【作者】 秦海鹏；

【导师】 吴才来；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2012， 博士

【摘要】 北祁连造山带经历了多期次、多阶段的复杂构造演化,形成了不同类型的花岗岩类。本文选择了祁连山地区东部13个花岗岩体,进行了系统的岩石地球化学、锆石U-Pb年代学和锆石原位Hf同位素、全岩Rb-Sr、Sm-Nd同位素地球化学研究,并在前人工作的基础上,结合花岗岩类岩石地球化学特征及年代学结果,划分了花岗质岩浆活动的期次,阐述了各期次花岗岩类的岩石成因、源区物质特征及其形成的地球动力学背景。北祁连造山带的花岗质岩浆活动可划分为4期共5个阶段：第1期(505.4-498.6Ma)属晚寒武世,主要为出露于北祁连造山带东南侧的石英闪长岩、花岗闪长岩,具有Ⅰ型花岗岩的特点,属于VAG型花岗岩。第Ⅱ期(464.6-440.9Ma)属早中奥陶世,该期岩浆活动规模最大,分布范围最广,具有I-S过渡型花岗岩的特征。早阶段(464.6-450.6Ma)形成的石英闪长岩—花岗闪长岩—花岗岩具有低硅、高FeOT、∑REE、Y,弱过铝质的特征；晚阶段(450.3-440.9Ma)形成具有低∑REE、Y特征的两类花岗岩：一类具有高Sr、低Y的特征；另一类为低Sr、低Y的特征。第Ⅲ期(435-421.3Ma)属晚奥陶世,在整个祁连山地区不同的构造块体上均有分布,具有低硅、高铝、高Sr、高Y,富碱的特征,属于造山带根部的岩石圈发生拆沉,引起造山带上不同块体的伸展、滑塌环境下形成的造山后Ⅰ型花岗岩类。第Ⅳ期(422～418.1Ma)属中晚志留世,主要为分布在走廊过渡带的A型花岗岩,具有高硅、低铝、低Na2O/K20、低Sr,轻重稀土具有弱分馏、强负铕异常(δEu=0.11-0.49)的雁行式配分特征,标志着北祁连俯冲造山带造山过程的结束。锆石原位Lu-Hf同位素及全岩Sr-Nd同位素研究表明,第Ⅰ期花岗岩类的εHf(t)=-5.7--2.5、二阶段模式年龄(TDM2)=1.31-1.83Ga,其源岩为壳幔混合物质,主要为中元古代幔源物质。第Ⅱ期早阶段(464.6-450.6Ma)分布在北祁连俯冲造山带南侧的花岗岩类的εHf(t)=-1.4～12、TDM2=0.68-1.53Ga,推测其源岩主要为新元古代幔源物质,并混入少量中元古代壳源物质。分布在北祁连俯冲造山带北侧的早阶段(464.6-450.6Ma)花岗岩类(样品07CL113、114)的πNd(t)值分别为-8.2、-12.0,TDM2分别为1.86、2.17Ga,表明源岩主要为古老陆壳物质；晚阶段(450.3-440.9Ma)形成的花岗岩类εNd(t)(-5.5～-0.1)、翻f(t)(-1.8～4.8)值增大、模式年龄减少(TDM2=1.2-1.6Ga、TDM2=1.12-1.54Ga),具有壳幔混合源区的特征。第Ⅲ期花岗岩类的εHf(t)变化范围较大(-12.2-1.8)、TDM2=1.3-2.19Ga,表明源岩为新元古代幔源与古元古代壳源物质的混合源。第Ⅳ期形成的A型花岗岩,推测其源岩可能为定位于陆—陆碰撞后造山环境的下地壳变质火成岩。在上述研究的基础上,探讨了古生代北祁连俯冲造山带的构造演化特点：(1)寒武纪(512-501Ma)北祁连洋向南俯冲；(2)早—中奥陶世(464.6-440.9Ma)北祁连洋向南俯冲受阻,转为向北俯冲,并最终导致洋盆闭合；(3)晚奥陶世(435Ma-421.3Ma)进入晚造山阶段；(4)中晚志留世(422～418.1Ma)造山作用结束,进入陆内演化阶段。更多还原

