U-Pb Age and Hf Isotopic Compositions of Magmatic Zircons from a Rhyolite Flow in the Porcellanite Formation in the Vindhyan Supergroup, Son Valley (India): Implications for Its Tectonic Significance

The Porcellanite Formation in the basal Semri Group of the Proterozoic Vindhyan Supergroup, exposed along the Son River Valley, India, primarily consists of intercalated silicified felsic tuff beds (porcellanite), rhyolitic breccia, and shale. Lensoid patches of very coarse-grained rhyolite occur in a stretch of about 10 km near the eastern end of the valley. These are newly interpreted as rhyolite flows, with flow structures including aligned volcanic fragments and phenocrysts, a few of which are similar to 10-cm-long feldspars. Microphenocrysts consist of fragments of patchy perthite and quartz, with subordinate Fe-oxide minerals set in a very fine-grained groundmass of quartz and feldspar. Volcanic rock fragments and fiamme are common. Bulk chemical compositions of nine samples also identify them as rhyolite (SiO2 = 75.24%; Na2O + K2O = 7.06%, CaO = 1.66%). Rare earth element distribution is typical of granitic rocks, with a negative Eu anomaly of 0.57. The samples are enriched in U, Th, La, and Rb and depleted in Cr and Ni relative to upper continental crust and total crust. New U-Pb isotopic analyses of magmatic zircons in the rhyolitic flow give a concordia intercept age of 1640 +/- 4 Ma and a weighted-mean Pb-207/Pb-206 average age of 1642 +/- 7 Ma. These are essentially identical with those determined previously from zircons in porcellanite outcrops to the west. Lu-Hf isotopic compositions show that all but one epsilon(Hf) value at 1640 Ma are less than chondrite uniform reservoir values, indicating a significant contribution from older, felsic crust. Crustal models suggest derivation from about 2.5 Ga middle to lower crust. The new data do not support Semri sedimentation in an active arc setting or derivation of the rhyolite magma from a putative and now-concealed subduction zone with contribution from the mantle. We suggest that far-field effects of one or more of the ca. 1650 Ma collisions of continental blocks caused crustal fracturing as the basin subsided, which induced melting and the consequent rhyolitic volcanism.