东天山黄山南镁铁-超镁铁质岩体锆石U-Pb年龄及岩浆演化过程探讨

黄山南镁铁-超镁铁质岩体位于东天山造山带觉罗塔格构造带内,与构造带内一系列镁铁-超镁铁质岩体构成了东天山镁铁-超镁铁质岩带。黄山南岩体锆石U-Pb同位素定年得到293.7±3.1Ma的加权平均年龄,显示其形成年龄为早二叠世。岩石样品镁铁比值m/f比值介于4.45~6.37之间,属于铁质超基性岩;利用橄榄石最高Fo值计算得到黄山南岩体母岩浆的Mg~#值为0.70,表明其母岩浆成分与原生岩浆较为接近,具有高镁拉斑玄武岩质岩浆的性质,且演化程度较低或母岩浆有过剩橄榄石堆晶的加入。Ba/La、Ba/Nb、Ba/Th、Rb/Nb、Th/Nb和Sm/Yb等微量元素比值表明黄山南岩体的岩浆源区可能为一被俯冲板片沉积物和流体改造过的较富集的岩石圈地幔。主量元素变化特征显示了岩浆结晶分异作用的一般特征;较高的Ba/Nb比值,相对高的La/Nb和低La/Ba值以及Ni-Ta-(Ti)元素的负异常表明岩浆上升过程中可能经历了地壳物质的混染作用。黄山南岩体年龄及岩石地球化学特征综合表明其能够代表东天山地区地幔岩浆演化早期阶段的产物,具有较好的成矿潜力。

Zicon U-Pb age and the magma evolution process of the Huangshannan mafic-ultramafic intrusion in the East Tianshan Mountains

The Huangshannan mafic-ultramafic intrusion is located in the Jueluotage tectonic zone in East Tianshan Mountains, which is part of the large-scale mafic-ultramafic rock belt in this area. Zircon U-Pb isotopic dating of this intrusion yields a weighted average age of 293.7±3.1Ma, indicating it was generated in the Early Permian. The m/f ratios of samples from the Huangshannan intrusion range from 4.45 to 6.37, implying ferric ultrabasic rock. Using the maximum of the Fo olivine, we estimate that the parental magma has a Mg# of 0.70, showing a similar composition with primary magma and a characteristic of high-Mg tholeiitic basaltic magma. The parental magma has a low evolution degree, or it has been added by surplus olivine crystal. The ratios of Ba/La, Ba/Nb, Ba/Th, Rb/Nb, Th/Nb and Sm/Yb reveal that the source of the Huangshannan intrusion is relatively enriched lithospheric mantle which was reformed by slab-derived fluids and subducting sediment. The variation features of major elements show a crystallization differentiation process; relatively high Ba/Nb, La/Nb and low La/Ba ratios as well as the negative anomaly of Ni-Ta-(Ti) indicate that the magma of Huangshannan has likely suffered crustal contamination during its emplacement. Combining the age and the geochemical characteristics of the Huangshannan intrusion, we consider that this rock could represent the product of mantle magmatic evolution of early stage, having a good metallogenic potential.