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The Otanmäki area in central Finland hosts two occurrences of Nb-Zr-REE mineralization, Katajakangas and Kontioaho, within a suite of ca. 2.04–2.06 Ga (A1-type) gneissic granites, syenite and monzonite-monzodiorite. They exhibit trace element characteristics and whole-rock εNd(2050 Ma) values (from +2.6 to -1.3) consistent with derivation by differentiation of mantle-derived mafic magmas with variable interaction with crustal material. The mineralization is localized in 0.1to 1.4-m-thick dikes (Katajakangas) and a 30to 50-m-thick sheet-like body (Kontioaho), containing allanite-(Ce), zircon, titanite, and Nb-REE-Th-U oxides. Their wall rocks are composed of ca. 2.06 Ga peraluminous monzogranite, which is genetically unrelated to the mineralized rock units, as evidenced by whole-rock chemical and Sm-Nd isotope data and zircon U-Pb geochronology. Instead, the mineralization is linked to the nearby peralkaline to metaluminous alkali feldspar granite magmatism dated at ca. 2.04–2.05 Ga. The development of REE-HFSE enrichment in the mineralized rock units required extensive crystallization of a peralkaline granite magma, producing residual metaluminous, highsilica melts enriched in REE-HFSE, Ca, and Fe relative to Na, K, and Al. The REE-HFSE and Ca enrichment was further promoted by volatile complexing with dissolved F-, CO3 2and SO4 2-. These highly evolved melts were parental to the mineralized dikes and the sheet-like intrusive body, which were emplaced into the monzogranite capping the intrusions of peralkaline granite.

Age and origin of the Nb-Zr-REE mineralization in the Paleoproterozoic A1-type granitoids at Otanmäki, central Finland