What is an alkaline complex in geology ?

Alkaline complex : definition

An alkaline complex is a set of magmatic rocks, usually plutonic, associating several types of undersaturated rocks rich in alkalis placed in anorogenic zones (unrelated to the process of formation of terrestrial reliefs). We note for example : Khibina and Lovozero (Kola Peninsula, Russia), Ilimaussaq (Greenland), Phalaborwa (South Africa), Jacupiranga (Brazil), etc...

The alkaline complexes frequently adopt a zoned structure which results from the interlocking of different intrusions : carbonatites, nepheline syenites, ijolites. They frequently include rocks and original varieties, sometimes unique in the world (khibinite from Khibiny, lujavrite, kakortokite and naujasakite from Ilimaussaq).

The particular chemistry of these alkaline complexes leads to the appearance of a very original mineralogy, dominated by feldspathoids (nepheline, noséane, etc.), alkaline pyroxenes and alkaline amphiboles (aegyrine, riébeckite, etc.). They commonly include large amounts of rare or scarce minerals in other magmatic groups : zircons, rare earth minerals, Nb, Ta, Ti, apatite, etc...

These alkaline complexes are frequently exploited deposits for these rare metals and sometimes for apatite (Kola deposit, in Russia), which is then the source of the phosphorus necessary for the preparation of fertilizers.