First identification of Carboniferous mafic dikes during a period of arc magmatic quiescence in the Northern Qiangtang terrane, central Tibet: Back-arc extension and implications for opening of the Xijir Ulan Ocean

Numerous studies have focused on the effects of subduction zones on the development of back-arc basins. However, few have considered whether or how back-arc extension affects subduction zones. Back-arc spreading is thought to have resulted in reduced arc magma production in many Cenozoic subduction systems, but there are no documented pre-Cenozoic examples of this process. This paper reports new geochronological, mineralogical, geochemical, whole-rock Nd, and zircon HfO isotope data for Carboniferous mafic dikes (diabase and gabbro) from the Tuotuo River area, Northern Qiangtang terrane (NQT), central Tibet. The dikes were intruded during the middle Carboniferous (330–325 Ma). The dikes are typical arc-like basalts with enrichments of large-ion lithophile elements and depletions of high-field-strength elements. Depleted mantle-like Nd − Hf − O isotopes indicate the mafic rocks were formed by partial melting of metasomatized asthenospheric mantle. Clinopyroxene compositions follow the trends of both back-arc basin basalts (BABBs) and rift cumulates, and the whole-rock compositions are also similar to BABBs. Based on the tectonic evolution of the Paleo-Tethys Ocean in the Tibet segment, we propose that this Carboniferous mafic magmatism was caused by back-arc extension during a period of magmatic quiescence of the main Paleo-Tethyan subduction system. This back-arc extension may have led to the opening of the Xijir Ulan Ocean.