Radioactive minerals from murmanite-lorenzenite tinguaite at Mt. Selsurt, Lovozero complex, Kola Peninsula

A sill of murmanite-lorenzenite tinguaite is exposed on the northern slope of Mt. Selsurt, Lovozero alkaline complex (Kola Peninsula). The sill emplac ed into the differentiated series represents a hypabyssal facies of murmani te lujavrites, one of the latest intrusive phases at Lovozero. The tinguait e has a pseudo-porphyritic texture resulting from large oikocrysts of Na-Ti (Zr) silicates set in a matrix of microcline, nepheline, magnesio-arfvedsen ite and aegirine. In common with other agpaitic rocks at Lovozero, the ting uaite at hit. Selsurt is enriched in Tn and U. In this rock, radioactive mi nerals are represented by pyrochlore-group phases and huttonite. Thorian py rochlore (10-12 wt.% ThO2) is one of the earliest phases and found only loc ally as inclusions in the groundmass silicates. However, during the early c rystallization stage, most of Th was retained in the melt, and subsequently concentrated in a residual fluid enriched in alkalies, Ti, Zr and Nb. Duri ng the late crystallization stage, this fluid precipitated the oikocryst-su ite minerals and huttonite (essentially ThSiO4). In contrast to Th, most of U was incorporated in uranoan pyrochlore (10.4-19.8 wt.% UO2) 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 Tn and U during the evolution of agpaitic alkaline magmas.