Tianshan And Its Adjacent Areas Ophiolite, Paleozoic Ocean Basin Evolution

The Tianshan and Junggar orogenic belt, located in northwestern China, are parts of the Central Asian mega-composite orogenic belt, in which many ophiolites outcrop. This paper focuses on the study of some ophiolites, especially Karamaili ophiolite in Eastern Junggar, Darbut ophiolite in Western Junggar, Kulehu ophiolite and pertinent igneous rocks of Bayinbluke in Southern Tianshan, discusses the spatio-temporal relations between them and other else ophiolites in these areas, and finally traces the evolutionary processes of Paleozoic oceans in Tianshan and adjacent areas on the basis of these studies and references to recent areal geological survey and research productions on Tianshan and Junggar orogenic belts.(1) The Karamaili ophiolite in Eastern Junggar and Darbut ophiolite in Western Junggar contained some basalts with intermediate N-MORB/IAT composition should have formed in back-arc basin of Supra-subduction zone (SSZ). The SSZ-type ophiolite of Karamaili and Aermantai, another ophiolite in Eastern Junggar, and some island arc volcanics between the two ophiolites constituted the Early Paleozoic intraoceanic "dual arc-basin systems" in Eastern Junggar, which collapsed and closed in the late period of Early Paleozoic, and assembled to southern margin of the Siberian continent in order as the accretionary complex; The Darbut ophiolite in Western Junggar, whose U-Pb isotopic age by LA-ICP-MS zircon dating is 398±10 Ma, was the early Devonian intraoceanic arc-basin system resulted from rollback of the subducted oceanic slab after closing of the arc-basin of Gangou to Tangbale close to the margin of Kazakstan-Yili-Central Tianshan continent.(2) The Silurian igneous rocks of Bayinbluke, Southern Tianshan, with U-Pb isotopic age of 446—431 Ma by TIMS zircon dating, are the intergradation between normal island arc basalt (IAB) and Nb-enriched arc basalt (NEAB). Both fluid-sediments and silicate melts derived from subducted oceanic slab are involved in the genesis of these rocks of Bayinbluke; The Kulehu ophiolite in Southern Tianshan, including two groups of basalts with different geochemical types, formed in back-arc basin. The first group of basalts with intermediate N-MORB/IAT composition was formed at the early stage of the back-arc basin, and the second group characterized by E-MORB was the products of more extension of the back-arc basin. The LA-ICP-MS zircon U-Pb isotopic age of gabbro in Kulehu is 418.2±2.6 Ma.(3) The subduction polarity of Paleo-Southern Tianshan Ocean should be bidirectional. The subduction northwards resulted in the arc-type granites aged from 416-355 Ma on the active margin of Yili-Central Tianshan continent. The subduction southwards induced to the Silurian infra-Southern Oceanic arc-basin system preserved in Southern Tianshan areas.(4) The occurrence of every ophiolite belt does not mean the open of a new ocean in the north of Xinjiang. All these ophiolites, except for Bayingou ophiolite in north Tianshan, should be the oceanic crust and lithosphere segments of Junggar-Northern Tianshan Ocean and Southern Tianshan Ocean resulted from the breakup of Rodinia supercontinent at the late Neoproterozoic. These segments with different characters and different ages, produced by spread and subduction of the two oceans at the different stages of Paleozoic, were emplaced to different sections of Tianshan and Junggar orogenic belts during the later tectonic evolution. The ubiquity of unconformity contact between lower-Carboniferous formations and pre-Carboniferous formations in the north of Xinjiang, and these formations upper and lower interface are different in sedimentary context, degree of metamorphism and deformation, which suggested that the two oceans had already closed by Carboniferous.