Petrography and Geochemistry of Jungel Metavolcanics, Bijawar Greenstone Belt, Central India : A Picritic Basalt Low K-Andesite Association

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.