早古生代古亚洲洋俯冲作用:来自新疆哈尔里克侵入岩的锆石U-Pb年代学、岩石地球化学和Sr-Nd同位素证据

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似。高放射成因Nd(εNd(t)=+4.4^+5.9)和低初始Sr(0.7032~0.7044)同位素组成表明偏基性的岩浆很可能起源于年轻的俯冲带岩石圈地幔,生成的镁铁质岩浆一部分与壳源长英质熔体发生混合形成各类岩浆,另一部分则是经历演化后(分离结晶)直接添加到地壳中。因此,壳幔岩浆混合作用和幔源物质的直接加入是古亚洲洋俯冲阶段陆壳增生的两种主要方式。Sr-Nd同位素模拟排除了区内存在古老前寒武纪陆壳的可能,陆壳主体应该是年轻的古生代大洋岛弧和洋壳。哈尔里克早古生代岛弧的确立改变了该带是泥盆纪岛弧或弧后盆地的原有认识,将古亚洲洋俯冲作用的时间追溯至奥陶纪。

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 -450Ma （Late Ordovician）. All rocks show H2O-riched and high fo2 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 εNd（t） （ ＋ 4. 4 - ＋ 5.9） and low （^87 Sr/^86Sr）i （0. 7032 - 0. 7044） values suggest mafic magmas originated from a depleted lithospheric mantle source, which was being metasomatized by subducting oceanic slab. Mantle-derived marie 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.