Carboniferous Sedimentary Basin Analysis And Tectonic Significance In The Baogutu-Halaalate Mountain, Western Junggar, Xinjiang

The West Junggar region, an essential part of the Central Asian Orogenic Belt(CAOB), is located at the southern margin, in which the late Paleozoic strata outcropped is relatively completed, and it is a pivotal area for the research of the tectonic evolution of orogenic belt in late Paleozoic. Controversies exist on the tectonic setting, the stratigraphic framework as well as the sedimentary basin prototype when it comes to the widely distributed Carboniferous. Thus, the investigation is based on the Carboniferous in the Baogutu-Halaalate mountain area. Combined with the previous data, the dissertation has set up a framework for the Carboniferous stratigraphic to analyze and discuss the stratigraphic sequence, the volcanic cycle and the basin model, providing a sound evidence for the further understanding of the late Paleozoic tectonic evolution process in the Western Junggar.The research believes that the strata sequence of the Carboniferous from bottom to top in the Baogutu-Halaalate Mountain, Western Junggar is the Baogutu Formation, the Xibeikulasi Formation, the Chengjisihanshan Formation, the Halaalate Formation and the Aladeyikesai Formation successively. The natural field contact relationship shows that the Xibeikulasi Formation is overlying on the Baogutu Formation unconformity, and overlain by the Chengjisihanshan Formation uncomfortably, the Halaalate Formation is contact with the Chengjisihanshan Formation comfortably. The Aladeyikesai Formation, the youngest of the marine strata in this area, is contact with the Jiamuhe Formation uncomfortable. The volcanic LA ICP-MS zircons U-Pb chronology has obtained the result in age that the Baogutu Formation was 331.2Ma~331.2Ma, the Chengjisihanshan Formation is 312.3Ma~319Ma, the Halaalate Formation is 304.5Ma~306.9Ma and the Aladeyikesai Formation is 303.8Ma respectively. The detrital zircon U-Pb chronology indicates that the lower limit of the Xibeikulasi Formation age is 324.3Ma.According to the comparison and validating of the unconformity, the research of the Carboniferous stratigraphy sequence is based on the detailed comparison of the stratigraphic section. Dividing the Early Carboniferous into one I sequenceas a whole, it includes two II sequences and four III sequences; and the Late Carboniferous strata belong to an II sequence, including four III sequences.In addition, this research divided the Early Carboniferous volcanic rocks into 10 volcanic eruption cycles, and the volcanic magmatism events have erupted at least 17 times, as a result, formed the basic volcanic rocks and volcanic clastic rocks, mostly the volcanic-sedimentary facie. The geochemical studies show that OIB and E-MORB geochemical characteristics, a result of partial melting by the mantle wedge, are formed in the island arc tectonic setting. The Late Carboniferous volcanic rock can be divided into 13 volcanic cycles, and the strong volcanic magma has erupted at least 22 times. Therefore, the volcanic rocks are mainly basic, basic-neutral and acidic, with the erupted facies and flowed facies in prior, While, neutral volcanic rocks are in a small amount, showing the characteristic of the bi-model volcanic activity. The geochemical features indicate the volcanic rocks were formed in the continental intra plate. The slab experienced metamorphic dehydration and detachment at last, triggered the asthenosphere mantle upwelling and the basaltic magma invasion. Besides, a part of the island arc geochemical features of the volcanic rocks are inherited from the magma source.The order of the Early Carboniferous regional tectonic configuration in the study area from the south to the north is the Daerbut-Karamay ophiolite belt, the Baogutu-Halaalate mountain fore-arc basin, and the Boshchekul-Chingiz arc, in which the basement of the fore-arc basin is island arc with a part of oceanic crust, the cover of the basin showing a strong fold but weak metamorphism. The Boshchekul-Chingiz arc in the north and the continental margin arc of Tacheng microcontinental in the west are mainly provenances for the fore-arc basin. In the Late Carboniferous, the subduction stopped and the basin changed into the residual sea basin evolution stage, with the basin basement is mainly the Early Carboniferous fold strata, while the strata are formed in shallow marine environment, with two provenances of both north and south.Research shows that the Baogutu-Halaalate mountain area in the Early and Late Carboniferous has a distinct tectonic setting, tectonic basin presenting a significant distinction of filling and volcanic activity characteristics. Thus, based on the research of predecessors’, this paper puts forward such a tectonic evolution model that two different stages, namely, the fore-arc basin tectonic evolution stage in the Early Carboniferous and the residual sea evolutionary stage in the Late Carboniferous. The shifting of plate edge tectonic to the plate tectonic took place in the late of the Early carboniferous. Though the geotectonic background has changed, the sedimentary response in the Late Carboniferous show an obvious lag, which is still the marine deposit. From the regional perspective, the sedimentary facies changes from marine to continental facies occurred at the end of the Late Carboniferous, in the mark of Jiamuhe Formation Early Permian of continental molasses.After the Carboniferous, the West Junggar region has experienced a large-scale intra-continental strike-slip, especially the left lateral slip along the Daerbut fault, leading to the destruction and transformation of the structure of the ancient forearc basin and the ancient subduction zone. The most prominent sign is that what originally an ophiolite belt broke into two parts, and the zonal distribution of granitic batholith in the Late Carboniferous was converted into planar, making it hard to recognize and recover the original face of the orogenic belt and the tectonic basin which is closed to the orogenic. Based on the Carboniferous stratigraphic sequence study, together with volcanic chronology and geochemistry research, this dissertation provides new evidences for a better understanding of the Carboniferous tectonic evolution of CAOB in the Western Junggar.