地壳深俯冲与富钾火山岩成因

富钾火山岩是一类兼具壳幔双重地球化学特征的特殊岩石组合 ,它们不可能由亏损或原始地幔所派生 ,成岩过程中必须有地壳物质的参与 ,将地壳物质引入富钾火山岩成岩过程的主要动力机制即是深俯冲作用。洋壳和陆壳均可以通过俯冲进入地幔 ,俯冲地壳物质析出流体对地幔岩石的交代作用是导致富钾火山岩具特殊地球化学特征的主要原因。根据对大别—苏鲁造山带南北两侧晚中生代富钾火山岩的实例研究 ,表明该区火山岩的形成均受到了俯冲洋壳析出流体的交代作用 ,但造山带北侧富钾火山岩的形成还叠加了俯冲的扬子陆壳析出流体的交代作用 ,是多次富集事件综合作用的结果。文中还对富钾火山岩成因研究中值得进一步深入探索的问题进行了讨论。

DEEP SUBDUCTION OF CRUST MATERIALS AND ITS IMPLICATION TO THE GENESIS OF POTASH-RICH VOLCANIC ROCKS

Potash-rich volcanic rocks are a unique igneous rock association, which show both crust and mantle geochemical features. This type of rocks can not be produced through partial melting of depleted or primary mantle. Crustal materials must have contributed to the formation of these rocks, and deep subduction of crustal materials is thought to be the main dynamic mechanism. Both oceanic and continental crust can be subducted into the deep mantle, and mantle metasomatism caused by fluids expelled from dehydration of subducted crustal materials is the most likely process that produced the peculiar geochemical features. Based on a case study of the late Mesozoic potash-rich volcanic rocks along both north and south sides of the Dabie-Sulu orogenic belt, we suggest that the formation of these rocks is related to mantle metasomatism by fluids from dehydration of subducted oceanic crust materials. However, potash-rich volcanic rocks in the north side of the Dabie-Sulu orogenic belt had also been superimposed by the involvement of subducted Yangtze continental crust materials, i.e., their source region had been affected by multiple enriched events. Besides, further research directions for petrogenetic study of potash-rich volcanic rocks have also been discussed in this paper.