天山北部上石炭统前峡组火山岩地球化学特征及构造意义

上石炭统前峡组火山岩位于早石炭世巴音沟蛇绿岩北侧,为一套以火山碎屑岩为主夹熔岩和陆源碎屑岩建造。岩石及地球化学特征显示,该组火山岩是以安山岩为主的玄武岩+玄武安山岩+安山岩+英安岩组合,里特曼指数介于1.20～3.43,属钙碱性系列。稀土总量(ΣREE)为69.53×10-6～111.32×10-6,(Ce/Y)N=1.12～2.25,(La/Yb)N=1.86～5.39,δEu=0.79～1.21,为轻稀土略富集、Eu亏损不明显的火山岩类型。岩石显著富集大离子亲石元素K、Rb、Ba、Th,亏损高场强元素Nb、Ta、Hf、Ti、Y,其中Nb含量绝大多数介于1.89×10-6～2.65×10-6,与大洋地壳Nb含量平均值2.2×10-6接近。锶初始比值为0.70425～0.70428,与岛弧区玄武岩的0.70437接近,这些特征均与俯冲带大陆岛弧火山岩一致。与之伴生的滨浅海-半深海相沉积地层和南部早石炭世巴音沟蛇绿岩的存在,表明前峡组火山岩是巴音沟蛇绿岩所代表的早石炭世洋壳在晚石炭世时期向北俯冲消减的产物。

Geochemistry of Upper Carboniferous Volcanics of Qianxia Formation in North Tianshan Mountains and Its Tectonic Significance

The Upper Carboniferous volcanics of Qianxia Formation are distributed at the north side of lower carboniferous ophiolite near Bayingou,which are mainly composed of pyroclastic rocks and interbeded lava and terrestrial clastic sedimentary rocks.According to the lithology and geochemistry,the volcanics are mainly andesite suite including trachyandesite,basaltic andesite,andesite and dacite,its Rittman index σ=1.20-3.43,attached to calc-alkaline,and ΣREE=69.53×10-6-111.32×10-6,ΣCe/ΣY=1.12-2.25,（La/Yb）N=1.86-5.39,δEu=0.79-1.21,these data show that LREE are concentrated and europium depletion is not obvious.Moreover li elements such as K,Rb,Ba,and Th are rich in the rocks but the high field strength element（HFSE） such as Nb,Ta,Hf,Ti,and Y are depleted,of which the Nb content is almost 1.89×10-6-2.65×10-6,similar to the average value of Nb in oceanic crust which is 2.2×10-6.The initial ratio of Sr is up to 0.704 15-0.704 19,almost equal to 0.704 37 belonging to basalt in the island-arc province,these features are associated with continental island-arc volcanics near benioff zone.In addition the subtidal-bathyal facies sedimentary and lower carboniferous ophiolite near Bayingou exist at the same period,all have been confirmed that the Qianxia Formation volcanics are products generated from lower carboniferous ophiolite near Bayingou when the oceanic crust are subducted into the continental mass northward in late carboniferous age in the convergence zone.