新疆阿克纳瓦山晚泥盆世火山岩及其构造环境

Properties and Tectonic Environments of Late Devonian Volcanic Rocks in Akenawa Hills, Xinjiang

新疆北部阿克纳瓦山晚泥盆世中基性火山岩以全碱含量较高为其最突出的特点。岩相学、矿物学、岩石化学、稀土元素和微量元素地球化学的研究表明,火山岩属于碱性玄武岩系列,以碱性玄武岩组岩石为主,同时有少量橄榄玄粗岩组岩石,表现为碱性玄武岩演化的肯尼迪趋势。火山岩是地幔部分熔融作用的产物,但在岩浆上升过程中发生过橄榄石和(或)辉石的分离结晶作用。火山岩产出的构造环境为具有陆壳基底的岛弧或活动大陆边缘火山弧,不可能是洋中脊。岛弧或活动大陆边缘火山弧的产生是以阿尔曼泰—扎河坝蛇绿混杂岩带为代表的次生洋盆,自晚泥盆世起向北东方向(现代方向)俯冲作用的结果。

The Late Devonian volcanic rocks of Akenawa Hills in northern Xin--jiang, as a suite of intermediate and basic rocks, are characterized by hightotal alkali content which ranges from 3.55 to 7.55%. On the basis of theirpetrology, mineralogy, petrochemistry and REE as well as trace element geoche-mistry, it is believed that these volcanic rocks are of alkaline basaltic seriesdominated by rocks of alkaline basalt group with a small part belonging toshoshonite group, exhibiting a Kenedy trend of differentiation in alkalinebasalts. The volcanic rocks might be products of partial melting of the mantle;nevertheless, fractionation and crystallization of olivine might have occurred inmagma during the upward migration of magma through the crust. The tectonicenvironment of the volcanic rocks was probably an island arc or a volcanicarc along the active continental margin underlain by continental crust basementinstead of a mid--oceanic ridge. The formation of an island arc or a volcanicarc in the active continental margin seems to have been related to Late Devo-nian northeastward subduction of the secondary coeanic crust formed duringEarly and Middle Devonian when the studied area was in extention and nowrepresented by Aermantai--Zhaheba Ophiolitic melange zone.