Triassic Paleo-Tethyan slab break-off constrained by a newly discovered 211 Ma dacite–rhyolite suite in the Qiangtang terrane, central Tibet

Abstract

• A newly discovered Late Triassic volcanic suite in the eastern Qiangtang subterrane includes dacites and A-type rhyolite. • This suite was emplaced at 211 Ma and was generated via partial melting of the Qiangtang continental crust. • The formation of this volcanic suite reflects the slab break-off of the Paleo-Tethyan in the Late Triassic. Triassic igneous rocks are widely exposed in the central Qiangtang terrane, Tibet. However, Triassic tectonic–thermal evolution in the central Qiangtang terrane and the evolution of the Paleo-Tethyan Ocean are still unclear. In this study, we report zircon U–Pb ages, and whole-rock major- and trace- element compositions, and Sr–Nd isotope data for the newly discovered Late Triassic Tumen volcanic rocks. The volcanic rocks consist of interlayered dacite and rhyolite (SiO 2 = 63.8–69.7 wt%) with a thickness of approximately 150 m. New zircon U–Pb dating reveals the eruption of this volcanic suite at 211 Ma. Geochemical data show that the volcanic suite has a wide range of K 2 O (1.36–5.46) values and high-molar Al 2 O 3 /(CaO+K 2 O+Na 2 O) values (0.97–1.36). All samples clearly show the enrichment of light rare earth element, with (La/Yb) N = 6.79–11.03 and (La/Sm) N = 3.23–4.44, and distinct negative Eu anomalies (Eu/Eu* = 0.44–0.70). All samples exhibit the depletion of high field strength elements (e.g., Ti, Nb, and Ta), and enrichment in large-ion lithophile elements (e.g., Rb and K). The dacite–rhyolite suite has a narrow range of ( 87 Sr/ 86 Sr) i ratios (0.7102–0.7132) and ε Nd (t) values (−10.2 to −10.0), with Nd isotope model ages of 1.76–1.82 Ga. Zircon saturation temperatures (860℃–921℃) and geochemical characteristics show that the samples are A-type granitoids and were likely generated by partial melting of the Qiangtang continental crust. The rise of the asthenospheric mantle caused by the slab break-off provides a large amount of heat and materials for the igneous rocks in the Shuanghu–Longmucuo suture zone during the Late Triassic.