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Mingguo Zhai - One of the best experts on this subject based on the ideXlab platform.
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formation of 2 5 ga sittampundi Anorthosite complex in southern india implications to lower crustal stabilization of the dharwar craton
Precambrian Research, 2021Co-Authors: Yuquan Wang, P M George, K Sajeev, Jinghui Guo, Chunkit Lai, Mingguo ZhaiAbstract:Abstract Mantle-derived magmas at the base of the lower crust exerted a key control on late Archean cratonization in many continents. Since well-preserved, complete lower crustal section is rarely exposed, direct studies on the genetic link between mantle-derived magmas and cratonic lower crustal stabilization are inadequate. Cratonic lower crustal section is well-preserved in the southern margin of the Dharwar Craton (southern India), with a number of late Archean anorthositic-gabbroic complexes. Among these complexes, the Sittampundi Anorthosite complex (SAC) consists of white- and dark-Anorthosite (>60 vol.%), gabbros, and ultramafic rocks. In this study, SIMS zircon U-Pb dating of the Anorthosite revealed a minimum emplacement age of 2522 ± 12 Ma, similar to the chromite Os model ages (2528–2563 Ma) of the Anorthosite-hosted chromitite. In-situ plagioclase (87Sr/86Sr)i ratios (0.70079–0.70100) of the dark Anorthosite and the chromite γOs (T) values (-0.2 to -0.4) of the chromitite suggest that the SAC was derived from a depleted mantle source. From the dark to white Anorthosite, the (87Sr/86Sr)i ratios increase while the An contents decrease, suggesting crustal assimilation occurred during fractionation. Similarly, the mantle-like zircon δ18O values and relatively-wide eHf(T) (-2.1 to +8.4) range of the SAC Anorthosite suggest that the parental magma had assimilated the ancient mafic lower crust. Emplacement ages of the SAC and published ages of the mafic/felsic granulites and charnockites altogether indicate that the Anorthosites were formed during the Dharwar cratonization, and that the mantle-derived magma underplating may have led to extensive lower crustal melting. We argued that during underplating, high-density olivine-pyroxene cumulates (from fractionation of the mantle-derived magma) and partial-melting residues (in the overlying lower crust) mostly sank back to the underlying mantle. In contrast, the lower-density plagioclase and minor amphibole remained in the lower crust to form anorthositic-gabbroic sills. The magmas underplating and subsequent lower-crustal melting have likely made the cratonic lower crust more refractory and buoyant, which facilitated cratonization.
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lower crust mantle interactions in the massif type Anorthosite formation new evidence from zircon u pb hf o isotopes of the neoproterozoic kadavur complex southern india
Lithos, 2021Co-Authors: Mingguo Zhai, Yan Zhao, Chengli Zhang, P M George, K Sajeev, P Rajkumar, Longlong GouAbstract:Abstract The Kadavur massif-type Anorthosite, which intruded the basement of the Madurai Block in southern India, comprises Anorthosite, leucogabbro, and (noritic) gabbro with minor Fe-Ti oxide ores. The U-Pb zircon SIMS dating indicates that the magmatism occurred during the Neoproterozoic period (Tonian), i.e., noritic gabbro (790.4 ± 5.1 Ma), gabbro (793.0 ± 4.1 Ma), leucogabbro (784.9 ± 4.1 Ma), and Anorthosite (790.7 ± 4.3 Ma). Combined with the published age data from the coeval charnockite and A-type granite, we conclude that the Anorthosite-charnockite-rapakivi granite suite was emplaced during ca. 785–805 Ma. In-situ zircon Hf-O isotopic data of the Kadavur gabbros and Anorthosite provide insights on the source and evolution of the parental magmas of the massif-type Anorthosites. Zircons from the Anorthosite are characterized by ancient crustal eHf(T) values (−11.4 to −6.5) and higher δ18O values (5.92 to 6.4‰). In contrast, zircons from the coarse-grained gabbro have relatively primitive eHf(T) values (−6.3 to −2.3) and mantle-like δ18O values (4.86–5.73‰). Moreover, zircons from the noritic gabbro and leucogabbro have eHf(T) values (−11.1 to −3.7) and δ18O values (5.35 to 6.77‰) distributed between those of the Anorthosite and coarse-grained gabbro. The new zircon Hf-O data demonstrate that the parental melt of the Anorthosite was subjected to crustal contamination during the early evolution stages, producing an evolved magma with crustal isotope and trace element signatures. In contrast, the gabbros show less crustal influence and like to represent original magma source of the Kadavur Complex. The zircon Hf-O isotope compositional array from the primitive gabbros to the more-evolved Anorthosite demonstrates that the parental magmas were derived from partial melting of the upper mantle with varying crustal input, which can be up to 30–40% for the Anorthosite formation. Contamination of the ponded basaltic magmas by the felsic crust can effectively increase the SiO2, Al2O3, Na2O, and Sr contents in the magmas, which was likely essential for enormous plagioclase fractionation in the massif-type Anorthosites.
