Amphibolites and basic metasomatites of the Belomorian belt: Similarities and differences

The paper presents a comparative mineralogical-geochemical characterization of amphibolites and basic metasomatic rocks in the Belomorian belt. The metasomatites are spread much more locally than the amphibolites. The metasomatic rocks compose bodies of very diverse morphology, some of which are similar to the morphologies of amphibolite bodies (podiform and tabular bodies and lenses). The metasomatites are characterized by a great diversity of their petrographic varieties, whereas the usual assemblage of the amphibolites is amphibole ± garnet + plagioclase + quartz. Although one of the varieties of both rock types contains the same assemblage (garnet-hornblende), the metasomatites contain no quartz, have different quantitative proportions of their minerals, their compositions, and different overall contents of amphibole and garnet. The metasomatites are more mafic and contain 85–100% mafic minerals, whereas the amphibolites contain only 60–65% mafic minerals. The rocks have different concentrations of most major and some trace elements, including REE. The basic metasomatites are poorer than the amphibolites in SiO₂, Al₂O₃, Na₂O, K₂O, LREE, and HREE but richer in MgO and Cr₂O₃. The Cr concentrations in some of the metasomatites are higher than the clarkes of this element in crustal ultrabasic rocks. The main Cr concentrator in the rocks was determined to be amphibole: some grains of this mineral contain as much as 6800 ppm Cr. The element is unevenly distributed even within single grains. Amphiboles of the amphibolites do not contain Cr at all. Another difference of amphiboles from the metasomatic rocks and amphibolites is the depletion of this mineral in the former rocks in Al and Fe, the absence of K, but high concentrations of MgO and 1.5–2.0 times higher. A remarkable feature of the compared rocks is their geneses. The amphibolites of the Belomorian belt are metamorphosed mafic Archean rocks, whereas the basic metasomatites were produced by Fe-Mg-Ca metasomatism (basification) closely related to the granitization of various rocks, first of all, amphibolites. It is important to distinguish between these rocks of various genesis and age, particularly when geological survey is conducted.