Petrogenesis of the Zenong Group volcanics in the central Lhasa subterrane, Tibet: Geodynamic implications of the scissor-like southward subduction of the Shiquanhe-Jiali Tethyan Ocean

Abstract

The widespread Mesozoic volcanics in the central Lhasa subterrane have been a subject of debate concerning their genetic mechanisms. Leveraging geochronology, element geochemistry, and Hf isotopes, we have systematically investigated the origin and dynamic mechanisms of the Zenong Group volcanics in the Bengnazangbu and western Dangreyongcuo areas in the east-middle part of the central Lhasa subterrane. Comprising primarily of andesites, dacites, and rhyolites, the Zenong Group volcanics display distinct geochemical signatures. In the Dangreyongcuo and Bengnazangbu areas, the andesite, dacite, and rhyolites show medium to high-K calc-alkaline characteristics. These rocks share similarities in trace element distribution, exhibiting enrichment in Rb, U, Th, Pb, and K, but depletion in Nb, Ta, Ti, Ba, and Sr. These characteristics accord with the typical composition of arc volcanic rocks. Zircon UPb dating has revealed that the volcanic rocks in the Dangreyongcuo and Bengnazangbu areas have ages of 127–126 Ma and 112–103 Ma, respectively. Further, the distribution of zircon UPb ages suggests a west-east younging trend both in the Zenong Group and the Shiquanhe-Jiali suture zone. The ε_Hf(t) values of andesite, dacite, and rhyolite are all below the chondrite standard line, ranging from 0 to −15, and the crustal model ages range from 2.0 to 1.2 Ga. Our data, combined with the geological setting, whole-rock geochemistry, Hf isotopes, and geochronology, suggest that the Zenong Group volcanic rocks are the result of the mixing of mantle wedge melts formed by the subduction of the Shiquanhe-Jiali Ocean with the lower crust of the Lhasa terrane, and undergoing fractional crystallization of amphibole ad plagioclase. And we believe that the Shiquanhe-Jiali Ocean was sequentially closed from west to east in a scissor-like fashion during southward subduction.