian–Tonian age. Structurally intercalated with these are sheets of Neoarchean gneiss (∼2.5Ga) and Neoproterozoic metaclastic rocks (Molo Group). Like the Itremo Group, quartzite of the Ikalamavony Group has detrital zircons of Paleoproterozoic age (2.1–1.8Ga). The southern domain of Anosyen–Androyen consists of a newly recognized suite of Paleoproterozoic igneous rocks (2.0–1.8Ga), and stratified supracrustal rocks also having Paleoproterozoic detrital zircons (2.3–1.8Ga). The contact between the Anosyen–Androyen and Ikalamavony-Itremo domains, formerly known as the Ranotsara–Bongolava shear zone, is a tightly folded and highly flattened boundary that was ductilely deformed in Ediacaran time. It is roughly equivalent to the Palghat–Cauvery shear zone in south India, and it defines approximately the boundary between the Archean Greater Dharwar Craton (to the north) and the Paleoproterozoic terrane of Anosyen–Androyen (to the south).
We report single zircon 207Pb/206 Pb evaporation and SHRIMP ages, combined with whole-rock Nd isotopic systematics for granitoid rocks from the Antananarivo Block (terrane), one of five tectono-metamorphic units making up the Precambrian basement of central and northern Madagascar. Our data reveal three distinct age groups at 560 to 530, 820 to 720, and 2520 to 2500 Ma respectively that reflect major magmatic events and correlate with similar events in various parts of East Africa and Sri Lanka but not in southwestern India. A widespread high-grade metamorphic event at 550 Ma transformed many of the earlier granitoid gneisses into enderbite-charnockite assemblages. This granulite-facies event is common to Madagascar, East Africa, and southernmost India/Sri Lanka and reflects the final amalgamation of East and West Gondwana. Contrary to previous interpretations, there is a distinct lack of Kibaran-Grenvillian magmatism or metamorphism in Madagascar, making it unlikely that the island played a major role in the accretionary history and amalgamation of the supercontinent Rodinia. The widespread and voluminous granitoid magmatism at 824 to 720 Ma remains enigmatic, and the tectonic scenario with which it is associated is difficult to reconstruct due to severe tectonic transposition of most gneisses. The Nd isotopic systematics as well as abundant zircon xenocrysts attest to extensive remelting of Archean and Paleoproterozoic crust. On presently available data the 740 to 820 Ma granitoids are either related to magmatic underplating following plume generation, subcrustal mantle delamination during break-up and dispersal of Rodinia, or to continental arc magmatism related to subduction of the Mozambique ocean. They were emplaced into the ancient crust of central Madagascar as it lay either attached to East Africa or formed a microcontinent within the Mozambique ocean.
This paper is based on 465 new analyses of Ni, Cu, S and PGE from the 19 chromitite horizons between the LG-1 and UMG-2 from 6 sectors around the Bushveld Complex, along with microprobe analyses of representative samples of 41 chromites. Two trends in chromite composition, A and B, are distinguished on a plot of cation% Mg/(Mg + Fe2+) versus Cr/(Cr+Al). Trend A, that has a negative slope, is close to that predicted as the result of the reciprocal exchange substitution of Cr and Fe2+ for Mg and Al between spinel and liquid affecting the Mg-Fe2+ spinel-liquid Kd2. Trend B, that has a positive slope and is defined primarily by the LG-5 to MG-2 chromitites, is the result of the progressive increase in the activity of Al2O3 as a result of the fractional crystallization of orthopyroxene. Overall, the average PGE concentrations in massive chromitite increase upward. The LG-1 to LG-4 chromities have low (Pt+Pd)/(Rh+Ru+Ir+Os) ratios (0·1 to 0·3), above which there is an abrupt jump to higher ratios in the LG-5 (0·9 to 10) and all overlying chromitites (also documented by Scoon and Teigler). The Pt/Ru and Pd/Ru ratios are very variable, but the Ru/Ir, Ru/Rh and Ru/Os ratios of all chromitites are relatively constant, indicating that Pt and Pd respond to different concentration mechanisms to the other PGE. Rh, Ru, Ir and Os were likely concentrated by chromite itself, probably as grains of laurite and alloys incorporated in growing chromite crystals, but the bulk of the Pt, Pd along with lesser proportions of the other PGE were concentrated by sulphide liquid. Most chromitites now have very low contents of S, but mineragraphic and chemical data support the suggestion of Naldrett and Lehmann that vacancies in chromite forming above 900°C were filled by Fe2+ derived from the destruction of interstitial sulphide