Evaluation of Deformation Temperatures in Carbonate Mylonites at Low Temperature Thrust-Tectonic Settings via Micro-Raman Spectroscopy

Carbonaceous materials (CMs) have been widely used to assess temperatures in sedimentary and metamorphic carbonate rocks. The use of Raman spectroscopy of carbonaceous material (RSCM) is largely devoted to the study of deformed rocks hosted in thrust-tectonic settings. Raman spectroscopy of carbonaceous material successfully allows the study of carbonate rocks at a temperature as high as 650 °C. In this study, a set of carbonate-mylonite rocks (Italian Alps) were investigated using micro-Raman spectroscopy, in order to infer the deformation conditions associated with the Alpine thrusts, expected to occur at T < 350 °C. Micro-Raman spectra were collected using two sources: green (532 nm) and red (632.8 nm) lasers. Several deconvolution procedures and parameters were tested to optimize the collected spectrum morphologies for the laser sources, also in accordance with the low temperature expected. The obtained temperatures highlight two clusters: one at 340–350 °C for the samples collected in the axial part of the Alpine chain, and the other at 200–240 °C for those collected in the external thrust-and-fold belt. These results agree with the independent geological and petrological constraints. Consistent results were obtained using 532 and 632.8 nm laser sources when the appropriate deconvolution approach was used.