Origin of Triassic mafic magmatism in the North Qiangtang terrane, central Tibetan Plateau: implications for the development of a continental back-arc basin

Abstract

This paper presents a comprehensive study of geochronology, in situ clinopyroxene mineralogy, geochemistry and isotope geology for the Triassic mafic magmatism of the central Tibetan Plateau. Moreover, the results are used to understand the development of a continental back-arc basin and the generation of back-arc basin magmatism. Zircon U–Pb isotope analyses indicate that these rocks were formed during the Middle Triassic (c. 231 Ma). Both the mafic rocks and the trapped melt calculated by the clinopyroxene/melt partition coefficient (Dcpx/melt) exhibit distinct enrichments in light rare earth elements (LREE) and Th with pronounced depletions in Nb and Ta, which are similar to the geochemical composition of typical continental back-arc basin basalts. Geochemical features and isotopic variations indicate that the mafic melts originated from a depleted mantle modified by c. 10% crustal components. The ratios of Zn/Fet and REE further suggest a spinel-bearing peridotite source. Considering other regional geological data, we argue that the origin of Triassic mafic magmatism should be attributed to the extension of a continental back-arc basin. Slab rollback and mantle plume activity played vital roles in the formation and development of a continental back-arc basin in the central part of the Tibetan Plateau.