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lower crustal contribution to the magma formation of the damiao massif type Anorthosite north china craton evidence from zircon hf o isotopes
Precambrian Research, 2019Co-Authors: Xieyan Song, Mingguo ZhaiAbstract:Abstract The formation of massif-type Anorthosites and the associated mangerite, charnockite and (rapakivi-) granite (AMCG suite) are mostly restricted in the “Earth’s Middle Age” (~1.70–0.75 Ga), and the magma sources of massif-type Anorthosites are highly debated between the depleted mantle and the mafic lower crust. The ~1.7 Ga Damiao massif-type Anorthosite in the North China Craton consists mainly of Anorthosite and leuconorite with minor melanorite, mangerite, Fe-Ti-(P) ores and abundant ferrodioritic or gabbroic dykes. In-situ zircon Hf-O isotopic data from Anorthosite reveal decoupling of mantle-like δ18O values (5.1–7.4) and ancient crustal-like eHf(T) values (−4.0 to −8.9). Since the significant 177Lu/176Hf reduction in the lower mafic crust, the mafic lower crustal materials derived from depleted mantle could display an ancient crustal medium to high negative eHf(T) values after long period of time (e.g., 0.5–1.0 Ga). In contrast, if no supracrustal materials were added, these mafic crustal materials would retain the mantle δ18O features. Thus, the decoupling of mantle-like δ18O and ancient crustal-like eHf(T) values suggest a lower crustal origin of the Damiao massif-type Anorthosite. Compilation of Sr-Nd-O isotopic data of massif-type Anorthosites reveal decoupling of eHf(T), initial 87Sr/86Sr and δ18O values, which signifies important lower crustal input in magmas sources of most massif-type Anorthosites. We proposed that zircon U-Pb age of the Damiao massif-type Anorthosite (~1.70 Ga) and regional age correlation suggest that the Anorthosite was emplaced in an intraplate, anorogenic extensional setting after the Paleoproterozoic orogeny had ended.
Nilanjan Chatterjee - One of the best experts on this subject based on the ideXlab platform.
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mineralogy and geochemistry of the bengal Anorthosite massif in the chotanagpur gneissic complex at the eastern indian shield margin
Journal of The Geological Society of India, 2008Co-Authors: N C Ghose, Nilanjan Chatterjee, Dipankar Mukherjee, R W Kent, A D SaundersAbstract:The Bengal Anorthosite is a narrow 40 km long, "tadpole-shaped" massif that occurs within granulite facies rocks of the Proterozoic Chotanagpur Gneissic Complex at the eastern margin of the Indian shield. The core of the massif consists of grey Anorthosite with coarse-grained cumulus plagioclase megacrysts showing magmatic flow-related alignment and the periphery consists of a mixture of the megacrysts and medium-grained, equigranular white Anorthosite. Repetitive graded layers of grey and white Anorthosite and cyclic variation of elemental concentrations characterize the massif at depths. These features are consistent with emplacement of the Bengal Anorthosite through episodic magma pulses. Labradorite (An 57-58 ) is the major constituent with clinopyroxene (Mg# 62), hornblende (Mg# 36-44), ilmenite and occasional orthopyroxene occurring as minor phases. Thermobarometric pressure-temperature estimates of the Anorthosites (4.1-7.3 kbar and 593-795 o C) are similar to earlier studies achieved from the metabasic and gneissic country rocks, and correspond with the last high-grade (Grenvillian) metamorphism. Similar Zr/Nb, Zr/Hf and Th/Ce ratios of the Anorthosites and oceanic island basalt possibly indicates derivation from a mantle source. Lower crustal interaction is evident from similar Zr/Y, La/Nb and Th/Ce ratios of the Anorthosites and lower continental crust. Anatectic upper crustal melts probably contaminated the Anorthosites as indicated from an enriched LILE pattern of the Anorthosite. Metabasic rocks associated with the Anorthosites have lower crustal Zr/Y, Nb/Y and Zr/Nb ratios. Minor gabbroic Anorthosites within the massif, rich in iron and incompatible elements, were perhaps derived by differentiation of a coeval mafic parental magma. Proximity of the Anorthosite to the Damodar Graben indicates that the Bengal Anorthosite may have been emplaced in an extensional tectonic setting.
