天山及其邻区蛇绿岩研究与古生代洋盆演化

【作者】 马中平；

【导师】 夏林圻；

【作者基本信息】 西北大学 ， 矿物学、岩石学、矿床学， 2007， 博士

【摘要】 天山及准噶尔造山带是中亚复合造山系的组成部分，是研究和理解中亚造山带构造演化的关键地区之一，而蛇绿岩的系统研究对于揭示新疆北部乃至整个中亚地区的早期地质演化、重建古构造环境和板块拼合过程等具有十分重要的科学意义。本文重点对东准噶尔卡拉麦里、西准噶尔达拉布特、南天山库勒湖蛇绿岩及其相关的巴音布鲁克火山岩等进行了研究，对它们与其它相邻蛇绿岩带的时空关系进行了讨论，并结合近年来的区域地质调查新资料和前人研究成果，对新疆北部古生代洋盆演化过程进行了总结恢复。(1)东准卡拉麦里、西准达拉布特蛇绿岩都含有弧后初始扩张所形成的兼具IAT和N-MORB特征的基性熔岩，应形成于消减带之上(SSZ)的弧后盆地环境。卡拉麦里蛇绿岩形成于早古生代，它与阿尔曼太蛇绿岩、以及两者之间早古生代岛弧火山岩，构成了东准噶尔地区早古生代洋内“双重弧-盆系”。早古生代晚期，该双重弧-盆系由北至南顺次拼贴于西伯利亚古陆南缘。达拉布特蛇绿岩的锆石LA-ICP-MS测年结果为398±10Ma，它是由于奥陶纪末准噶尔-北天山洋向哈萨克斯坦-伊犁-中天山陆块之下强烈俯冲，使得靠近活动陆缘的洋内弧-盆系(中天山北缘干沟—唐巴勒)发生挤压关闭，其后随着俯冲洋壳向洋一侧的后退式迁移，于早泥盆世时所形成的洋内弧-盆系的记录，其拼贴于哈萨克斯坦-伊犁-中天山古早古生代活动大陆边缘，并经后期逆冲、走滑，最终就位于现今所在位置。(2)南天山巴音布鲁克火山-侵入岩形成于岛弧环境，为正常岛弧玄武岩与富铌岛弧玄武岩(NEAB)之间的过渡系列，其源区是受俯冲洋壳板片流体与熔体双重交代的亏损地幔源。其中的含石英辉长岩锆石TIMS测年结果为446—431Ma；库勒湖蛇绿岩应形成于SSZ环境的弧后盆地，其中一组兼具IAT与N-MORB特征的玄武岩形成于弧后盆地打开的早期阶段。另一组为E-MORB型，是由于弧后次级地幔对流的加剧、弧后盆地进一步扩张的产物，其中辉长岩锆石LA-ICP-MS测年结果为418.2±2.6 Ma。(3)南天山洋可能存在南北双向的俯冲作用：向北的洋-陆俯冲，形成了中天山南缘集中于泥盆纪(416-355 Ma)的陆缘岩浆弧型花岗岩；向南的洋内消减，形成了南天山志留纪洋内“弧-盆系”。(4)新疆北部，并非每条蛇绿岩带的出现都代表着一次新的洋盆的打开与闭合，它们(巴音沟蛇绿岩除外)应都是由于新元古代末作为Rodinia超大陆组成部分的新疆北部统一古陆的裂解，所出现的准噶尔-北天山洋和南天山洋，在古生代扩张和多阶段俯冲-消减过程中，于不同区段、不同阶段、不同构造环境所形成的洋壳-岩石圈的组成部分，只是在晚期构造演化中，这些蛇绿岩侵位于天山及准噶尔造山带的不同部位而被分隔开来。洋盆于泥盆纪末闭合，其直接证据是新疆北部未(或浅)变质、弱变形的下石炭统地层与强变形、强变质的前石炭褶皱(或结晶)基底之间普遍存在着不整合接触。更多还原

【Abstract】 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.更多还原