AbstractThe studied rock samples belong to the classic 1615–1645 Ma Wiborg Rapakivi granite terrane of southeastern Finland. Geochemical studies show that the rapakivi granites and associated rocks form metaluminous to peraluminous A-type granites and plot in the "within plate granites (WPG)" field on the tectonic discriminate diagrams from the Onkamaa, Suomenniemi and Luumäki. The rapakivi granite displays enrichment of light over heavy REE (LREE/HREE = 2-34) and usually negative Eu anomalies (Eu/Eu* = 0.01 - 1.4). Enrichment in REE in some studied samples is confined to highly fractionated portions of the Rapakivi granite. Fractional crystallization of the evolving fluorite-rich peraluminous granitic magma was accompanied, particularly at later stages by fluid fractionation, which plays an important role in the genesis of the REE-mineralization. The studied rapakivi granites host REE-minerals including monazite-(Ce), allanite (Ce), bastnäsite (Ce), xenotime, thorite and REE-bearing mineral apatite. Monazite and allanite are the most important REE carriers in the studied granites and these minerals are strongly enriched in the LREE.Monazites are hosted in apatite, quartz, plagioclase, K-feldspar, and biotite. Grain size of monazite is variable ranging from 50 to >100 μm.Monazite contains 48-68 wt% REE2O3, 24.3-29.3 wt% P2O5 and low Th<1.5 wt%ThO2. The Y, REE, U, Th-bearing minerals are not commonly associated with the primary minerals except for Th-bearing minerals, which occur as silicates (e.g. thorite, ThSiO4); and/or replace other elements in the structure of some accessory mineral, especially xenotime, brabantite, zircon, and apatite. Electron probe microanalysis (EPMA) provides an indication of solid solution series between thorite-xenotimezircon, which are related to hydrothermal solutions enriched in REE, Y, P, U, F, and Zr.
Geoheritage Meaning of Artificial Objects: Reporting Two New Examples from Russia