liquid. Data on En composition through the Bushveld CriticalZone, indicate that the LG-1 to LG-4 chromitites formed at a stage when influxes of magma into the chamber were rapid and primitive, and overrode the effect of fractional crystallization, whereas above this, fractionation mostly overrode influxes of new magma. Irvine's model of mixing of resident magma with influxes of more primitive magma is invoked as the origin of the chromitite horizons. It is shown, using the equation for sulphur solubility and the programme MELTS, that influxes and mixing of fresh primitive magma from depth with that in the chamber (i.e. as envisaged for the LG-1 to LG-4) would not have caused sulphide immiscibility along with chromitite crystallisation, but that influxes and mixing of slower-ascending magma, that fractionated en route, could give rise to sulphide liquid segregating along with the chromitite (i.e. the scenario for the LG-5 and overlying chromitites). The modelling also shows that the more fractionated the magma in the chamber becomes, the more sulphide will form, accounting for the overall upward increase in Pt and Pd above the LG-5.This paper is based on 465 new analyses of Ni, Cu, S and PGE from the 19 chromitite horizons between the LG-1 and UMG-2 from 6 sectors around the Bushveld Complex, along with microprobe analyses of representative samples of 41 chromites. Two trends in chromite composition, A and B, are distinguished on a plot of cation% Mg/(Mg + Fe2+) versus Cr/(Cr+Al). Trend A, that has a negative slope, is close to that predicted as the result of the reciprocal exchange substitution of Cr and Fe2+ for Mg and Al between spinel and liquid affecting the Mg-Fe2+ spinel-liquid Kd2. Trend B, that has a positive slope and is defined primarily by the LG-5 to MG-2 chromitites, is the result of the progressive increase in the activity of Al2O3 as a result of the fractional crystallization of orthopyroxene. Overall, the average PGE concentrations in massive chromitite increase upward. The LG-1 to LG-4 chromities have low (Pt+Pd)/(Rh+Ru+Ir+Os) ratios (0·1 to 0·3), above which there is an abrupt jump to higher ratios in the LG-5 (0·9 to 10) and all overlying chromitites (also documented by Scoon and Teigler). The Pt/Ru and Pd/Ru ratios are very variable, but the Ru/Ir, Ru/Rh and Ru/Os ratios of all chromitites are relatively constant, indicating that Pt and Pd respond to different concentration mechanisms to the other PGE. Rh, Ru, Ir and Os were likely concentrated by chromite itself, probably as grains of laurite and alloys incorporated in growing chromite crystals, but the bulk of the Pt, Pd along with lesser proportions of the other PGE were concentrated by sulphide liquid. Most chromitites now have very low contents of S, but mineragraphic and chemical data support the suggestion of Naldrett and Lehmann that vacancies in chromite forming above 900°C were filled by Fe2+ derived from the destruction of interstitial sulphide liquid. Data on En composition through the Bushveld CriticalZone, indicate that the LG-1 to LG-4 chromitites formed at a stage when influxes of magma into the chamber were rapid and primitive, and overrode the effect of fractional crystallization, whereas above this, fractionation mostly overrode influxes of new magma. Irvine's model of mixing of resident magma with influxes of more primitive magma is invoked as the origin of the chromitite horizons. It is shown, using the equation for sulphur solubility and the programme MELTS, that influxes and mixing of fresh primitive magma from depth with that in the chamber (i.e. as envisaged for the LG-1 to LG-4) would not have caused sulphide immiscibility along with chromitite crystallisation, but that influxes and mixing of slower-ascending magma, that fractionated en route, could give rise to sulphide liquid segregating along with the chromitite (i.e. the scenario for the LG-5 and overlying chromitites). The modelling also shows that the more fractionated the magma in the chamber becomes, the more sulphide will form, accounting for the overall upward increase in Pt and Pd above the LG-5.
The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U-Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil-Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo-Mesoarchean (3.32-3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55-2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the "Betsimisaraka suture" is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro-Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560-520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo-Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.
