苏鲁造山带胡家林(石榴)单斜辉石岩成因——来自单斜辉石矿物化学的证据

Origin of garnet clinopyroxenite in Hujialin,Sulu orogen:Evidence from mineral chemistry of clinopyroxene

胡家林超镁铁质杂岩体位于苏鲁造山带中部日照市附近,主要由橄榄岩及(石榴)辉石岩组成。为探究(石榴)辉石岩成因,在岩相学研究的基础上,系统总结了(石榴)辉石岩的微量元素及其中单斜辉石的主-微量元素特征。结果显示,(石榴)辉石岩富集轻稀土、亏损重稀土,其原岩为单斜辉石岩。单斜辉石富MgO(Mg^(#)为90.4~97.1),贫Na_(2)O(质量分数0.14%~0.94%)、TiO_(2)(质量分数<0.38%)、Al_(2)O_(3)(质量分数0.48%~3.73%),部分不相容元素和高场强元素富集、部分亏损,暗示了单斜辉石岩为熔体-橄榄岩反应成因。单斜辉石Ca/Al比值及Mg^(#)较高,显示出碳酸盐熔体交代的特征;而Zr/Hf、Y/Ho比值较低、Ti/Eu-(La/Yb)N协变关系不明显,与典型碳酸盐熔体交代特征不同。结合岩相学特征,胡家林(石榴)辉石岩的原岩应是由橄榄岩与富Si熔体反应后,叠加了碳酸盐熔体的交代作用。

The Hujialin ultramafic complex in the middle of Sulu orogen mainly consists of dunite and garnet clinopyroxenite.To explore the origin of garnet clinopyroxenite,I carried out the petrographic analysis,and analyzed the major and minor elements characteristics of clinopyroxene,as well as minor elements features of garnet clinopyroxenite.The result showed that garnet clinopyroxenite contained high concentrations of light rare earth elements(LREEs)and low concentrations of heavy rare earth elements(HREEs),indicating the protolith of garnet clinopyroxenite was clinopyroxenite.Clinopyroxene contained high MgO with Mg^(#)ranging from 90.4 to 97.1,low Na_(2)O(0.14%~0.94%),TiO_(2)(<0.38%)and Al_(2)O_(3)(0.48%~3.73%).Some incompatible and high field-strength elements were enriched,and some were depleted,indicating clinopyroxenite was formed by melt-peridotite interaction.High Mg^(#)and Ca/Al in clinopyroxene were consistent with carbonate melt metamorphism.However,the low Zr/Hf and Y/Ho ratios,and no obviously covariant relationship between Ti/Eu and(La/Yb)N were inconsistent with typical carbonate melt metamorphism.Taken the petrographic characteristics of rocks together,the protolith of garnet clinopyroxenite was formed by peridotite-siliceous melt interaction,and then overprinted by carbonate melt metasomatism.