Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

Michel Faure,Hugues Raimbourg,Clément Montmartin

doi:10.1051/bsgf/2020043

Abstract

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400 °C near the dome, and 230 °C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont color alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming event. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300 °C and 360 °C could be explained by the presence of granitic plutons under the foreland basin.

Highlights

The knowledge of quantitative constraints, such as pressure, temperature, duration of heating, strain, strain rate, exhumation rate, and uplift rate, is essential to understand the formation and evolution of a mountain belt
Its applicability has been further extended towards low temperatures (Rahl et al, 2005; Lahfid et al, 2010; Kouketsu et al, 2014), using different calibrations of the Raman spectra based on other geothermometers, such as vitrinite reflectance, fluid inclusions microthermometry or illite crystallinity (Rahn et al, 1995, Hara et al, 2013) or indirect data on temperature provided by fission-tracks and U-Th/He method (Rahl et al, 2005)
The 72 Raman Spectroscopy of Carbonaceous Material (RSCM) measurements acquired in this study combined with the results of several previous studies, derived from different methods, allow us to reconstruct the thermal history of the Montagne Noire southern flank

Summary

Introduction

The knowledge of quantitative constraints, such as pressure, temperature, duration of heating, strain, strain rate, exhumation rate, and uplift rate, is essential to understand the formation and evolution of a mountain belt. In the inner domain of an orogen, temperature and pressure conditions experienced by metamorphic rocks can be approached by petrological investigations based on mineral parageneses. Several methods have been developed to overcome this problem and to reach a quantitative knowledge of the conditions of low-grade metamorphism, including illite crystallinity, conodont alteration colour, calcite-dolomite thermo-barometer (e.g. Kübler, 1968; Dunoyer de Segonzac et al, 1968; Epstein et al, 1977; Frey, 1987). The Raman Spectroscopy of Carbonaceous Material (RSCM) method based on carbonaceous material crystallinity has been developed for the temperature range 330–650 °C (Beyssac et al, 2002). Its applicability has been further extended towards low temperatures (Rahl et al, 2005; Lahfid et al, 2010; Kouketsu et al, 2014), using different calibrations of the Raman spectra based on other geothermometers, such as vitrinite reflectance, fluid inclusions microthermometry or illite crystallinity (Rahn et al, 1995, Hara et al, 2013) or indirect data on temperature provided by fission-tracks and U-Th/He method (Rahl et al, 2005)

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