On the Genesis of Lestivarites from Lestivara (Khibiny)

Abstract

The collected material proves with sufficient clarity the widespread development of metasomatic processes at the contacts of large pegmatite veins. The width of the alteration zones and the intensity of the process are directly dependent on the thickness of the vein, and therefore on the reserves of components moving into the host rock. At the contacts of small veins or at some distance from them we encounter light‑colored, granite‑like rocks, so to speak, that have not yet reached the stage of typical lestivarite. Changes in them consist of greater or lesser albitization of the rock, a decrease or disappearance of quartz, and the appearance of alkali bisilicates. The original minerals – orthoclase, biotite and oligoclase – are present in them in greater or lesser quantities. The process of formation of lestivarites at Lestivar is thus a process of alkaline metasomatism with the introduction of alkalis (Na) and alumina, and in the presence of volatiles, mainly water. Owing to these and to the initial composition of the rock, intense albitization occurs, the formation of alkali amphibole further from the contact and aegirine closer to the contact; finally, apatite, eudialyte, and sphene appear near the contact. When the rock is completely replaced, macroscopically white and sugary rocks are formed, often found at Lestivar and in other places of the external contact within fields of migmatites intruded by alkaline pegmatite veins. This metasomatic process is similar to the phenomena of alkaline metasomatism described by Goldschmidt, Brøgger and others in the contact zones of alkaline intrusions.

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