Akhtar R Mir, Shabber H Alvi, V. Balaram, Fayaz A. Bhat, Sumira Z. and Shamim A. Dar
Newer Dolerite Dykes (NDD) traverses the Singhbhum Granitoid Complex (SGC) in distinct orientations viz., NNE-SSW, NNW-SSE and E-W, among which the NNE-SSW trend is the most dominant. These dykes are subalkaline in nature and show compositional variation from basalt through basaltic andesite to andesite. The relationship shown by various oxides against MgO is consistent with fractional crystallization of a mafic magma. Variations in major elements, particularly SiO2, Al2O3, CaO, TiO2 contents, and CaO/TiO2 and Al2O3/TiO2 ratios in the studied dykes indicates that their Ca and Al are held in the residual mantle phases such as clinopyroxene, plagioclase, spinel and garnet. Low Ni and highly variable Cr values suggest that olivine and clinopyroxene fractionation controlled the abundances of Ni and Cr in these rocks. The high (La/Yb)N and (Gd/Yb)N in combination with relatively low HREE abundance of the Newer Dolerites dykes suggest that they may have formed by low degrees of partial melting of a garnet bearing source. The higher Th/Zr, Rb/Y, Ba/Nb and Ba/Th ratios of these dykes suggest that their mantle source was enriched/ metasomatised by slab derived fluids. Their geochemical characteristics particularly Ti/Y, Zr/Y, Th/Nb, Ba/Nb, La/Nb, (La/Sm)PM are similar to subduction zone basalts that occur along the plate margin. The enriched LREE-LILE and depletion of high-field strength elements (HFSE) especially Nb, P and Ti and detectable slab-derived components of the studied dykes are all features typical for subduction zone-related petrogenesis.
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