REE-bearing minerals in the Rapakavi Grante, Southern Finland

Abstract

Abstract The studied rock samples belong to the classic1615–1645 Ma Wiborg Rapakivi granite terrane of southeasternFinland. Geochemical studies show that the rapakivigranites and associated rocks form metaluminousto peraluminous A-type granites and plot in the "withinplate granites (WPG)" field on the tectonic discriminate diagramsfrom the Onkamaa, Suomenniemi and Luumäki.The rapakivi granite displays enrichment of light overheavy REE (LREE/HREE = 2-34) and usually negative Euanomalies (Eu/Eu* = 0.01 - 1.4). Enrichment in REE in somestudied samples is confined to highly fractionated portionsof the Rapakivi granite. Fractional crystallization ofthe evolving fluorite-rich peraluminous granitic magmawas accompanied, particularly at later stages by fluid fractionation,which plays an important role in the genesis ofthe REE-mineralization.The studied rapakivi granites host REE-minerals includingmonazite-(Ce), allanite (Ce), bastnäsite (Ce), xenotime,thorite and REE-bearing mineral apatite. Monazite and allaniteare the most important REE carriers in the studiedgranites and these minerals are strongly enriched in theLREE.Monazites are hosted in apatite, quartz, plagioclase,K-feldspar, and biotite. Grain size of monazite is variableranging from 50 to >100 μm.Monazite contains 48-68 wt%REE2O3, 24.3-29.3 wt% P2O5 and low Th<1.5 wt%ThO2. TheY, REE, U, Th-bearing minerals are not commonly associatedwith the primary minerals except for Th-bearingminerals, which occur as silicates (e.g. thorite, ThSiO4);and/or replace other elements in the structure of some accessorymineral, especially xenotime, brabantite, zircon,and apatite.Electron probe microanalysis (EPMA) provides an indicationof solid solution series between thorite-xenotimezircon,which are related to hydrothermal solutions enrichedin REE, Y, P, U, F, and Zr.