Eclogites have been recently discovered in the Xitieshan area in the middle segment of the northern marginof the Qaidam basin. These eclogites, together with those recognized earlier in the Yuka area of the western segment of thenorthern margin of the Qaidam basin and in the Dulan area of the eastern segment of the northern margin of the Qaidambasin, form an eclogite belt with a length of 350 km. A comparison of the eclogites from the Yuka and Xitieshan areassuggests that they show different country rocks, microtextures, mineral assemblages, and especially, different peak tem-peratures, PT paths during decompression and isotopic features. Eclogites from the Yuka area bear evidence of progrademetamorphism, such as prograde mineral relics in garnet and growth zoning of garnet, and hairpin-shaped PT paths withcoincidence of the baric and thermal peaks of metamorphism, which reflect rapid burial and uplift. Sm-Nd isotopic de-termination shows obvious Sm-Nd disequilibrium, and no isochron ages of Early Paleozoic metamorphism was obtained.On the contrary, the Xitieshan eclogite was subjected to metamorphism of higher eclogite-facies and post-eclogite-faciestemperatures, showing an obvious granulite-facies overprint. A tentative PT path for the Xitieshan area suggests that thebaric peak preceded the thermal culmination. Whole rock-garnet-omphacite Sm-Nd isotopic data give an isochron age inthe Early Paleozoic. The differences between the two eclogites imply their different tectonic-thermal history during theirburial and uplift.
Ultrahigh pressure (UHP) metamorphism refers to metamorphism that has occurred at pressures for the stability of coesite. The polycrystalline quartz inclusions showing the characteristic texture within garnets of eclogites indicates the pre-existence of coesites under the peak meta-morphic condition. The unusual exsolution textures in om-pacites and apatites, and the pressure estimations of phengite-bearing eclogites have been taken to provide further proof of eclogite formation under the UHP conditions. Combined with the fact that coesites have been observed in country rocks of eclogites in North Qaidam Mountains, another UHP metamorphic belt cut by the large-scale strike-slip fault in the Altun-North Qaidam area of China is confirmed.
Jianxin ZhangJingsui YangZhiqin XuFancong MengHaibing LiRendeng Shi
