Late Triassic granites (ca. 230 Ma) from Cairi area in eastern Qiangtang subterrane, central Tibet: Product of a mid-oceanic ridge subduction

Although a large number of Triassic magmatism were found in the Qiangtang terrane, central Tibet, the tectonic evolution of the Shuanghu Palaeo-Tethys Ocean remains controversial. In this study, we present the zircon U-Pb age and major- and trace-element data for the Late Triassic pluton from the Cairi area in the eastern Qiangtang terrane. Zircon U-Pb dating indicates that the samples were emplaced at ca. 230 Ma. The magmatism from the Cairi area primarily consists of granites (SiO2 = 67.28-77.74), with the characteristics of medium-K calc-alkaline, metaluminous, and peraluminous. The Cairi granites are enriched light rare earth elements ([La/Yb](N) = 5.01-10.56), with obvious Eu anomaly (Eu/Eu* = 0.32-0.47). The Cairi granites are characterized by low 1,000 x Ga/Al (2.10-2.64), FeO*/MgO (3.41-5.03), Ce (57.95-114.36 ppm), Y (27.30-45.19 ppm), and negative correlation between P2O5 and SiO2. These geochemical features reveal that the Cairi granites are calc-alkaline, I-type granites. The four samples show relatively restricted (Sr-87/Sr-86)(i) ratios (0.70598-0.70724), Nd-143/Nd-144 ratios (0.51192-0.51196), and epsilon Nd(t) of -8.19 to -7.54. Based on the variations of Rb-Sr-Ba and Th-La-Nb and the zircon saturation of temperatures (806-868 degrees C), we suggest that the primary magmas of the Cairi granites were generated through various degrees of mixing between sediments and mantle-derived components. Petrological and geochemical characteristics, as well as regional geological characteristics indicate that the Cairi granites formed in a continental volcanic arc setting. Our results, in combination with the coeval granites and elcogite, indicate that the generation of Late Triassic (ca. 230 Ma) Cairi granites is related to ocean ridge subduction and subsequent development of a slab window.