LATE PALEOZOIC POST-COLLISIONAL MAGMATISM IN THE SOUTHWESTERN TIANSHAN OROGENIC BELT, TAKE THE BALEIGONG PLUTON IN THE KOKSHAL REGION AS AN EXAMPLE

Abstract

Southern Tianshan is one of the regions with most complex structural evolution history in the Tianshan Orogenic belt. Compared with other regions in the northern Xinjiang, studies on problems of continental dynamics of the post-collision procedure and the magma genesis are relatively weak. Baleigong pluton, which is located in the southwestern section of the Southern Tianshan, is carefully studied in this study. Based on geochemistry and geochronology, the petrogenesis, characteristics of the magma source, as well as the geodynamic are discussed, in order to provide some constraints for the further studies on the evolutionary patterns of the ocean-crust system in the region such as closure of the oceanic crust and timing of the collisional orogeny. The Baleigong pluton located in the Southwestern Kokshal area, the western segment of the Southwestern Tianshan; and emplaced into the south of the Baleigong ophiolitic m lange. Lithologically, the pluton consists mainly of biotite moyites. Geochemically, the rocks are high-K calc-alkaline to shoshonitic series, and rich in alkali (K2O + Na2O 8.25% - 8.72%) and K (K2O/Na2O 1.34 - 1.56), with the A/CNK values between 0.94 and 1.05, which fall into the range of metaluminous to slightly peraluminous rocks. The rocks are also characterized by enrichment of LREE and LILE (Cs, Rb, Th and Ba) and depletion of Sr, P, Nb and Ti, with moderate negative Eu anomaly (δEu 0.49 - 0.59), showing the transitional features between high-K catc-alkaline and A-type granites. The elemental ratios of the Baleigong pluton, such as Nd/Th (1.64 - 3.19), Th/U (5.95 - 7.11), Nb/Ta (7.26 - 9.17, combined with the high K2O/Na2O and low Sr/Ba ratios, indicate the partial melting of middle to lower crust with intermediate magmatic rocks as protoliths, with plagioclase amphibolite as the restite. In situ zircon U-Pb LA-ICP-MS dating on the magmatic genetic zircons yielded an age of the 273 ± 2Ma, representing the emplacement age of the pluton. Combined with the previous studies, it is concluded that the Late Paleozoic post-collisional granitic magmatism in the South Tianshan took place in the period between 282 and 259Ma and obviously later than that of other areas in the Northern Xinjiang. Together, these studies also indicate that the post-collisional granitic magmas evolved from high-K calc-alkaline series (282 - 266Ma) to alkaline series (266 - 259Ma) successively, which suggest the process of orogenic collapse and continuously extensional tinning of the continental crust during the post-collisional stage. The Baleigong high-K granite was probably formed during the first stage of the collapse after the collisional orogeny or during the transitional stage between collision and post-collision, by partial melting of the middle to lower crust materials. The post-collisional magmatism in the South Tianshan indicates that before the Middle Permian, the Paleozoic South Tianshan Ocean has been closed and the South Tianshan was in a post-collisional setting, which probably represents the last closure of southern part of the Paleo-Asia oceanic crust, as well as the ending of the accretionary orogeny in the southern section of the Central Asia. These conclusions provide some new constraints to the further study on the timing and petrogenesis of the post-collisional magmatism in the Central Asia.