Geochemistry of metamorphic processes in mafic rocks of the Krasnaya Guba area, Belomorian Mobile Belt

Abstract

New data are obtained on the major-component and minor- and trace-element composition of metamorphosed gabbronorites and Fe-Ti gabbro in the Krasnaya Guba area, Belomorian Mobile Belt. These two magmatic complexes, which are spatially combined but are of different age and were produced in different geodynamic environments, were simultaneously metamorphosed in the Paleoproterozoic to parameters transitional between the amphibolite and eclogite facies. Eclogite metamorphism was associated with perceivable changes in the concentrations of alkalis and alkali earth elements, and the metamorphic transformation of the plagioclase eclogite (which can be metamorphosed gabbronorite and/or gabbro) into amphibolite and biotite amphibolite was associated with changes in the concentrations of practically all trace elements. This implies that the metamorphic processes were not isochemical and were accompanied by significant removal and introduction of incompatible elements. However, some trace-element ratios (such as Ti/Y, Ti/Zr, Zr/Y, La/Sm, and Nb/Th) remained unmodified in the course of metamorphism and remained the same as in the pristine gabbronorite or gabbro and likely can be employed as indicators of the composition of the probable protolith and the conditions under which it was produced.