ON THE AGE OF THE ORTHOGNEISSES IN THE NORTHWESTERN WHITE SEA AREA

Abstract

The gneisses, granite-gneisses and migmatites on the shore of Kandalksha Bay, in the northwestern White Sea area, are traditionally assigned to the very old Saami complex of the Baltic shield. This article will discuss the results of radioisotope age determinations on the gneisses of plagiogranite composition from the earliest substrate of the migmatites. The marked banding of the gneisses of this substrate is manifested in the subparallel disposition of the dark minerals (epidote, biotite and garnet) in the medium-crystalline quartz-feldspar mass. The larger crystals of magmatogenic andesine, which contain cryptomicroscopic fluid (?) inclusions, are deformed and recrystallized in the grain margins. Together with epidote, quartz and orthite, the andesine in shear zones is replaced by biotite and microcline; garnet sometimes appears. Still later biotite, muscovite and secondary finely-crystalline epidote intensify the metamorphic banding of the primary granitoids, reflecting the heterogeneity of the deformation and metamorphic transformations.