东昆仑祁漫塔格拉陵灶火中游基性变质岩年代学、地质及地球化学特征

Zircon U-Pb Chronology,Geological and Geochemical Characteristics of Metamorphosed Basic Rocks in the Middle Reaches of Lalingzaohuo,Qimantage Area of East Kunlun Mountains

拉陵灶火中游基性变质岩体原岩侵位于古元古界金水口岩群白沙河岩组片麻岩之中。文章对基性变质岩体开展了岩相学、年代学和岩石地球化学研究,探讨了其原岩及源区性质、成岩构造环境、年龄及地质意义。结果表明,基性变质岩为斜长角闪岩、变质辉绿岩和角闪岩,原岩应属于亚碱性系列岩石,既有拉斑系列,又有钙碱性系列和钾玄质系列。岩石中SiO2含量为44.04%~54.39%,全碱(Na_(2)O+K_(2)O)含量为1.97%~6.53%、且Na_(2)O>K_(2)O,MgO含量为5.01%~9.58%,TFe_(2)O_(3)含量为7.71%~13.46%,TiO_(2)含量为0.56%~1.49%;ΣREE平均为83.60×10^(-6),δEu为0.89~1.55,平均为1.14,呈轻微正异常;稀土元素球粒陨石标准化配分模式图上呈稍右倾的较平滑曲线,具有轻、重稀土轻微分异的轻稀土略富集特征;岩石明显富集大离子亲石元素Cs、Rb、Ba、K、Sr、U、Pb,显著亏损高场强元素Nb、Ta、Zr,略微亏损P和Ti。采用SIMS锆石U-Pb定年,结合岩石地球化学和区域地质背景,认为拉陵灶火中游地区斜长角闪岩记录了新元古代早期(928±25 Ma)、早寒武世末期(514±11 Ma)、中晚奥陶世(451±11 Ma)和早中泥盆世时期(392±12 Ma)的岩浆—变质事件,其中928±25 Ma和514±11 Ma是斜长角闪岩捕获的锆石年龄,451±11 Ma是斜长角闪岩的原岩结晶年龄,392±12 Ma是斜长角闪岩的变质年龄。拉陵灶火中游基性变质岩体原岩是俯冲板片脱水交代的岩石圈地幔部分熔融的产物,应形成于岛弧环境。

The metamorphosed basic body in the middle reaches of Lalingzaohuo intruded into the gneiss of Baishahe formation of the Paleoproterozoic Jinshuikou group.The petrography,chronology and geochemistry of basic metamorphic rocks are investigated in this study.The protolith,nature of source region,tectonic environment,ages and geological significance of these rocks are discussed.The results show that the basic metamorphic rocks are plagioclase amphibolite,metamorphosed diabases and amphibolite.The protolith belongs to subalkalic series,including tholeiite series,calc-alkaline series and shoshonitic series rocks.The contents of silica and alkali are 44.04%-54.39%,1.97%-6.53%,and Na_(2)O are more than K_(2)O.The contents of MgO,Fe_(2)O_(3),and TiO_(2) are 5.01%-9.58%,7.71%-13.46%,and 0.56%-1.49%,respectively.The average values ofΣREE is 83.60×10^(-6) andδEu values range from 0.89 to 1.55 with an average value of 1.14,showing slightly positive Eu anomaly.The chondrite-normalized REE distribution patterns show a relatively smooth slightly right inclined curve,which has the features of slightly enriched LREE-and slightly differentiated HREE.The rocks are clearly enriched in LILEs(e.g.,Cs,Rb,Ba,K,Sr,U,Pb)and significantly depleted in HFSE(e.g.,Nb,Ta,Zr),slightly depleted in P and Ti.Combining SIMS zircon U-Pb dating with rock geochemical and regional geological background,it is believed that the magmatic-metamorphic events in early Neoproterozoic(928±25 Ma),the end of early Cambrian(514±11 Ma),middle-late Ordovician(451±11 Ma),and early-middle Devonian(392±12 Ma)have been recorded in the amphibolite.The zircon ages of 928±25 Ma and 514±11 Ma represent the ages of captured zircons,the crystallization age of the protolith is 451±11 Ma,the metamorphism of the amphibolites occurred at 392±12 Ma.The protolith of basic metamorphic rocks in the middle reaches of Lalingzaohuo was product result of partial melting in lithosphere mantle ever dehydrating by subduction slab and should be formed in an island-arc environment.