Cretaceous volcanic rocks in south Qiangtang Terrane: Products of northward subduction of the Bangong–Nujiang Ocean?

Abstract

The subduction of the Bangong–Nujiang Ocean is a long-standing problem in the study of the geological evolution of the Tibetan Plateau. Based on recent data acquired from the Abushan volcanic rocks in the Qiangtang Terrane, we will discuss the relationship between the Cretaceous volcanism and the evolution of the Bangong–Nujiang Ocean. The Abushan volcanic rocks are mainly composed of trachy-andesites, dacites, and rhyolites. Zircon U–Pb dating constrains the time of emplacement as the Middle Cretaceous (102.6±1.6–96.1±2.4Ma). Major element geochemistry shows that the volcanic rocks belong to the high-K calc-alkaline and calc-alkaline series. All of the volcanic rocks are enriched in light rare earth elements (LREE) and some large ion lithophile elements (LILE, Rb, Th, and U) and are depleted in heavy rare earth elements (HREE) and some high field strength elements (HFSE, Nb, Ta, and Ti). The geochemical characteristics of the Abushan volcanic rocks are correlated with those of the volcanic arc rocks in the subduction zone. The petrogenesis of the Abushan volcanic rocks suggests that they were derived from the partial melting of the relict subducted oceanic crust combined with the input of oceanic sediments. Oceanic crust will melt upon reaching its hydrous solidus in the amphibolite facies after plate collision; this induces the formation of the Abushan volcanic rocks. Our data, compared with the previous studies on volcanism in the Lhasa Terrane, leads us to propose that the Bangong–Nujiang Ocean crust was subducted beneath the Qiangtang Terrane and underwent bidirectional subduction during the Cretaceous.