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crystallization history of a massif Anorthosite in the eastern indian shield margin based on borehole lithology
Journal of Asian Earth Sciences, 2005Co-Authors: Dipankar Mukherjee, N C Ghose, Nilanjan ChatterjeeAbstract:Abstract The Bengal Anorthosite occurs as a large tadpole-shaped pluton (250 km 2 ) in the granulite facies terrain of the Proterozoic Chotanagpur Gneiss Granulite Complex at the northeastern edge of the Indian peninsular shield. Its axis of elongation conforms to the general strike (ENE–WSW) of the country rocks. It is bounded by a (Gondwana) basin margin fault in the north and it shows an interfingering contact with the country rocks at its eastern and western edges. Deep drilling, attaining a maximum depth of 622.85 m, reveals a cyclic order of grey, white and mottled Anorthosites of variable thickness. The possibility of Anorthosite extending further below contradicts the 200 m thickness of Anorthosite previously estimated from gravity modeling. Chemical data also indicate a cyclic variation of elemental concentrations and their ratios with depth. In each chemical cycle, the grey plagioclase megacrysts apparently floated over a relatively denser white granular plagioclase with higher anorthite contents. The base of a chemical cycle also contains higher concentrations of transition elements—a feature arising plausibly by sinking of Fe–Ti oxides. The chemical cyclicity possibly indicates derivation of melt in batches and emplacement of the crystal laden-melt by impulses. Minor presence of orthopyroxene in the Anorthosite suggests a tholeiitic source.
Birger Rasmussen - One of the best experts on this subject based on the ideXlab platform.
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role of fluids in fe ti p mineralization of the proterozoic damiao Anorthosite complex china insights from baddeleyite zircon relationships in ore and altered Anorthosite
Ore Geology Reviews, 2019Co-Authors: Birger Rasmussen, Stephen Sheppard, Simon A Wilde, Jie MengAbstract:Abstract The Damiao Fe–Ti–P deposit offers a rare opportunity for studying late-stage Fe–Ti ore-forming processes in Proterozoic Anorthosites. The orebodies are hosted in Anorthosite and commonly show chlorite-dominated alteration in the contact zone on both sides, but the nature and origin of fluids in Fe-Ti-P mineralization remains contentious. Baddeleyite is a common accessory mineral in Anorthosites and Fe–Ti–P orebodies, and typically occurs as blebs and lamellae in primary ilmenite reflecting decreasing solubility of Zr in ilmenite during slow cooling and consequent exsolution of ZrO2. Two types of zircon are identified in the Fe–Ti–P orebodies and altered Anorthosite at Damiao, and both are related to hydrothermal replacement of baddeleyite by Si-rich fluids. The type-I zircon shows subhedral to anhedral shapes with variable sizes (5–50 μm) in Fe–Ti–P orebodies, and coexists with magnetite–rutile symplectite formed by ilmenite breakdown. In contrast, the type-II zircon typically occurs as tiny aggregates in chlorite–quartz–titanite replacement fronts of altered Anorthosite, indicative of a hydrothermal origin. The type-I zircon yielded an age of 1739 ± 16 Ma, similar to the age of baddeleyite previously reported for the orebodies. Formation of the type-I zircon is related to the replacement of baddeleyite in the presence of Si-enriched hydrothermal fluids evolved from magma. Ti-in-zircon geothermometry indicates a fluid temperature of >700 °C for the formation of the type-I zircon. However, homogenization temperature of fluid inclusions in co-precipitated apatite suggests that the type-II zircon in altered Anorthosite may have formed by later hydrothermal fluids at temperature of ~350 °C. Our results indicate that the Fe–Ti–P mineralization of the Damiao Anorthosite complex involved hydrous melts and magmatic–hydrothermal processes, with the Fe–Ti oxides being formed at the magmatic stage and apatite at the hydrothermal stage.
