苏鲁地体南缘超高压变质带朐山二云花岗片麻岩的地球化学特征及其构造意义

二云花岗片麻岩是组成苏鲁地体南缘超高压变质带朐山杂岩体的重要岩石类型,虽然经历了超高压变质作用,但仍保留了花岗岩的岩石学特征。常量元素和微量元素分析结果表明该片麻岩具有高w(K2O)、低w(CaO)、高w(TFeO)/w(MgO)比值、铝饱和指数偏高的A型花岗岩的特征,岩石类型为高钾碱性过铝质A型花岗岩。稀土元素中轻稀土富集、分馏程度高、Eu强烈亏损,微量元素中Ba、P、Ti、Sc具有明显的负异常,w(Sr)/w(Y)、w(La)/w(Yb)和w(Rb)/w(Sr)、w(Rb)/w(Ba)较高,尤其是w(TiO2)<0.2%和w(Y)/w(Nb)>1.2的特征,以及在w(Rb)-w(Yb+Ta)和w(Rb)-w(Y+Nb)判别图解上样品投点位于板内环境区等,表明该片麻岩的原岩形成于板内与裂谷有关的非造山环境。锆石SHR IMP U-Pb测年结果揭示其侵位时代为新元古代中晚期((722±32)M a),与杂岩体中早期侵位的二长花岗片麻岩及杂岩体上覆地层中的变质火山岩同为Rod in ia大陆裂解、扬子地块陆内裂谷形成过程中伴随的岩浆活动的产物。

Geochemical Characteristics of Qushan Mica Granitic Gneisses from Ultrahigh Pressure Metamorphic Belt in the Southern Sulu Terrane and Its Tectonic Significance

The mica granitic gneisses is an important rock type for Qushan complex from UHP belt in the southern Sulu terrane,which had suffered UHP metamorphism,but still had the petrological features of granite.The geochemical analysis data indicate that the mica granitic gneisses have the characteristics of A-type granitoid with high K_(2)O contents and TFeO/MgO ratios,and with low CaO contents,and high peraluminous coefficient.The mica granitic gneisses have the characteristics of the REE patterns with steeply fractionated and strongly negative Eu anomalies,the trace elements patterns with negative Ba, P,Ti and Sc anomalies,high Sr/Y,La/Yb,(Rb/Sr),Rb/Ba ratios,and TiO_(2)<0.2%,Y/Nb >1.2,and all of the samples plots in the Within Plate Field in the Rb vs(Yb+Ta) and Rb vs(Y+Nb) discrimination diagrams,which show that the mica granitic gneisses formed in non-orogenic environment but within plate related with rifting.The zircon SHRIMP U-Pb(dating) inferred that the time of magmatic emplacement was middle-late Upper Proterozoic((722±32)Ma),it was resulted from the magmatic activity that was identical early emplaced feldspar granitic gneisses in Qushan complex and the metamorphic volcanics overlying on Qushan complex during Rodinia continent breakup and within plate rifting in Yangtz block.