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

Vinod Kumar ¹, P. S. Agarkar ²

Affiliations
1 Geological Survey of India, Petrology Division Aliganj Extension, Lucknow 226020, IN
2 Department of Geology, Government Science College, Jabalpur, 482001 M.P., IN

Abstract

Alkali olivine basalts of Jungel metavolcanic suite in Bijawar greenstone belt of Central India display various morphologies of quench crystals indicative of rapid cooling or supercooling. There is resemblance between crystal growths of the present area and those described from submarine basalts and experimentally produced crystals under disequilibrium conditions. Stratigraphy, structural and petrographic characters of these rocks demonstrate that they represent an ancient oceanic crust.

Keywords

Petrology, Genesis, Quench-Texture, Basalt, Jungel-Valley, Bijawar Greenstone Belt, Mirzapur, Uttar Pradesh.

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Vinod Kumar ¹

Affiliations
1 Petrology Division, Geological Survey of India, Aliganj Extension, Lucknow-226 020, IN

Abstract

Spectacular array of volcanic structures shown by Jungel metavolcanics include pillow structures, pahoehoe flows and toes, groove lava and shark tooth projections. Some of these features are indicative of small volume phreato-magmatic eruptions under shallow water conditions. The style of igneous activity is inferred to be moderately explosive, fragmental and spasmodic, supported by substantial. amount of volatiles. Based on volcanological, geochemical and tectonic evidences it is suggested that the present volcanic rocks were erupted in island-arc environment and that the southern margin of the Bundelkhand protocontinent formed a zone of subduction.

Keywords

Bijawar Greenstone Belt, Volcanogenic, Island-Arc Volcanism.

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Vinod Kumar ¹

Affiliations
1 Petrology Division, Geological Survey of India, Aliganj Extension, Lucknow-226 020, IN

Abstract

Jungel valley is known to have dominant magmatic rocks of basaltic affinity evolved on ancient oceanic crust. It consists of alkali picritic basalt (APB) - alkali olivine basalt (AOB) - low potassic andedsite (LKA) association besides those earlier reported lava flows, tuffs, agglomerates and lamprophyres. These rocks were emplaced along numerous fractures in the earth's crust caused by N-S compressional forces at the time of Satpura orogeny in a rifted back are tectonic setting.

Chemically the APB and AOBs are characterized by high MgO, mg values, CaO, TiO₂ and P₂O₅ contents and depleted in SiO₂, Al₂O₃ and K₂O The APB is considered to be parental/primitive magma formed by varying degree of melting of garnet peridotite from the mantle. This source, however, was heterogeneous in respect of K, P and Rb contents similar to Sunda-Banda arc. The geochemical evolution of these rocks is shown to have analogy with some modem island arcs eg. Dunedian and Sunda-Banda arcs. Major elements, SiO₂, Al₂O₃, Na₂O, K₂O, TiO₂ and FeO¹ show increase with increasing differentiation from APB to LKA while CaO and MgO have inverse relationship.

The basalts are enriched in both transitional and incompatible elements, whereas, andesite is depleted in these. Cu has followed silicate phase, while Pb and Zn have remained in residual liquids as in Dunedian arc rocks.

Keywords

Bijawar Greenstone Belt, Jungel Metavolcanics, Picritic Basalt, Andesite.

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Vinod Kumar ¹

Affiliations
1 Operation U.P., Geological Survey of India. Aliganj Extension, Lucknow - 226 020, IN

Abstract

A systematic petrographic and geochemical study of three, previously unreported carnptonite dykes from Bijawar of Son valley is presented with a brief discussion on their petrogenesis. These larnprophyres show transition from alkali basalt and owe their origin through fractionation combined with local enrichment of volatiles, largely derived from mantle instead either by hybridization or assimiIation of basic magma with granite/late stage granitic fluids or mixing with an andesitic magrna as proposed by some earlier workers.

Keywords

Lamprophyres, Geochemistry, Igneous Petrology, Bijawar, Son Valley, Uttar Pradesh, Madhya Pradesh.

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