Metasomatic Epidote in Amphibolite-Replacing Rocks: An Indicator of Tectonic Processes Related to the Exhumation of Rocks of the Belomorian Belt. Petrography and Geochemistry.

The dominant rocks in the area between the village of Kovda and township of Zelenoborskii in northern Karelia, Russia, are amphibole–biotite and biotite plagiomigmatites, which develop after amphibolites. These plagiomigmatites contain epidote that replaces the amphibole and biotite and is found in reaction relations with these minerals. The epidote occurs as thin rims around mineral grains it replaced and in large euhedral crystals. The rocks contain 5–15% epidote. The epidote contains relatively little FeO (no more than 10.57 wt %) but elevated concentrations of CaО (up to 27.53 wt %) and Al2O3 (up to 31.28 wt %), i.e., the mole fraction of the pistacite end member is 0.14–0.22. The mineral crystallized at 400–600°C and 5 kbar. It was formed by metasomatic processes, when components of the replaced minerals interacted with components of the fluid. The amphibole–biotite and biotite rocks in which the epidote crystallized resemble amphibole–biotite and biotite gneisses. Geological, petrographic, and geochemical characteristics of the rocks indicate that they are plagiomigmatites that developed after amphibolites. The paper reports features that enable distinguishing between the plagiomigmatites and similar gneisses. The transformations of the amphibolite at its migmatization starts with the replacement of the hornblende by biotite, up to the complete replacement of the former at a certain progress of this process. Chemically, the original rocks in their migmatization sequence are gradually depleted in Fe, Ti, Mg, and Ca and enriched in Si, Na, and K. The normalized REE patterns of the plagiomigmatites are strongly fractionated: (La/Yb)n = 18.3 for the biotite plagiomigmatite and 30.9 for the biotite–hornblende plagiomigmatite. The rocks of the area occur in a tectonized zone, which is the boundary between two domains in the Belomorian Belt exhumed at 1.8 Ga according to the mechanism of metamorphic cores. This was favorable for the intense inflow of fluids, created a certain pressure (both total and that of H2O + CO2), temperature, and elevated oxidation potential, i.e., conditions required for epidote crystallization. [ABSTRACT FROM AUTHOR]