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

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,