Recent work in central and northern Madagascar has identified five tectonic units of the East African Orogen (EAO), a large collisional zone fundamental to the amalgamation of Gondwana. These five units are the Antongil block, the Antananarivo block, the Tsaratanana sheet, the Itremo sheet, and the Bemarivo belt. Geochronological, lithological, metamorphic, and geochemical characteristics of these units and their relationships to each other are used as a type area to compare and contrast with surrounding regions of Gondwana. The Antananarivo block of central Madagascar, part of a broad band of pre-1000-Ma continental crust that stretches from Yemen through Somalia and eastern Ethiopia into Madagascar, is sandwiched between two suture zones we interpret as marking strands of the Neoproterozoic Mozambique Ocean. The eastern suture connects the Al-Mukalla terrane (Yemen), the Maydh greenstone belt (northern Somalia), the Betsimisaraka suture (east Madagascar), and the Palghat-Cauvery shear zone system (south India). The western suture projects the Al-Bayda terrane (Yemen) through a change in crustal age in Ethiopia to the region west of Madagascar. Our new framework for the central EAO links the Mozambique belt with the Arabian/Nubian Shield and highlights the power of tectonic analysis in unraveling the complex tectonic collage of the EAO.
About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex
on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial
cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are
concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites
are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions.
Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite
are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex.
Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different.
The Mg/(Mg+Fe2+) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in
the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO
(26wt%), Na2O (2.4wt%) and H2O (2.6wt%), and low CaO (1.1wt%) and FeO (4.4wt%). The δ18O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent
with the previous estimates for the Bushveld magma and with the δ18O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent
with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich
melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich
fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution
of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with
the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al
in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along
with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions
were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.
Chromitite segregations in dunites of the Uktus Uralian-Alaskan-type complex (Central Urals, Russia) display large variation of the chromite composition: Cr/(Cr+Al)=0.46-0.77, Fe2+/(Fe2++Mg)=0.28-0.66, and Fe3+/(Fe3++Fe2+)=0.23-0.59. Three types of PGM assemblages have been recognized, varying in accordance with chromite composition: type I, dominated by Ru-Os-Ir (sulfides), is associated with magnesiochromite having Fe3+/(Fe3++Fe2+)<0.30, in the southern dunite body. Type II, containing abundant Pt-Ir (alloys, minor sulfides), is found in magnesiochromite with Fe3+/(Fe3++Fe2+)=0.40-0.44; type III, consisting of Ir-Rh-Pt-Pd (alloys, sulfarsenides, antimonides) in Fe-rich chromite having Fe2+/(Fe2++Mg)=0.66 and Fe3+/(Fe3++Fe2+)=0.59. Positive anomalies of Ir and Pt, and a negative peak of Ru characterize the PGE patterns of chromitites with type II and III PGM assemblages, whereas a positive Pt anomaly is observed in their dunite host. Intensive fractionation of Pt-Fe alloys in the Uktus chromitites reflects the anomalous behavior of Pt which is decoupled from Rh and Pd. Among other factors, the high iron activity and oxygen fugacity in the parent melt appear to exert a major control on precipitation of Pt-Fe alloys, below sulfur saturation. The strong Pt anomaly in chromitites from Uktus may indicate that Uralian-Alaskan-type magmas were derived from a Pt-rich mantle source.
We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73-77 in the banded type to 81-86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd-Rh-As and Pd(Pt)-(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite-erlichmanite series and irarsite (8%). Os-Ir alloy, geversite, an unnamed Pt-Pd-Bi-Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir-Os, Os-Ir, Pt-Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh)2S4, braggite, unnamed BM-Ir and BM-Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.
Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an FeNi core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at ∼ 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an FeNi alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence (∼ 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk of ∼ 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth (∼ 14%) and the core (∼ 86%). Chondritic meteorites display a range of ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.
