Neoarchaean felsic volcanic rocks from the Shimoga greenstone belt, Dharwar Craton, India: Geochemical fingerprints of crustal growth at an active continental margin

• Potassic rhyolites of Shimoga greenstone belt with continental arc signatures.
• Intracrustal fractional crystallization of basaltic magma and assimilation of continental crust.
• Slab dehydration-wedge melting and slab melting-wedge hybridization account for arc signatures.
• Neoarchean active continental margin process for crustal growth in western Dharwar Craton.

The felsic volcanic rocks of Neoarchaean Shimoga greenstone terrane of western Dharwar Craton in India are dominantly represented by rhyolites occurring at stratigraphically upper horizons. The Shimoga rhyolites are associated with conglomerates, quartzites, argillites, limestones, cherts, basalts and intermediate volcanic rocks clearly suggesting an accretionary package. The rhyolites of Daginkatte and Shikaripura areas are potassic, with porphyritic alkali feldspar and quartz as essential minerals and chlorite, biotite and opaques as accessory phases. Geochemically, the rocks show enrichment in LILE and depletion in HFSE relative to primitive mantle values, with negative Nb-Ta, Zr-Hf anomalies and positive Th anomalies. These features of the Shimoga rhyolites compare well with the geochemical characteristics of magmas generated in subduction-related tectonic settings. Their alkaline compositions, intermediate to low HFSE abundances, moderate to high Zr/Y values (1.5–8.3), with La/Ybn (2–28), pronounced negative Eu anomalies, and variable LREE/HREE fractionation trends resemble the FI and FII rhyolites of Wabigoon and Uchi belts of Superior Province, Canada. The Shimoga rhyolites are interpreted to be products of melting of thick basaltic crust metamorphosed to amphibolite/eclogite grade, with garnet- and amphibole-bearing mantle residue. The rhyolites show prominent negative Eu and Ti anomalies, moderate to strong LREE fractionation, flat to mildly fractionated HREE patterns and are geochemically analogous to Type 1 and Type 3 rhyolites of Superior Province, Canada suggesting their derivation from intracrustal melting and fractional crystallization of basaltic liquids with prominent contribution from mantle wedge and slab components. Our data suggest the contribution of Neoarchaean active continental margin processes for the growth and evolution of continental crust in the western Dharwar Craton.

Figure optionsDownload as PowerPoint slide