Multiple sources of the Early Mesozoic Gouli batholith, Eastern Kunlun Orogenic Belt, northern Tibetan Plateau: Linking continental crustal growth with oceanic subduction

Abstract

Orogenic belts have been among the most important locations to investigate the growth of continental crust (CC). The Eastern Kunlun Orogenic Belt (EKOB), which contains widespread Permian–Triassic granitoids, is volumetrically comparable to the Cenozoic Gangdese magmatic belt in the Tibetan Plateau and is an ideal region to investigate the mechanism of the Paleozoic–Mesozoic CC growth in this region. The Gouli batholith at the eastern end of the EKOB consists of the synchronous Xiangride granodiorite, Asiha quartz diorite (ca. 242Ma) and adamellite. The granodiorite and quartz diorite, both of which contain magmatic enclaves, show medium–high K, calc-alkalic and metaluminous signatures and have similar rare earth element and trace element patterns to those of bulk CC. Besides, the Xiangride granodiorite displays distinct adakitic signatures (average Sr/Y of 47). The Sr-Nd isotopic values for the different types of rocks are roughly similar ((87Sr/86Sr)i=0.708167–0.713553, εNd(t)=−6.8 to −5.3), while Hf isotopes are distinguishable, with εHf(t)granodiorite=0.3 to 5.1 and εHf(t)diorite=−1.6 to 0.7. These geochemical and petrographic signatures suggest that the granodiorite originated from the partial melting of subducting oceanic crust and terrigenous sediments, and the quartz diorite and enclaves formed via the mixing of slab-derived magma and enriched mantle-derived melt. Further comprehensive analyses of the spatial and temporal distribution of regional magmatic rocks, metamorphism and sedimentary facies reveal that the Gouli batholith and most of the Permian–Triassic granitoids in the EKOB formed during the subduction of the Paleo-Tethys Ocean instead of subsequent syn-collision setting. Thus, we contend that the Permian–Triassic CC growth of the EKOB occurred in a slab subduction setting and that both oceanic slab and subcontinental mantle significantly contributed to this process.