Early Paleozoic Subduction Imprints of the Proto-Tethys Ocean: Evidence from the Appinite–Diorite–Granodiorite Complex in East Kunlun, Northern Tibet

The East Kunlun Orogenic Belt (EKOB), on the northern margin of the Tibetan Plateau, forms a part of the western segment of the orogenic system in Central China. The Paleozoic tectonic evolution of the EKOB, along the western margin of Gondwana, is closely related to Paleozoic subduction and closure of the Proto-Tethys and is key to the palinspastic reconstruction of the Central Asia and Tethys domains. Numerous studies have been conducted on the Early Paleozoic granitoids in the EKOB. However, a convincing magmatic proxy to indicate the timing of oceanic subduction during the Early Paleozoic is yet to be well constrained, partially because the arc geochemical signature of the calc-alkaline granitoids is not a reliable indicator of oceanic subduction. Herein, we present new zircon U–Pb ages, whole-rock major and trace element analyses, and Nd–Pb and zircon Hf isotopic data for Early Ordovician appinite (mafic), diorite (intermediate), and granodiorite (felsic) intrusive rocks in the Dundegou (DDG) area of the EKOB, northern Tibet Plateau. The zircon U–Pb ages suggest this complex was emplaced at approximately 477–472 Ma. Abundant 557–510 Ma zircon xenocrysts and/or inherited zircons were found in the granodiorite and appinites, suggesting two earlier magmatic episodes during the late Ediacaran–Cambrian. Appinites and diorites are characterized by depletion in high field strength elements, with negative initial εNd(t) values (appinites: −6.5 to −9.4; diorites: −8.2 to −9.1) and negative zircon εHf(t) values (appinites: −2.5 to −10.3; diorites: −1.0 to −9.1). These features suggest that appinites and diorites were most likely derived from the partial melting of enriched mantle wedge peridotites with the addition of sediment-derived melts from the sedimentary veneer of the subducting slab, along with variable degrees of fractional crystallization and a limited extent of crustal contamination. The granodioritic magma was contributed by (a) the ancient mafic lower crust-derived magma, (b) the derivative felsic magma from a dioritic parent, and (c) a minor mantle-derived magma component, resulted in the formation of granodiorites with highly unradiogenic initial Nd and Hf isotopic compositions (εNd(t) values of −8.5 and εHf(t) values from −0.6 to −7.3). Our study suggests that the subcontinental lithospheric mantle beneath the EKOB is likely heterogeneous in terms of its isotopic composition. The DDG complex is the oldest appinite-bearing complex found so far in the EKOB and a good indicator of proto-Tethys oceanic subduction.