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Petrology, geochemistry of hornblende gabbro and associated dolerite dyke of Paharpur, Puruliya, West Bengal: Implication for petrogenetic process and tectonic setting
- ADITI MANDAL, ARIJIT RAY, MAYUKHEE DEBNATH, SANKAR PRASAD PAUL
- Journal of earth system science 2012 v.121 no.3 pp. 793-812
- zirconium, fractionation, geochemistry, Earth system science, tectonics, strontium, melting, plagioclase, barium, gneiss, India
- Paharpur gabbroic intrusive is an arcuate body running eastâwest paralleling the foliation of Chhotanagpur Granite Gneiss which acts as country rock. The main gabbroic body is intruded by a number of dolerite dykes running northâsouth. It is composed of clinopyroxene (Wo48En40Fs12âWo51En40Fs09, mg no. 72â82), plagioclase (An52âAn90), hornblende (magnesian hornblende to ferro-tschermackite), orthopyroxene (En76âEn79) and ilmenite. Hornblende occurs as large poikilitic grain and constitutes around 60% of the rock. Both gabbro and associated dolerite dykes, show relatively primitive character (mg no. 65â73). Primitive mantle-normalized and MORB-normalized spider diagrams indicate enrichment in Rb, Ba, Th, La, Sr and depletion in Nb, Zr, Y, Ti and Nd. The LILE enrichment and Nb, Ti, Zr, Y depletion suggest arc like geochemical signature for the gabbroic and doleritic rocks of Paharpur. Flat to slightly LREE fractionated pattern and variable degree of REE enrichment is observed. An early stage fractionation of clinopyroxene, plagioclase, orthopyroxene, ilmenite and late stage reaction of cumulate pile and evolved melt/hydrous fluid is suggested for magmatic evolution of gabbro. Associated dolerite dykes, which are geochemically similar to the gabbro, have tholeiitic with boninitic character. The mineralogical and chemical compositions of intrusive rocks also have some similarity with mafic rocks of ophiolite complex of subduction zone.