Early Paleozoic granitic rocks of the South Qiangtang Terrane, northern Tibetan Plateau: Implications for subduction of the Proto- (Paleo-) Tethys Ocean

Abstract

Knowledge of the early Paleozoic tectonic framework of the northern margin of Gondwana continent is critical for reconstructions of the Gondwana supercontinent. Early Paleozoic leucogranite and gneissic granite units within the South Qiangtang Terrane, northern Tibetan Plateau, can constrain the timing and geological processes that occurred along the northern margin of Gondwana continent at this time. Here, we present new field, petrological, zircon geochronological, geochemical, and Nd–Hf isotopic data for this suite of granitic rocks, and use these data to provide insights into the evolution of this Gondwanan margin. Zircon U–Pb dating yielded two groups of ages at 469–453 and 486–473 Ma. All of the granitic rocks in the Gemuri area are high-K calc-alkaline and peraluminous, and record the fractionation of variable amounts of plagioclase, K-feldspar, and biotite. These leucogranites formed from magmas generated by the dehydration melting of muscovite within Paleoproterozoic–Mesoproterozoic metapelites under relatively low-temperature conditions (TZr = 616 °C–695 °C). In contrast, the protoliths of the Duguer gneissic granite formed from magmas generated by the partial melting of Paleoproterozoic–Mesoproterozoic metapelites contaminated by mantle-derived material under relatively high-temperature conditions (TZr = 730 °C–884 °C). The coeval nature and linear distribution of the igneous rocks within the northern Gondwanan margin, and the fact that the granites in the Gemuri area are enriched in large-ion lithophile elements (LILEs; e.g., Rb and K) and depleted in high-field-strength elements (HFSEs; e.g., Zr, Nb, Ta, and Ti), suggest that this magmatism occurred in an active continental margin setting involving the formation of arc-type rocks. The presence of an early Paleozoic suprasubduction zone-type ophiolite within the South Qiangtang Terrane provides evidence of the southward subduction in the Paleo-Tethyan Ocean at this time, suggesting that the two stages (i.e., the Gemuri leucogranites and Duguer gneissic granites) of arc-type magmatism in the Gemuri area may both be related to this subduction event.