苏鲁超高压变质带中蛇纹岩的地球化学特征及岩石成因

摘要

青岛的仰口和日照的梭罗树是苏鲁超高压变质带中重要的蛇纹岩出露区.系统的岩石地球化学分析表明,仰口和梭罗树地区的蛇纹岩具有高Mg、低Al2O3/SiO2、贫高场强元素( Nb、 Th、 Zr和Hf)和重稀土元素(HREE)的特征,其原岩是经历了不同程度部分熔融后的地幔残余,符合弧前地幔橄榄岩的特征.经模拟估算,仰口蛇纹岩的原岩熔融程度＜15%,梭罗树蛇纹岩的原岩熔融程度在20%～25%之间.蛇纹岩富流体迁移元素(U、 Pb和LREE),且LREE和HFSE无明显相关性的地球化学特征表明,研究区蛇纹岩原岩在弧前地幔楔环境下,受到俯冲板片释放的含H2O流体作用发生蛇纹石化,形成了流体迁移元素富集的蛇纹岩.%Yangkou in Qingdao and Suoluoshu in Rizhao are the important outcrops of serpentinites in the Su-lu ultrahigh-pressure metamorphosed belt.Through systematic analysis of petrogeochemistry, it can be found that serpentinites from Yangkou and Suoluoshu are with high contents of Mg, low ratios of Al2O3/SiO2 and relatively de-pleted HFSE ( Nb, Th, Zr and Hf ) and HREE, which illustrate characteristics of the forearc mantle peridotites. It reveals that protoliths of these studied serpentinites are residual mantle peridotites formed from different degrees of partial melting.Estimation from modelling suggests that the partial melting degree is less than 15%for protoliths of Yangkou serpentinites, and 20%～25%for the Suoluoshu serpentinites.Serpentinites are rich in fluid-mobile ele-ments and show no obvious correlation between LREE and HFSE, which indicates that protoliths of studied serpen-tinites have undergone serpentinization by hydrous fluids released from the subducted slab and formed serpentinites enriched in fluid-mobile elements in the forearc mantle wedge.