Identification of the eclogites with different ages and their tectonic significance in central Qiangtang, Tibetan Plateau: Constraints from 40Ar-39Ar geochronology

The high pressure metamorphic belt in central Qiangtang consists of eclogite, glaucophane schist and garnet-muscovite schist. It constitutes part of Longmucuo-Shuanghu-Lancang river plate suture zone, together with Late Paleozoic epirock and ophiolite melange. Recently, eclogites have been found in Pianshishan area, Guoganjianianshan area and Gangmacuo area in turn. In this paper, we focus on the eclogites in Pianshishan and Guoganjianianshan area. The Pianshishan eclogites enclosed within garnet-muscovite schist have peak metamorphic p-T conditions at T=500℃ and p=2.3GPa. The ages ranging between 230 and 244Ma are given by U-Pb dating of zircons and Lu-Hf datings, and were interpreted as the peak metamorphic age of eclogites. Ar-Ar datings of eclogites and their country rocks gave similar ages ranging between 210Ma and 220Ma, representing the cooling age of the eclogites during retrograde metamorphism. On the other hand, the Guoganjianianshan eclogites have similar field occurrence, mineral assemblage, p-T conditions and country rocks to those of Pianshishan eclogites. However, Ar-Ar datings show that the peak metamorphic age of the Guoganjianianshan eclogites obviously differ from that of Pianshishan eclogite. These data suggested that the Guoganjianianshan eclogites had been exhumated to shallow crust lever at about 240Ma.