Middle–Late Triassic bimodal intrusive rocks from the Tethyan Himalaya in South Tibet: Geochronology, petrogenesis and tectonic implications

Abstract

This paper presents new geochronological and geochemical (whole-rock major and trace elements and Sr–Nd–Pb isotopes) data on intrusive rocks exposed in the Qiongduojiang area, the northern part of the Tethyan Himalaya. The Qiongduojiang intrusive rocks are mainly composed of mafic rocks (diabase and gabbro) with subordinate felsic rocks (monzonite), indicating bimodal component characteristics. In zircon U-Pb dating of monzonite, diabase and gabbro samples, the weighted mean ages of magma emplacement were determined to be 230.5 ± 2.9 Ma to 227.4 ± 4.7 Ma, showing Middle–Late Triassic magmatic activity in the northern part of the Himalayan belt. The monzonitic rocks share most of the geochemical features of A-type granites, and their Nd isotopic compositions are consistent with those of coeval diabase rocks. This suggests that they are likely generated by mantle-derived magmas, which had assimilated significant amounts of continental crust. The gabbro samples exhibit the geochemical composition of ocean island basalt (OIB) features and therefore assumingly originate from a depleted mantle source with insignificant crustal contamination. In contrast, the diabase samples display enriched mid-ocean ridge basalt (E-MORB) features and were derived from spinel bearing peridotite that was metasomatised by OIB-like components with limited crustal contamination. The Qiongduojiang bimodal magmas can be considered as the products of the opening of the Neo-Tethyan ocean. Considering the magmatic petrology, paleomagnetism and paleontology data, we further propose that the Neo-Tethyan ocean began opening before ~230 Ma.