Physicochemical model for the genesis of Svecofennian plagiomigmatites of the Belomorian Complex, northern Karelia

Abstract

A mechanism is proposed to explain the genesis of biotite-quartz-feldspar plagiomigmatite leucosomes as a consequence of the reaction of an amphibolite protolith with ascending fluid. The model is based on the analysis of the mineralogical and geochemical zoning of plagiomigmatite bodies and their outer contact aureoles. The model is underlain by the physicochemical simulations of interactions between a solution and rock in a system of sequential flow-through reactors. The development of the plagiomigmatites was related to the relief of mechanical strain in amphibolite strata and the ensuing origin of shear fractures. In these fractures, brittle homblende was metasomatically replaced (under the effect of the fluid) by ductile quartz-biotite-plagioclase plagiomigmatite aggregates. Vein plagiomigmatites could have been produced at relatively low temperatures (590–630°C), when highly concentrated alkaline-silicic solutions affected amphibolites. The optimum fluid/rock ratio was equal to 30/1. The condition controlling the origin of the predominant mineral assemblage of the plagiomigmatites (Qtz + Pl + Bt + Ttn) was a local pressure decrease in shear fractures from 3.75–4.65 to 1.5–2.5 kbar. In regions with lower pressures (<1.5 kbar), the Bt + Ttn + Qtz assemblage was formed. The solution in equilibrium with this assemblage of the plagiomigmatites had pH 4.5–5.5. An increase in the KCl and NaCl concentrations in the fluid was favorable for the expansion of the stability field of plagiomigmatites toward lower temperatures and higher pressures. The structural and textural heterogeneities in plagiomigmatite veins resulted from local silica dissolution at sites with higher mechanical strain and the redeposition of this component where the strain was relieved. The development of the mineralogical and geochemical zoning revealed in the outer-contact zones of the plagiomigmatite bodies and within these bodies themselves was controlled by the pressure gradient.