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

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 highaluminum metamorphic and metasomatic rocks of the socalled Keivy Series (4). It contacts by shear zones with rocks of the lower Leb� yazhinskii nappe mainly composed of biotite and gar� net-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 sepa� rated from each other by shear zones and often zones of silicification with variable thicknesses. There is a clear connection between repeated metasomatism and narrow fluidtransporting shear zones, elongated for many tens of kilometers. Among the main varieties of metasomatic rocks in the Bolshie Keivy nappe are kyaniterich rocks with variable con� tents of quartz, plagioclase, staurolite, biotite and muscovite, containing kyanite-quartz veins, as well as the areas of plagioclasites and plagioclaserich crystal� line schists interpreted as protolith relics. They are mainly concentrated in the shear zones of the lower and upper parts of nappe, at the contact with metavol� canites of the Lebyazhinskii nappe. These metasoma� tites were the objects of isotope-geochemical investi� gations aimed at solution of the problem of the origin and composition of fluid responsible for metasomatic processes in the shear zones of the Bolshie Keivy tec� tonic nappe. Based on the analysis of parageneses and thermo� dynamic calculations, we determined the physico� chemical conditions of the formation of metasoma� tites and host metamorphic rocks. The calculated val� ues of the water activity in dense hydrous-saline fluid vary from 0.4-0.6 in aluminiferous gneiss to 0.6-0.8 in metasomatites. Petrogenetic diagrams of the stabil� ity of parageneses in P-T coordinates were plotted for the most typical rocks of the KFMASH system for dif� ferent water activities. As a result, the general P-T trend (Fig. 1) was obtained for the Bolshie Keivy nappe reflecting the evolution of metamorphic and metaso� matic petrogenesis in the anticlockwise direction, first by the line of isobaric cooling (6.3-7.2 kbar and 640- 490°C) and, then, isothermal decompression (up to 5-4 kbar and ~500°C). Analysis of the P-T evolution is limited by the area of stability of staurolite paragene� ses. The more lowtemperature alterations in the con� sidered zones were not reflected in considered estima� tions. The age of metasomatites (1762-1720 Ma) is determined by local methods (SHRIMP II) (2). The area of isothermal decompression is confirmed by the results of fluid inclusion study (Table 1). More detailed analysis of these results will be made in a spe� cial paper. In this study we mention only two peculiar� ities of the fluid. The first of them implies that in the period of fluid capture and the formation of primary and refilled (primary-secondary) inclusions, as well as during the later processes (secondary inclusions), fluid was heterogeneous consisting of at least phases: pure CO 2 and solutions of calcium and sodium chlorides with the maximal salinity of >40 wt %. The second fea� ture, the most important for us, implies that in addi� tion to the abovementioned phases, most of the stud� ied samples contained inclusions of methane and