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the link between an Anorthosite complex and underlying olivine ti magnetite rich layered intrusion in damiao china insights into magma chamber processes in the formation of proterozoic massif type Anorthosites
Contributions to Mineralogy and Petrology, 2019Co-Authors: Birger Rasmussen, Stephen Sheppard, Jie Meng, Zhe SongAbstract:Mafic–ultramafic intrusions comagmatic with Proterozoic massif-type Anorthosites can provide insights into the parental magma from which large volumes of hyper-feldspathic rocks are produced. Recent deep drilling has unveiled a large olivine–Ti-magnetite-rich layered intrusion (named Dawusunangou) beneath the Damiao massif-type Anorthosite complex in the North China Craton. The layered intrusion is composed of alternating olivine–Ti-magnetite-rich dark layers and plagioclase-rich light layers (ca. 35–80% plagioclase), with the latter also containing pod- or lens-shaped pyroxene–Ti-magnetite-rich aggregates. This layered intrusion shows low Mg# and REE patterns similar to the overlying Damiao Anorthosite complex. Baddeleyite Pb–Pb geochronology yielded indistinguishable crystallization ages of ca. 1735 Ma for both the Dawusunangou layered intrusion and the Damiao Anorthosite complex, suggesting coeval emplacement. Using the average bulk compositions of the two intrusions, mass balance calculations assuming 30–40% Dawusunangou and 70–60% Damiao would give a composition similar to high-Al basaltic magma. Collectively, these features indicate that the Dawusunangou layered intrusion represents the mafic residues after the segregation of the Damiao Anorthosites from high-Al basaltic parental magma. A short-lived magma chamber is thought to have supplied the two intrusions. In situ crystallization with variable nucleation rates for plagioclase combined with the mafic minerals crystallizing in equilibrium proportions resulted in the formation of repeated dark and light layers of the Dawusunangou layered intrusion. The two intrusions are interpreted to have formed by multiple magma injections, instead of continuous differentiation of one melt. The parental magma was derived from a depleted mantle source with significant crustal contribution during magma evolution. The large Nd–Hf isotopic variations suggest contamination by Paleoarchean to Neoarchean crust.
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the kunene Anorthosite complex namibia and its satellite intrusions geochemistry geochronology and economic potential
Economic Geology, 2013Co-Authors: Wolfgang Maier, Birger Rasmussen, Ian R Fletcher, Sarahjane Barnes, Hannu HuhmaAbstract:The Kunene Complex of Namibia-Angola is one of the largest Anorthosite massifs on Earth (up to 18,000 km2), consisting of several distinct Anorthosite and leucotroctolite intrusions. The Namibian portion of the Kunene Complex measures ~80 × 50 km, ~4,000 km2, and is dominated by the Zebra Mountain lobe, a ~16-km-thick dome-like mass of interlayered, relatively unaltered dark leucotroctolite with relatively altered, “white,” Anorthosite. Past studies and the present work have found evidence for intrusion of two distinct phases of dark leucotroctolite into the white Anorthosite, namely a relatively early, deformed, phase dated at 1363 ± 17 Ma (U-Pb in baddelyite), and a relatively later and undeformed phase whose absolute age remains unknown. The Kunene leucotroctolites are among the least evolved troctolites known from Anorthosite complexes, with olivine containing 59 to 77 mol % forsterite and up to 1,700 ppm Ni, and plagioclase containing 56 to 69 mol % Anorthosite. Our isotope data from the troctolites indicate a relatively small crustal component (δ18O, ~5.3–7.3; δ34S, 0.5–1; and ɛNdT, 0.9–1.8), whereas Nd and oxygen isotope data from the white Anorthosites, published by other