Cambrian ultrapotassic rhyolites from the Lhasa terrane, south Tibet: Evidence for Andean-type magmatism along the northern active margin of Gondwana

Abstract

The petrogenesis and tectonic significance of early Paleozoic magmatic rocks in south Tibet is debated. In this paper, we report Cambrian ultrapotassic felsic volcanics from the Lhasa terrane. Petrographic and whole-rock geochemical studies indicate that these rocks are rhyolite and rhyolitic ignimbrite, and have varying SiO 2 (69.56–82.09 wt.%), Al 2 O 3 (9.52–16.61 wt.%) and Fe 2 O 3 t (1.0–7.9 wt.%). The rocks are ultrapotassic (high K 2 O 4.19–6.90 wt.%, and very low Na 2 O 0.01–0.06 wt.%) and peraluminous (A/CNK = 1.41–2.22). They are enriched in Rb, Th, U, Pb, Zr and Y, and showed negative Ba, Sr, P, Nb, Ta and Ti anomalies, and slightly fractionated REE patterns with moderately negative Eu anomalies, typical of A-type granitoids. Zircons from six samples yielded crystallization ages of ca. 512 Ma. These zircons show ε Hf (t) values in the range of −4.7 to +1.3, and two- stage Hf model ages of 1734–1392 Ma. Combining with previous results, we propose that these ultrapotassic rhyolites were derived from the partial melting of middle Proterozoic crustal rocks in the lower crust under high-temperature and water-absent conditions in an extensional environment of active magmatic arc. Thus, this study provides new insights into early Paleozoic Andean-type orogeny along the northern margin of Gondwana supercontinent.