Cambrian volcanism in the Lhasa terrane, southern Tibet: Record of an early Paleozoic Andean-type magmatic arc along the Gondwana proto-Tethyan margin

Abstract

The present study reports new zircon LA–ICP–MS U–Pb ages, trace element and Hf isotope data, and whole-rock major and trace element data from Cambrian metarhyolites from Zhaqian and Zhakang in the central Lhasa subterrane of southern Tibet. One sample from Zhakang provides a weighted mean 206Pb/238U age of 510.4±4.0Ma and two samples from Zhaqian yield weighted mean 206Pb/238U ages of 510.6±2.6Ma and 524.8±2.9Ma, indicating that the Zhaqian and Zhakang metarhyolites were contemporaneous. Both are characterized by high SiO2 and K2O and low Na2O. They are also primarily high-K calc-alkaline, are enriched in Th, U, and light rare earth elements (LREEs), and are depleted in Nb, Ta, Ti, and P. Thus, they are geochemically similar to typical arc volcanic rocks. Moreover, the Zhaqian metarhyolites exhibit varying zircon εHf(t) values (−3.8 to +0.3) that are comparable to those of the Zhakang metarhyolites (−4.9 to −1.0). Both metarhyolites are interpreted as resulting from partial melting of Proterozoic metasedimentary rocks with mantle-derived magma contributions. Contemporaneous magmatism in the early Paleozoic has also been recognized in other microcontinents along the Gondwana proto-Tethyan margin. The emplacement of these magmatic rocks and the development of a Cambro–Ordovician angular unconformity in the central Lhasa subterrane can be attributed to subduction of proto-Tethys Ocean lithosphere in a Andean-type magmatic arc setting following the assembly of various continental components within the Gondwana supercontinent.