Neoproterozoic eclogitic metamorphic age of the Beishan eclogite of Gansu, China：Evidence from SHRIMP U-Pb isotope dating

Abstract：The Beishan orogen is located at the junction between the Tarim-Sino-Korean and Kazakhstan plates, and its main orogenic event occurred in the Early Paleozoic. Eclogites are associated with ultramafic-mafic rocks and collisional granites in the south belt of the Beishan orogen, which suggests that this belt might represent a plate boundary. SHRIMP U-Pb isotope dating of zircons from the Beishan eclogite yielded an eclogite-facies metamorphic age of 819 ± 21 Ma and a protolith age of 1007 ± 20 Ma, indicating an important plate breakup and subduction-collision event in the Neoproterozoic. Recently an event of Neoproterozoic plate breakup and ophiolitic oceanic crust formation has also been reported in the North Qaidam marginal area in the southern Qilian Mountains adjacent to the Beishan, suggesting the occurrence of a regional breakup event in a Neoproterozoic oceanic basin in western China, which is called the Rodinian Supercontinent breakup event. However, it should be emphasized that the eclogitic-facies metamorphism in the Beishan took place in the Neoproterozoic, while that for the North Qaidam marginal eclogite occurred in the Early Paleozoic (500-440 Ma) though its protolith is the Neoproterozoic oceanic crustal rock, suggesting another subduction-collision event after the Neoproterozoic subduction-collision event in the Beishan.