Geochronology and geochemistry of the Yagan granite in the northern margin of the Alxa block: Constraints on the tectonic evolution of the southern Altaids

The Yagan granites, which are located in the northern margin of the ALxa block, mainly consist of mzonogranites. The zircon SIMS U-Pb age of the granites is 283. 2 +/- 2. 2Ma (n = 14, MSWD = 0. 016). The Yagan granites have relatively low SiO, contents (66. 96%similar to 70. 71%), and high Al2O2 (15. 05%similar to 16. 05%) and Na2O + K2O (7. 24%similar to 9. 19%) contents. They also exhibit high K calc-alkaine and metaiuminous-peraluminous characteristics. Total rare earth element (REE) contents of the Yagan granites range from 101. 9 x 10(-6) to 133. 1 x 10(-6). They show enrichments of LREEs, and exhibit relatively moderate-strong negative Eu anomalies (delta Eu = 0. 59 similar to 0. 77) in the chondrite-normalized BEE patterns. They display depletions of high field strength elemens (Nb, Ta, P, Ti) and enrichments of large ion lithophile elements (Ba, Rb, Cs, Th, K) in the primitive mantle-normalized trace element spiderdiagram. The granites also have high (Sr-87/Sr-86)(i) values (O. 707654 similar to 0. 710235) and negative epsilon(Nd) (t) values. Based on the geochemical characteristics and related sedimentary formations, it is concluded that the Yagan granites should be formed in the post-collisional setting. The age of the Yagan granites (283. 2 +/- 2. 2Ma) implies that the branch of the Paleo-Asian Ocean in the northern Alxa region had been closed before the Early Permian.