滇西云县地区中二叠世火山岩的发现及其对昌宁-孟连古特提斯洋俯冲作用的约束

Discovery of the Middle Permian volcanic rocks in the Yunxian area, western Yunnan and constraints on subduction of the Changning-Menglian Paleo-Tethys Ocean

多年来,对滇西云县-南涧一带广泛出露的中晚三叠世火山岩的形成时代及大地构造属性一直存在争议。出露于云县漫湾地区的原划中三叠统忙怀组中新发现有厚数米的英安岩、英安质凝灰岩夹流纹岩、凝灰岩组合。本次研究采集英安岩D0001进行锆石U-Pb LA-ICP-MS定年,获得262.5±3.1Ma的岩浆结晶年龄和447.4±9.1Ma的继承性年龄,时代厘定为中二叠世晚期,而非前人认为的中三叠世。对岩石组合、同位素测年及地球化学的详细研究表明,这套火山岩属石炭-二叠系龙洞河组,为一套中酸性火山岩组合;SiO_(2)含量68.14%-74.62%,Eu负异常,在TAS图解中全部落入英安岩-流纹岩区;微量元素蛛网图明显富集Rb、Th、Ce等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,具典型火山岛弧特征,是目前临沧岩浆弧发现的最为年轻的岛弧型酸性火山岩记录,其岩浆源区为受壳源物质混染的下地壳镁铁质岩石部分熔融;应为早中二叠世昌宁-孟连洋向东俯冲的产物。本次的发现,为西南三江地区昌宁-孟连洋向东俯冲提供有利的岩石学记录及时代约束,也为石炭-二叠系龙洞河组北延提供有利支撑,澜沧江断裂以西区域前人划分的忙怀组的分布范围和规模需要重估。

For many years, there have been controversies over the formation time and tectonic attributes of the volcanic rocks of the late Triassic that are widely exposed from Yunxian to Nanjian, western Yunnan. A few meters of thick dacite, dacite tuff intercalated with rhyolite and tuff were newly discovered in the original Middle Triassic Manghuai Formation in the Manwan area of Yunxian County. In this study, the zircon U-Pb LA-ICP-MS dating was carried out on D0001 andesite, and the magmatic crystallization age of 262.5±3.1 Ma and the inherited age of 447.4±9.1 Ma were obtained. The age was determined to be late Middle Permian, not the Middle Triassic as considered by predecessors. The detailed study of rock assemblage isotopic dating and geochemistry shows that the volcanic rocks belong to the Carboniferous-Permian Longdonghe Formation and are a set of intermediate-acid volcanic rock assemblages. The content of SiO_(2)(68.14%-74.62%) has weak deficit in varying degrees and falls into the dacite-rhyolite area in the TAS diagram. The trace element cobwebs are obviously enriched in Rb, Th, Ce and other large ion lithophile elements and depleted in high field strength elements such as Nb, Ta, Ti, which are characterized by volcanic island arc.It is the youngest island-arc acid volcanic record within the Lincang magmatic arc. The magma source area is the partial melting of mafic rocks in the lower crust mixed with crust source materials. It should be the product of the eastward subduction of the early and middle Permian Changning-Menglian Ocean. This discovery provides favorable petrological records and chronological constraints for the eastward subduction of the Changning-Menglian Ocean in the Sanjiang area in the southwest, and also provides favorable support for the north extension of the Carboniferous-Permian Longdonghe Formation. The distribution and scale of the Manghuai Formation needs to be reassessed.