Geochemistry and Sr–Nd Isotope Systematics of Apatite from Corundum-Bearing Metasomatites of the Belomorian Mobile Belt

Abstract

The geochemical characteristics (REE, trace elements) and Sr and Nd isotopic composition of apatite from corundum-bearing metasomatites of the Khitoostrov occurrence (Belomorian Mobile Belt), associated plagioclasites, and host rocks represented by garnet amphibolites and kyanite–garnet–biotite gneisses of the Chupa sequence have been studied. Apatites from the corundum-bearing metasomatites and kyanite–garnet–biotite gneisses are enriched in medium REE and have a negative Eu anomaly (Eu/Eu* 0.20–0.35). Apatite from the corundum-bearing rocks differs from apatite from the gneisses of the Chupa sequence in the increased content of Sr, LREE, decreased content of HREE, as well as a lower ⁸⁷Sr/⁸⁶Sr(t) ratio and an increased ɛ_Nd(T) value: 0.70 865–0.70 896 and –9.3 ± 0.2 compared to 0.72 533 and –8.1, respectively. Apatite from the garnet amphibolites is enriched in MREE, lacks Eu-anomaly (Eu/Eu* 0.98), and has a low ɛ_Nd(T) = –9.3 and the lowest ⁸⁷Sr/⁸⁶Sr(t) ratio of 0.70 560. The Sm-Nd age estimate of apatite is 1.80 ± 0.15 Ga, which coincides with the Svecofennian metamorphism in the Belomorian Mobile Belt. The geochemical features of the apatite and ⁸⁷Sr/⁸⁶Sr(t) ratios indicate that the metasomatic alteration of the gneisses was caused by the lower crustal fluid and was accompanied by the influx of LREE and the removal of HREE. The slightly lower Eu anomaly and higher Ce vs Th and REE vs La/Sm relations reflect the fact that apatite from the corundum-bearing metasomatic rocks was formed in a more oxidizing environment than apatite from host rocks. Neither the corundum-bearing metasomatites and plagioclasites, nor the host rocks revealed any Sr-isotopic and REE-geochemical traces of interaction with surface (meteoric) waters.