Graphite Crystallinity and Metamorphic Temperature Estimates Using RSCM Geothermometry in the Loberget Area, Bergslagen Lithotectonic Unit, Sweden

Sammanfattning

Graphite-rich paragneiss from Loberget, situated near the Storsjön-Edsbyn deformation zone in Sweden, provides valuable insights into the region’s metamorphic history. Transformation of carbonaceousmaterial into graphite in sedimentary rocks is an irreversible process, allowing graphite to retaininformation about the conditions under which it was formed. Thus, graphite crystallinity is widely usedin Raman spectroscopy of carbonaceous matter (RSCM) to estimate peak metamorphic temperatures. This thesis aims to determine peak temperatures in a graphite-rich paragneiss from Loberget using RSCM geothermometry. Graphite grains representing multiple generations were identified based ontextural position and were subsequently analyzed. The RSCM results indicate that all graphite types arehighly crystalline, suggesting peak metamorphic temperatures exceeding 650°C, regardless of theirtextural position. This suggests that both regional metamorphism and later deformation phases reached temperatures exceeding 650 °C. The presence of high-grade metamorphic minerals such as sillimanite and garnet in close spatial association further supports this interpretation. The findings are consistent with the metamorphic history of the area, which includes multiple episodes of high-grade deformation during the 2.0–1.8 Ga Svecokarelian orogen. However, these estimated temperatures exceed the uppercalibration limit of the applied geothermometer (330–650°C), making precise temperature calculations impossible. While these results provide valuable insights into the metamorphic history of the area, further analysis using geothermometers suitable for higher temperature ranges is necessary to obtain more precise peak temperature estimates in Loberget.