High-temperature amphibolization synchronous with enderbite migmatization of mafic granulites in granulite-enderbite-charnockite complexes in Karelia

Detailed data on the area of lakes Notozero-Kovdozero and Lake Kerchug are used to analyze petrological aspects (mineral replacement reactions Opx + Cpx + Pl → Hbl with plagioclase of various composition, mineral compositions, and P-T parameters) of the high-temperature amphibolization of mafic granulites in the Archean granulite-enderbite-charnockite (GECH) complexes of Karelia. It was concluded that the amphibolization of the mafic crystalline schists in GECH complexes occurs at the peripheral zones of enderbite migmatization. In the course of pyroxene replacement by hornblende, the primary two pyroxene-plagioclase granulites are transformed into two pyroxene-amphibole-plagioclase crystalline schists with more sodic plagioclase and, if the process is more intense, into amphibolites. Our results testify that equilibrium in mafic crystalline schists at the peak of granulite metamorphic and the inflow of Na-rich fluids begins to shift toward an increase in the content of Na-bearing mafic minerals (amphiboles) instead of pyroxenes. At the same time, more sodic plagioclase is formed in place of labradorite-bytownite. The process of amphibolization in these rocks differs from that related to either retrograde metamorphism or younger diaphthoresis under amphibolite-facies conditions in geochemical regime, P-T parameters, and the type of hornblendes.