伊犁地块北缘早古生代构造热事件:来自托克赛石榴角闪岩及其围岩的约束  

The Early Paleozoic tectonic-thermal events in northern margin of Yili Block:Constraints from Toksai garnet amphibolite and its surrounding rocks

作　　者：陈有炘[1] 朱生强 裴先治[1] 王盟 周海 李佐臣[1] 李瑞保[1] CHEN YouXin;ZHU ShengQiang;PEI XianZhi;WANG Meng;ZHOU Hai;LI ZuoChen;LI RuiBao(School of Earth Science and Resources,Chang'an University,MOE Key Laboratory of Western China's Mineral Resources and Geological Engineering,Xi'an 710054,China)

机构地区：[1]长安大学地球科学与资源学院,西部矿产资源与地质工程教育部重点实验室,西安710054

出　　处：《岩石学报》2022年第10期3165-3190,I0016-I0021,共32页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41802234、42072267);陕西省自然科学基础研究计划项目(2019JQ-209、2019JQ-090);中央高校基本科研业务费专项资金项目(300102271207、300102279104、300102279201);中国地质调查局项目(12120115041301)联合资助。

摘　　要：伊犁地块是中亚造山带西南缘的重要组成部分,其南北缘出露前寒武纪变质基底,其中伊犁地块北缘温泉岩群中识别出奥陶纪岛弧型岩浆岩,被认为是准噶尔洋向南俯冲于伊犁地块之下的物质记录,但相应的变质作用记录很少有报道。近期,从温泉县托克赛地区温泉岩群中识别出的石榴角闪岩具有顺时针P-T-t轨迹,可能记录了伊犁地块北缘早古生代俯冲、碰撞事件,但其形成时代和构造属性尚不清楚。本研究在详细野外地质调查和岩相学研究的基础上,对托克赛石榴角闪岩及其围岩开展详细的岩石地球化学和年代学研究。结果表明,托克赛石榴角闪岩呈透镜状分布于温泉岩群中,原岩为拉斑玄武岩,岩石TiO_(2)(3.10%~3.89%)含量较高,K_(2)O+Na_(2)O(0.76%~2.01%)含量低,富集大离子亲石元素,亏损高场强元素,具有陆内裂谷拉斑玄武岩的特征(Th/Ta=1.70~2.76、Ta/Hf=0.23~0.37),来源于具有OIB特征的幔源岩浆。岩石较低的MgO(4.82%~6.40%)、Cr(70.8×10^(-6)~224×10^(-6))、Ni(9.68×10^(-6)~65.7×10^(-6))含量及Mg^(#)值(34.0~41.3)、Nb/U值(14.3~36.3)、Nb/Ta值(9.70~16.2)和La/Sm值(2.01~3.28),表明岩浆经历了较强的分离结晶作用和少量的地壳混染。锆石U-Pb测年结果显示石榴角闪岩原岩形成于新元古代中晚期,峰期变质时代为晚奥陶世末期(450~440Ma),围岩中的锆石和独居石均记录了同时期的构造热事件。综合研究认为,石榴角闪岩原岩形成于板内裂谷环境,是伊犁地块响应罗迪尼亚超大陆裂解事件的物质记录,暗示伊犁地块是原塔里木地块的一部分;石榴角闪岩及其围岩中识别出的奥陶纪末期高角闪岩相变质作用事件,记录了阿克套-温泉微陆块向南与伊犁地块或哈萨克斯坦微陆块的拼贴-碰撞过程。The Yili Block(YB)is one microcontinent in southwest of Central Asian Orogenic Belt(CAOB)with Precambrain basement rocks exposed in its northern and southern margins.The Middle to Late Ordovician arc-type magmatic rocks were identified from the Wenquan Group in the northern margin of the YB,which has a subduction-related calc-alkaline affinity of a continental arc environment.Although a southward subduction of the Junggar Ocean beneath the Yili Block was suggested,the related record of metamorphism was rarely reported.The Toksai garnet amphibolite identified from the Wenquan Group in the northern margin of YB records a clockwise P-T-t path,but the age and tectonic attributes of them are still unclear.Here,we presented systematic study of whole-rock geochemistry and zircon/monazite U-Pb age dating on the Toksai garnet amphibolite and its surrounding rocks in the northern margin of YB.The garnet amphibolites outcrop as tectonic lenticles in the Wenquan Group,which mainly contains garnet,hornblende and plagioclase.Its protolith belongs to tholeiite,with high TiO_(2)(3.10%~3.89%)and low K_(2)O+Na_(2)O(0.76%~2.01%)contents,enrichment of large ionic lithophile and rare earth elements and depletion of high field strength elements,showing the geochemical characteristics of tholeiite in intra-continental rift setting(Th/Ta=1.70~2.76,Ta/Hf=0.23~0.37).The geochemical characteristics of the magmatic rocks reveal that they are derived from an OIB-like mantle source.The garnet amphibolites also have low contents of MgO(4.82%~6.40%),Cr(70.8×10^(-6)~224×10^(-6)),Ni(9.68×10^(-6)~65.7×10^(-6))and low values of Mg^(#)(34.0~41.3),Nb/U(14.3~36.3)and Nb/Ta(9.70~16.2),indicating that their protoliths are not primitive magma,but formed by separate crystallization of different mineral phases with a small amount of crustal contamination.The zircon U-Pb dating results suggest that the protolith of the garnet amphibolite was formed in the Middle to Late Neoproterozoic with a metamorphic age ending in the Late Ordovician(450~440Ma).The z

关 键 词：石榴角闪岩 锆石 独居石 地球化学 奥陶纪 构造意义 伊犁地块北缘 

分 类 号：P588.34[天文地球—岩石学] P597.3[天文地球—地质学]