西准噶尔玛依勒蛇绿混杂岩锆石U-Pb年代学、地球化学及源区特征

Zircon U-Pb geochronology and geochemistry of the Mayile ophiolitic mélange in West Junggar and implications for source nature

西准噶尔地区出露多条蛇绿混杂岩带,对其进行精确的锆石U-Pb年代学及岩石地球化学研究可以为揭示西准噶尔地区古大洋形成与演化过程、恢复古构造格局及追溯岩浆源区物质来源提供线索。本文对玛依勒蛇绿混杂岩中的辉长岩及玄武岩进行了LA-ICP-MS锆石U-Pb年代学及全岩地球化学研究,获得辉长岩中锆石的加权平均206Pb/238U年龄为572.2±9.2Ma,属于早震旦纪,该年龄是准噶尔乃至北疆地区报道的最古老的蛇绿混杂岩年龄。玛依勒蛇绿混杂岩中的枕状玄武岩为碱性玄武岩,岩石具有高Ti(TiO2=1.65%～3.13%)、高Fe(FeOT=8.93%～18.11%)、高Mg(MgO=3.95%～5.27%)及高P(P2O5=0.17%～0.51%),Th/Ta比值相对较高(=1.1～1.9),LREE和HREE分异较为明显((La/Yb)N=2.5～7.4)等特征,这些特征与洋岛玄武岩类似,可能形成于大洋板内的洋岛或海山环境。其中的辉长岩地球化学特征不同于玄武岩,可能形成与俯冲有关的环境。玛依勒蛇绿混杂岩中玄武岩与EMI型洋岛玄武岩具有相似的地球化学特征,表明其岩浆源区可能为EMI型富集地幔。岩石成因与软流圈地幔关系密切,软流圈的上涌导致尖晶石相二辉橄榄岩地幔源区大比例部分熔融,是岩石圈-软流圈地幔相互作用的产物。

Precise zircon U-Pb dating and geochemical composition analysis on the ophiolitic mélange belts in West Junggar. It is a clue for forming and evolution of the pale-ocean, rebuilding palaeotectonics and tracing the source of magma source. LA-ICP-MS zircon U-Pb weighted mean 206Pb/238U age of the gabbros from the Mayile ophiolitic mélange in the West Junggar is 572.2±9.2Ma, belong to Early Sinian, this is oldest age from ophiolitic mélange in the Junggar as well as North Xinjiang. The alkalic pillow basalts from Mayile ophiolitic mélange are similar with the ocean island basalt （OIB）, characteristics by high Ti （TiO2=1.65%~3.13%）, Fe （FeOT=8.93%~18.11%）, Mg （MgO=3.95%~5.27%） and P （P2O5=0.17%~0.51%）, and high Th/Ta ratios （1.1~1.9）, and clear differentiation between LREE and HREE （（La/Yb）N=2.5~7.4）. It is assumed that the magma extruded into ocean island or seamount of the ocean floor. However, the geochemical characteristics of the gabbros different from basalts, may be form subduction-related setting. The geochemical features of the basalts show they have ocean island basalts, derived from EMI type enriched-mantle. Petrogenesis is closely related to asthenosphere mantle, asthenospheric mantle upwelling led to a large proportion of spinel lherzolite mantle partial melting to forming the basalt magam. It is a product of asthenosphere interacted with lithosphere.