碧口群形成的地质构造环境探讨

碧口群是一套浅变质的中晚元古代火山－沉积岩建造，主要分布于川陕甘三省交界地区。其中火山岩主要由细碧－石英角斑岩系列火山岩组成，还有少量的玄武－流纹岩系列火山岩和富钾流纹岩。下亚群以酸性火山岩为主，上亚群既有基性火山岩，又有酸性火山岩，中性火山岩出露较少。火山财变质很浅，基本保持原岩的结构和化学组成。岩石化学、微量元素和稀土元素等一系列图解，都一致判别为大肆秒火山岩主要是钙碱系列，也有少量的拉斑系列

GEOTECTONIC ENVIRONMENT OF FORMATION OF THE BIKOU GROUP AS VIEWED FROM THEIR GEOCHEMICAL CHARACTERS

The Bikou Group strata belong to a suite of Middle Late Proterozoic volcano sedimentary rocks of light metamorphism and great thickness which are widely distributed in the juncture area of Shaanxi, Gansu and Sichuan provinces. The volcanic rocks are mainly associated with spilite quartz keratophyre series, and a few volcanic rocks of the basalt rhyrolite series and potassic rhyrolite. There are mainly acid volcanic rocks at the lower subgroup, basic volcanic rocks at the middle subgroup, and there are not only basic volcanic rocks, but also are acid volcanic rocks at the upper subgroup. There are a few intermediate volcanic rocks in the Bikou Group strata. Petrochemical, microelement, rare earth element data and the character of bimodal volcanic rocks show that the Bikou Group strata were formed in an island arc rift valley geotectonic environment.