Compositional zoning patterns in garnet porphyroblasts from kyanite-bearing samples of the Devonian Littleton Formation, north-central Massachusetts, reveal complex patterns of growth that are related to multiple deformation and metamorphic events. Garnet porphyroblasts exhibit asymmetrical and irregular zoning patterns in XMn, XCa and Fe/(Fe + Mg). Zoning reversals in Mn and Fe/(Fe + Mg) and patch distribution in Ca appear to occur around the boundaries of the textural zones. Also, the compositions of the garnet at the textural boundaries are variable for all traverses. These observations suggest that the garnet zoning was not only modified from diffusion processes, but was also influenced by pre-existing microfabrics through the effects of preferential dissolution and resorption in partial disequilibrium. Relationships between chemical and textural truncations indicate that the zoning patterns of garnet were strongly modified from preferential dissolution and precipitation during the development of successive foliations that occurred in zones of high strain/stress (cleavage seams) and zones of low strain/stress, respectively.
Bibliographical noteFunding Information:
I thank T.H. Bell for initiating this project and reviewing drafts of this paper. I acknowledge M.J. Rubenach, K.A. Hickey and D. Areden for their careful reviews and constructive suggestions. I also thank H.W. Lee for enlightening discussions on microstructure. This work was partially supported by the Doctoral Merit Research Scheme (DMRS) of James Cook University.
- Preferential dissolution and precipitation
- Zoning reversal
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