Origin and Composition of Fluid Responsible for Metasomatic Processes in Shear Zones of the Bolshie Keivy Tectonic Nappe, Baltic Shield: Carbon Isotope Composition of Graphite

摘要

The formation of the observed structure of the Keivy terrain is related to the nappe formation during the Paleoproterozoic collisional processes at the southwestern boundary of the Kola fragment of the Archean Craton. The Bolshie Keivy tectonic nappe with a length of > 160 km is the upper part of the tec-tonostratigraphic section of the Keivy terrain [1-3] and composed of plates and lenses of blastomylonitic, predominantly high-aluminum metamorphic and metasomatic rocks of the so-called Keivy Series [4]. It contacts by shear zones with rocks of the lower Leb-yazhinskii nappe mainly composed of biotite and garnet-biotite gneisses, and, to a lesser extent, biotite-amphibole gneiss and amphibolite gneisses and amphibolites (interpreted as metavolcanites) with numerous bodies of alkaline metasomatites. Plates and lenses of rocks in the considered nappe are separated from each other by shear zones and often zones of silicification with variable thicknesses.