Paleozoic granitic magmatism and tectonic evolution of the South Altun block, NW China: Constraints from zircon U-Pb dating and Lu-Hf isotope geochemistry

The South Altun orogenic belt (SAOB), consisting of the South Altun block (SAB) and the South Altun continental block (SACB), is one of several important orogenic belts in NW China. Numerous granite bodies are widely distributed in the SAB. However, the petrogenesis and tectonic environment of these granites, and their relationships to those of the SACB, Central Altun Continental block (CACB), North Altun block (NAB), Qilian block (QLB), and North Qaidam block (NQB) remain unclear. In this investigation, we selected the SAB granites for detailed study, and explore the petrogenesis employing bulk-rock geochemistry, zircon U-Pb dating, and Hf isotopic analysis. Zircon U-Pb LA-ICP-MS dating of the granites from the SAB identified seven spatially distinct Paleozoic plutonic suites: (a) Yigan-352-343 Ma; (b) Baiganhu-448-444 Ma; (c) west Mangya-444-435 Ma; (d) Aketishan-265 Ma; (e) Chaishuigou-406 Ma; (f) Changchungou-469 Ma; and (g) north Mangya-462 Ma. Based on previous research combined with our new U-Pb dating and bulk-rock geochemical investigations in the SAB, we identdified six distinct episodes of granitoid emplacement: episodes 1 and 2, quartz diorite + granodiorite + monzogranite with I-type affinities, and ages of 503-497 Ma and 483-458 Ma, respectively; episode 3, granodiorite + monzogranite + syenogranite with I- and S-type affinities, and an age of 456-432 Ma; episode 4, monzogranite + syenogranite + alkali feldspar granite with A-type affinities, and an age of 424-385 Ma; episode 5, granodiorite + monzogranite with S-type affinities, and an age of 352-338 Ma; and episode 6, quartz diorite + monzogranite + syenogranite with I-type affinities, and an age of 265-264 Ma. Zircon Lu-Hf isotope analyses from various episodes show that epsilon(Hf)(t) values are positive in most cases, but negative for a small number of inherited (xenocrystic) zircons. Source rocks were mainly juvenile crust, evidently mixed with minor ancient continental material. Based on our new results integrated with regional geological data, we conclude that episodes 1 and 2 granitic magma generations were related to the subduction of oceanic crust, episode 3 to post-collisional partial melting, episode 4 to the isostatic adjustment of orogenic blocks after plate suturing, episode 5 to petrotectonic thickening of the Altun crust, and episode 6 to Altun fault transcurrent slip attending Kunlun Paleo Tethys subduction.