Ultramafic–mafic complexes that intruded into Archaean ortho- and paragneisses in the Andriamena region of north-central Madagascar fall within the domain of a 900-km-long belt consisting of gabbroic and granitoid plutons that has been interpreted as the roots of a Neoproterozoic (700–800 Ma) continental arc. This Andean-type arc is reported to extend from north-central Madagascar through the Seychelles into India. The ultramafic bodies (∼ 2 km × 1 km) are crudely zoned, with dunite forming the cores surrounded by harzburgite, lherzolite, pyroxenite, pegmatoidal pyroxenite and fine-grained gabbroic rocks on the margins of the intrusions. One sampled contact between an ultramafic entity and the basement is chilled and sharp, indicating an intrusive rather than a sheared contact; this argues against allochthonous emplacement as in ophiolite suites. The composition of this chilled basaltic margin is similar to volcanic lithologies from well-known continental Andean-type arcs. PGE data from dunites, harzburgites, lherzolites and websterites of the Lavatrafo body show enrichment in the Pd-group PGEs with significant Pt-enrichment and prominent negative Ru anomalies. [Ru/Ru*]N ranges from 0.037 to 0.31 with average [Ru/Ir]N = 0.17. These features are characteristic of zoned Alaskan–Uralian-type complexes ([Ru/Ru*]N = 0.029–0.15), which generally form the roots of volcanic arcs. Dunites, forming the cores of some intrusions, show enrichment in the LREE, are depleted with respect to chondrites (0.1× chondrite) and display negative Eu anomalies possibly caused by orthopyroxene fractionation. The Andriamena complexes are suggested to represent the roots or feeder conduits of a continental Andean-type arc and provide new evidence, in the form of ultramafic intrusions, for a convergent margin on the western edge of the Rodinia supercontinent. This tectonic setting is consistent with published data suggesting eastward subduction, on the eastern edge of the Mozambique Ocean, underneath the continental terrains of Madagascar, the Seychelles and NW India.
Platinum-group minerals (PGM) are present as very small grains (<10 μm) in chromitites and their dunite host in the Niquelandia layered intrusion. The chromite-hosted PGM occur as primary inclusions of sulphides (laurite, erlichmanite) and alloys (iridosomine, platinum-iron alloy). The compositional field of published laurite analyses from dunite-hosted stratiform chromitites is extended towards Os-rich compositions by the data presented here. The Os content of laurite increases upsection with a corresponding increase in Ti in the host chromite, possibly as a result of the increased degree of differentiation of the parental magma. The PGM in the dunite consists of Pt-Fe alloys included in orthopyroxene and Pd- and Pt-bearing tellurides, bismuthides and antimonides, some of these appear to be related to the magmatic segregation of an immiscible sulphide liquid, and some were possibly formed by the mobilization and re-deposition of Pt and Pd minerals during the serpentinization process. -from Authors
Platinum-group minerals (PGM) are present as very small grains (<10μm) in chromitites and their dunite host in the Niquelandia layered intrusion. The chromitehosted PGM occur as primary inclusions of sulphides (laurite, erlichmanite) and alloys (iridosomine, platinum-iron alloy). Laurite has a low Ir content (3–6 at%) and a Ru ratio (Ru/(Ru + Os + Ir)) in the range 0·90–0·53. The compositional field of published laurite analyses from dunite-hosted stratiform chromitites is extended towards Os-rich compositions by the data presented here. The Os content of laurite increases upsection with a corresponding increase in Ti in the host chromite, possibly as a result of the increased degree of differentiation of the parental magma.
The PGM in the dunite consist of Pt–Fe alloys included in orthopyroxene and Pdand Pt-bearing tellurides, bismuthides and antimonides, some of these appear to be related to the magmatic segregation of an immiscible sulphide liquid, and some were possibly formed by the mobilization and re-deposition of Pt and Pd minerals during the serpentinization process.
