Petrogenesis of Early Cambrian granitoids in the western Kunlun orogenic belt, Northwest Tibet: Insight into early stage subduction of the Proto-Tethys Ocean

Abstract

The west Kunlun orogenic belt, located on the northwest margin of the Tibetan Plateau, represents a crucial tectonic junction between the central Asia and Tethys domains. Its evolution was closely related to the Paleozoic subduction and closure of the Proto-Tethys Ocean, which was formed by the breakup of the Rodinia supercontinent following the Neoproterozoic. However, the early evolution of Proto-Tethys oceanic subduction (e.g., subduction initiation timing, polarity, and process) remains controversial. The source of the Early Cambrian granitoids is also unclear. To explore these questions, four Cambrian plutons (i.e., two Tianshuihai monzogranites and south Kunlun diorite and monzogranite) were chosen for geochronological and geochemical studies. Zircon U-Pb dating reveals that these plutons formed at ca. 533–513 Ma and thus represent the oldest arc-related magmatism in the west Kunlun orogenic belt. The Tianshuihai monzogranites have positive εNd(t) values (+0.76 to +1.34) and zircon εHf(t) values of +0.25 to +6.42, with low δ18Ozrn values of +5.11‰ to +7.38‰, suggesting that their source includes juvenile material. These rocks are weakly peraluminous and have relatively old Hf model ages of 1.09–1.48 Ga. Mass balance calculations show that the Tianshuihai monzogranites were derived from partial melting of Mesoproterozoic meta-igneous rocks with the addition of 22% of juvenile material. The south Kunlun monzogranites in this study are weakly peraluminous, and their lowest εNd(t) values are –9.24 to –9.27 and zircon εHf(t) values are –7.80 to –11.2. The oldest Hf model ages are 1.97–2.18 Ga, and the highest zircon δ18Ozrn values are +8.11 to +9.73‰. Their isotopic compositions are different from those of the magmas derived from partial melting of just Paleoproterozoic and Mesoproterozoic basement rocks but can be produced by a mixing source of 32% meta-igneous rock and 68% meta-sedimentary rock. The south Kunlun diorites are characterized by high Sr contents and relatively high Sr/Y (52–63) ratios but low Y, Yb, Cr, and Ni contents, like those of the thickened continental crust-derived adakites. Their Sr-Nd–Hf-O isotopic compositions indicate that their parental magma was derived from a Mesoproterozoic metaigneous basement in the garnet stability field. Based on the newly identified, oldest island arc magmatic records in the west Kunlun orogenic belt, the subduction initiation of the Proto-Tethys oceanic slab must have occurred prior to the Early Cambrian (>533 Ma). Our results, with previously published data, show that the west Kunlun orogenic belt was in an extensional setting during the Early Cambrian and that the magmatism migrated northeastward along the axis of the south Kunlun terrane between 533 Ma and 513 Ma. Therefore, considering the spatial and temporal distribution and petrogenesis of the Early–Middle Cambrian plutons in the west Kunlun orogenic belt, we propose that the Early Cambrian magmatism was most plausibly triggered by asthenospheric upwelling in response to the rollback of southward-subducted Proto-Tethys oceanic slab.