Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China
The Dulan eclogite–gneiss region is located in the eastern part of the North Qaidam eclogite belt, NW China. Widespread evidence demonstrates that this region is a typical ultrahigh-pressure (UHP) metamorphic terrane. Eclogites occur as lenses or layers in both granitic and pelitic gneisses. Two distinguished sub-belts can be recognized and differ in mineralogy, petrology and geochemistry. The North Dulan Belt (NDB) has tholeiitic protoliths with high TiO2 and lower Al2O3 and MgO contents. REE patterns and trace element contents resemble those of N-type and E-type MORB. In contrast, eclogites in the South Dulan Belt (SDB) are of island arc protoliths with low TiO2, high Al2O3 and show LREE-enriched and HFSE-depleted patterns. Sm–Nd isotope analyses give isochron ages of 458–497 Ma for eclogite-facies metamorphism for the two sub-belts. The ages are similar to those of Yuka and Altun eclogites in the western extension of the North Qaidam-Altun eclogite belt. The Dulan UHP metamorphic terrane, together with several other recently recognized eclogite-bearing terrenes within the North Qaidam-Altun HP-UHP belt, constitute the key to the understanding of the tectonic evolution of the northern Tibetan Plateau. The entire UHP belt extends for more than 1000 km from the Dulan UHP terrane in the southeast to the Altun eclogite–gneiss terrane in the west. This super-belt marks an early Paleozoic continental collision zone between the Qaidam Massif and the Qilian Massif.