滇西澜沧江南段眼球状花岗岩锆石U-Pb年龄及其地质意义
Augen granites from the southern Lancangjiang tectonic zone in western Yunnan: Zircon U-Pb dating and its geological implications
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摘要: 滇西澜沧江构造带与昌宁-孟连古特提斯缝合带空间配置关系密切,其花岗质岩浆作用的研究对于揭示三江地区古特提斯演化过程及其动力学背景具有重要的地质意义。本文针对澜沧江南段时代不明的眼球状花岗岩开展了高精度的锆石U-Pb年代学、元素地球化学和Sr-Nd同位素组成研究。定年结果显示,2个眼球状花岗岩和1个糜棱岩化花岗岩分别给出了213 Ma、216 Ma和211 Ma的岩浆结晶年龄,与公认的晚三叠世临沧花岗岩、小定西组和芒汇河组"双峰式"火山岩时代一致,而不是1:20万地质图上标绘的古生代或者中三叠世岩浆岩。地球化学特征上,眼球状花岗质岩石与临沧花岗岩基主体相似,显示出典型S型花岗岩的特征,可能来源于中上地壳变沉积岩的部分熔融。结合区域上的研究资料,认为该期岩浆作用是澜沧江构造带古特提斯演化晚期阶段-碰撞后伸展构造背景下的产物。Abstract: The wide-spread magmatic rocks are developed in the southern Lancangjiang tectonic zone, especially in the Changning-Menglian tectonic zone in western Yunnan as a major suture zone of the Palaeo-Tethyan Ocean. The approaches of the magmatism in this area will be helpful to the better understanding of the evolutionary processes and relevant geodynamic mechanisms of the Palaeo-Tethyan Ocean. In this paper, the authors present the highresolution zircon U-Pb and Sr-Nd dating results for the deformed augen granites from the southern Lancangjiang tectonic zone. Two pieces of augen granite samples 13YJ-45-1 and 13YJ-45-3 and one piece of mylonitized granite sample 13YJ-46 give the respective ages of 213 Ma, 216 Ma and 211 Ma of magmatic crystallization, which should be assigned to the Late Triassic rather than to the Palaeozoic or the Middle Triassic in age. Geochemically, the augen granites have the features of the S-type granites, suggesting that they may be originated from the partial melting of the middle and upper crust metasedimentary rocks, and represent the products generated in the postcollisional-extensional tectonic settings during the late stages of the Palaeo-Tethyan evolution in the Lancangjiang tectonic zone.