workers, showed a slightly larger crustal component (e.g., ɛNdT as low as −3; δ18O up to 7.5‰). In the periphery of the Kunene Complex are several, relatively small (<10 km2), mafic-ultramafic intrusions comprising peridotite, pyroxenite, gabbro, troctolite, and Anorthosite. Some of these bodies are Ni-Cu-PGE mineralized, including the Ohamaremba troctolite, the Oncocua pyroxenite, and the Ombuku peridotite-gabbronorite. The latter additionally contains a massive chromitite layer. A new U-Pb baddelyite age of 1220 ± 15 Ma for Ohamaremba indicates that the latter postdates the main Kunene Complex by ~140 Ma. The relative enrichment in MgO, Cr, and Ni, and the O, Nd, and S isotope characteristics of Kunene magmatism suggest that the primary magmas were predominantly mantle-derived picrites or basalts. The massif-type Anorthosites formed through ascent of feldspathic slurries followed by downward draining of residual liquid. Subsequent magma pulses formed troctolitic sills within the Anorthosite plutons and mafic-ultramafic satellite intrusions in the periphery of the Anorthosites. The recurring nature of Kunene mafic-ultramafic magmatism results from several successive mantle upwellings. Partial mantle melts ascended through reactivated translithospheric lineaments along the southern margin of the Congo craton.
Robert F Dymek - One of the best experts on this subject based on the ideXlab platform.
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Comparative Petrology of the Montpelier and Roseland Potassic Anorthosites, Virginia
The Canadian Mineralogist, 2016Co-Authors: Brent E. Owens, Robert F DymekAbstract:Abstract The Montpelier and Roseland plutons are the southernmost, and among the youngest and smallest, examples of massif Anorthosite in North America. They occur in separate lithotectonic terranes, but strong similarities between them suggest common factors in their petrogenesis. Both massifs represent “alkalic Anorthosite” dominated by antiperthitic plagioclase that yield reconstituted K-rich compositions averaging ∼An 26 Or 19 and ∼An 29 Or 16 , respectively. Concentrations of Rb (∼10–30 ppm), Sr (∼1100–1300 ppm), and Ba (∼800–1400 ppm) are likewise high in both, and Montpelier is unique in being the only known Anorthosite in which Ba levels exceed those of Sr. Montpelier and Roseland are the most potassic of all massif Anorthosites on Earth and their feldspar compositions appear unique among plutonic igneous rocks. In addition to such alkalic compositions (and in contrast to most other massif Anorthosites), quartz is a common accessory mineral, belying any direct link to undersaturated alkali basalt. Furthermore, compositions of co-occurring plagioclase and pyroxene in Montpelier and Roseland (%An versus %En) lie well off trends defined for basaltic intrusions. Another distinctive feature of Montpelier and Roseland is their Fe,Ti-oxide assemblage of ilmenite + rutile, which contrasts with the ilmenite ± magnetite assemblage typical of most mafic intrusions. This ilmenite + rutile assemblage also contrasts with the widespread occurrence of hemo-ilmenite in similarly young, alkalic Anorthosites in the Grenville Province of Quebec ( e.g ., St-Urbain, Labrieville, Château-Richer, Mattawa). Such differences indicate that the Virginia Anorthosites crystallized under low f O 2 conditions compared to their Quebec counterparts, despite higher silica activity. We suggest two alternatives for the origin of the compositionally distinct magmas that gave rise to these Anorthosites: (1) melting of an atypically alkalic source; or (2) significant contamination of mantle-derived magmas by evolved crust. Regardless of mechanism, the origin of these Anorthosites is probably linked to their timing as late- to post-tectonic intrusions, and may be related to orogen-scale extension following crustal thickening.
