LANTHANIDES AS INDICATORS IN INTERPRETING THE ORIGIN OF ALBITITE FROM THE OUTER CONTACT OF A RAPAKIVI PLUTON

Abstract

At the nearest outer contact of a rapakivi pluton in one area of a crystalline shield are albitites containing either diopside and amphibole, or biotite and with a mosaic-tabular recrystallization microtexture. Interpretations of the origin and, consequently, the petrographic association of contact albitites are inconsistent. The geologic and structural position of both metasomatite types, as well as the fact that they grade into each other in terms of mineral composition and microtexture, allowed us earlier to assume that the fault-associated uraniferous albitite was converted by processes of contact and thermal alteration to albitite with a mosaic-tabular recrystallization texture. This hypothesis is confirmed by the distribution pattern of rare-earth elements (REE) and yttrium in accessory apatities from a suite of samples forming a series transitional between the two rock types. Given its geologic position, we can classify the contact albitite as a product of recrystallization of fault zone alkali carbonate metasomatites with a blastocataclastic microtexture located around the pluton. The finding of recrystallized (contact-metamorphosed) albitite indicates that potentially ore-bearing carbonatealkali metasomaties, associated with a deep fault zone, pinch out, and that they were formed before the rapakivi was intruded.