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M. E. A. Mondal ¹, R. Chandra ², T. Ahmad ³

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh - 202 002, IN
2 Department of Geology, Bundelkhand University, Jhansi - 284 128, IN
3 Department of Geology, University of Delhi, Delhi - 110 007, IN

Abstract

Precambrian mafic magmatism of the Bundelkhand craton is represented by the mafic dyke swarms that have extensively traversed the gneisses and granitoids and metamorphosed mafic-ultramafic rocks associated with greenstone-gneiss, occurring as small linear isolated basins. The mafic dykes were emplaced (during ∼2.1 to ∼2.0 Ga) in three phases, two in NW-SE direction and the third in NE-SW direction, depicting episodic events subsequent to the stabilization of the craton (at ∼2.5 Ga). The former represent culmination of tectono-magmatic event in the Bundelkhand region. The mafic-ultramafic suite of rocks associated with greenstone-gneiss units, range in composition from basalt through basaltic-andesite to basaltic-komatiite. The basaltic-komatiites display remarkable geochemical similarity with boninites. The dykes are of varying compositions and are mostly low-Ti tholeiites and range from sub-alkaline basalt through basaltic andesite and dacite. They show enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE). Incompatible trace elemental ratios indicate that crustal contamination alone cannot be the dominant process for the evolution of the dykes. Ti/Zr and Ti/Y ratios of the dykes reflect near primary chemical characteristics of the mantle sources. Large variations observed in Nb/La and La/Sm ratios possibly reflect varying influence of the crustal material and also some LREE-LILE enrichment processes. To explain the observed geochemical characteristics, it is proposed that the mantle source region was contaminated by sediments/crustal components probably via subduction processes. Geochemical characteristics of the dykes swarm probably reflects enriched mantle source characteristics followed by fractional crystallisation of the magma. Our study indicates important role of subduction processes in the evolution of the Precambrian crust and subcontinental lithosphere in the Bundelkhand and neighbouring shield and Himalayan areas.

Keywords

Precambrian Mafic Dykes, Bundelkhand Craton, Enriched Mantle Sources, Crustal Contamination, Subduction Processes.

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R. Chandra ¹, B. K. Gupta ¹, D. K. Srivastava ¹, R. R. Sarin ¹

Affiliations
1 Postgraduate Department of Medicine, G.S.V.M. Medical College, Kanpur, IN

Abstract

No Abstract.

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Anant Shastry ¹, Rajesh K. Srivastava ², R. Chandra ², George A. Jenner ³

Affiliations
1 Department of Earth Sciences, University of Asmara, Asmara, ER
2 Department of Geology, Banaras Hindu University, Varanasi 221 005, IN
3 Department of Earth Sciences, Memorial University of Newfoundland, NF A1 B 3x5, CA

Abstract

Plagiogranite is a leucocratic low-K member of the Andaman Ophiolite Suite (AOS) that occurs as bands (dykes?) of irregular shapes and sizes, particularly in the cumulate sequence of the suite. It is medium- to coarse-grained and commonly shows hypidiomorphic granular texture. At places granophyric texture (intergrowth between feldspar and quartz) is also seen. Thc main mineral constituents are plagioclasc (mainly albite), quartz and minor mafic minerals. Mineralogically and chemically these rocks are classified as low-K dacite (trondhjemite/tonaIite type). Low Rb concentrations (< 5) and low RblSr ratios (<0.01 5), observed in studied plagiogranites and associated Fe-Ti enriched rock, are characteristic of oceanic derivatives. All elements, plotted on the primordial mantle normalized multi-element spidergrams show enriched patterns. Variations in the concentrations of Nb, Sr, Zr, and Y arc limited but other elements show wide variations. Strong negative anomalies for Nb are also observed. These rocks show characters analogous to those of orogenic granites, emplaced in volcanic arc settings. On the basis of observed petrological and gcochernical characteristics and their spatial association with Fe-Ti enriched mafic rocks, it is concluded that the Andaman plagiogranites are the integral part of AOS having crystallized from late stage immiscible silicate liquid.

Keywords

Plagiogranite, Fe-Ti enriched mafic rocks, Geochemistry, Liquid Immiscibility, Andaman Ophiolite Suite.

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