甘肃北山碱泉子火山岩地球化学特征及地质意义

甘肃北山碱泉子地区志留系公婆泉群岩石组合为石英杂砂岩、含砾粗砂岩夹薄层灰岩、凝灰质砂岩、粉砂岩、砂质板岩和少量火山岩,粉砂岩中局部具鲍马层序。火山岩岩石化学分析数据显示,其主量、微量元素特征与岛弧火山岩地球化学特征有明显差异,说明其不是岛弧环境的产物。前人认为结晶灰岩中产出的珊瑚化石是Altaja,将时代定为志留纪。此次工作重新采集鉴定为Dohophyllum reticulatum Fomitchev(网状顿河珊瑚),? Paracarruthersella sp.(? 拟卡鲁特珊瑚)并鉴定出有孔虫,时代应为晚石炭世。因此,结合化石时代和火山岩的地球化学特征,认为整个北山地区,红柳河-牛圈子-洗肠井蛇绿岩带以南没有岛弧火山岩的出露,预示着该地区古生代构造演化过程中不存在有向南俯冲的火山岩证据,这对完善北山古生代构造演化格架具有重要的指示意义。 

Geochemistry of Jianquanzi Volcanic Rocks in Beishan Area,Gansu Province

The Jianquanzi area of Beishan in Gansu Province is designated as Silurian Gongpoquan Group originally.Lithology combination includes quartz greywacke,conglomeratic gritstone interbedded thin-layer limestone,siltstone with Bouma sequenced,and volcanic rock.Chemical analysis data of volcanic rock show that the major elements and microelements are very different from island-arc volcanic rocks,it indicates that they are not derived from the island-arc environment.Crystalline limestone produces the corals fossil(designated as the Silurian Altaja).This study collects data and redefines the volcanic rock as Dohophyllum reticulatum Fomitchev,? Paracarruthersella sp.and foraminifer,and age is Late Carboniferous.The age of fossil and geochemical characteristics of volcanic rock reveal that there is no island arc volcanic rocks exposed on the south side of the Hongliuhe-Niujuanzi-Xichangjing ophiolite belt in the entire Beishan region.In conclusion,there is no volcanic evidence of the southward subduction in the Paleozoic tectonic evolution in the study region,which has an important foundation of improving the framework of Beishan Paleozoic tectonic evolution.