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

摘要

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 205 Ma. The emplacement time is similar to the Ar-Ar ages of phengites and sodic amphiboles of the edogites 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 (223-219 Ma) includes diorite from the Baohu area and some volcanic rocks from the Juhuashan area. These rocks have high Sr (291-1367 ppm) and low Y (5.5-13.9 ppm) 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 (644-729 Ma). The second and younger group (215-210 Ma) comprises volcanic rocks from the Jiangai and Guoganjianian areas. They show high initial ~(87)Sr/~(86)Sr 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 (1658-2178 Ma) and zircon Hf model ages (1251-2012 Ma). 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.