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geochemistry of massif Anorthosite and associated rocks adirondack mountains new york
Geosphere, 2010Co-Authors: Karl E Seifert, Robert F Dymek, Philip R Whitney, Larry A HaskinAbstract:Geochemical data for a comprehensive suite of over 700 samples of massif Anorthosite and associated rocks from the Adirondack Mountains, New York, exhibit a ubiquitous mixed tholeiitic and calc-alkaline signature indicating a complex petrogenesis. The origin and relationship of massif Anorthosite to associated rocks has been a major petrological problem despite decades of study. The Adirondack Mountains is one of the best areas to study these rocks because it contains one of the world9s most abundant occurrences of massif Anorthosite and associated rocks forming bedrock over thousands of square kilometers. The suite of rocks analyzed includes both Anorthosite suite rocks, consisting of Anorthosites, leucogabbros, gabbros, oxide apatite gabbronorites (OAGNs), and oxide gabbronorites (OGNs) (defined in text), and mangerite suite rocks consisting of jotunites, monzodiorites, mangerites, and charnockites. Representative major-element compositions were determined largely by X-ray fluorescence (XRF) analysis for 352 massif Anorthosites and associated rocks and a variety of trace elements were determined by XRF, instrumental neutron activation analysis (INAA), and inductively coupled plasma mass spectroscopy (ICP-MS) for 296 massif Anorthosites and associated rocks. All rock types show a mixture of tholeiitic and calc-alkaline compositional characteristics with major elements exhibiting a strong iron enrichment tholeiitic trend and trace elements showing a depletion of Nb and Ta characteristic of calc-alkaline rocks. Prior to this study the geochemistry of these rocks in the Adirondacks has been only poorly characterized from scattered local studies. The Anorthosite suite of rocks exhibits two distinct compositional trends. Massif Anorthosites, leucogabbros, gabbros, OAGNs, OGNs, along with jotunites, separate into two distinct compositional trends on P 2 O 5 -MgO and TiO 2 -MgO diagrams, whereas monzodiorites, mangerites, and charnockites have only one compositional trend. Two trends in Anorthosites are caused by two varieties of Anorthosite: one type with a characteristic mineralogy dominated by plagioclase plus pyroxene and another type dominated by plagioclase plus oxide minerals and apatite. Mafic enclaves at some localities near the margins of Anorthosite masses contain gabbro, OGN, and OAGN in close spatial association, suggesting they represent crystallization from the same or similar parental magmas at different stages of evolution.
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Rediscovery of the Mattawa Anorthosite Massif, Grenville Province, Quebec
Canadian Journal of Earth Sciences, 2005Co-Authors: Brent E. Owens, Robert F DymekAbstract:We present new field observations and laboratory data confirming the presence of the Mattawa Anorthosite Massif (MAT), whose existence in south-central Quebec was hinted at more than 35 years ago. MAT thus represents a newly recognized member of the late- to post-tectonic ~10601010 Ma andesine Anorthosite belt that includes the Château-Richer, St. Urbain, and Labrieville massifs. The dominant rock type at MAT is foliated andesine Anorthosite or leuconorite, and orientations of foliations indicate that the pluton has the shape of a dome. MAT contains separate core and border zones, which are distinguished on the basis of plagioclase composition and concentrations of Ga, Rb, Sr, and Ba. Xenoliths of labradorite Anorthosite having Ga, Sr, and Ba concentrations different from those of the host andesine Anorthosites occur sporadically throughout the pluton as well. Lastly, rocks enriched in Fe, Ti, and P (jotunite, oxideapatite gabbronorite, nelsonite, ilmenitite) also occur at MAT, primarily near the coreb...
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A Geochemical Reconnaissance of the Roseland Anorthosite Complex, Virginia, and Comparisons with Andesine Anorthosites from the Grenville Province, Quebec
Proceedings of the International Conferences on Basement Tectonics, 1999Co-Authors: Brent E. Owens, Robert F DymekAbstract:The occurrence of the Roseland Anorthosite in Grenville-aged basement of the Blue Ridge Province of Virginia invites comparison to massif Anorthosites of similar age (~1000-1160 Ma) and setting found in the Grenville Province of Quebec (Fig. 1). Indeed, more than 50 years ago, Ross (1941) drew attention to certain similarities between the titanium deposits associated with the Roseland Anorthosite and those found with the St. Urbain Anorthosite, Quebec. Decades later, Herz (1969) pointed out that the Roseland and St. Urbain bodies are both highly alkalic, each containing antiperthitic andesine as the dominant mineral. Herz (1984) and Herz and Force (1984, 1987) also described a suite of Fe-, Ti-, and P-rich rocks at Roseland, including ferrodiorite and the renowned nelsonites (ilmenite-apatite rocks) of Watson and Taber (1913). Similar suites of “FTP” rocks — some called by different names — are also found at St. Urbain, as well as with virtually all other Anorthosites in Quebec.
