桂北元宝山地区超镁铁岩的年代、源区及其地质意义

Geochronology,Mantle Source and Geological Implications of Neoproterozoic Ultramafic Rocks from Yuanbaoshan Area of Northern Guangxi

桂北元宝山地区超镁铁岩的SHRIMP锆石U-Pb年龄为（841±22）Ma（2σ），与桂北三防-何家湾地区出露的镁铁-超镁铁侵入岩年龄（约825Ma）接近，表明桂北地区出露的镁铁-超镁铁侵入岩具有基本一致的结晶年龄。超镁铁岩的岩石学和主量元素特征表明超镁铁岩是堆晶岩，其微量元素质量分数较低（〈NMORB），蛛网图上除了Nb、Ta、Zr、Hf亏损外，一般表现为平坦的分配曲线。超镁铁岩εNd（f）的变化范围为-1．0～6．5，表明超镁铁岩的母岩浆遭受了一定程度的地壳混染。元宝山地区超镁铁岩低的w（Th）／w（Nb）比值与原始地幔非常类似，而与大陆弧玄武岩浆的比值明显不同，推测元宝山地区超镁铁岩应形成于板内环境，而不是活动大陆边缘，其形成很可能与导致新元古代Rodinia超大陆裂解的发生于约825Ma的华南地幔柱有关。

SHRIMP U-Pb zircon age,geochemical and Sm, Nd isotopic results are reported for the Neoproterozoic ultramafic rocks in Yuanbaoshan area of northern Guangxi. The ultramafic rocks have a crystallization age of（841±22）Ma（2σ） gained in this paper, similar to the age about 825 Ma of mafic intrusions in Sanfang-Hejiawan area of northern Guangxi. Petrological characteristics show that ultramafic rocks have remarked olivine peritectic texture with Fo values of 78-83 in olivine. The magma, equilibrated to the olivine,may have the calculated MgO of 7.3 % at most. So the ultramafic rocks are cumulus rocks. Trace elements of these rocks are low in contents and less than NMORB. On the spidergram,the Nb,Ta, Zr and Hf are depleted but Ti is not. The spidergram has a horizontal-like pattern excepting the depletions of some HFSEs. The most incompatible element ratios have wide ranges, such as w（Nb）/w（La） （0.11 - 0.96） and w（Th）/w（Nb）（0.18-0. 57）. The εNd （t） Of these rocks ranges from -1.0 to ±6.5 and most have high εNd （t）. These characteristics indicate the magma of the ultramafic rocks may have undergone some crustal contamination. By comparison with simultaneous mafic-ultramafic rocks in northern Guangxi,the ultramafic rocks in Yuanbaoshan area have the highest εNd （t） and suffer the least contamination. The study has found that the mantle source of the ultramafic rocks has comparability with that of the Australia Gairdner mafic dykes （GDS）, which is related to the Neoproterozoic mantle plume. For example, the εNd （t） and w（Th）/w（Nb） of them are alike and the lowest w（Th）/w（Nb） is close to that of primitive mantle. The lowest w（Th）/w（Nb）, close to primi tramafic rocks should be generated wi sis of the ultramafic rocks in Yuanba tive mantle and far from the continental arc basalt implies that the ulthin a continental plate and not an arc setting. Hence, the petrogeneoshan area is possibly related to the 825 Ma mantle plume beneath South China of Rodinia.