Metamorphic evolution of graphite in the Paleoproterozoic Savo Schist Belt (SSB), Central Finland: Constraints from geothermetric modeling

Abstract

Flake graphite deposits of the Savo Schist Belt (SSB), Central Finland are mainly hosted by quartz-mica schist, hornblende biotite gneiss, and to a minor extent in amphibolite. These rocks represent high-grade metamorphic conditions of the SSB. Raman spectroscopy measurements on graphite flakes provide quantitative information on the degree of graphitization, which can be correlated to the grade of metamorphism. The metamorphic conditions of the graphite-bearing metapelites have been determined by P-T pseudosection modelling and three types of geothermometers: garnet-biotite (GB), Ti-in biotite and Raman spectroscopy of carbonaceous material (RSCM) geothermometry. The calculated P-T pseudosections indicate peak P-T conditions of ca. 5–6 kbar and 650 °C for two garnet-bearing hornblende biotite gneiss samples, and slightly higher conditions of ca. 5–6 kbar and 700 °C for a quartz mica schist sample. The GB thermobarometer yielded a temperature and pressure range of 600–725 °C and 5.2–8.0 kbar. The Ti-in-biotite applied to the same samples produced temperature ranging between 570 and 735 °C, and the Raman (RSCM) geothermometers indicated temperatures between 550 and 670 °C. For the garnet-bearing hornblende biotite gneiss samples there is no significant (<10–50 °C) difference between temperatures determined using the P-T pseudosections and mean temperatures calculated using RSCM, GB and Ti-in-biotite methods. Although the peak temperature in some highly crystalline graphite flakes of studied samples calculated by (RSCM) geothermometer yield temperatures of 641 °C with the Beyssac et al. 2002 method and 737 °C with the Rahl et al. (2005) method, which are the highest temperature for this calibration. Raman spectroscopy is now part of a wide selection of tools that can be used to identify the ordering of the graphite crystallinity, which is including the number of layers, quality of layers, doping level and confinement in graphene structures. The results of this work show that Raman spectroscopy is a simple and fast way to determine the peak metamorphic temperature of carbonaceous material CM in amphibolite facies metasedimentary rocks.