浙江平水群细碧角斑岩形成环境及其物质来源探讨

THE FORMATION ENVIRONMENT AND MATERIAL SOURCE OF SPILITE-KERATOPHYRES OF THE PINGSHUI GROUP IN ZHEJIANG

平水群细碧角斑岩建造,出露于江山—绍兴断裂带北东端。通过对该群岩石学、岩石地球化学及同位素地质学的系统研究,认为细碧角斑岩系来自上地幔的富钠质岩浆,且以钙碱性系列为主。它生成于晚元古代早期的岛弧环境,属于古扬子板块东南缘陆壳增生的产物。平水群物质来源和组成为:上地幔成分占95.36％,上、下地壳成分混杂部分占4.64％。根据K.C.康迪(1973)计算公式求得当时地壳厚度为23.2～29km,应属于过渡型陆壳性质,岩浆来源深度大约97.3km。此外,还查明在晚元古代时期,江山—绍兴断裂带为一条会聚型板块边界,并非是华南裂谷系的主裂谷槽。

The spilite-keratophyres of the Pingshui Group occur at the northeastern end of the Jiangshan-Shaoxing fault. Systematic studies of the petrology, petrogeochemistry and isotope geology show that the spilite-keratophyre sequence of the group, composed mainly of calc-alkaline series, was derived from the sodium-rich magma in the upper mantle.It originated in an islandarc environment in the early Late Proterozoic from continental crustal accretion on the southeastern margin of the Yangtze paleoplate. The material source and composition of the Pingshui Group are as follows: the upper mantle materials account for about 95.36% and the mixed upper and lower crustal materials about 4.64%. The sequence belongs to transitional continental crust. The thickness at that time is estimated at 23.2—29 km. The depth of the source of magma is about 97.3 km. It has been also ascertained that the Jiangshan-Shaoxing fault was a converging plate boundary in the Late Proterozoic rather than a main rift trough of the South China rift system,