【作者】 李平；

【导师】 徐学义；

【作者基本信息】 长安大学 ， 矿物学、岩石学、矿床学， 2011， 硕士

【摘要】 中国天山为中亚巨型复合造山系的中国境内部分,由于其物质组成和结构的相对复杂性,天山造山带的构造演化过程一直是地学研究的重点,尤其是古生代南天山洋闭合时限这一科学问题仍存有较大争议。本文通过中天山中-西段花岗岩的岩石学成因研究对天山古生代洋盆的洋-陆转换过程进行了探讨,进而限定了南天山洋的闭合时限。根据区内花岗岩的时空分布、变形程度和LA-ICPMAS锆石U-Pb定年资料,中天山中西段花岗质岩浆作用可以划分为四个阶段：第一阶段花岗质岩浆活动发生在495-460Ma之间,此阶段花岗岩主体分布在那拉提山西段,一般规模较小,多呈带状分布,显示有较强的区域变形特征,岩石组合为闪长岩-花岗闪长岩-花岗岩系列；第二阶段花岗质岩浆活动主要发生在440-390Ma之间,此阶段花岗岩主体分布在那拉提山中西段和巴伦台地区,成带状或不规则面状分布,具有区域变形,在构造带发育部位变形强烈。岩石类型主要为花岗岩,伴有少量的闪长岩和花岗闪长岩；第三阶段主要发生在370～310Ma之间,该阶段形成的花岗岩主要出露在那拉提山东段,多呈不规则面状或椭园状产出,无显著变形特征(除强构造变形带外)。岩石组合为花岗闪长岩-二长花岗岩-碱长花岗岩系列,含有少量的闪长岩。第四阶段为300-260Ma,该期花岗岩多为未变形的碱长花岗岩,在区内零星分布。中天山中西段不同时段花岗岩体具有不同的地球化学特征差异,在碱含量、铝饱和指数、Eu、Nd和Ti等元素的亏损程度上表现的较为明显。Sr-Nd同位素多反映其为地壳物质熔融的产物。研究区内花岗岩自269.7Ma至494.2Ma之间均显示有Nb、Ti、P等元素的亏损,前人多以此作为天山古生代洋盆持续俯冲的地球化学依据。但本文认为此种地球化学特征也可源自地壳物质的部分熔融,是源区与形成构造环境共同作用的产物。同时,中天山南缘花岗岩的Hf同位素研究显示其物质源区较为复杂,包含有前寒武纪古老地壳物质和古生代新生地壳两部分。形成于370Ma之前的花岗岩,一部分具有负的εHf(t)值,且T2DM远大于岩体的形成年龄,因而为前寒武纪古老地壳部分熔融的产物。其余形成年代大于370Ma的花岗岩具有正εHf(t)值,为亏损地幔形成的新生地壳部分熔融的产物(且可能含有古老地物质的加入),这种新生地壳可能为弧岩浆作用的产物；360Ma后,伴随花岗质岩浆的大规模活动,形成有具有正的εHf(t)值的花岗岩,反映了石炭纪之后中天山南缘侵入岩带的物质源区可能有幔源物质加入。本文认为在花岗质岩浆宁静期前后,花岗岩锆石Hf同位素所反映的两次地幔物质的加入,极有可能同石炭纪前后天山地区地壳的侧向增生和垂向增生有关。通过综合分析,本文认为370-360Ma为天山造山带的主碰撞期；370Ma之前为古生代南天山洋及更早的帖尔斯克依洋俯冲消亡阶段形成；360Ma之后南天山洋完成闭合,研究区形成一定数量和后碰撞阶段伸展环境的花岗岩。尤其是石炭纪初期区内显示的大规模花岗质岩浆活动和幔源物质加入事件,暗示360Ma之后,天山全区已经进入碰撞后大陆伸展的阶段。因而本文认为,南天上山洋的闭合时限应为晚泥盆世至早石炭世。更多还原

【Abstract】 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.更多还原