Platinum-group elements (PGE) are typically associated with mafic and ultramafic intrusive rocks and the main exploration targets are layers and zones rich in PGE-bearing sulphides. Some PGE occurrences, however, are in sulphide-poor situations and this raises the possibility that PGE deposits may be present in parts of mafic and ultramafic intrusives currently considered to have low exploration potential. A multidisciplinary study was undertaken on four subeconomic deposits of platinum-group metals to develop a model of formation for PGE deposits lacking significant base-metal sulphides. Two of the deposits occur in Albania, in the Tropoja and Bulqiza massifs, and are part of an ophiolitic belt created in an oceanic environment during the Upper Jurassic. The other two deposits occur in Madagascar, in the Andohankiranomena and Lavatrafo ultramafic massifs, and are within a Pan-African rifted zone. A Pt-rich chromitite style of mineralization was identified in the Andohankiranomena and Tropoja deposits, where the PGE are mostly included in chromite. A Pt- and Pd-rich silicate (dunite) style of mineralization was identified in the Lavatrafo and Bulqiza massifs, where PGE mineralization is associated with interstitial material between olivine grains. The four deposits have contrasting patterns of PGE distribution and individual element ratios, suggesting that different mineral species (alloys, arsenides and sulphides) host the PGE. No primary geochemical halos were detected around any of the deposits and weathering has little effect on the distribution of the PGE. The study showed that alloys and arsenides are the main carriers for platinum in all the deposits. A comparison of experimental results with natural phases in the deposits suggests that fluid-assisted exsolution of Pt, Ir and other elements from original higher-temperature solid solutions could be widespread. This supports the fluid-driven multistage mineralization concept suggested by field data. From the proposed genetic model it follows that the presence of chromite layers in an basic-ultrabasic complex increases the prospecting potential for PGE, that silicate rocks above the upper chromite reef are an exploration target and that the absence of significant base-metal sulphides does not preclude the presence of Pt and Pd concentrations.
Chromite, spinel and magnetite compositions from a wide range of geological environments have been accumulated from published and unpublished sources in order to better characterize the spinel-group mineral as a function of environment of formation. Approximately 17 000 analyses have been included in a spinel database. The factors that control the crystallization of spinel-group minerals from a basaltic melt are discussed, and their composition is compared for rocks from various terrestrial environments, including mid-ocean-ridge basalts, ocean-island basalts, boninites, ophiolites, mantle xenoliths and kimberlites. The variation in composition of the spinel-group minerals from terrestrial sources are then compared to that in chondritic meteorites. The composition of chromite in unequilibrated chondrites is consistent with crystallization from a chondritic melt, whereas its composition in equilibrated chondrites is consistent with metamorphism at temperatures in the range of 600-800°C. -Author
This paper presents new data on platinum-group-mineral (PGM)-bearing chromitites from the Ranomena mine, the only past-producing chromite mine in the North Toamasina chromite district of Eastern Madagascar. Such data can provide an insight on tectonic setting that led to emplacement of their host ultramafic body and can help to highlight the differences with other chromite deposits of Madagascar. Chromitite host rocks are harzburgite and pyroxenite and gangue minerals are orthopyroxene and minor olivine. Chromite mineral chemistry shows a strong affinity with chromitites from layered intrusions and PGE patterns are similar to those from the stratigraphically lowest chromitites in the Bushveld Complex. PGMs are dominated by laurite and are free of alloys. PGE mineralogy is consistent with early crystallization of laurite–erlichmanite at low fS2. Previous studies have suggested that the North Toamasina ultramafic bodies are fragments of dismembered ophiolites and that their presence within the so-called suture zone supports the suggestion that they mark the closure of an ocean between east and west Gondwana. Even though these ultramafic bodies have been interpreted as ophiolitic, our data better support an origin as small layered intrusions, rather than as fragments of oceanic crust.
The Gaositai complex is a typical Alaskan-type intrusion, emplaced in the northern margin of the North China Craton at about 280Ma. It is a concentrically zoned intrusion, with dunite in the core, rimmed by wehrlite, clinopyroxenite and hornblendite outwards. To understand the chemical compositions of the parent magma to the ultramafic complex, and thus that of the mantle source, we conduct petrological and geochemical studies on the dunite core and high-Mg clinopyroxene (cpx) inclusions in the chromitite from the rock unit, including the major and trace elements of cpx inclusions and whole-rock Re–Os isotope of the chromitites and chromite-rich dunites. The cpx inclusions have chemical compositions similar to the high Mg# cpx of primitive ankaramite, suggesting that the cpx inclusions could have formed from a primitive parent magma. The calculated chemical compositions of the primitive parent magma are characterized by high Mg# and CaO, with CaO/Al2O3 ratios >1, and enriched in Sr and other large ion lithophile elements compared to high field strength and rare earth elements. This, together with the high fO2 (up to FMQ+2.83) of chromitite, suggests that the parent magma was a typical arc ankaramite (wet and CaO-rich picrite), originated probably from a hydrous-carbonatite fluxed sub-continental lithospheric mantle above a subduction zone that formed as a result of the subduction of the paleo-Asian oceanic slab beneath the north China craton in late Paleozoic.
