The Cause Evolution And Tectonic Implications Of The Late Carboniferous-early Permian Intermediate-mafic Magmatic Rocks In The East Tianshan And Its Adjacent Area, Xinjiang

The eastern Tianshan and its adjacent area is located on the eastern part of the Tianshan orogen in the north of Xinjiang, it is the convergence belt between the Siberia plate and the Tarim plate, and experienced three important stages: Paleo-Asian Ocean’s extension and subduction in the early-Paleozoic, Plates’ collision and the post-collision stage in the late-Paleozoic, and stacking the mantle column magmatism in the Permian. Late-Paleozoic is the most important period in which the Tianshan orogen completed the conversion from ocean to inner continent, and especially in the late Carboniferous-early Permian, based on the large scale of these complicated structural background, it is widely formed all kinds of magma rocks of this period in the whole Tianshan orogen. The study of these rocks’ ages and genesis are significant in the investigation of the regional tectonic evolution and geodynamic setting.Because of the intermediate-mafic magma rock has more remarkable features aout the composition of its magmatic source and the environment of its deep dynamics than the middle-acidic rock, we take the intermediate-mafic magma rocks for the research object in four areas which includes Bingdaban-Houxia and Kangguertage-Huangshan in the junction part of the plates, Heishan ridge in the north of the Trim plate and Balikuntage in the south of the Junggar-Kazachstan plate. Among them, for the restriction of the geographical environment in the Balikuntage and Heishan ridge, there are lower research degree which mainly manifested as the lack of geochemistry and isotopic dating data for the most magmatic rocks. While the higher research degree of the Bingdaban-Houxia and Kangguertage-Huangshan does not mean that they have not existed any dispute on the genesis of the magmatic rocks and its tectonic evolution, based on a complicated structure background in the sutured boundary between two plates. On the basis of above, this thesis is concentrated on the petrology, geochemistry, Sr-Nd-Pb isotope and zircon SHRIMP U-Pb ages for the intermediate-mafic magma rocks of the four Regions in the eastern Tianshan and its adjacent area, made it clear what the age of the magma active is, and what the magma rocks’ source characteristics and procession are. Then, through the comprehensive comparison of the intermediate-mafic magma rocks in the four Regions, conclude and analyze the time-space evolution to limit the the closure age of the Paleo-Asian Ocean to ±290Ma in the the eastern Tianshan and its adjacent area. The main results of this study are as followings:(1) Zircon SHRIMP U-Pb method has been used to accurately determine the formation age of magma rocks in southeastern of Heishan ridge and Bingdaban-Houxia region. 2volcanic samples’ zircon ages are 298±5 Ma and 277±5.8 Ma respectively in southeastern ofHeishan ridge. 1 basalt sample and 5 intrusive rocks’ zircon ages are 310~291Ma. These age data are initially defined that the forming age of the magma rocks is the late Carboniferous-early Permian.(2) The 310~302Ma intermediate-mafic intrusive rocks of the Bingdaban-Houxia show calc-alkaline and high-K calc-alkaline series, LILE and LREE enriched, but negative HFSE anomaly. The similar right deviation characters of the REE types reflect the same source position, which is the depleted mantle for the( 87Sr/86Sr)ⅰ=0.70318~0.70359 and εNd(t)=6.5~7.0. The tectonic environment is the island that is formed in the subduction process of the Paleo-Asian Ocean in the late Carboniferous. But the 272 Ma basalt showing calc-alkaline series have been formed in the intra-plate setting, which is the production of the post-collision stage after the Paleo-Asian Ocean closed, and the addition of the mantle wedge material metasomatized by fluid in subduction belt lead to the basalts have the similar characteristics as the island-arc volcanic rocks in the trace elements.(3) In the Kangguertage-Huangshan area, most of the intermediate-mafic magma rocks pointed on the both sides of the plate suture zone are formed in the late- Carboniferous(300~290Ma), composed of basalts and andesites mainly, which shows calc-alkaline series,LILE and LREE enriched, but negative HFSE anomaly. The similar right deviation characters of the REE types reflect the same source position. These magma rocks are the production in the subduction process of the Paleo-Asian Ocean between the northern Junggar-Kazachstan plate and the southern Tarim plate. Then the Paleozoic volcanic molasse formation represents the end of the orogenesis and the beginning of the intra-plate evolution stage.(4) In the Kangguertage-Huangshan area, the late Carboniferous-early Permian volcanic rocks exhibit a bimodal feature, and the intermediate-mafic volcanic rocks are dominated by tholeiite series, and the intermediate-mafic intrusive rocks are mainly calc-alkaline and high-K calc-alkaline series. All the rocks are formed in the inter-plate environment that is the part of the Bogeda intra-continental rift in the Carboniferous. In the aspect of the trace elements, these intermediate-mafic magma rocks are all enriched in the LILE and LREE, but negative HFSE anomaly, which are the similar characteristics as the island-arc volcanic rocks.The petrogenic mechanism is the mantle material rising in the intra-plate rifting with the contamination of the Haerlike island-arc material in the earlier stage.(5) The intermediate-mafic volcanic rocks of the Heishan ridge have the different geochemical characteristics between the first and the second phase in the early-Permian. The volcanic of the first phase(298±5Ma) are the same as the other areas, but the second one(277±5.8Ma) has the obvious positive Nb abnormal and affinity with OIB, without the earth’scrust material contamination. The causes of these volcanic is related to the Tarim mantle plume(LIPs). The intrusive rocks has the similar characters with the first phase volcanic, they are both the production of cognate magma evolution.(6) Based on the comprehensive comparison analysis of the late Carboniferous-early Permian intermediate-mafic magma rocks’ ages and tectonic evolutionary environment in the four regions, during 310~290 Ma, Bingdaban-Houxia, Kangguertage-Huangshan and Heishan ridge are all the island-arc tectonic environment, shows that the subduction of the Paleo-Asian Ocean is still existed. During 290~270Ma, these four regions are all the intra-plate evolution environment, tthat means the Paleo-Asian Ocean has been closed completely. While the age of the basalts formed in the mantle plume is 277 Ma, the result show that the Trim LIPs develops into the east Tianshan and its adjacent area in the late Early Permian.