新疆东准噶尔晚石炭世六棵树组火山机构类型与时限

研究区位于新疆东准噶尔卡拉麦里大断裂以南地区, 晚石炭世中-晚期大地构造活动增强, 发生海、陆交互相的火山喷发, 尤其以晚石炭世六棵树组发育最好, 保存状态最佳。在依孜恩德阿德尔一带, 发育串珠状火山口, 野外共识别出3个火山口。针对保存最好的“1”号火山口测制“十”字型剖面, 结合spot5影像、岩石化学特征, 该旋回火山活动多属中心式喷发, 同时兼有裂隙式多中心喷发, 喷发方式以喷溢式为主;剖面上为一漏斗状, 四周岩层倾向中心, 由外到内, 倾角逐渐变缓, 为典型“岩蕈”构造;中心为火山颈相的霏细斑岩, 内环为大量安山质角砾凝灰岩、流纹质角砾凝灰岩、安山质熔结凝灰岩、安山质火山尘凝灰岩、火山角砾岩等充填, 外环以角砾凝灰岩、火山角砾岩、角砾安山岩不整合于晚石炭世石钱滩组之上;对火山机构中玄武岩进行了锆石SHRIMP U-Pb测年, 得到加权平均年龄为(305±21)Ma;下伏石钱滩组中产晚石炭世(宾夕法尼亚纪)浅海相动物化石, 上覆孔雀屏组中产晚石炭世陆相植物化石, 进一步将该火山机构喷发时限限定为晚石炭世卡西莫夫期。

Types and Time of Late Carboniferous Liukeshu Formation Volcanic Edifices in Eastern Junggar,Xinjiang

The study area is located south of Kalamaili great fault, east Junggar. Geotectonic environment activities were very strong in Kasimovian-Gzhelian, while volcanoes interacted by marine and continental facies erupted frequently, especially in Liukeshu formation. Three craters which combined as beaded were recognized in Yiziende'ader area. X-shaped profile of the best preserved No.1 crater was measured; combined with spot5 images and rock chemical characteristics, the activities of the polycyclic volcano are mostly central eruption, with both multi-center fissure eruption; the eruption manner are mainly effusive. The profile is a typical rock mushroom structure; the surrounding rocks centered to the funnel-shaped middle from outside to inside with the dip angle gradually slowing down. The center is felsitic volcanic porphyry neck; inner ring is a large number of andesitic tuff breccia, rhyolitic tuff breccia, andesitic ignimbrite, andesitic volcanic tuff dust and volcanic breccia filling; for the outer ring, brecciated tuff, volcanic breccia, brecciated andesite unconformably overlies the late Carboniferous Shiqiantan formation. Zircon SHRIMP U-Pb dating of volcanic bodies basalts shows a weighted average age of 305±21 Ma. Shallow marine animal fossils in Moscovian are rich in Shiqiantan formation of the underlying layer, while continental plant fossils in Gzhelian are rich in Kongqueping formation of the upper layer. In view of the above, the age of the volcanic eruption mechanism is limited to Kasimov of late Carboniferous.