新疆中天山南缘库米什地区蛇绿岩的锆石U-Pb同位素定年:早古生代洋盆的证据
U-Pb ages of zircons from ophiolite and related rocks in the Kumishi region at the southern margin of Middle Tianshan, Xinjiang: Evidence of Early Paleozoic oceanic basin
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摘要: 新疆中天山南缘库米什地区的榆树沟和铜花山蛇绿混杂岩包括地幔橄榄岩,辉石岩、辉长岩、斜长岩等堆晶岩,辉绿岩墙和基性熔岩,以及上部的硅质岩等。岩石地球化学研究表明,蛇绿岩的岩石类型来自MORB型和SSZ型两种构造背景。蛇绿岩及有关岩石的锆石U-Pb同位素年代学的研究表明,与中天山南缘洋盆扩张和闭合有关的事件至少可以分为4期: (1)奥陶纪-志留纪的洋盆形成事件,证据来自蛇绿岩斜长花岗岩和斜长岩,两者的年龄分别为435.1±2.8Ma、439.3±1.8Ma;(2)志留纪的岛弧岩浆作用,获得岛弧火山岩英安岩年龄422.1±2.6Ma 和花岗闪长岩年龄423.1±1.8Ma;(3)泥盆纪的剪切变形和糜棱岩化变质作用,由于板块斜向俯冲和碰撞作用,产生大规模的走滑作用和与之伴生的由剪切作用形成的糜棱岩,糜棱岩的形成年龄为402.8±1Ma,为早泥盆世;(4)俯冲碰撞后的造山带伸展阶段的岩浆作用,在俯冲碰撞作用之后发生与垂直主受力面张裂作用伴生岩浆作用,获得石英正长斑岩294.8±1.2Ma年龄,即晚石炭世。 此外,认为榆树沟蛇绿岩北部出露的麻粒岩是一个很特殊的构造岩块,岩石的锆石中普遍存在500~1800Ma的老核,表明其原岩很复杂,不属于蛇绿岩的组合 。Abstract: The Yushugou and Tonghuashan ophiolitic mélange, which occurs in the Kumishi region at the southern margin of the Middle Tianshan, Xinjiang, consists of mantle peridotite, cumulate pyroxenite, gabbro and anorthosite, diabase dikes and mafic lavas overlain by chert. Geochemical data indicate that the ophiolitic units originated in both MORB and SSZ tectonic settings. U-Pb isotope ages of zircons from the ophiolitic and related rocks record at least four tectonic events related to oceanic spreading and basin closure: (1) Ocean basin formation in the Ordovician-Silurian, based on the ages of oceanic crust (439.3±1.8Ma for plagiogranite and 435.1±2.8Ma for anorthosite; (2) Island arc magmatism in the Silurian, based on the ages of island arc volcanic rocks and granites (422.1±2.6Ma for dacite and 423.1±1.8Ma for granodiorite); (3) Shearing in the Devonian (400Ma for mylonite), which was probably related to oblique subduction and collision between plates that formed large-scale, strike-slip faults; (4) Extensional magmatism of the orogen in the Late Carboniferous (294.8±1.2Ma for quartz syenite porphyry that formed perpendicular to the direction of subduction and collision). In addition, some granulites in the Yushugou area are regraded as a separated block from the ophiolite, because most of zircons from granulites have old cores with ages of 500~1800 Ma and appear to have had a complex protolith and a very different metamorphic history from the ophiolite. The granulites appear to have had a complex protolith and a very different metamorphic history than the ophiolite .
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Key words:
- Ophiolite /
- Yushugou /
- Tonghuashan /
- Kumishi /
- Tianshan
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