Carbonatite Genesis: The Example of the Oceanic Nephelinite-Phonolite-Carbonatite Volcanic Suite on Brava, Cape Verde Islands

佛得角布拉瓦岛发现了第四纪榴辉石-白云石-碳酸岩火山碎屑层序。主要的空闪岩的喷发样式与典型的长英质岩浆的喷发样式明显不同，绘制这些矿床的分布图将增加我们对高碱性岩浆喷发行为的认识。phonolite是典型的稀土元素富集岩，与伴生的富稀土碳酸盐火山具有互补关系。最可能的机制是碳酸盐液体与硅酸盐岩浆的不混溶分离，碳酸盐在火山堆中表现为碳酸盐熔岩和凝灰岩。这些空纹石似乎是由橄榄石-云母石-云母石-空纹石这条下降线的分馏结晶产物，但主要的问题仍然存在，特别是关于硫酸盐长石在空纹石成因中的作用。岩石成因建模将基于矿物化学和主要的、微量元素和同位素的测定，这些测定是对经过仔细研究的火山碎屑单元中从新鲜浮石碎屑中分离出来的一套完整的岩石和矿物进行的。在一个暴露良好的第四纪中心，辉白云岩、玄武岩、声母岩和碳酸盐岩火山单元的紧密结合为进一步了解碳酸盐岩岩浆的成因和行为以及它们与伴生的碱性硅酸盐岩浆的关系提供了难得的机会。

The island of Brava, Cape Verdes, eshibits a Quaternary nepheline-phonolite-carbonatite pyroclastic sequence. Eruptive styles of the dominant phonolites depart markedly from those typical of felsic magmas, and mapping the deposits will add to our knowledge of the eruptive behavior of highly alkaline magmas. Phonolites are typically strongly depleted in rare-earth elements, and bear a complementary relationship to associated rare-earth-enriched carbonatite volcanics. The most probable mechanism for this is the immiscible separation of a carbonatite liquid from silicate magma, the carbonate being represented in the volcanic pile as carbonatite lavas and tuffs. The phonolites appear to be products of fractional crystallization via the line of descent, olivine nephelinite - nephelinine - phonolite, but major questions remain, especially regarding the role of the sulphate feldspathoid nosean in phonolite geneses. Petrogenetic modelling will be based on mineral chemistry and major, trace element, and isotopic determinations of a suite of whole rocks and mineral separates from fresh pumice clasts in carefully studied pyroclastic units. The close association of nephelinitic, basanitic, phonolitic and carbonatitic volcanic units at a well-exposed Quaternary center provides an unusual opportunity to advance understanding of the genesis and behavior of carbonatite magmas, and their relationship to associated alkaline silicate magmas.