Textural changes of graphitic carbon by tectonic and hydrothermal processes in an active plate boundary fault zone, Alpine Fault, New Zealand

Martina Kirilova, Virginia G. Toy, Nick Timms, Angela Halfpenny, Catriona Dorothy Menzies, Dave Craw, Olivier Beyssac, Rupert Sutherland, John Townend, Carolyn Boulton, Brett M. Carpenter, Alan Cooper, Jason Grieve, Timothy Little, Luiz Morales, Chance Morgan, Hiroshi Mori, Katrina Sauer, Anja M. Schleicher, Jack WilliamsLisa Craw

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Abstract

Graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization. We examine processes of graphitic carbon emplacement and deformation in the active Alpine Fault Zone, New Zealand by analysing samples obtained from Deep Fault Drilling Project (DFDP) boreholes. Optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain. Raman spectroscopy allowed interpretation of the degree of graphite crystallinity, which reflects both thermal and mechanical processes. In the amphibolite-facies Alpine Schist, highly crystalline graphite, indicating peak metamorphic temperatures up to 640°C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions (500–600°C), resulting in clustered (in protomylonites) and foliation-aligned graphite (in mylonites). In cataclasites, derived from the mylonitized schists, graphite is most abundant (<50% as opposed to <10% elsewhere), and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon. Tectonic–hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences. The increasing abundance of graphite towards the fault zone core may be a significant source of strain localization, allowing fault weakening.
Original languageEnglish
Pages (from-to)205-233
Number of pages29
JournalGeological Society Special Publications
Volume453
Early online date15 Nov 2017
DOIs
Publication statusPublished - 2018

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Keywords

  • graphite
  • hydrothermal
  • tectonic
  • fault
  • cataclasite
  • Raman
  • Alpine Fault
  • Deep Fault Drilling Project (DFDP)

Cite this

Kirilova, M., Toy, V. G., Timms, N., Halfpenny, A., Menzies, C. D., Craw, D., Beyssac, O., Sutherland, R., Townend, J., Boulton, C., Carpenter, B. M., Cooper, A., Grieve, J., Little, T., Morales, L., Morgan, C., Mori, H., Sauer, K., Schleicher, A. M., ... Craw, L. (2018). Textural changes of graphitic carbon by tectonic and hydrothermal processes in an active plate boundary fault zone, Alpine Fault, New Zealand. Geological Society Special Publications , 453, 205-233. https://doi.org/10.1144/SP453.13