溧阳戴埠火山岩开放体系岩浆活动探讨

AN APPROACH TO THE OPEN SYSTEM MAGMATIC ACTIVITY IN DAIBU VOLCANIC ROCKS,LIYANG

戴埠下火山岩系层序—岩相—成分演化之间关系研究表明，火山产物是从十个喷发中心经深部岩浆脉动式补充、快速分异、间歇式喷发所形成。第一和第二旋回之间具沉积间断，而且喷发中心迁移与新生岩浆补充相对应。在两个旋回内部，中性和酸性喷发产物都具宽的或较宽的成分范围和快速分异特点。中性溢流相之后，从中酸性火山屑碎流相至酸性溢流相或侵出相的喷发强度降低，是岩浆房萃取深度降低的结果。分离结晶和同化作用是岩浆演化的主要原因。

The Mesozoic volcanic basin in Liyang county, Jiangsu Province is composed of andesite, trachyandsite, dacite and rhyolite of high potassium cafe-alkaline serices. These rocks may be divided into three eruptive (intrusive) cycles, nine active stages and six factes. The lower volcanic sequence is located in Daibu volcanic-tectonic depression, and is produced seperately by the pulsative replenish of intermidiate parental magma, rapid differentiation and intermittent eruption from ten volcanic organs (i. e. eight calderas and two reactive domes). There is a sedimentary intermittence between Ⅰ and Ⅱcycle, and the eruptive center are occurred transmigration toward the center of basin. The each cycle shows the evolutional regularity from intermediate to acid. There is a brife intermitterce after the intermidiate eruption, soon afterwards acidic magma erupts. The acidic products always transfer from petroclastic factes to effusive factes or extrusive factes, and eruptive strength progressively decreases. The diorite-quartz monzoodiorite-granodiorite of intrusive facies in Ⅱ cycle represents the base of shallow magma chamber. The geochemical characteristics of rocks show that various rock types and various factes have the characteristics of some souces, and fractional crystallization and assimilation are main mechanism of magmatic evolution in the region. The eruptive products both the intermidiate and acid show the compositional ranges, therefore these rocks posses the rapidly differential features. The high silicon rocks with higher ratio of the crustal hybridism in the late effussion of each cycle represent the top of shallow magma chamber. This is the consequence of decrease of eruptive rate resulted from vent migration from caldera to reactive dome, as well as the decrease of magma extractive depth.