Zircon U-Pb age and geological significance of retrograded eclogites from Mengku area in western Yunnan Province

The retrograde eclogite, which is a structural lens, is located in the Wanhe ophiolitic melange zone in the Shuangjiang area of western Yunnan Province. This discovery makes up the blank that high pressure-ultra-high pressure metamorphic rocks have not been found in Yunnan for the Eastern Tethys orogeny. On the basis of petrological observation, accurate U-Pb dating was performed on the selected zircons using laser ablation plasma mass spectrometry (LA-ICP-MS). The three samples tested here were collected from three different outcrops. Sample PM011-9-1 was taken from the Kongjiao profile, sample PM038-15-4 was taken from the boundary profile of the Mengku area, and sample GH1612-1-1 was picked up from Dazha Village. The dating results show that there are two sets of ²⁰⁶Pb/²³⁸U ages in the 23 points of sample PM011-9-1, which are 801.0±9.8Ma and 227.0±12Ma; there are two groups of relatively concentrated ²⁰⁶Pb/²³⁸U ages at the 26 sites of sample PM038-15-4, which are 447.5±3.6Ma and 291.7±6.3Ma for sample GH1612-1-1, there is a group of relatively concentrated ²⁰⁶Pb/²³⁸U ages at 30 sites, being 229.0±1.3Ma. According to the zircon cathodoluminescence image and regional geological data, 801.0±9.8Ma may represent the protolith age, which is consistent with Rodinia supercontinent initial breakup; and the ages of 447.5±3.6Ma, 291.7±6.3Ma and 229.0±1.3Ma may represent the ages of retrograde eclogites which underwent three stages of metamorphism. Combined with the current research results, the authors hold that Mengku retrograde eclogite was probably formed in the Neoproterozoic, and was an initial oceanic crust of Proto-Tethys Ocean basin; Proto-Tethys Ocean basin subduction ended in the Late Ordovician, the subduction oceanic crust experienced lawsonite eclogites facies metamorphism in the mantle at the depth of ca 100~150km; in the early Permian period, large-scale eastward subduction of the Paleo-Tethys oceanic basin took place along Changning-Menlian Suture Zone; and a retrograde metamorphism of hornblende eclogites facies-high pressure granulite facies occurred, which was hot-exhumation phase of the temperature rise and pressure drop; in the Middle-Late Triassic, large-scale decompression of retrograde eclogite occurred, following the arc-continental collision orogenesis, and formed a large amount of amphibole minerals. The late retrograde metamorphism of the amphibolite facies eclogites laid the foundation for main features of retrograde eclogite in this area.