Abstract

A sill of murmanite-lorenzenite tinguaite is exposed on the northern slope of Mt. Selsurt, Lovozero alkaline complex (Kola Peninsula). The sill emplaced into the differentiated series represents a hypabyssal facies of murmanite lujavrites, one of the latest intrusive phases at Lovozero. The tinguaite has a pseudo-porphyritic texture resulting from large oikocrysts of Na-Ti(Zr) silicates set in a matrix of microcline, nepheline, magnesio-arfvedsonite and aegirine. In common with other agpaitic rocks at Lovozero, the tinguaite at Mt. Selsurt is enriched in Th and U. In this rock, radioactive minerals are represented by pyrochlore-group phases and huttonite. Thorian pyrochlore (10-12 wt.% ThO 2 ) is one of the earliest phases and found only locally as inclusions in the groundmass silicates. However, during the early crystallization stage, most of Th was retained in the melt, and subsequently concentrated in a residual fluid enriched in alkalies, Ti, Zr and Nb. During the late crystallization stage, this fluid precipitated the oikocryst-suite minerals and huttonite (essentially ThSiO 4 ). In contrast to Th, most of U was incorporated in uranoan pyrochlore (10.4-19.8 wt.% UO 2 ) early in the crystallization history. Subsequent alteration of this mineral, involving cation leaching and hydration, resulted in conversion of uranoan pyrochlore to uranpyrochlore. The above example illustrates different behavior of Th and U during the evolution of agpaitic alkaline magmas.

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