Abnormally high-hafnium zircon from rare-metal pegmatites of the Vasin-Mylk deposit (the northeastern part of the Kola Peninsula)

The deposit Vasin-Mylk is located in NE Kola Peninsula and represents the pegmatites of Li-Cs-Ta specialization (LCT-type). The studied zircon grains from aplitic zone of pegmatites contain av. 0.34-0.45 apfu of Hf, while the rims of grains contain 0.52-0.54 apfu Hf and correspond to hafnon. Zones with high hafnium content are brighter in BSE and CL comparing to central parts of crystals. The crystallization temperatures of high-Hf zircon were estimated as about 550 degrees C using Ti-in-zircon thermometer. A significant change in crystallization conditions led to isomorphic substitution of Zr by Hf to form hafnon. This change is related to precipitation of aluminous minerals (spodumene, lepidolite) from alumina-oversaturated melt, combined with flux elements (B, Li and F) contributed to the strong fractionation of the Zr/Hf ratio and crystallization of high-Hf zircon at the late (hydrothermal) stage of rare-metal pegmatite evolution. During this stage KDHf between zircon and granitic melt increases rapidly.