The Petrogenesis Of Paleozoic Granites In The Middle And West Segment Of The Central Tianshan And Constrain To The Process Of The Ocean-continent Transition Of The Tianshan

Chinese Tianshan is one of the representatives of Central Asian Orogenic Belt in China, due to its relative complicated composition and structure, the process of tectonic evolution has been controversially discussed. Especially, there are still some debates on point of closure of the South Tianshan Ocean. By the study of the granitoids exposed near the area of the South Margin Fault of the Central Tianshan and the Baluntai Block, this paper discussed the tectonic evolution of the Paleozoic Ocean and then limited the closure time of the South Tianshan Ocean.According to deformation features, rock assembles, spatial and temporal distribution of granitoids which exposed in the western segment of the Central Tianshan, the granitic magmatism can be divided into four stages:the first stage of granitic magmatism occurred between 495～460Ma. Granitoids formed in this period maninly distributed in the west segment of the Nalati areas and showed a strong deformation characteristics, including diorite-granodiorite-granite series; the second stage of granite magmatism occurred between 440～390Ma, the granitoids which are formed in the period located in the middle- west segment of the Nalati and Baluntai regions, which have a different level of deformation characteristics. The instrusions are mainly granite, with a small amount of diorite and granodiorite; the third stage is between 370～310Ma, granitoids exposed mainly in the east segment of the Nalati whith no significant deformation features, which include granodiorite, monzonite,alkali feldspar granite and a small amount of diorite. The fourth stage is between 300～260Ma, the instusion is mainly alkali feldspar granite.Granitoids in the research area have no similar geochemical characteristics, however, the Sr-Nd isotopes research on granites supports that all of them are the product of crustal or island arc materials and. All the granitoids formed between 269.7Ma and 494.2Ma are depleted Nb and Ti, which is regarded as a sign of the subduction of the ocean. In fact, it's the product the crustal materials and can not be seemed as a useful tool for the granitoids to judge the geological settings. And the Hf isotope studies of granite reveal that both of the ancient crust and the Paleozoic juvenile crust can be their sources. Part of the deformed granites has obviously negativeεHf (t) value, and T2DM is much larger than their formation age, all of which support the Precambrian crust is the source of this kind granitoids. Other deformed granites have positiveεHf (t) values, which suggests that they are formed by the partial melting product of juvenile crust which evolved from the depleted mantle (may incude the ancient crustal materials also). The granites with no deformation characteristics still have positiveεHf (t) values, reflecting some mantle-derived materials may be added in Carboniferous again. Via zircon Hf isotopic research, this paper argues that before and after the quiet period of granitic magma there may exit two events about large-scale mantle materials added.Through the comprehensive analyzed above, the paper suggest that the 370～360 Ma may be the collision period of the Tianshan orogenic belt. Before 370Ma, the subduction of the early Palaeozoic Terskey Ocean and the South Tianshan Ocean cause the formation of the subduction-related deformed granites. After 360 Ma, the South Tianshan Ocean has closed and a great mounts of post-collision granites with no deformed features formed in an extensive setting. In short, through the study about granitoids exposed along the South Margin Fault of the Central Tianshan the paper reveals that the South Tianshan Ocean has completed closed in the Late Devonian-Early Carboniferous.