Sr, Nd and O isotopic characters of quartz syenite in the Weiya magmatic complex from eastern Tianshan in NW China: Melting of the thickened juvenile lower crust

抄録

New Sr-Nd-O isotopic data are reported for the early Triassic (early Indosinian) Weiya quartz syenite from eastern Central Tianshan, northwestern China, with the aim of documenting its possible source(s), constraining its petrogenesis and discussing its tectonic implications. Whole-rock Rb-Sr isochron of the Weiya quartz syenite rock yielded an age of 246.5 ± 4.8 Ma, which is well consistent with zircon SHRIMP U-Pb age. Additionally, the rock exhibits positive ε_Nd(t) value (+1.36 to +1.66), medium I_sr (0.7088 to 0.7090), “normal” δ¹⁸O value (6.9 to 8.1‰) and young Nd model ages (T_DM = 0.88 to 0.91 Ga). Combined with their petrogaphic and geochemical characters, the Weiya quartz syenite is suggested to have been generated by partial melting of juvenile lower crust rocks at high temperature. The partial melting is attributed to intra-continental subduction, which resulted in thickening of the continental crust, delamination of the lithosphere root, underplating of mantle-derived magma and subsequent regional extension. Heat from underplated mantle magma caused melting of the overlying lower crustal rocks and consequent generattion of the quartz syenitic liquid. It is supposed that the intra-continental subduction was induced by tectonic conversion from the Paleo-Asian subduction-collision system to the Paleo-Thethys regime during the latest Permian or earliest Triassic in the eastern Tianshan.