摘要
Eclogites have been recently discovered in the Xitieshan area in the middlesegment of the northern margin of the Qaidam basin. These eclogites, together with those recognizedearlier in the Yuka area of the western segment of the northern margin of the Qaidam basin and inthe Dulan area of the eastern segment of the northern margin of the Qaidam basin, form an eclogitebelt 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 temperatures, PT paths during decompression and isotopic features. Eclogites fromthe Yuka area bear evidence of prograde metamorphism, such as prograde mineral relics in garnet andgrowth zoning of garnet, and hairpin-shaped PT paths with coincidence of the baric and thermal peaksof metamorphism, which reflect rapid burial and uplift. Sm-Nd isotopic determination shows obviousSm-Nd disequilibrium, and no isochron ages of Early Paleozoic metamorphism was obtained. On thecontrary, the Xitieshan eclogite was subjected to metamorphism of higher eclogite-facies andpost-eclogite-facies temperatures, showing an obvious granulite-facies overprint. A tentative PTpath for the Xitieshan area suggests that the baric peak preceded the thermal culmination. Wholerock-garnet-omphacite Sm-Nd isotopic data give an isochron age in the Early Paleozoic. Thedifferences between the two eclogites imply their different tectonic-thermal history during theirburial and uplift.
Eclogites have been recently discovered in the Xitieshan area in the middlesegment of the northern margin of the Qaidam basin. These eclogites, together with those recognizedearlier in the Yuka area of the western segment of the northern margin of the Qaidam basin and inthe Dulan area of the eastern segment of the northern margin of the Qaidam basin, form an eclogitebelt 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 temperatures, PT paths during decompression and isotopic features. Eclogites fromthe Yuka area bear evidence of prograde metamorphism, such as prograde mineral relics in garnet andgrowth zoning of garnet, and hairpin-shaped PT paths with coincidence of the baric and thermal peaksof metamorphism, which reflect rapid burial and uplift. Sm-Nd isotopic determination shows obviousSm-Nd disequilibrium, and no isochron ages of Early Paleozoic metamorphism was obtained. On thecontrary, the Xitieshan eclogite was subjected to metamorphism of higher eclogite-facies andpost-eclogite-facies temperatures, showing an obvious granulite-facies overprint. A tentative PTpath for the Xitieshan area suggests that the baric peak preceded the thermal culmination. Wholerock-garnet-omphacite Sm-Nd isotopic data give an isochron age in the Early Paleozoic. Thedifferences between the two eclogites imply their different tectonic-thermal history during theirburial and uplift.
基金
the National Natural Science Foundation of China(49732070 ,49902018)
the National Key Project for Basic Research on the Tibetan Plateau(G1998040805)
the Key Project of the Ministry of Land and Resources of China(20010201).
