Early Paleozoic subduction of the Paleo-Asian Ocean: Zircon U-Pb geochronological, geochemical and Sr-Nd isotopic evidence from the Harlik pluton, Xinjiang

新疆哈尔里克造山带的形成时间(早古生代/晚古生代)及其构造属性(岛弧/弧后盆地)一直是有争议的问题。本文选择位于哈尔里克带内的奥尔达乌台克侵入体,对其成因及所揭示的地质意义进行了探讨。奥尔达乌台克岩体由一套成份连续的岩石组合构成,包括辉长岩、辉长闪长岩、闪长岩和花岗闪长岩。对闪长岩及其内部暗色包体进行的LA-ICP-MS锆石U-Pb定年结果显示两者同时形成于~450Ma,为晚奥陶世岩浆活动的产物。岩石学和地球化学证据反映奥尔达乌台克岩体普遍具有富H2O、高fO2的湿岩浆性质,富集LREE、Sr、Ba等大离子亲石元素(LILE)而亏损Nb、Ta、Ti等高场强元素(HFSE),与典型的岛弧岩浆岩类...It is hotly debated whether the Harlik orogenic belt from the Xinjiang was constructed by island arc system or back-arc basin of the Early or Late Paleozoic time. The Ordautec pluton occurring at the Harlik orogenic belt comprises a wide spectrum of rocks types from gabbro, gabbroic diorite, diorite to granodiorite. LA-ICP-MS Zircon U-Pb dating results indicate diorite and enclaves formed at similar to 450Ma (Late Ordovician). All rocks show H2O-riched and high f(O2) characteristics for their magmas, and exhibit an island arc affinity, such as enrichment in LILE (e.g., LREE, Sr and Ba) and depletion in HFSE (e.g., Nb, Ta and Ti). High epsilon(Nd)(t) (+4.4 similar to +5.9) and low (Sr-87/Sr-86), (0.7032 similar to 0.7044) values suggest mafic magmas originated from a depleted lithospheric mantle source, which was being metasomatized by subducting oceanic slab. Mantle-derived mafic magmas underplated to the bottom of the lower crust and mixed with the crustal felsic melts, while some evolved mafic magmas added to the crust directly. Therefore, crust growth is probably caused by magma mixing and/or immediate addition of mantle materials. Sr-Nd isotopic modeling indicates the continental crust in the Harlik area mainly compose of young Paleozoic island arc and oceanic crust. The Harlik belt was constructed by island arc system of Late Ordovician, rather than back-arc basin or island arc system of Devonian.SCI(E)中文核心期刊要目总览(PKU)中国科技核心期刊(ISTIC)中国科学引文数据库(CSCD)0maxh@pku.edu.cn; binchen@pku.edu.cn0189-1043