| The time of Carboniferous to Permian is an important turning period for tectonic regimeof the Junggar Basin, in which Junggar Basin or even the Central Asia experienced the demiseof the ancient Asian Ocean to the process of continent-continent collision, Magmatic andtectonic activity was intense, widely distributed sedimentary and volcanic constructionsprovides a good basis to investigate the tectonic framework and evolution of this periodprovides. This paper will provide a detailed study on the Carboniferous and Permian strataand volcanic rocks in Qiada area of West Junggar, combined with existing research data,discussed the basic geological features and depositional environment of Carboniferous andPermian and studied petrology, petrographic, zircon LA-ICP-MS U-Pb age, geochemicalcharacteristics and tectonic setting of the volcanic rocks of Carboniferous and Permian. Thepaper also discussed the tectonic evolution of this area during Carboniferous and Permian.The study area only exposed the Lower Carboniferous, including Jiangbasitao andHeishantou Formation. Clastic rocks take the most part of Jiangbasitao Formation. After acomprehensive analysis it is determined its depositional environment belongs to paralic.Heishantou Formation is divided into two segments, the lower segment is mostly made up ofvolcanic rocks, with a small amount of tuff and clastic rocks (Containing coral and brachiopodfossils). And the upper segment is the combination of clastic terrigenous (Containing corals,brachiopods, and plant fossils) and clastic rock containing volcanic ash (tuffaceous).Heishantou Formation was formed in land-sea transition sedimentary facies. The increasing ofterrigenous and the appearance of plant fossils indicate that there was a shallowing trend fromthe lower segment to the upper one.Permian in the study area includes Haerjiawu Formation and Kujiertai Formation. Thereis a small amount of limestone which indicates the marine environment are exposed in theHaerjiawu Formation, and pebbly sandstone, conglomerate, siltstone and fine-grainedsandstone are all well developed. Terrigenous clastic was fully supplied. Volcanic rocks whichmainly are intermediate are well developed too. It is concluded that the formationenvironment was paralic, in tectonic active regions. The most part of Kujiertai Formation iscoarse clastic sedimentary rocks, so it is supposed that the depositional environment wascontinental, lacustrine.The Heishantou Formation volcanic rocks consisted of gray purple (vesicular) almondandesite and almond shaped basaltic andesite, belonging to sub-alkaline series. High-precisionLA-ICP-MS zircon U-Pb dating has revealed that they were formed at the age of349±8Ma, MSWD=0.14(95%conf.), the time belongs to the Early Carboniferous. All samples arecharacterized by large ion lithophile elements (LILEs, eg., Ba, Th, K and Sr) enrichment,especially Ba and K, high field-strength elements (HFSEs, eg., Nb, Ta, and Ti) depletion,while Zr, Hf slightly enriched. Geochemical features indicate that the volcanic rocks areproducts of metasomatism between subduction-zone fluids and lithospheric mantle.Comprehensive analysis indicates that the volcanic rocks were derived from active continentalmargin.The volcanic rocks of Haerjiawu Formation consisted of gray green or purple (vesicular)(almond) andesite, and outcrops of dacite, gray and black basalt, and amount of volcanicbreccia and volcano agglomerate. High-precision LA-ICP-MS zircon U-Pb dating hasrevealed that they were formed at the age of295±7Ma, MSWD=0.95(95%conf.), the timebelongs to the Early Permian. Comprehensive analysis of its main elements(mainly A12O3、CaO、MgO), large ion lithophile elements, high field-strength elements, combined withtectonic environment discrimination diagrams and its geological background, it is concludedthat the forming environment was intraplate.On the whole, the Carboniferous and Permian and the volcanic rocks in them in the studyarea recorded the demise of the residual basin as well as marine strata and the completetermination of the oceanic development, and the important tectonic regime transformationfrom dive squeeze to orogeny and the beginning of stable continental development happenedin the study area or even the entire west Junggar. |
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