Geology and geochemistry of archaean felsic metavolcanic rocks of the Eastern Part of the Kolar greenstone belt, Dharwar craton, India Implications for their petrogenesis and geodynamic setting
A.G.Ugarkar, S.N.Solankar, V.N.Vasudev
Journal of the Geological Society of India 81(2): 192-202
In the Kolar greenstone belt of the Dharwar craton, felsic metavolcanics are encountered prominently in its eastern region around Surapalli and Marikoppa. These felsic volcanic rocks are essentially homogeneous and their bulk mineralogy is almost the same. They consist of phenocrysts of quartz and feldspar, set in a fine-grained quartzo-feldspathic groundmass. They are calc-alkaline rhyolite in composition, and are characterized by high SiO2 (av. 75.74 wt.%), moderate Al2O3 (av. 11.84 wt.%), Na2O (av. 3.55 wt.%), K2O (av. 3.26 wt%) contents and low Mg# (av. 6.07), Cr (av. 8 ppm), Ni (av. 8 ppm), Sr (av. 331 ppm.), Y (av. 7 ppm), Yb (av. 0.87 ppm) and Nb/Ta (av. 6.40) values, suggesting Tonalite-Trondhjemite-Granodiorite (G) affinity for these felsic volcanics. They are strongly fractionated [(La/Yb)N? = 14.41 48.70] with strong LREE enrichment [(La/Sm)N = 2.50-3.59] and strong HREE depletion [(Gd/Yb)N = 1.34 2.77] with positive Eu anomaly. The regional geological set-up, petrographic and geochemical characteristics suggest that these felsic volcanics probably were derived by partial melting of a subducting basalt slab at shallow depth without much involvement of mantle wedge in an island arc geodynamic setting.