Dipankar Mukherjee - One of the best experts on this subject based on the ideXlab platform.
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mineralogy and geochemistry of the bengal Anorthosite massif in the chotanagpur gneissic complex at the eastern indian shield margin
Journal of The Geological Society of India, 2008Co-Authors: N C Ghose, Nilanjan Chatterjee, Dipankar Mukherjee, R W Kent, A D SaundersAbstract:The Bengal Anorthosite is a narrow 40 km long, "tadpole-shaped" massif that occurs within granulite facies rocks of the Proterozoic Chotanagpur Gneissic Complex at the eastern margin of the Indian shield. The core of the massif consists of grey Anorthosite with coarse-grained cumulus plagioclase megacrysts showing magmatic flow-related alignment and the periphery consists of a mixture of the megacrysts and medium-grained, equigranular white Anorthosite. Repetitive graded layers of grey and white Anorthosite and cyclic variation of elemental concentrations characterize the massif at depths. These features are consistent with emplacement of the Bengal Anorthosite through episodic magma pulses. Labradorite (An 57-58 ) is the major constituent with clinopyroxene (Mg# 62), hornblende (Mg# 36-44), ilmenite and occasional orthopyroxene occurring as minor phases. Thermobarometric pressure-temperature estimates of the Anorthosites (4.1-7.3 kbar and 593-795 o C) are similar to earlier studies achieved from the metabasic and gneissic country rocks, and correspond with the last high-grade (Grenvillian) metamorphism. Similar Zr/Nb, Zr/Hf and Th/Ce ratios of the Anorthosites and oceanic island basalt possibly indicates derivation from a mantle source. Lower crustal interaction is evident from similar Zr/Y, La/Nb and Th/Ce ratios of the Anorthosites and lower continental crust. Anatectic upper crustal melts probably contaminated the Anorthosites as indicated from an enriched LILE pattern of the Anorthosite. Metabasic rocks associated with the Anorthosites have lower crustal Zr/Y, Nb/Y and Zr/Nb ratios. Minor gabbroic Anorthosites within the massif, rich in iron and incompatible elements, were perhaps derived by differentiation of a coeval mafic parental magma. Proximity of the Anorthosite to the Damodar Graben indicates that the Bengal Anorthosite may have been emplaced in an extensional tectonic setting.
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crystallization history of a massif Anorthosite in the eastern indian shield margin based on borehole lithology
Journal of Asian Earth Sciences, 2005Co-Authors: Dipankar Mukherjee, N C Ghose, Nilanjan ChatterjeeAbstract:Abstract The Bengal Anorthosite occurs as a large tadpole-shaped pluton (250 km 2 ) in the granulite facies terrain of the Proterozoic Chotanagpur Gneiss Granulite Complex at the northeastern edge of the Indian peninsular shield. Its axis of elongation conforms to the general strike (ENE–WSW) of the country rocks. It is bounded by a (Gondwana) basin margin fault in the north and it shows an interfingering contact with the country rocks at its eastern and western edges. Deep drilling, attaining a maximum depth of 622.85 m, reveals a cyclic order of grey, white and mottled Anorthosites of variable thickness. The possibility of Anorthosite extending further below contradicts the 200 m thickness of Anorthosite previously estimated from gravity modeling. Chemical data also indicate a cyclic variation of elemental concentrations and their ratios with depth. In each chemical cycle, the grey plagioclase megacrysts apparently floated over a relatively denser white granular plagioclase with higher anorthite contents. The base of a chemical cycle also contains higher concentrations of transition elements—a feature arising plausibly by sinking of Fe–Ti oxides. The chemical cyclicity possibly indicates derivation of melt in batches and emplacement of the crystal laden-melt by impulses. Minor presence of orthopyroxene in the Anorthosite suggests a tholeiitic source.