Abstract

Representative rocks of gabbro, diorite, lamprophyre, and syenite from the Girnar complex, Deccan volcanic province, India, and associated silicic porphyritic rocks have been analyzed for major and trace (including rare earth) elements and for strontium and oxygen isotopic composition. Variation diagrams of the major elements against the Fe/Mg index suggest a systematic evolution in the complex. Silicic porphyry, on the other hand, forms a separate group in the variation diagrams. Rare earth elements (REEs) increase in abundance from gabbro to syenite and become increasingly fractionated. REE patterns and Th/Ta ratios in the silicic porphyritic rocks differ from the others. The initial 87Sr/86Sr ratios of the rocks show bimodal distribution — that is, silicic porphyry with an average value of 0.7275, whereas the other rocks have a range from 0.7051 to 0.7080. δ18O values range from +5.37 per mil to +8.68 per mil relative to standard mean ocean water. Variation of the strontium isotopic composition within the gabbro-syenite group is inferred to be related to crustal contamination. The silicic porphyry may be derived by partial fusion of the Precambrian granitic basement. New K-Ar age determinations show a range from 63.8 to 56.2 m.y. and suggest that the igneous complex was emplaced immediately after the eruption of the Deccan basalts.

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