First- and second-order Raman spectra of carbonaceous material through successive contact and regional metamorphic events (Ryoke belt, SW Japan)

Abstract

The evolution of both the first- and second-order Raman spectra of carbonaceous material (CM) through successive contact and regional metamorphic events is explored in the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Thirty-two metasedimentary rock samples were collected along a N-S gradient locally affected by contact metamorphism before and after the main regional tectono-metamorphic event (DP1). First-order spectra document a decreasing peak area ratio R2 and an increasing temperature TCM towards the south and the surrounding granitoids. Domains with intermediate (535–600 °C) TCM values match the extent of the pre-DP1 contact aureole but also image a so far unclear post-DP1 aureole. The axial part of the belt, likely unaffected by granite intrusions, preserves southward increasing TCM from 425 to 660 °C. Second-order spectra show a single S1 band that splits into two peaks (S1- and S1+) whose frequency difference ΔS1 increases stepwise towards the south. The spatial distribution of ΔS1 follows that of the E–W trending regional metamorphic zones. The splitting of S1 indicates a transition from two-dimensional to three-dimensional CM and occurs at ~500 °C, which seems to be common to all metamorphic belts worldwide. Despite regional metamorphism CM was able to record the post-DP1 contact overprint and there is no clear observation of delayed CM recrystallization, which likely depends on the crystallinity of the CM precursor. A discrepancy between first- and second-order Raman parameters suggests that they partly record the influence of different factors; R2 gives an account of thermal events, particularly those related to localized contact metamorphism, whereas ΔS1 potentially yields information on regional variations in heating duration and pressure. This demonstrates the potential of the full Raman spectrum of CM for deciphering the complex thermal history of orogenic systems.