Crustal Thickening of the Central Tibetan Plateau prior to India–Asia Collision: Evidence from Petrology, Geochronology, Geochemistry and Sr–Nd–Hf Isotopes of a K-rich Charnockite–Granite Suite in Eastern Qiangtang

Abstract

It has been intensely debated whether late Mesozoic subduction–collision contributed to the growth of the Tibetan Plateau prior to the Indo-Asian collision. Here we present zircon U–Pb geochronology, mineral chemistry, whole-rock and Sr–Nd–Hf isotope geochemistry for a suite of latest Cretaceous K-rich charnockite and granite samples from the eastern Qiangtang terrane in central Tibet. Zircon U–Pb dating suggests that these two plutons were emplaced at ∼68 Ma. Both plutons display K-rich and magnesian affinities, with enrichment of light rare earth elements and large ion lithophile elements, and pronounced negative anomalies in Nb, Ta, P, and Ti, as well as positive anomalies in Th, U, and Pb. The charnockite and granite have (87Sr/86Sr)i = 0·7056 − 0·7059 and 0·7068–0·7070, εNd(t) = −0·92 to 0·33 and −2·37 to −1·45, and εHf(t) = 0·3 − 4·5 and −0·8 to 4·2, respectively. Geochemical characteristics suggest that the two plutons are co-magmatic, although the granite experienced a greater extent of magmatic differentiation during its emplacement; the parental magmas were generated by partial melting of a thickened lower continental crust at 850–1000°C and 11–15 kbar, corresponding to depths of 40–50 km. The charnockite–granite suite provides important evidence for the presence of thickened crust in central Tibet, formed during the late Cretaceous Lhasa–Qiangtang terrane collision.