Late Triassic granitic magmatism in the Eastern Qiangtang, Eastern Tibetan Plateau: Geochronology, petrogenesis and implications for the tectonic evolution of the Paleo-Tethys

Abstract

Triassic granites are widely exposed in the central Qiangtang, northern Tibetan Plateau. They, therefore, made an important contribution to the growth of the Tibetan Plateau. In this paper, we present zircon U–Pb dating and Hf isotopic results, and whole-rock elemental and Sr–Nd isotopic analyses of the Dongdashan (DDS) batholith in the central Qiangtang in order to understand their petrogenesis and tectonic setting. LA-MC-ICP-MS zircon U–Pb results show that the DDS batholith was emplaced at 220.3±0.7Ma. All the granites in the DDS possess high A/CNK values (>1.0) and display peraluminous characteristics, similar to S-type granite. Their strongly fractionated REE patterns ((La/Yb)N=6.98–13.9) with conspicuous negative Eu anomalies (Eu*/Eu=0.51–0.67), together with negative εNd(t) and εNd(t) values, and depleted Nd and Hf model ages, suggest that the DDS granitic magma had a dominantly crustal source, likely the Paleoproterozoic basement (i.e., the Ningduo and Caoqu group metasediments) in the area.In combination with regional studies, our new geochemical data and geochronological results demonstrate that the Late Triassic magmatism was generated in a post-collisional tectonic setting. The spatial distribution pattern of the Mesozoic igneous rocks, coupled with the exhumation of high-pressure metamorphic rocks in the central Qiangtang, favors a slab breakoff model, which resulted in post-collisional extension and asthenospheric upwelling that induced large-scale partial melting of the middle-lower crust to produce voluminous amounts of felsic magma. Therefore, the occurrence of the Late Triassic post-collisional magmatism, and particularly the exhumation of high-pressure eclogites and blueschists as well as their presence in the lowermost portion of the Late Triassic volcanoclastics in the Qiangtang basin, clearly indicate that the final closure of the of Paleo-Tethys Ocean and associated continent–continent collision between the Gondwana-derived Western Qiangtang and Eastern Qiangtang terranes had not been completed until the early mid-Triassic.