The Precambrian rift-related metamagmatic rocks of the Southern and Western Lithuania

Abstract

The main geological unit of the Archaean-Early Proterozoic crust of Southern Lithuania is Dzukia greenstone belt. The metamagmatic sequence of this greenstone belt is composed of mafic and ultramafic rocks sometimes with komatiitic-like and tholeiitic affinities and was emplaced at about 2.5 Ga ago. The uprising of a mantle diapir initiated the breaking of the preexisting sialic crust and induced the formation of the greenstones in a proto-oceanic rift geodynamic environment. We consider that imbricated thrusts are the main structural feature of greenstone belt. The volcano-sedimentary supracrustal Vidmantai complex is part of the Western Lithuanian Precambrian basement, which was formed by extensive mantle-derived crustal growth during the Early Proterozoic. The rocks range from medium and high-K tholeiitic dacites to high-Ti shoshonites resemble those of present-day mature volcanic arcs or arcs near or at active continental margins. The high-Ti shoshonites may mark the existence of temporal or local extensional environments. This is in agreement with the models assigning the Svecofennian province to convergent plate margin environments. The basalts and lamprophyres were found almost in the each borehole of the Southern and Western Lithuania. The dyke swarm includes tholeiitic continental basalts and kersantites, which clearly suggest a continental rift setting. Our results suggest that the Translilhuanian dyke swarm represents an abortive attempt at the Early Riphean break up the Archean-Lower Proterozoic thick continental crust, which was continued a little later by formation of the Veivirzhenai volcano-sedimentary sequence in the graben-synclines. It must be pointed out that, in course of time, petrogenetic processes in Precambrian have changed from ensimatic to ensialic, implying a major reworking of the preexisting crustal materials.