Geological position and mineral composition of kyanite amphibolites of the Chupinskiy paragneiss belt (Belomorian province of Eastern Fennoscandia)

Abstract

The Lyagkomina occurrence of kyanite amphibolites (the Belomorian Province of the Fennoscandian Shield) is confined to an amphibolite boudin among kyanite-garnet-biotite and garnet-biotite gneisses of the Chupa Sequence. The main rock types at the Lyagkomina occurrence are meso- and melanocratic garnet-kyanite amphibolites with large kyanite (more than 1 cm in length) and kyanite-garnet amphibolites with small kyanite. There are also leucocratic garnet-biotite-amphibole-plagioclase rocks with kyanite, melanocratic epidote-staurolite-amphibole rocks, zoisite plagioclasites with staurolite and kyanite. The country gneisses around the boudin were metasomatically altered: quartz disappears in kyanite gneisses, and amphibole appears in garnet-biotite gneisses. Calcic amphiboles in the kyanite amphibolites are represented by high-alumina tschermakite; biotite — by intermediate-alumina phlogopite; garnet belongs to the pyrope-almandine series with a high grossular content. Plagioclases have an intermediate and basic composition. Intermediate plagioclases are common in kyanite amphibolites without staurolite and in kyanite amphibolites with staurolite rims, while the basic plagioclases are found in staurolite amphibolites. Many rocks are characterized by reaction structures, representing the change of the mineral assemblages. Metasomatic zoning has been reconstructed in the occurrence with a regular sequence of the mineral assemblages: Hbl + Grt + Pl + Qtz ± Bt (garnet amphibolite) → Hbl + Grt + Pl + Ky + Qtz ± Bt → Hbl + Grt + Pl + Ky ± Bt → Hbl + Grt + Pl + Ky + St ± Bt → Hbl + Grt + Ky + Pl + St + Czo ± Bt → Hbl + Grt + Pl + St + Czo ± Bt → Hbl + St + Czo + Pl ± Bt. In the described series, the alumina content increases and the Mg content of the main rock-forming minerals decreases, and desilication of the rock occurs. The described mineral zoning was formed during metamorphism as a result of the reaction between the initial amphibolite and fluid flow in the shear zone (i. e., at synmetamorphic metasomatism). Petrological and isotope data indicate that the Lyagkomina kyanite amphibolites are genetically related to the occurrences of corundum-bearing rocks in the Belomorian Province of the Fennoscandian Shield.