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

凡尔德斯角的布拉瓦岛隐伏着一个第四纪霞石-响岩-碳酸盐火山碎屑序列。占主导地位的声石的喷发风格明显不同于长英质岩浆的典型喷发风格，绘制矿床图将增加我们对高碱性岩浆喷发行为的了解。白云石通常强烈亏损稀土元素，并与伴生的富含稀土的碳酸盐岩火山岩具有互补关系。最可能的机制是碳酸盐液体与硅酸盐岩浆不相容地分离，碳酸盐在火山堆中表现为碳酸盐岩熔岩和凝灰岩。声石似乎是通过下降线分离结晶的产物，橄榄石霞石-霞石-声石，但主要问题仍然存在，特别是关于硫酸盐长石鼻石在声岩成因中的作用。岩石成因模拟将基于矿物化学和从仔细研究的火山碎屑单元中的新鲜浮石碎屑中分离出的一套完整岩石和矿物的主元素、微量元素和同位素测定。在暴露良好的第四纪中心，霞石、玄武岩、声橄榄岩和碳酸盐岩火山单元的紧密组合为进一步了解碳酸岩岩浆的成因和行为及其与伴生的碱性硅酸盐岩浆的关系提供了不同寻常的机会。

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.