滇西勐库地区退变质榴辉岩锆石U-Pb年龄及其地质意义

滇西双江县勐库地区退变质榴辉岩呈构造透镜体产于湾河蛇绿混杂岩带内,该发现弥补了东特提斯造山带高压-超高压变质岩在云南境内的空缺。在岩石学观察的基础上,借助激光剥蚀等离子体质谱(LA-ICP-MS)技术,对退变质榴辉岩中的锆石开展了精确的U-Pb年龄测定。所测试的3件样品分别采自3个不同的露头:样品PM011-9-1采自勐库控角剖面,样品PM038-15-4采自勐库地界剖面,样品GH1612-1-1采自勐库根恨大寨。测年结果显示,样品PM011-9-1的23个测点中存在2组较集中的~(206)Pb/~(238)U年龄,分别为801.0±9.8Ma和227.0±12Ma;样品PM038-15-4的26个测点中存在2组较集中的~(206)Pb/~(238)U年龄,分别为447.5±3.6Ma和291.7±6.3Ma;样品GH1612-1-1的30个测点中存在一组较集中的~(206)Pb/~(238)U年龄,为229.0±1.3Ma。结合区域资料及锆石阴极发光图像分析,801.0±9.8Ma应属退变质榴辉岩的原岩年龄,可能代表了Rodinia超大陆裂解早期出现的初始洋壳;而447.5±3.6Ma、291.7±6.3Ma和229.0±1.3～227.0±12Ma这3组年龄可能代表了退变质榴辉岩经历的3期变质作用年龄:分别为峰期硬柱石榴辉岩相的变质作用;中期角闪石榴辉岩相-高压麻粒岩相的退变质作用,为一个降压-增温的"热折返"过程;主期角闪岩相的退变质作用,是一个大幅度的降温-减压过程,奠定了勐库地区退变质榴辉岩的主体面貌。

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.