Allanite-(Ce): a Typical Mineral of Metakimberlite from the Lake Kimozero Area, Karelia

Resumo

Paleoproterozoic kimberlite from the Kimozero area in Karelia is the oldest rock of this type in Russia. It is strongly tectonized, metamorphosed, and it finally transformed into metakimberlite of the prehnite-pumpellyite facies with widespread lanthanide minerals: allanite-(Ce), bastnaesite-(Ce), bastnaesite-(La), parisite-(Ce), and monazite-(Ce). The contacts between their crystals and other metamorphic minerals, e.g., titanite, antigorite, and tremolite, are characterized by induction surfaces of concerted growth. Among lanthanide minerals, allanite-(Ce) is the most abundant. It occurs close to clinochlore pseudomorphs after phlogopite or as intergrowths with titanite in aggregates of tremolite–actinolite, calcite, and dolomite. Allanite crystals from the Kimozero area are not zonal, but vary in lanthanide contents and the Fe³⁺/Fe²⁺ ratio in grains no more than tens of microns from one another. Kimozero allanite mostly belongs to the allanite–ferriallanite series (up to 30% of ferriallanite endmember); the lesser amount corresponds to the allanite–Cr-bearing allanite series. At the late stage of metamorphism, allanite was partly replaced with parisite, bastnaesite, or monazite.