Preliminary Study On Formation Setting And Geological Implications Of Pyroclastic Rock In West Junggar, In Late Paleozoic

The West Junggar region, belonging to the Central Asian orogenic belt, is located between the secondary orogenic belts of the Altai and Tianshan orogenic belts.The West Junggar region attributes to the Paleozoic continental accretion fold zone and mainly be formed by the Variscan fold system. The study area is located in the northwestern Junggar basin and the strata exposed incomplete. This paper is to compile the strata in Tuoli County, West Junggar according to the solid field geology and mineral resources investigation, the indoor detail analysis of data, regional stratums, the characteristic of volcanic cycle and the characteristic of petrology. Combined with the data of size analysis from rocks, fossils and age of intrusive rocks in the West Junggar Toli County, a systematic study on the tectonic setting and sedimentary environment in the old outlet of strata has been carried out, and some new understanding has been made, which provides important scientific basis on in-depth study of sedimentary in the West Junggar. The results show that:(1) Malasu group (D1ml):Through the analysis of the zircon U-Pb ages of monzodiorite porphyry in the Akeqiaoke rock body(410.5±2.9Ma) and those of monzogranite in Seerdeng rock body (309.9±2.4Ma), which both invaded the Malasu group, the sedimentary age of Malasu group was earlier than the formation age of monzodiorite porphyry in the Akeqiaoke rock body(410.5±2.9Ma); Combined with volcanic formation, the characteristics of volcanic eruption cycle, and the regional stratigraphic correlation, it showed that the strata is in the Early Devonian. Volcanic activity in this period was from weak to strong; and it showed that this region belonged to shallow marine environment at that time on the basis of rock grain size data and fossils feature.(2) Kulumudi group (D2k):Through the analysis of the zircon U-Pb ages of monzogranite in Hatu rock body (319.6±3.2Ma), which invaded the Kulumudi group, the sedimentary age of Kulumudi group was earlier than the formation age of monzogranite in the Hatu rock body(319.6±3.2Ma); Combined with volcanic formation, the characteristics of volcanic eruption cycle, and the regional stratigraphic correlation, it showed that the strata is in the Middle Devonian. Volcanic activity in this period was from strong to weak to strong, and it formed a series of volcanic construction, which was pyroclastic rocks mainly, clipping a small amount of andesitic volcanic lava; and it showed that this region belonged to shallow marine environment at that time on the basis of rock grain size data.(3) Jiabasitao group (C1j):Combined with volcanic formation, the characteristics of volcanic eruption cycle, and the regional stratigraphic correlation, it showed that the strata is in the Early Carboniferous.Volcanic activity in this period was weak, and it formed a series of volcanic construction, which was pyroclastic rocks mainly, clipping a small amount of andesitic volcanic lava; and it showed that this region belonged to shallow marine environment at that time on the basis of rock grain size data and fossils feature.