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)M...

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.