Petrogenesis of Jurassic Xietongmen intrusive rocks at the southern margin of the Lhasa terrane: implications for intra-oceanic arc evolution

Abstract

The Gangdese batholith, Tibet, records the opening and closing of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. The Mesozoic magmatic rocks play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. This study focuses on Jurassic intrusive rocks in the Xietongmen area of the southern margin of the Lhasa terrane adjacent to the Yarlung–Tsangpo suture. Zircon U–Pb dating yielded Middle Jurassic dates for ca 170 Ma hornblende gabbro and ca 173 Ma granodiorite intrusions. All of the samples are medium- to high-K calc-alkaline, and the majority are metaluminous and enriched in the large ion lithophile elements and depleted in the high-field-strength elements. This indicates the magma was generated in a subduction-related tectonic setting. The intrusive rocks have high and positive εHf(t) values (hornblende gabbro: 13.3–18.7; granodiorite: 14.2–17.6) that yield Hf model ages younger than 312 Ma. These new data, combined with the results of previous research, suggest that the Jurassic igneous rocks were derived from a metasomatised region of an asthenospheric mantle wedge. Extremely depleted Sr–Nd–Pb–Hf isotope compositions are similar to the Yarlung ophiolite and igneous rocks within other intra-oceanic island arcs. Together with the existence of sandstone that is identified as the product of the oceanic island arc environment, we suggest formation in an intra-oceanic island arc.The new zircon U–Pb dating has yielded Middle Jurassic ages for the ca 170 Ma hornblende gabbro and ca 173 Ma granodiorite phases of the Xietongmen intrusion.Jurassic igneous rocks formed from a metasomatised asthenospheric mantle wedge by northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane.Late Triassic–Jurassic igneous rocks, which are characterised by highly depleted isotopic compositions within the Southern Lhasa sub-terrane, record residual intra-oceanic island arcs in the eastern Tethyan belt.