Middle Triassic arc magmatism along the northeastern margin of the Tibet: U–Pb and Lu–Hf zircon characterization of the Gangcha complex in the West Qinling terrane, central China

<p>The tectonic setting of Mesozoic magmatic complexes in the northeastern margin of the Tibet plateau is disputed, and hence gives rise to uncertainty concerning the tectonic evolution of the northeastern Tibet Plateau and the timing of the closure of the Palaeo-Tethys ocean. The Gangcha complex is typical of these complexes, consisting of andesite, dacite, gabbro, gabbroic diorite, granodiorite, quartz diorite, and diorite with typical chemical traits of continental margin arc rocks. Andesite, gabbroic diorite, and mineralization-associated potassic-altered diorite yield weighted mean ²⁰⁶Pb/²³⁸U ages of 242.1 ± 1.2 Ma, 243.8 ± 1.0 Ma and 234.0 ± 0.6 Ma respectively. Zircon ϵ_Hf(t) for magmatic grains ranges from −3.5 to +5.7, interpreted to demonstrate that the Gangcha complex contains crustally contaminated mantle magmas. Inherited zircons in the complex yield similar U–Pb ages (777–310 Ma) to the A’nyemaqen composite ophiolite assemblage with ϵ_Hf(t) of −17.4 to +11.6. This suggests that components of this older ophiolite melted and contributed to the Gangcha complex magmas. Hence the Gangcha complex is considered to have formed as a continental margin arc in northeastern Tibet by northward subduction during consumption of the Palaeo-Tethys ocean. Regionally, it corresponds to the arc magmatism along the eastern and western Kunlun sutures to the west and the Mianlue suture to the east. </p

Assembly and dispersal history of continental blocks within the Altun-Qilian-North Qaidam mountain belt, NW China

The continental arc and intraplate magmatic rocks occur widely within the Altun-Qilian-North Qaidam (AQNQ) region of NW China and play a key role in reconstructing the assembly and dispersal history of micro-continental blocks within a complex orogenic system. In the South Qilian belt, amphibolites from the Hualong Complex can be subdivided into four types based on their lithologies, ages, and tectonic affinities. Type I is characterized by garnet and Proterozoic detrital zircons, indicating a metasedimentary origin. Type II and IV amphibolites are tholeiitic and calc-alkaline, respectively, and characterized by enrichment in LREES, Th, U, and Ba, accompanied by depletion in Nb and Ta, indicating a continental arc setting on an active continental margin. Type III is alkaline with ocean island basalt affinity and considered to have intraplate, rift-related origins. SHRIMP U-Pb dating and previous studies suggest that type II, III, and IV amphiboltes are formed at 1126 Ma, 882 Ma, and 470 Ma, respectively, which corresponds to 1126–895 Ma continental arc magmatism, 882–580 Ma rifted-related magmatism, and 522–440 Ma continental arc magmatism within the AQNQ. The three episodes of magmatism further demonstrate the processes of assembly and breakup of the Proterozoic supercontinent Rodinia and reassembly of the Altun-Qilian-North Qaidam orogenic belt.</p