Abstract: For the first time, the isotopic composition of oxygen, the content of trace elements, and the U–Pb zircon age were determined for kyanite–gedrite metasomatites from the Dyadina Gora ore occurrence, Belomorian Mobile Belt, Fennoscandian Shield. The zircon cores preserved from the protolith are characterized by a magmatic REE distribution and an average value of δ¹⁸O of about 3.0‰, and their age is close to 2400 Ma. The inner zones around the cores and rims of zircon were formed during metasomatic processes at the time of the Svecofennian metamorphism about 1870–1880 Ma ago, experienced strong fluid reworking, and are characterized by a decreased δ¹⁸O value to –0.9‰.