Abstract:

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 (SiO249.16%—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.

Key words: North Qinling, ultrahighpressure metamorphic eclogite, LAICPMS zircon dating, geochemistry, continental crust deep subduction