新疆西天山松湖铁矿床火山岩年代学和岩石地球化学

Geochronology and rock geochemistry of volcanic rocks from Songhu iron deposit in West Tianshan Mountains, Xinjiang

新疆西天山阿吾拉勒铁成矿带内众多铁矿床的围岩火山岩时代、岩石成因和构造背景迄今尚未得到很好地约束。文章对该铁成矿带内的松湖铁矿床围岩粗面英安岩和流纹岩进行了锆石LA_ICP_MS U_Pb测年和地球化学研究。粗面英安岩和流纹岩的锆石206Pb/238U加权平均年龄分别为（326.8±2.7）Ma和（327.3±1.7）Ma。粗面英安岩和流纹岩准铝质,均富集轻稀土元素、Rb、K、Zr、Hf,显示中等Eu负异常,亏损Ba、Sr、P、Ta、Nb和Ti。锆石饱和温度计算结果显示,粗面英安岩的母岩浆温度较高（774～812℃,平均792℃）,流纹岩母岩浆的温度较低（713～790℃,平均750℃）。粗面英安岩具有非常低Sr高Yb的特征,Nb/Ta比值为10.8～11.4,具有较高的w（Th）（≥8.1×10^（-6））和高的Th/Ce比值（≥0.31）;流纹岩具有低Sr低Yb特征,Nb/Ta比值为8.5～9.7,w（Th）（≥5.3×10^（-6））较高,Th/Ce比值为0.14～0.75,据此推测粗面英安岩母岩浆可能是上地壳部分熔融的产物,流纹岩母岩浆可能源于比粗面英安岩母岩浆更深的地壳的部分熔融。松湖铁矿区粗面英安岩和流纹岩具有弧火山岩的地球化学特征,结合构造环境判别图解和区域地质情况,推断其形成于大陆弧环境。

The Awulale Iron Metallogenic Belt（ AIMB） in West Tianshan Mountains hosts huge amounts of rich iron ore deposits. However, the age, genesis and tectonic setting of the wall rocks of these rich iron ore deposits have beenpoorly known. This paper reports the zircon LA_ICP_MS U_Pb dating result and rock geochemistry of the trachydacite and rhyolite which host the Songhu rich iron orebodies in the AIMB. The weighted average age of206 Pb/238 U for trachydacite and rhyolite is（ 326. 8 ± 2. 7） Ma and（ 327. 3 ± 1. 7） Ma, respectively. Both the trachydacite and rhyolite are metaluminous, rich in light REE, Rb, K, Zr, Hf, but display medium Eu negative anomaly and strong depletion of Ba, Sr, P, Ta, Nb and Ti. The zircon saturation temperature calculation shows high temperatures（ 774℃ to 812℃, averagely 792℃） for the parent magma of trachydacite, but low temperatures（ 713℃ to 790℃,averagely 750℃） for that of rhyolite. The trachydacite is characterized by very low Sr content but high Yb content with Nb/Ta ratio between 10. 8 to 11. 4, high Th content（ ≥8. 1 × 10^（-6）） and Th/Ce ratio（ ≥0. 31）. The rhyolite has features of low Sr and Yb content, Nb/Ta ratio between 8. 5 to 9. 7, high Th content（ ≥5. 3 × 10^（-6））, and Th/Ce ratio between 0. 14 to 0. 75. Based on the above evidence, it is held that the parent magma for trachydacite originated from partial melting of the upper crust, which was deeper than the parent magma of the trachydacite. The trachydacite and rhyolite in the Songhu iron deposit have geochemical characteristics similar to those of volcanic arcs. In combination with results from tectonic environment discrimination diagrams and other regional geological facts, the authors consider that there existed a continental arc setting for the formation of trachydacite and rhyolite.