Spinel is often used as a magmatic indicator of crystallization processes, without considering the effects of metamorphic alteration on spinel geochemical features. Serpentinized melanges in the southern Urals host different kinds of disseminated to massive chromitite mineralization. In melange environments, intense metamorphic alteration above 300 degrees C leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chromian-chlorite. Based on textural and chemical analyses, melange-hosted Kalkan chromitites exhibit a hydration and oxidation reaction that can explain the formation of ferritchromite and chromian-chlorite from chromite and serpentine: [GRAPHICS] Textural analyses fit well with the proposed reaction and show that it usually proceeds very close to completion. The degree of alteration of chromite into ferritchromite is controlled by the initial chromite to serpentine ratio. In chromitites, high ratios prevent complete transformation of chromite into ferritchromitc. The most likely environment for such reaction is a prograde metamorphic event post-dating serpentinization of the Kalkan ophiolite, possibly related to emplacement within an accretionary wedge.
The Nurali lherzolite massif is one of the dismembered ophiolite bodies associated with the Main Uralian Fault (Southern Urals, Russia). It comprises a mainly lherzolitic mantle section, an ultramafic clinopyroxene-rich cumulate sequence (Transition Zone), and an amphibole gabbro unit.The cumulate section hosts small chromitite bodies at different stratigraphic heights within the sequence. Chromitite bodies from three different levels along a full section of the cumulate sequence and two from other localities were investigated. They differ in the host lithology, chromitite texture and composition, and PGE content and mineralogy. Chromitites at the lowest level, which are hosted by clinopyroxenite, form cm-scale flattened lenses. They have high Cr# and low Mg# chromites and are enriched in Pt and Pd relative to Os and Ir. At a higher, intermediate level, the chromitites are hosted by dunite. They form meter thick lenses, contain low Cr# and high Mg# chromites, have high PGE contents (up to 26,700 ppb), and are enriched in Os, Ir and Ru relative to Pt and Pd, reflecting a mineralogy dominated by laurite–erlichmanite and PGE–Fe alloys. At the highest level are chromitites hosted by olivine–enstatite rocks. These chromitites have high Cr# and relatively low Mg# chromites and very low PGE content, with laurite as the dominant PGE mineral.The platinum group minerals (PGMs) show extreme zoning, with compositions ranging from erlichmanite to almost pure laurite and from Os-rich to Ru-rich alloys, with variable and irregular zoning patterns.Two chromitite bodies up to 6 km from the main sequence can be correlated with the latter based on geochemistry and mineralogy, implying that the variations in chromitite geochemistry are due to processes that operated on the scale of the massif rather than those that operated on the scale of the outcrop.Pertsev et al. [Pertsev, A.N., Spadea, P., Savelieva, G.N., Gaggero, L., 1997. Nature of the transition zone in the Nurali ophiolite, Southern Urals. Tectonophysics 276, 163–180.] propose that the Transition Zone formed by solidification of a series of small magma bodies that partially overlapped in time and space. The magmas formed by successive partial melting of the underlying mantle. We suggest that this process determined the changing PGE geochemistry of the successive batches of magma. The PGE distribution fits a model of selected extraction from the mantle, where monosulphide solid solution–sulphide liquid equilibrium was attained until complete melting of the monosulphide solid solution. Later and localized variations in fS2 resulted in the formation of different PGMs with complex zoning patterns.
With a few notable exceptions, ophiolitic chromite deposits are scattered in a vast number of small bodies of irregular shape and distribution within ophiolite massifs. This has favoured bloom of hypotheses regarding their origin. During the last two decades, the interest in ophiolite studies has been fostered by their analogues with oceanic lithosphere; new concepts and approaches, in particular the structural and kinematic, have improved our understanding of ophiolites and of their chromite deposits. The object of this paper is to present, or recall, and discuss these new ideas concerning chromite concentration in ophiolites. -from English summary
New detrital zircon U–Pb age data obtained from various quartzite units of three spatially separated supracrustal packages in central and northern Madagascar, show that these units were deposited between 1.8 and 0.8Ga and have similar aged provenances. The distribution of detrital zircon ages indicates an overwhelming contribution of sources with ages between 2.5 and 1.8Ga. Possible source rocks with an age of 2.5Ga are present in abundance in the crustal segments (Antananarivo, Antongil and Masora Domains) either side of a purported Neoproterozoic suture (“Betsimisaraka Suture Zone”). Recently, possible source rocks for the 1.8Ga age peak have been recognised in southern Madagascar. All three supracrustal successions, as well as the Archaean blocks onto which they were emplaced, are intruded by mid-Neoproterozoic magmatic suites placing a minimum age on their deposition. The similarities in detrital pattern, maximum and minimum age of deposition in the three successions, lend some support to a model in which all of Madagascar's Archaean blocks form a coherent crustal entity (the Greater Dharwar Craton), rather than an amalgamate of disparate crustal blocks brought together only during Neoproterozoic convergence. However, potential source terranes exist outside Madagascar and on either side of the Neoproterozoic sutures, so that a model including a Neoproterozoic suture in Madagascar cannot be dispelled outright.
