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

D. V. Subba Rao ¹, M. W. Y. Khan ², D. N. Sridhar ¹, K. Naga Raju ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 560007, IN
2 Pt. Ravishankar Shukla University, Raipur (C. G)., IN

Abstract

Younger and fine-Grained unmetamorphosed dolerite dykes intrude the Raipur Group sediments in the central part of the Meso-Neoproterozoic Chhattisgarh Basin in Central India. Petrologically these dykes consist of unaltered microphenocrysts of intermediate pigeonite (X_Mg=0.40), augite (X_Mg=0.595), calcic plagioclase (X_ca=O.64), titanomagnetite and ilmenite. These rocks show high Fe₂O₃^T (14.43 to 15.19%), TiO₂ (2.75 to 2.87%) and P₂O₅(0.27 to 0.29%) contents with an unusual Continental Flood Basalt (CFB) affinity. Significantly these dykes show lower K₂O content (0.43%). The trace element abundance such as Ba, Sr, Rb, Zr, Nb and trace element ratios Sm/Nd, Ba/Nb, Zr/Nb, Y/Nb, Th/Yb, Ta/Yb and K/Ba in these rocks chemically resemble the least contaminated Deccan Basalts. Chondrite/OIB normalized REE patterns of these rocks also show striking similarities with the Deccan tholeiitic basaltic flows. The available petrological, mineralogical, major, trace and rare earth element data indicate that these within basin basaltic dykes of Raipur are intraplate basalts, which were derived from an enriched mantle source.

Keywords

Basaltic Dykes, Chhattisgarh Basin, Geochemistry, Continental Flood Basalts.

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D. V. Subba Rao ¹, V. Balaram ¹, K. Naga Raju ¹, D. N. Sridhar ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

Boninite-Like rocks represented by high-Ca boninitic dykes, melanogabbro dykes, recrystalized plagioclase bearing high MgO dykes and high-Mg nonte suites occur at few places in the vicinity of Meso to Neoproterozoic Chhattisgarh sedimentary basin in the northern Bastar craton in Central India. These rocks are formed in an intracratonic setting, not at convergent margin, similar to Archaean bonmitic rocks reported from intracratonic settings such as Mallina Basin, Northwest Australia and Abitibi, Optica regions of Canada. These high-MgO mafic dykes show a strong boninitic affinity with high SiO₂ (>52%), high MgO (9-15%), low TiO₂ (0 30-0 54 wt%) and strong LILE enrichment. These unusual dykes show distinct mineralogical, petrological and geochemical charactenstics and are totally different to that of the normally occurring abundant metadoleritic and metagabbroic dykes in Chhattisgarh region. The generation of bonmite magmatism requires unique thermal conditions such as shallow melting, elevated geothermal gradient and subducted slab flux. On the basis of field, geological, petrological and geochemical inferences on these Chhattisgarh boninitic and nontic dykes, a two-stage melting model and derivation from a strongly depleted mantle source, enriched later by metasomatic events is suggested.

Keywords

Boninitic-Noritic Dykes, Chhattisgarh, Geochemistry, Bastar Craton.

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