Distribution of rare-earth elements in rock-forming minerals of corundum-bearing rocks of the Khitoostrov deposit (North Karelia)

The SIMS method was used to study the distribution of rare-earth elements in minerals from the rocks of the Khitoostrov occurrence (Belomorian mobile belt of Eastern Fennoscandia): corundum-bearing metasomatites with anomalous isotopically light oxygen and hydrogen and apogabbro garnet amphibolites with normal isotopic composition. The study was accompanied by estimates of PT-parameters of rock formation using multi-equilibrium thermobarometry (TWEEQU method). Temperatures calculated for apogabbro garnet amphibolites fall within the range of 730-770 degrees C, pressures - 13-14 kbar; for corundum-bearing rocks, temperatures were 680-710 degrees C, pressures - 6.5-7.5 kbar. Corundum-bearing rocks were formed at slightly lower temperatures and at significantly lower pressures than apogabbro garnet amphibolites. The REE distribution spectra in garnets from apogabbro amphibolites are characterized by a clearly pronounced slope from light to heavy REE, while in garnets from corundum-bearing rocks they have a less pronounced positive slope, which is associated with a noticeable enrichment of garnets in LREE and an insignificant depletion of HREE. Calcium amphiboles from corundum-bearing rocks are significantly enriched in rare-eart elements as compared to amphiboles from apogabbro garnet amphibolites, especially LREE (by more than an order of magnitude) and, to a lesser extent, MREE. Plagioclases from corundum-bearing rocks are also enriched in LREE against the background of garnet amphibolites. Thus, in all the studied minerals of corundum-bearing rocks, LREE enrichment is recorded. It isn't manifested in the minerals of amphibolites and, obviously, isn't related to the difference in PT-parameters of rock formation. Consequently, LREE was transferred by a specific fluid during the mineral-forming processes, which led to the formation of metasomatites with an anomalous isotopic composition of oxygen and hydrogen.