Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Naresh C. Ghose ¹, Nilanjan Chatterjee ², Dipankar Mukherjee ³, Ray W. Kent ⁴, A. D. Saunders ⁵

Affiliations
1 Department of Geology, Patna University, Patna - 800 005, IN
2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, US
3 Geological Survey of India, Kankarbag, Patna - 800 020, IN
4 Research Office, Rutland Hall, Loughborough University, Loughborough, Leicester LEI 3TU, GB
5 Department of Geology, Leicester University, Leicester LEI 7RH, GB

Abstract

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 Labradonte (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 (41-73 kbar and 593-795°C) are similar to earlier studies achieved from the metabasic and gneissic country rocks and correspond with the last high-grade (Grenvilhan) 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 m an extensional tectonic setting.

Keywords

Bengal Anorthosite, Chotanagpur Gneissic Complex, Labradonte, Cyclic Layers, Damodar Graben.

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Nilanjan Chatterjee ¹, Somdev Bhattacharji ²

Affiliations
1 Department of Earth, Atmospheric and Planetary Sciences, Room 54-1216, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, US
2 Department of Geology, Brooklyn College and Graduate Center of the City University of New York, Brooklyn, New York 11210, US

Abstract

The variation in incompatible trace element contents of lower basaltic dykes and flows from different parts of the Deccan province was assessed through modelling of partial melting of primitive mantle and fractional crystallization of the parental melts coupled with crustal assimilation (AFC). Variations in Ba/Zr, Rb/Y and Nb/Y can be attributed to different degrees of partial melting of mantle. Variations in Zr, rare earth elements and Rb/Nb indicate fractional crystallization and significant coupled crustal assimilation away from the Deccan plume center. The Zr variation near the plume center can be explained through only fractional crystallization without the involvement of crust. AFC modelling of available Sr-Nd isotope data requires very high and unreasonable amounts of crustal assimilant implying that the parental magmas may have acquired their isotopic characteristics before AFC occurred in crustal magma chambers. The occurrence of AFC away from the plume center in the Narmada-Tapti rift region may be related to longer and greater magma-Wall rock interaction in shallow crustal magma chambers due to crustal extension-Related enlargement of the magma chambers, recharge with fresh, hot magma and convective mixing.

Keywords

Deccan, Basalt, Trace Element, Partial Melting, Fractional Crystallization, Crustal Assimilation.

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