Triassic arc magmatism in the Qiangtang area, northern Tibet: Zircon U-Pb ages, geochemical and Sr-Nd-Hf isotopic characteristics, and tectonic implications

Felsic magmatic rocks of Triassic ages occur on the north side of the Qiangtang metamorphic belt. Zircon dating using the SHRIMP and LA-ICP-MS techniques reveals that these rocks were emplaced between 223 and 205Ma. The emplacement time is similar to the Ar-Ar ages of phengites and sodic amphiboles of the eclogites and blueschist from the Qiangtang metamorphic belt. The felsic magmatic rocks can be divided into two groups with distinct geochemical and Sr-Nd-Hf isotopic compositions. The first and older group (223-219Ma) includes diorite from the Baohu area and some volcanic rocks from the Juhuashan area. These rocks have high Sr (291-1367ppm) and low Y (5.5-13.9ppm) contents with high Sr/Y ratios (28-117). They are characterized by slightly negative ε_Nd(t) values (-1.9 to -5.7), positive ε_Hf(t) values (+3.2 to +5.4) and young zircon Hf model ages (644-729Ma). The second and younger group (215-210Ma) comprises volcanic rocks from the Jiangai and Guoganjianian areas. They show high initial ⁸⁷Sr/⁸⁶Sr ratios (0.708-0.714), distinct negative ε_Nd(t) (-9.6 to -7.9) and ε_Hf(t) values (-15.3 to -10.1) and much older bulk-rock Nd model ages (1658-2178Ma) and zircon Hf model ages (1251-2012Ma). Geochemical arguments led us to suggest that the rocks of the first group were generated by the melting of subducted oceanic crust and associated sediments, and the second group rocks were derived from anatexis of the Proterozoic continent crust. In conclusion, the Late Triassic magmatic rocks from the northern Qiangtang have recorded the process from subduction to break-off of an oceanic slab. The Qiangtang metamorphic belt to the south represents a northward oceanic subduction zone.