天宇铜镍矿床的岩相学、锆石U-Pb年代学、地球化学特征：对东疆镁铁-超镁铁质岩体源区和成因的制约
Lithological, chronological and geochemical characteristics of Tianyu Cu-Ni deposit: Constraints on source and genesis of mafic-ultramafic intrusions in eastern Xinjiang
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摘要: 天宇镁铁-超镁铁质岩体位于新疆中天山地块与觉罗塔格构造带的分界断裂——沙泉子深大断裂的南侧约5km。岩体呈向北陡倾的岩墙状，地表出露面积约0.056km2。该杂岩体主要由辉长岩、辉石岩、辉橄岩和橄榄岩相组成，橄辉岩、辉橄岩和橄榄岩是主要的Cu、Ni赋矿岩相。镁铁-超镁铁岩的主量元素含量显示，天宇岩体属拉斑玄武岩成因系列，m/f为1.44~3.61，平均值为2.25，为铁质超基性岩（m/f=2~6.5）。稀土元素标准化配分模式为轻稀土富集型，Eu弱负异常或无异常，相似的稀土元素配分模式说明岩体的同源性。富集大离子亲石元素，亏损高场强元素Nb、Ta，说明源区可能有陆壳物质的混染。Nb/Hf、Ce/Pb、Nb/U、La/Sm、Th/Nb比值显示岩体可能经受上地壳物质的混染。锆石的LA-ICP-MS U-Pb年代学研究表明，岩体形成年龄为290.2±3.4Ma，与东天山地区镁铁-超镁铁质杂岩体的形成时间一致。锆石的Lu/Hf比值比较均一，且(176Hf/177Hf) i变化很小，说明源区性质比较单一。锆石的(176Hf/177Hf) i值为0.282625~0.282770，相应的εHf(t)均为正值（1.10~6.06），Lu-Hf的单阶段模式年龄（tDM1）为674~884.7Ma，平均年龄为799Ma，比岩体的U-Pb年龄大，显示岩浆来源于亏损地幔或者源区有壳源物质的加入。上述岩石地球化学、年代学特征，为晚古生代古亚洲洋向南俯冲提供了佐证。Abstract: Tianyu mafic-ultramafic intrusions are located in the south of the Shaquanzi deep-seated fault. Shaquanzi fault is the boundary between the Central Tianshan Massif and Jueluotage tectonic belt. It covers an area of 0.056km2in shape of dyke pitched to the north. The intrusions consist of gabbro, pyroxenite, olivine pyroxenite, pyroxene peridotite and peridotite. Olivine pyroxenite, pyroxene peridotite and peridotite are the main host rocks for the Cu-Ni ores. Major-element contents of Tianyu complex show tholeiitic evolution trend and m/f values range from 1.44 to 3.61 with 2.25 on average, falling into the range of ferrous-ultramafic rocks. Similar LREE-rich pattern and negative or no Eu anomaly indicate that Tianyu complex have the same magma source. Trace elements show LILE-rich and HFSE-depleted pattern. The depletion of Nb、Ta implies the contamination of continental material. Nb/Hf, Ce/Pb, Nb/U, La/Sm, Th/Nb values suggest the contamination of upper crust during upward of magma. Zircon LA-ICP-MS U-Pb dates yield 290.2± 3.4Ma which is consistent with the age of other mafic-ultramafic complex in East Tianshan. Uniform Lu/Hf values and the (176Hf/177Hf) i reveal that the characteristics of source are simple. The (176Hf/177Hf) i are in the range of 0.282625~0.282770 with corresponding εHf(t) values of 1.10~6.06. Lu-Hf single-stage modal ages (tDM1) vary from 674.0 to 884.7Ma, mean age（799Ma）is older than U-Pb age. These characteristics indicate that the parental magma derived from depleted mantle or had been contaminated by crustal material. These geochemical and chronological characteristics also constrain the southern subduction of Paleo-Asia Ocean in Paleozoic.
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Key words:
- Tianyu /
- Cu-Ni deposit /
- Geochemistry /
- Zircon LA-ICP-MS U-Pb date /
- Lu-Hf isotope /
- Crust contamination /
- Eastern Xinjiang
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