Petrogenesis of Late Carboniferous granitoids in the Chihu area of Eastern Tianshan, Northwest China, and tectonic implications: geochronological, geochemical, and zircon Hf-O isotopic constraints

This contribution presents new SIMS zircon U-Pb geochronology, major and trace element geochemistry, and zircon Hf-O isotope systematic on an example of Late Carboniferous granodiorite and porphyritic granodiorite intrusions from the Chihu area of Eastern Tianshan, Xinjiang. SIMS zircon U-Pb dating indicates that the Chihu granodiorite and porphyritic granodiorite formed at 320.2 +/- 2.4Ma and 314.5 +/- 2.5Ma, respectively. These rocks are metaluminous to weakly peraluminous with an A/CNK value of 0.92-1.58, as well as low 10000Ga/Al, Zr + Nb + Y + Ce, and Fe2O3T/MgO values, which suggest an I-type normal island arc magmatic suite. The porphyritic granodiorite has a slightly higher Sr/Y ratio (28-37) and lower Y (6.9-11.7ppm) and Yb (0.98-1.49ppm) contents, suggesting mild adakite affinities. In situ Hf-O isotopic analyses using LA-ICP-MS-MC and SIMS indicate that the epsilon(Hf)(t) and delta O-18 values of granodiorite zircons vary from +11.5 to +14.9 and 4.80 to 5.85 parts per thousand, respectively, similar to values for porphyritic granodiorite zircons, which vary from +11.9 to +17.2 and 3.78 to 4.71 parts per thousand, respectively. The geochemical and isotopic data imply that the Chihu granodiorite and porphyritic granodiorite share a common origin, most likely derived from partial melts of the subduction-modified mantle. Based on the regional geological history, geochemistry of the Chihu intrusions, and new isotopic studies, we suggest that the Late Carboniferous magma was generated during the period of the northward subduction of the Palaeo-Tianshan ocean plate beneath the Dananhu-Tousuquan island arc.