青海省同仁地区上二叠统石关组上部火山岩的新发现及其地球化学特征和构造环境意义

Discovery and Geochemistry of Upper Permian Volcanic Rocks in Tongren Area, Qinghai Province and Their Tectonic Significance

青海省同仁地区位于秦、祁结合部，构造演化历史悠久．在该区隆务河两侧广泛发育着二叠、三叠纪地层，通过两套地层岩石组合分析，我们认为晚二叠世晚期至早三叠世早期该区属于半深海浊积扇相沉积环境．在该区晚二叠世晚期地层中新发现一套火山岩，火山岩的岩石组合主要为玄武岩、安山岩、玄武安山岩等．根据岩石的常量元素特征，可以将本区火山岩分为两部分：下部岩石的SiO2含量为49．04％～52．43%,，平均为51．28％，TiO2含量为0．67％～1．11％，平均为0．9％，ALK值为2．64％～3.6％,通过常量元素投点图分析，本组火山岩岩性偏基性，属于拉斑玄武岩系列，显示高钛玄武岩特征．上部岩石的SiO2含量为56.25％～60．36％，平均为58．76％，TiO2含量为0．52％～0．63％，平均为0．58％，ALK值在5．26％～5．54％之间．岩性偏中性，属于钙碱性系列，显示低钛玄武岩特征．火山岩的稀土元素也有明显差异，下部稀土总量较低，∑REE=（61．7～97.17）×10^-6，（La／Lu）N比值平均为3．07，轻重稀土分馏不明显，显示与过渡型洋中脊玄武岩相似的特征；而上部稀土总量较高，∑REE=（107．51-143．32）×10^-6，（La／Lu）N比值平均为7.78，轻重稀土分馏明显，稀土分配曲线显示与岛弧安山岩相似的特征．该区火山岩的微量元素比值蛛网图显示与岛弧钙碱性玄武岩-过渡类型玄武岩具有较强的相似性．构造环境判别显示出本区火山岩具有从拉张的过渡型洋中脊环境到闭合的岛弧环境的演化特征．

Tongren area is located in the lower reaches of the the West Qinling and South Qilian orogenic belts. Along the Longwuhe River, east of Qinghai Province, which is between two sides of Longwuhe River broadly develops stratum in Permian and Triassic. Through the analysis of rock assemblages, this area is determined to be turbidite fan facies of hemipelagic from the end of Late Permian to the beginning of Early Triassic. Through the geological research, we found volcanic rocks in the Shiguan Formation of Upper Permian. The volcanic rock assemblage is mainly composed of basalts, basaltic andesites and andesites. According to the characteristics of the ordinary elements, the volcanic rocks can be divided into two parts. The lower part： SiO2 content is 49.04%-52. 43%, with the average of 51.28%; the TiO2 content is 0. 67% 1.11%, with the average of 0. 9% ; and the ALK value is 2.64%-3.6%. The rock type is mainly tholeiitic basalt series, with high titanium quantity. The upper part： SiO2 content is 56. 25%-60. 36%, the average being 58. 76N; The TiO2 content is 0. 52%-0. 63%, the average being 0. 58%; the ALK value is 5.26%-5.54%. The rock type is calc-alkaline series with low titanium quantity. REE distribution patterns of the volcanic rocks are different. The lower part has lower REE, with （La/Lu） N average being 3.07, and its distribution patterns are consistent with those of transitional mid-oceanic ridge basalt. REE is rich in the upper part, especially the LREE. And its distribution patterns are similar to those of island arc andesite. The trace elements have the same characteristics, the distribution type of which is something like that of island arc basalt rock. Tectonic setting distinction reveals that the volcanic rocks of this area were formed in transitional mid-oceanic ridge basalt to island arc tectonic setting.