Two Contrasting Eclogite Types on the Northern Margin of the Qaidam Basin, Western China

AbstractEclogites have been recently discovered in the Xitieshan area in the middle segment of the northern margin of the Qaidam basin. These eclogites, together with those recognized earlier in the Yuka area of the western segment of the northern margin of the Qaidam basin and in the Dulan area of the eastern segment of the northern margin of the Qaidam basin, form an eclogite belt with a length of 350 km. A comparison of the eclogites from the Yuka and Xitieshan areas suggests that they show different country rocks, microtextures, mineral assemblages, and especially, different peak temperatures, PT paths during decompression and isotopic features. Eclogites from the Yuka area bear evidence of prograde metamorphism, such as prograde mineral relics in garnet and growth zoning of garnet, and hairpin‐shaped PT paths with coincidence of the baric and thermal peaks of metamorphism, which reflect rapid burial and uplift. Sm‐Nd isotopic determination 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‐facies temperatures, showing an obvious granulite‐facies overprint. A tentative PT path for the Xitieshan area suggests that the baric peak preceded the thermal culmination. Whole rock‐garnet‐omphacite Sm‐Nd isotopic data give an isochron age in the Early Paleozoic. The differences between the two eclogites imply their different tectonic‐thermal history during their burial and uplift.