3-D assessment of peak-metamorphic conditions by Raman spectroscopy of carbonaceous material: an example from the margin of the Lepontine dome (Swiss Central Alps)

Abstract

International audienceThis study monitors regional changes in the crystallinity of carbonaceous matter (CM) by applying Micro-Raman spectroscopy to a total of 214 metasediment samples (largely so-called Bündnerschiefer) dominantly metamorphosed under blueschist- to amphibolite-facies conditions. They were collected within the northeastern margin of the Lepontine dome and easterly adjacent areas of the Swiss Central Alps. Three-dimensional mapping of isotemperature contours in map and profile views shows that the isotemperature contours associated with the Miocene Barrow-type Lepontine metamorphic event cut across refolded nappe contacts, both along and across strike within the northeastern margin of the Lepontine dome and adjacent areas. Further to the northeast, the isotemperature contours reflect temperatures reached during the Late Eocene subduction-related blueschist-facies event and/or during subsequent near-isothermal decompression; these contours appear folded by younger, large-scale post-nappe-stacking folds. A substantial jump in the recorded maximum temperatures across the tectonic contact between the frontal Adula nappe complex and surrounding metasediments indicates that this contact accommodated differential tectonic movement of the Adula nappe with respect to the enveloping Bündnerschiefer after maximum temperatures were reached within the northern Adula nappe, i.e. after Late Eocene time

Petrology and tectonic evolution of the Kiskunhalas-NE fractured hydrocarbon reservoir, South Hungary

“…On the basis of abundant measurements (Wiederkehr et al 2011) the methods of Beyssac et al (2002) and of Rahl et al (2005) provide similar temperature estimates, with discrepancies of less than 30 °C. Wiederkehr et al (2011) recognized that the relative uncertainties in temperatures derived using the calibration of Rahl et al (2005) are higher when compared to results using those of Beyssac et al (2002), especially at temperatures above 480 °C.…”

“…The process of graphitization has a strictly irreversible character; thus it primarily depends on the maximum temperature reached along a given P-T path (e.g. Beyssac et al 2002;Beyssac et al 2004;Wiederkehr et al 2011;Aoya et al 2010). Therefore the CM structure is insensitive to retrograde metamorphic overprint or polymetamorphic evolution (Wiederkehr et al 2011;Beyssac et al 2002;Wopenka and Pasteris 1993).…”

“…Beyssac et al 2002;Beyssac et al 2004;Wiederkehr et al 2011;Aoya et al 2010). Therefore the CM structure is insensitive to retrograde metamorphic overprint or polymetamorphic evolution (Wiederkehr et al 2011;Beyssac et al 2002;Wopenka and Pasteris 1993). Due to the high spatial resolution and the short acquisition time, which allows numerous spectra to be recorded and therefore to quantify the sample heterogeneity, Raman spectroscopy appears the most appropriate tool to characterize metamorphic CM.…”

“…Wiederkehr et al (2011) recognized that the relative uncertainties in temperatures derived using the calibration of Rahl et al (2005) are higher when compared to results using those of Beyssac et al (2002), especially at temperatures above 480 °C. A discussion of calibration possibilities at lower temperatures (~200-300 °C) is given in Lahfid et al (2010).…”

“…Beyond these factors the CM has a natural heterogeneity (Wiederkehr et al 2011), which may have an influence on the results of the measurement and temperature estimation. During deposition CM types of various organic precursors that underwent heterogeneous graphitization may mix (Wiederkehr et al 2011;Rantitsch et al 2004).…”