Late Triassic crustal growth in southern Tibet: Evidence from the Gangdese magmatic belt

Abstract

The Gangdese magmatic belt, located in the southern margin of the Lhasa terrane and carrying significant copper and polymetallic mineralization, preserves important information relating to the tectonics associated with Indian–Eurasian collision and the crustal growth of southern Tibet. Here we investigate the Quxu batholith in the central domain of the Gangdese magmatic belt and report the occurrence of hornblende gabbros for the first time. We present petrologic, zircon U–Pb–Hf isotopic and bulk-rock chemistry data on these rocks. The hornblende gabbros display sub-alkaline features, and correspond to tholeiite composition. They also show medium K calc-alkaline to low K affinity. The rocks show enrichment in LILEs and LREEs, but are depleted in HFSEs, indicating a subduction-related active continental margin setting for the magma genesis. Our computations show that the gabbroic pluton was emplaced in the middle-lower crustal depth of ca. 18km. Zircons from the hornblende gabbros yield crystallization age of ca. 210Ma, revealing a late Triassic magmatic event. Combined with available data from the Gangdese magmatic belt, our study suggests that the northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane might have been initiated not later than the Norian period of Triassic. Zircons from the hornblende gabbro show positive εHf(t) values of 9.56 to 14.75 (mean value 12.44), corresponding to single stage model ages (TDM1) in the range of 256Ma to 459Ma, attesting to crustal growth in the southern Lhasa terrane associated with the subduction of the Neo-Tethys oceanic crust.