Middle Triassic arc magmatism in the southern Lhasa terrane: Geochronology, petrogenesis and tectonic setting

Abstract

The voluminous Triassic magmatism within the Lhasa Terrane, Tibetan Plateau, has been studied for decades yet the petrogenesis of the magmas involved and the tectonic setting of this magmatism and associated events remain uncertain. This study presents new zircon U–Pb geochronological, Lu–Hf isotopic, and whole-rock geochemical data for the Middle Triassic volcanic rocks within the southern region of the Lhasa terrane. Zircon U–Pb dating of three samples from this area yield consistent ages of 233.9 ± 1.3, 232.5 ± 1.6, and 231.5 ± 1.6 Ma, representing the Middle Triassic magmatism in the southern Lhasa terrane. The whole-rock geochemistry of these volcanic rocks indicate they are enriched in the large ion lithophile elements (LILE) and the light rare earth elements (LREE). They have high εNd(t) (from +4.3 to +5.4) and zircon εHf(t) (from +16.6 to +21.8) values as well as low initial 87Sr/86Sr (0.7035–0.7037) values. These characteristics indicate that the Middle Triassic igneous rocks in this region formed from magmas generated by the partial melting of arc-type mantle wedge material. These magmas underwent fractional crystallization but did not assimilate significant amounts of ancient crustal material prior to emplacement and eruption. Combining our new data with the results of previous petrological, stratigraphic, paleomagnetic, and paleontological data from this region indicates that the Triassic magmatic rocks of the southern Lhasa terranes formed as a result of tectonism associated with the opening of the Neo-Tethyan Ocean. The driving force of the extension was most likely driven by correlated with the southward-subduction of the Banggonghu–Nujiang Oceanic plate.