东昆仑大灶火沟—万宝沟晚二叠世陆缘弧火山岩的发现及意义

The discovery of Dazaohuogou-Wanbaogou Late Permian epicontinental arc volcanic rocks in Eastern Kunlun Mountains and its significance

东昆仑地区物质组成复杂、构造演化历史悠久,现保存的建造物质记录了从古元古代以来该区发生的地质事件,在多次洋陆转化中中泥盆世—中三叠世造山阶段尤为重要,同时也是该区岩浆活动和成矿作用的主要时期之一,长期以来缺失二叠纪火山岩记录。最近开展的1∶25万大灶火幅区调工作新发现一套中酸性高钾钙碱性系列火山岩组合,用LA-ICP-MS技术测得次流纹英安岩锆石U-Pb年龄为254.7±0.6Ma。根据剖面研究和区域对比,将这套陆缘弧环境下喷发的火山岩命名为晚二叠世大灶火沟组,主要分布在大灶火沟—万保沟一带,地层严格受昆南断裂控制,出露面积大于74km^2。这一发现不仅为东昆仑岩浆弧长期争论的是否存在二叠纪火山岩提供了明确的证据,而且填补了东昆仑地区晚二叠世火山岩地层的空白。东昆仑地区从二叠纪开始昆南洋向北俯冲,形成了规模巨大的碰撞型花岗岩,大灶火沟组火山岩的形成标志着晚二叠世东昆仑处于区域性挤压阶段。

The material composition in Eastern Kunlun area is complex with long tectonic evolution history. The building materials have been preserved to record geological events that have occurred since the Paleoproterozoic. The orogenetic phase from Middle Devonian to Middle Triassic was particularly important in many ocean-land transformations, thus being a major period of magmatism and mineralization in the Eastern Kunlun area. For a long time, records of Permian volcanic rocks in the Eastern Kunlun area were lacking. 1∶250000 regional geological survey was carried out recently in Dazaohuo area and, as a result, a volcanic rock group of inter-mediate-acid, high potassium, calc-alkaline series was discovered, and U-Pb age of zircon in the subrhyolite-dacite was obtained through LA-ICP-MS, which is 254.7 ± 0.6Ma. The eruption of volcanic rocks under the continental margin arc environment was newly named Late Permian Dazaohuogou Formation to accord with section research and regional correlation. The Dazaohuogou For-mation mainly located in the Dazaohuogou-Wanbaogou area is strictly controlled by southern Kunlun fracture, with the outcropping area more than 74km2. This discovery not only provides definite evidence for the existence or nonexistence of Permian volcanic rocks in the Eastern Kunlun magmatic arc but also fills the gaps in Late Permian volcanic strata. The northward subduction of Kunnan Ocean started from Permian in Eastern Kunlun area, and formed huge collision-type granites. Volcanic rocks of the Dazaohuogou＆nbsp;Formation imply that East Kunlun area was still in the regional extrusion phase in Late Permian.