The UG2 chromitite and its immediate footwall have been mapped at the centimetre scale in a characteristic section exposed in a wall in the Middelpunt mine, northeast Bushveld Complex. In this region the UG2 chromitite and its coarse-grained footwall rocks are underlain and overlain by plagioclase pyroxenite. The UG2 chromitite itself is a massive, 70 cm-thick layer composed of 75 to 90% chromite with plagioclase as the dominant interstitial mineral. Immediately beneath the chromitite is the so-called mixed layer, a distinct, mappable layer, 50 to 70cm thick, consisting of chromite-bearing pegmatoidal pyroxenite and chromitite intimately mixed at scales of centimetres to decimetres. The mixture is chaotic with no megascopic fabric and contains in bulk about 15% chromite. The contact between the mixed layer and overlying chromitite is well defined but sinuous at scales of centimetres to decimetres. Based on observations of drill core and outcrop, the mixed layer appears to exist throughout the central sector of the E Bushveld and may be ubiquitous throughout the entire Complex. The mixed layer usually rests directly on the UG2 pegmatoid, which is a well known and widespread pegmatoidal plagioclase pyroxenite. The pegmatoid is much coarser grained than the underlying pyroxenite, but the two lithologic units are similar in terms of mineral modes and compositions. The contact between them is irregular at scales of decimetres to metres, with vertical fingers of pegmatoid extending into the pyroxenite. The mixed layer and UG2 pegmatoid contain rare lenses of massive anorthosite encased by continuous, centimetre-thick rims of chromitite. A model is proposed to account for the footwall sequence of the UG2 chromitite.
Detailed trace-element analyses of pure magnetite from four continuous borehole intersections through the main magnetitite layer from the upper zone of the Rustenburg Layered Suite of the Bushveld Complex are presented. One section has been analysed at one centimetre intervals. Rapid depletion of Cr occurs over short, vertical sequences near the base of the layer, which is due to bottom-crystallization and the resulting chemical depletion of a thin layer of liquid. Sudden increases in Cr content of magnetite are attributed to convection cells which bring undepleted magma into the zone or crystallization. We suggest that these cells individually have lateral extents no greater than hundreds of metres, but collectively may be traced at a specific stratigraphic horizon for several tens of km. This lateral traceability of the effect of convection cells at approximately uniform stratigraphic height demonstrates the long-held implicit assumption that time-planes are in general parallel to the layering, and does not support the hypothesis that layers in the Bushveld Complex grew laterally. The activity of these cells is highly variable, with long periods of quiescence interspersed with periods of rapid, small-scale overturn. Most convection cells do not impinge upon the floor, and the abruptness of the resulting chemical reversal is largely a function of the thickness of the layer of depleted liquid trapped between the cumulates and the sole of the convection cell. Occasionally, these cells do touch the cumulate pile and may even cause erosion. This material may be redeposited elsewhere in the magnetitite layer either as mineralogically distinct fragments if erosion penetrated below the layer or, in the present instance, as a chemically chaotic pile of magnetite. The abruptness of the chemical reversals severely restricts the extent to which post-cumulus redistribution of elements or re-equilibration with percolating trapped liquid (infiltration metasomatism) may have occurred. The appearance of disseminated plagioclase in magnetite layers in variable proportions and in a non-systematic manner in the four profiles is attributed to fluctuations in pressure.
