| Summary: | 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.
|
|---|
