Petrological and geochemical study of the Sylhet trap basalts, Shillong plateau, N.E. India: Implications for petrogenesis

Abstract

Sylhet Traps exposed along the southern margin of Shillong plateau, Northeast India are subalkaline tholeiitic basalts. The basalts are generally massive but occasionally contain large amygdules of zeolites and chalcedony. Microscopically, some basalts show porphyritic texture with olivine phenocrysts. Phenocryst assemblage of plagioclase ± clinopyroxene ± olivine implies crystallization at shallow level. SEM-EDX analysis shows occurrences of spinel with Ni and Cr within the basalts therefore indicating partial melting of the subcontinental lithospheric mantle as the possible source materials for the basalts. The multi-element plot for the basalts shows two distinct trends: one with significant enrichment of LILE and depletion of HFSE and plot similar to OIB (Type 1) while the other trends are chara cterized by slight enrichment of LILE and negative anomalies at Nb, P and Ti (Type 2). Chondrite-normalized REE patterns for Type 1 basalt shows very high enrichment of LREE and a strong right dip HREE pattern and also plots similar to typical OIB while Type 2 show a slight enrichment of LREE over HREE with small Eu anomaly. The geochemical signatures suggest crustal contamination by plume-derived magma produced by low degree of partial melting for Type 1 basalt. Type 2 basalt was produced by partial melting of subcontinental lithospheric mantle, which may be triggered by plume upwelling.