Geochronology, geochemistry and petrogenesis of the Jietebutiao A-type granites in West Junggar, Xinjiang

Late Paleozoic post-collisional granitoids are widespread in West Junggar and even in the whole northern Xinjiang. As a representative of these intrusions, the Jietebutiao granite occurs in the southwestern margin of West Junggar, mainly comprises middle-coarse-grained monzogranite and syenogranite, and provides important clues for petrogenesis of granites and evolution of tecton-ic-magmatism in West Junggar. This paper reports the results of high-precision zircon LA-ICP-MS U-Pb dating of the Jietebutiao granite, which yields weighted mean 206Pb/238U ages of (287±9) Ma (n=10, MSWD=0.92) and (278±3) Ma (n=14, MSWD=0.43) for monzogranite and syenogranite, respectively, corresponding to the Early Permian and implying that the granite is the product of post-collisional magmatic activity around the Junggar area approximately at 300 Ma. Petrogeochemical analyses suggest that the Jietebutiao pluton which has long been thought to be of I-type granite actually belongs to A-type. The syenogranite is characterized by high silica (SiO2: 76.11%~76.82%), alkali (Na2O+K2O: 8.47%~8.49%), and low titanium and calcium (TiO2: 0.04%~0.05%, CaO: 0.36%~0.42%). Moreover, the monzogranite is similar to the syenogranite in such aspects as high silica (SiO2: 68.35%~71.80%) and alkali (Na2O+K2O: 6.80%~7.86%) as well as low titanium and calcium (TiO2: 0.29%~0.82%, CaO: 1.76%~2.87%); nevertheless, they both belong to metaluminous or peraluminous (ACNK: 0.98~1.09) high-K calc-alkaline series. Compared with the monzogranite, the syenogranite has relatively low REE content (ΣREE: 23.8×10-6~49.3×10-6, 95.23×10-6~222.2×10-6) with significant negative Eu anomalies (Eu/Eu*: 0.01~0.02, 0.57~0.72). They are also enriched with large ion lithophile elements such as Rb, Th and K and high strength field elements such as Zr, Hf and Nb and strongly depleted in Ba, Sr, Eu and Ti, with high 10000Ga/Al ratios (>2.44). These characteristics are more clearly in syenogranite than in monzogranite. Based on trace element ratios and related discrimination diagrams, the Jietebutiao pluton can be further subdivided into A1 and A2 type granites, which are usually believed to have been formed in an post-collisional tectonic setting and derived from the lower crust composed of juvenile mantle-origin substance. In the early post-collisional magmatism, A2-type monzogranite magma characterized by island arc features was derived from partial melting of the lower crust and, with the further extension of the lithos-phere, probably formed a rift-like environment in local area, producing the A1-type syenogranite magma.