青海赛什塘矿区变质岩特征及其形成构造环境

赛什塘矿区地层为一套滨-浅海相碎屑岩、泥质岩夹碳酸盐岩,普遍遭受浅变质作用,各类岩石稀土元素配分曲线相似,均为轻稀土富集型,具中等铕负异常,具有一致性。岩相学及La/Yb—∑REE图解判别表明:矿区变质岩原岩属沉积岩类,以砂岩和杂砂岩为主,未显示与火山成因有关;前人所述"硅质岩"并不具备典型硅质岩的特征,其原岩应为石英砂岩类,矿区未见明显的热水沉积作用。应用w(Th)—w(Co)—w(Zr)构造背景判别图解以及与杂砂岩稀土元素特征值和典型构造背景砂岩配分模式对比等方法进行矿区构造环境分析,认为矿区形成环境同时具有活动大陆边缘型和大陆岛弧型的特点,推断其形成于三叠纪古特提斯洋陆缘的岛弧-弧后盆地环境。

Characteristics of metamorphic rocks and tectonic setting in Saishitang mining area, Qinghai Province, China

The sedimentary sequence of Saishitang district is composed of offshore-neritic clastic rocks and argillaceous rocks with carbonate streaks,which are commonly metamorphosed weakly,with similar REE patterns enriched in LREE and moderate Eu negative anomaly.The lithofacies features and La/Yb—∑REE diagram indicate that the protoliths of metamorphite are sedimentary rocks,with main types of sandstone and greywacke and unrelated with volcanic process;the ‘silicalite’ defined previously does not have the typical characteristics of silicalite,whereas the protolith of this ‘silicalite’ should be quartz sandstone,and no obvious evidence of hot water deposition was found in the district.According to the w(Th)—w(Co)—w(Zr) diagram and correlation between greywacke REE features and sandstone REE distribution patterns of typical tectonic settings,the tectonic environment of the district displays both active continental marginal and continental island arc characteristics,which indicates that the Saishitang district is developed in arc-back basin located in margin of the Triassic Paleotethys.