PRIMARY NATURE OF GARNET-ZOISITE GNEISS IN THE BELOMORIAN SERIES OF ARCHEAN AGE

Abstract

In aluminous gneiss of the Loukhi formation near the top of a small hill, a bed of garnet-zoisite gneiss was detected similar to rocks earlier reported in the same upper formation by N.G. Sudovikov. This garnet-zoisite gneiss bed is 50 to 65 cm thick and traceable from 38 to 40 m. The rock studied consists mainly of garnet, plagioclase, quartz and zoisite and differs sharply in color (grayish-white) from the host rust-brown garnet-biotite and kyanite-garnet-biotite gneiss of the Loukhi formation. Chemical analyses of garnet-zoisite gneiss showed that it is enriched in CaO and Al2O3 and is a sodipotassic rock with a relatively high mafic index. Additional properties and analyses indicate that the garnet-zoisite gneiss seems to be a derivative of sandy, calcareous and clayey sediments, deposited away from the coast in a basin where clastic material was supplied from chemically deeply weathered mantle, accumulating on a peneplaned landmass in a warm or hot climate with humid elements. Then those sediments were converted by diagenesis, catagenesis and medium-grade regional metamorphism (amphibolite facies) to garnet-zoisite gneiss.