Geochemical characterizations of mafic magmatic rocks of the Central Indian Shield: implication for Precambrian crustal evolution

Abstract

Geochemical and petrogenetic studies have been conducted on a set of basaltic rocks from Betul, amphibolites and dolerite dykes which intrude the Tirodi and Amgaon Gneissic Complex from Betul-Chindwara and Amgaon regions, respectively. Geochemically Betul mafic volcanics are classified as basalt to basaltic andesite with tholeiitic lineage in the form of Fe-enrichment trend. All the rocks are distinctly enriched in incompatible trace elements including light rare earth elements (LREE) and large ion lithophile elements (LILE), with continental signature in the form of negative Nb, P and Ti anomalies in their primitive mantle-normalised spidergrams. These chemical characteristics indicate their derivation from enriched mantle sources / sub-continental lithosphere. The dykes traversing the Tirodi Gneissic Complex (TGC) in Chindwara region have high abundances of Fe-oxides and alkali elements. These dykes are sub-alkaline, ranging in composition from basalt to basaltic andesite, with tholeiitic affinity. Doleritic dykes of TGC have enriched LREE-LILE with depleted high field strength elements (HFSE) patterns, which probably indicate their derivation by from enriched mantle sources, while the dykes of Amgaon Gneissic Complex (AGC) have lower LREE-LILE enrichment, which probably indicate higher degrees of partial melting of similarly enriched sources. Negative Sr-anomaly is conspicuous, probably indicating role of plagioclase fractionation in their genesis. Depleted mantle Sm-Nd model ages for the Betul mafic volcanics vary between 1951 Ma and 2320 Ma, indicating this period represents a major crustal evolution event in this region.