江苏溧阳火山岩的成因演化及构造环境探讨

THE LIYANG VOLCANIC ROCKS,EASTERN CHINA:THEIR GENESIS,EVOLUTION AND TECTONIC SETTING

江苏溧阳盆地火山岩主要由钙碱性的安山岩、英安岩和流纹岩等组成。岩石以富集钾族元素而贫高场强元素为特征。稀土元素配分型式表现为轻稀土的相对富集,并且Eu负异常。岩相学和主要元素及微量元素的变化都表明,分离结晶是溧阳火山岩岩浆演化的主要机理。笔者应用岩石学混合计算方法和热力学模式定量地模拟了这一过程。根据岩石组合及岩石化学的特点,认为溧阳火山岩是中生代时期中国东部大陆边缘板块俯冲作用的产物。

A Mesozoic volcanic basin is located in Liyang county, Jiangsu Province, East-ern China. Two eruptive cycles have been identified there. The Longwangshancycle consists of hornblende andesite--rhyolite and the Dawangshan cycle consistsof pyroxene andesite-dacite-rhyolite. Petrographic study shows that crystallizationof hornblende preceded that of pyroxene in the volcanic rocks, which may indicatehigher P_(H_2O) and lower f_(O_2) in the early stage of magmatism. These calc--alkaline volcanic rocks are characterized by a low TiO_2 contentand a REE distribution pattern with relative LREE enrichment and Eu negativeanomaly. The K--group element peaks and Ta and Ti valleys exist in the spidergrams of 17 elements, which have been recognized to be characteristic of subduc-tion--related magmas (Thompson et al., 1984). On the basis of the petrographicevidence and variation of major ande trace elements, it has been deduced that thefractional crystallization is a major mechanism of evolution of the Liyang volcanicrocks and that contamination of alkali--rich components might be involved in thelate stage of volcanic activity. The process has been satisfactorily simulated bythe petrological mixing calculation and the thermodynamic modelling (Nathan etal., 1978). Comparison has been carried out in rock associations and malor and traceelements between volcanic rocks from the Liyang basin and those in some typicaltectonic settings in the world. Petrological, mineralogical and trace element featuresall suggest that volcanic activities in the Liyang basin were probably relatedto the subduction of the Kula--Pacific plate at the margin of the East Asiancontinent during the Late Mesozoic.