【Abstract】 North Qilian Orogenic Belt (NQOB) underwent complex multi-phase and multi-stage tectonic evolution which yielded different types of granitoids. Thirteen granite bodies in the eastern part of Qilian Mount Area(QMA) were selected in this paper and their petrology, geochemistry, zircon geochronology and in situ zircon Hf isotope as well as whole rock Rb-Sr and Sm-Nd isotope were systematically analyzed,. On the basis of previous works, the granitic magma episodes for NQOB were divided in terms of geochemical characteristics and geochronology results, then petrogenesis of granitoids from NQOB, their source material and geodynamic setting were thereafter illustrated.Granitic magma activities of NQOB can be subdivided into four periods(or five stages altogether):The first period (505.4-498.6Ma),occurred in Late Cambrian, mainly consists quartz diorite and granodiorite which belongs to VAG(Volcanic Arc Granite) exposed in the southeast part of NQOB and has the characteristics of type-I granite. The second period (464.6-440.9Ma) belongs to the Early and Middle Ordovician which occurred the largest and most widely distributed granitoids with I-S transitional characteristics. Among which, the earlier stage (464.6-450.6Ma) mainly formed the peraluminous quartz diorite-granodiorite-granite with low SiO2and high FeOT,∑REE, Y content, while the Late stage (450.3-440.9Ma) predominantly generated two types of granitoids with low content of∑REE and Y:one type has the characteristics of the high Sr, low Y; another has low Sr and Y. The third period (453-421.3Ma) granitoids in Late Ordovician are widely distributed in the QMA with low content of SiO2, high Al2O3, Sr, Y and K2O+Na2O content. It is generated by the delamination of the orogen roots, as well as orogenic belt stretching and slump. The fourth period (422-418.1Ma) mainlyn formed A-type granites in the Late Silurian. It has the characteristics of high content of SiO2, low Al2O3, Na2O/K2O and Sr content, merely weak fractionation of LREE/HREE, and strong negative Eu anomalies (δEu=0.11-0.49) which demonstrated the late orogenic process of NQOB, and then entering the intracontinental evolution stage.Zircon Lu-Hf isotope and whole rock Sr-Nd isotopic results show that:the εHf) value of granitoids zircons in first period ranging from-5.7to2.5, their TDM2are1.31-1.83Ga, implying that the protolith of granitoids is mainly Neoproterozoic crust with small amount of mantle-derived component. The εHf(t) value of grantoids zircons from first stage of second period which mainly distributed in the south part of NQOB ranging from-1.4to12, their TDM2are0.68-1.53, suggesting that the protolith is mainly mantle-derived component with small amount of Mesoproterozoic crustal material. Futthermore the εND(t) value of sample 07CL113/114range from-12--8.2, their TDM2are1.86-2.17, implying that the protolith is mainly the ancient continental crust. The εHf(t)(-1.8-4.8) and εNd(t)(-5.5--0.1) value of grantoids zircons from second stage of second period indicated the protolith is crust-mantle mixture. The εHf(t) value of granitoids zircons from the third period range from-12.2to1.8, their TDm2are1.3-2.19Ga, suggesting that the protolith is mainly Paleoproterozoic crust with small amount of Neoproterozoic mantle-derived component. The protolith of fourth period A-type granites might be the metamorphic igneous material from the continent-continent collisional environment.Based on the investigations mentioned above, the characteristics of tectonic evolution of NQOB were discussed:(1)North Qilian Ocean(NQO) southward subduction in Cambrian(512-501Ma);(2) northward Subduction of NQO, and ultimately the closure of the ocean basin in the stage of Early-Middle Ordovician (464.6-440.9Ma);(3) Post-orogenic stage in435Ma-421.3Ma;(4) the end of the orogenic process and then came the intracontinental evolution stage(422-418.1Ma).更多还原