The Neoproterozoic global reorganisation that saw the demise of Rodinia and the amalgamation of Gondwana took place during an incredibly dynamic period of Earth evolution. To better understand the palaeogeography of these times, and hence help quantify the interrelations between tectonics and other Earth systems, we here integrate Neoproterozoic palaeomagnetic solutions from the various blocks that made up eastern Gondwana, with the large amount of recent geological data available from the orogenic belts that formed as eastern Gondwana amalgamated. From this study, we have: (1) identified large regions of pre-Neoproterozoic crust within late Neoproterozoic/Cambrian orogenic belts that significantly modify the geometry and number of continental blocks present in the Neoproterozoic world; (2) suggested that one of these blocks, Azania, which consists of Archaean and Palaeoproterozoic crust within the East African Orogen of Madagascar, Somalia, Ethiopia and Arabia, collided with the Congo/Tanzania/Bangweulu Block at ∼ 650–630 Ma to form the East African Orogeny; (3) postulated that India did not amalgamate with any of the Gondwana blocks until the latest Neoproterozoic/Cambrian forming the Kuunga Orogeny between it and Australia/Mawson and coeval orogenesis between India and the previously amalgamated Congo/Tanzania/Bangweulu–Azania Block (we suggest the name ‘Malagasy Orogeny’ for this event); and, (4) produced a palaeomagnetically and geologically permissive model for Neoproterozoic palaeogeography between 750 and 530 Ma, from the detritus of Rodinia to an amalgamated Gondwana.
The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U–Pb zircon analyses to provide an event history for its evolution.The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320 ± 14 Ma to 3231 ± 6 Ma and 3187 ± 2 Ma to 3154 ± 5 Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178 ± 2 Ma and subject to migmatisation at around 2597 ± 49 Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570 ± 18 Ma and 2542 ± 5 Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531 ± 13 Ma and 2513 ± 0.4 Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147 ± 6 Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton.The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792 ± 130 Ma to 763 ± 13 Ma and 553 ± 68 Ma. These events are interpreted as being the only manifestation of the Pan-African orogeny seen in the craton, which led to the assembly of the tectonic blocks that comprise the island.
Madagascar lay in an interesting position in Gondwana, straddling one of the largest orogens that formed as the supercontinent amalgamated. The Malagasy basement preserves a record of the timing and style of this amalgamation, and in addition contains much information as to the palaeogeography of the eastern Mozambique Ocean.Madagascar consists of a number of tectonic units that amalgamated in the Ediacaran–Cambrian. The tectonic units are: The Antongil Block; the Antananarivo Block; the Tsaratanana Sheet and the Bemarivo Belt. In addition to these, there are a number of regions dominated by Neoproterozoic metasedimentary rocks, including the Molo, Betsimisaraka, Vohibory and Androyen regions. In this review I outline these units, discuss their amalgamation history and implications for Neoproterozoic–Cambrian palaeogeography, and highlight a few key questions for future study.
Chromitite layers in the Critical Zone of the Bushveld complex display a systematic change in the chondrite normalized platinum group element (PGE) concentration patterns. The bell-shaped pattern of the lower layers of the lower group of chromitite layers suggests a very low sulphide content and that ruthenium, osmium and iridium are largely contained as magmatic platinum group minerals (PGM) in the chromite grains. Elevated rhodium concentrations compared to platinum and palladium suggests a possible solid solution component of rhodium in chromite. The upper of the lower group of chromitite layers and the middle group of layers have gentle positive patterns, which suggest the presence of a base metal sulphide component. Pt (Pt+Pd) ratios within these layers can be related to the thickness of the chromitite layer and total PGE content. Positive slopes for the upper group of chromitite layers and the Merensky Reef reflects sulphide dominant PGE mineralization of samples from these layers used for this investigation. It is also demonstrated that all chromitite layers of the Bushveld complex obtained their PGE content largely by a combination of enclosure in chromite of early, magmatic PGM's and subsequent sulphide collection from a smaller volume of magma as that from which the chromite crystallized. The results also indicate that the Ir Os ratio seems to be a sensitive petrogenetic indicator. Variations in this ratio over the sequence of chromitite layers suggest that the B2 B3 parental melts of the Bushveld had a higher Ir Os ratio than the earlier B1 parental magma.
The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO2 content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.
The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen.U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between ∼800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200–3000, ∼2650, ∼2500 and 800–700 Ma.The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.
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