Emplacement Mechanism Of Post-collisional Pluton,Western Junggar, Xinjiang

Western Junggar is located on the west of the Junggar Basin in Xinjiang, NW China, and bounded by the Irtysh-Zaysan accretionary complex in the north and the North Tian Shan accretionary complex in the south, respectively. Complex processes of accretion, convergence, subduction and collision of various continents (or blocks) took place during the rifting, spreading, subducting and closing of the PaleoAsian Ocean (hereafter referred to as PAO), which is between the Siberia plate and Tarim-North China plates. The formation and emplacement machanism of plutons is one of a significant constituent part of regional tectonic evolution. Late Carboniferous-Early Permian post-collision plutons extensively distribute throughout the Western Junggar. Knowledge of three-dimension configuration and emplacement mechanism of plutons in the West Junggar can provide excellent opportunities to explore the tectonic evolution and intracontinental dynamics of Western Junggar. However, scarce studies on the emplacement mechanism of pluton greatly obstruct our ability to clarify the above questions. In this paper, we choose Akebasitao,Kelamayi and Xiaoxihu pluton in the east part of West Junggar areas as target to answer following two questions:(1) What is the emplacement mechanism of these three plutons? and (2) What is the tectonic regine during the emplacement of these plutons in the Late Carboniferous to Early Permian? A multidisciplinary approach has been applied in this study emphasizing (ⅰ) the configuration of plutons and its country rock at the micro, macro and regional scales. (ⅱ) the audio magnetotelluric sounding analysis, and (ⅲ) the anisotropy of magnetic susceptibility(AMS).The surface configuration and audio magnetotelluric sounding investigations of Akebasitao pluton indicate that the pluton, with slightly oblique movement of magma from north-west to south-east and an asymmetrical mushroom-shape, probably controlled by the north-east-striking Anqi fault. The rock magnetism features reveal that the AMS patterns ascribed to the primary magmatic fabrics,and the pluton was formed in a relatively stable structure environment with no regional extrusion. Integrating the above analysis, our study favors a permissive emplacement mechanism for the pluton, i.e. the magma ascended along the crustal-scale fault system and actively emplaced in a manner of doming accompanied by stopping.In Kelamayi pluton,new field observations and audio magnetotelluric sounding data suggest that the pluton boundaries are weakly inclined and the pathway of magmas located in central part of the pluton. what is more,the anisotropy of magnetic susceptibility reveals that the tectonic strain is very weak during the emplacement time of the Kelamayi pluton and the pluton AMS fabic will be controlled by magma flow, which is in agreement with the Akebasitao pluton. We suggest that the pre-existing crustal-scale fault system underneath the Chenjisihan mountain paobably acted as a effective pathway for migration of the magma:In the earlier stage,high-viscous granodiotitic magas,with slightly oblique movement, migrated probably along the pre-existing faults. Subsequently, low-viscous granitic magas similarly ascented along the faults,and meanwhile, obliquely expand from central to NE and SE. However, the wallrock’s xenoliths in the marginal belt suggest that stoping was also still active at this time.The rock magnetism features,integrated surface configuration suggest that the pluton boundaries are highly inclined and the pathway of magmas weakly located in south part of the Xiaoxihu pluton. A possible emplacement mechanism involving a permissive intrusion along the pre-exsiting faults and slightly oblique movement of magma from S or SSW to N or NNE accommodated by a relatively stable tectonic setting.The emplacement mechanism and tectonic regine analysis reveal that there are no obvious regional extrusion during the emplacement time of Akebasitao,Kelamayi and Xiaoxihu pluton.Thus, we suggest that the tectonic setting is relatively stable in the Late Carboniferous to Early Permian, which reflects the termination of collision orogenesis and the closure of the Junggar Ocean took place before the later Late Carboniferous.Such an important transition of tectonic dynamic regime in Western Junggar is also supported by the regional tectonic palaeogeography,magmatite system and paleostress field.