4.4 Article

Contrasting degrees of recrystallization of carbonaceous material in the Nelson aureole, British Columbia and Ballachulish aureole, Scotland, with implications for thermometry based on Raman spectroscopy of carbonaceous material

Journal

JOURNAL OF METAMORPHIC GEOLOGY
Volume 37, Issue 1, Pages 71-95

Publisher

WILEY
DOI: 10.1111/jmg.12449

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Funding

  1. Natural Sciences and Engineering Research Council of Canada [037233]
  2. City of Paris
  3. Sorbonne Universite

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The degree of recrystallization of carbonaceous material (CM), as monitored by Raman microspectroscopy, was examined as a function of metamorphic grade in two well-studied contact aureoles containing carbonaceous pelites: the Nelson aureole, British Columbia and the Ballachulish aureole, Scotland. Here, we use (a) the R2 ratio extracted from the Raman spectrum of CM as a proxy for the degree of graphitization (0.0 in perfect graphite then increasing with structural defects) and (b) the second-order S1 band (similar to 2,700 cm(-1)) as a marker for the tridimensional ordering of CM. The Nelson aureole (garnet-staurolite-andalusite-sillimanite-K-feldspar sequence, similar to 550-650 degrees C, 3.5-4.0 kbar) was developed in rocks that were unmetamorphosed prior to contact metamorphism, whereas the Ballachulish aureole (cordierite-andalusite-K-feldspar-sillimanite sequence, similar to 550-700 degrees C, similar to.0 kbar) was developed in rocks that had been metamorphosed to garnet grade conditions (similar to 7 kbar, similar to 500 degrees C) c. 45 Ma before contact metamorphism. Thirty-one samples were examined from Nelson and 29 samples from Ballachulish. At Nelson, the R2 ratio steadily decreases from similar to 0.25 to 0.0 as the igneous contact is approached, whereas at Ballachulish, the R2 ratio remains largely unchanged from regional values (similar to 0.20-0.25) until less than 100 m from the igneous contact. The second-order S1 band reveals that carbonaceous material (CM) was transformed to highly ordered locally tridimensional graphitic carbon at Ballachulish by regional metamorphism prior to contact metamorphism, whereas CM was still a disordered turbostratic (bidimensional) material before contact metamorphism in the case of Nelson. Pretexturation of CM likely induced sluggish recrystallization of CM and delayed graphitization in the Ballachulish aureole. Temperatures of recrystallization of the CM in the two aureoles were estimated using different published calibrations of the thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM), with differences among the calibrations being minor. In the Nelson aureole, temperatures are in reasonable agreement with those indicated by the metapelitic phase equilibria (all within 50 degrees C, most within 25 degrees C). In the Ballachulish aureole, the retarded crystallization noted above results in increasing underestimates of temperatures compared to the metapelitic phase equilibria (up to similar to 75 degrees C too low within 200 m of the igneous contact). Our study calls for careful attention when using RSCM thermometry in complexly polymetamorphosed rocks to assess properly the meaning of the calculated temperature.

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