北秦岭榴辉岩及相关岩石年代学的进一步确定及其对板片俯冲属性的约束

在岩相学观察和锆石CL图像研究的基础上，利用LA ICP MS原位分析方法，对北秦岭官坡超高压榴辉岩和伴生的石榴石角闪岩(榴闪岩)进行了详细的锆石微区微量元素和U Th Pb同位素分析，在榴辉岩样品中得到变质年龄为(502±11)Ma，原岩结晶年龄>(657±18)Ma；在榴闪岩样品中得到原岩结晶年龄为(791±6)Ma，变质年龄为487~503 Ma，角闪岩相退变质年龄为(366±4)Ma。岩石地球化学研究显示，北秦岭官坡地区的榴闪岩具有低Si(SiO2质量分数为4916%~5078%)，高Ti(TiO2质量分数为228%~283%)、富集LREE、LILE和大部分的HFSE元素，不显Nb、Ta负异常的板内玄武岩特征，与北秦岭超高压榴辉岩地球化学特征一致。结合两者的野外产状、岩相学特征、锆石形貌和年代学研究结果，表明本文研究的官坡地区的榴闪岩是超高压榴辉岩在抬升过程中在角闪岩相条件下退变质的产物。综合两者的年代学研究结果，得到北秦岭地区超高压榴辉岩的变质年龄为(502±11)Ma，原岩结晶年龄为(791±6)Ma，角闪岩相退变质年龄为(366±4)Ma。研究得到的(502±11)Ma的榴辉岩相变质年龄与前人得到的该榴辉岩围岩超高压泥质片麻岩的变质年龄(507±38)Ma以及北秦岭松树沟地区的超高压长英质片麻岩的变质年龄485~514 Ma一致，表明它们经历了同期超高压变质作用。而且，榴辉岩(502±11)Ma的变质年龄与其原岩的结晶年龄(791±6)Ma存在近300 Ma的时间间隔，表明原岩具有板内玄武岩性质的北秦岭官坡超高压榴辉岩不可能是秦岭古生代大洋板块深俯冲的产物，而可能是已构造就位的古洋壳或裂谷火山岩在古生代随陆壳一起发生大陆深俯冲作用的产物。

New data on the chronology of eclogite and associated rock from Guanpo Area, North Qinling orogeny and its constraint on nature of North Qinling HP-UHP eclogite terrane

Based on the investigation of petrography and zircon CL imaging,LA-ICP-MS trace elements and U-Th-Pb isotope analyses were carried out on zircon grains of UHP eclogite and associated garnet amphibolite from Guanpo area,the North Qinling orogeny.Zircon LA-ICP-MS dating yields metamorphic age of(502±11)Ma,protolith age of>(657±18)Ma for the eclogite,and protolith age of(791±6)Ma,metamorphic age of(487—503)Ma and amphibolite facies retrograde metamorphic age of(366±4)Ma for the garnet amphibolite.Geochemical studies show that the garnet amphibolite from the Guanpo area have characteristics of low Si(SiO2 49.16%—50.78%) and high Ti(TiO2 ranging from 2.28% to 2.83%),rich in LREE,LILE,most HFSE and without any depletion of Nb and Ta,showing the feature of within plate basalts(WPB).These geochemical characteristics are the same as those of the adjacent UHP eclogite in this area.Combined the geochemical data with the field occurrence,petrographic features and chronological results,it suggests that the garnet amphibolite resulted from the recrystallization of the UHP eclogite under amphibolte facies during exhumation.Therefore,the ages of(502±11)Ma,(791±6)Ma and(366±4)Ma obtained from eclogite and garnet amphibolite should represent the metamorphic,the protolith and the retrograde metamorphic age of Qinling UHP eclogite,respectively.The metamorphic age of(502±11)Ma of eclogite is consistent with that of its country rock pelitic gneiss((507±38)Ma)and UHP felsic gneiss in Songshugou area(485—514 Ma)of the North Qinling.These nearly identical metamorphic ages suggest that the Guanpo eclogite and their host gneisses as well as the Songshugou UHP felsic gneiss underwent the same metamorphism during the Early Paleozoic.Additionally,the metamorphic age of(502±11)Ma is nearly 300 Ma younger than that of the protolith age of(791±6)Ma for eclogite;these age data together with the WPB characters of the protolith of eclogite suggest that the North Qinling UHP eclogite may not be the product of Paleozoic oceanic crust deep subduction but the Neo-proterozoic oceanic crust or rift igneous rocks subducted with country rocks into deep mantle during the Paleozoic.