The Metamorphic Ages of the Country Rocks of the Yukahe Eclogites in the Northern Margin of Qaidam Basin and Its Geological Significance

A combined study of Cathodoluminescence (CL) imaging, LA-ICP-MS in situ trace element analysis and U-Pb dating was carried out on zircon grains from garnet-bearing muscovite-amphibole-albite gneiss and kyanite-bearing garnet-muscovite schist, the country rocks of the ultrahigh-pressure (UHP) eclogites in Yukahe area in the northern margin of Qaidam basin. Zircons from garnet-bearing muscovite-amphibole-albite gneiss mainly exhibit rounded crystal habits with internal sector and fir-tree zoning. The Th/U ratio and the content of REE and HREE of zircons are low. They are generally less than 0.01, 50 and 40 μg/g, respectively. All analyzed points display the REE pattern of a flat to right-sloping HREE profile with a slight positive Eu anomaly. These patterns indicate the presence of garnet and the absence of plagioclase in the peak metamorphic mineral assemblage. Zircons from kyanite-bearing garnet-muscovite schist display core-rim internal texture. The cores are larger in size than the rims and exhibit irregular shape and distinct or vague oscillatory zoning. The HREE content and the Th/U ratio of the core show large variations, varying from 22 to 1728 μg/g and from 0.01 to 0.82, respectively. These indicate that the cores represent relicts of the protolith that contained the detrital zircon. The rims are generally too narrow to date, except for three grains which exhibit metamorphic characteristics indicated by the low Th/U ratio, decreasing HREE enrichment and Eu anomaly. LA-ICP-MS U-Pb dating yielded peak metamorphic ages of 431±3 Ma and 432±19 Ma for the albite gneiss and the pelitic schist, respectively. These ages are consistent, within errors, with each other and also with the metamorphic age of the adjacent UHP eclogites (436±3 Ma and 431±4 Ma), as well as the coesite-bearing pelitic schist (423–432 Ma) in Yematan area. The almost identical metamorphic age of UHP eclogites and their country rocks, together with their field relationship, permit us to conclude that the Yukahe UHP metamorphic block was formed by continental deep subduction-collision during 430–435 Ma in early Paleozoic.