Journal
JOURNAL OF METAMORPHIC GEOLOGY
Volume 33, Issue 6, Pages 557-577Publisher
WILEY
DOI: 10.1111/jmg.12132
Keywords
grain boundary sliding; metamorphic reaction; numerical modelling; rheology; strain localization
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Funding
- Australian Research Council [FT110100070, DP120102060]
- ARC LIEF
- DEST Systemic Infrastructure Grants, Macquarie University and Industry
- Australian Research Council [FT110100070] Funding Source: Australian Research Council
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Granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow (<20mm) felsic dykes, in Fiordland, New Zealand, forming similar to 10-80mm wide garnet reaction zones. The metamorphic reaction changed the abundance of minerals, and their shape and grain size distribution. The extent of reaction and annealing (temperature-related coarsening and nucleation) is greatest close to the dykes, whereas further away the reaction is incomplete. As a consequence, grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres. The aspect ratios of clusters of S1 pyroxene and pargasite in the orthogneiss, which are variably reacted to post-S1 garnet, decrease from high in the host, to near equidimensional close to the dyke. Post-reaction deformation localized in the fine-grained partially reacted areas. This produced a pattern of paired' shear zones located at the outer parts of the garnet reaction zone. Our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction. The weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage.
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