Late Cretaceous K-rich magmatism in central Tibet: Evidence for early elevation of the Tibetan plateau?

Abstract

Major and trace element, Sr–Nd–Pb and zircon U–Pb and Hf isotope data are presented for the newly discovered Abushan volcanic rocks in the southern Qiangtang Terrane, central Tibet. These results offer new insights into the evolution of the Tibetan plateau during Cretaceous. The Abushan volcanic rocks are composed mainly of andesites and trachy-andesites. Zircon U–Pb dating constrains the timing of emplacement as Late Cretaceous (79.9±2.7–75.9±0.49Ma). Major element geochemistry shows that the Abushan volcanic rocks belong to high-K calc-alkaline series. All the andesites and trachy-andesites are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE). The geochemical features and ages of the Abushan volcanic rocks relate them with post-collisional setting. Furthermore, the volcanic rocks display negative εNd(t) (−3.1 to −2.5), relatively constant 87Sr/86Sr ratios (0.70614–0.70735) and negative εHf(t) values (−5.8 to −2.1). The Sr–Nd–Pb and Hf isotope signature suggests that the andesites and trachy-andesites were derived from the anatexis of mafic lower crust by intrusion or underplating of mantle-derived basaltic magma. The petrogenesis of Abushan andesites provides robust evidence for the crustal thickening, delamination and early uplifting of central Tibet during Late Cretaceous.