THE MANGANESE SILICATE ROCKS OF THE EARLY PROTEROZOIC VITTINKI GROUP, SOUTHWESTERN FINLAND: METAMORPHIC GRADE AND GENETIC INTERPRETATIONS

Abstract

The manganese silicate rocks of the Vittinki Group, in southwestern Finland, are interbedded with metachert, and are associated with amphibolitic metavolcanic rocks that have been metamorphosed to a temperature of up to 740 ± 30°C and a pressure of between 4 and 5 kbar. They are interpreted as the metamorphosed equivalent of “distal” submarine hydrothermal deposits of Mn–Fe-bearing interlayered silica-rich and carbonate-rich sediments. The metachert comprises a dominant Fe-rich silicate facies (with quartz, fayalite, ferrosilite, pyrrhotite, magnetite, annite, grunerite) and subordinate calc-silicate intercalations (with calcite, dolomite, siderite, diopside, tremolite, fayalite, quartz), consistent with X (CO 2 ) exhibiting abrupt lithologically controlled changes during peak metamorphism. The manganese silicate lenses consist of rhodonite, pyroxmangite, manganocummingtonite, manganogrunerite, tephroite, rhodochrosite, calcite, pyrrhotite and magnetite. The metamorphic assemblages reflect low and unbuffered X (CO 2 ) values and intermediate- to low- f (O 2 ) conditions. Consistent with an origin by polysomatic transformation from a “protorhodonite” precursor, the rhodonite contains abundant lamellae of pyroxmangite and clinopyroxene defects that persisted metastably at peak conditions of metamorphism. The synmetamorphic quartz – amphibole ± braunite veins originated from oxidized fluids. The oxygen isotope exchange thermometer between quartz and manganoan cummingtonite yields equilibration temperatures of 640 ± 30°C.