Formation of garnet polycrystals during metamorphic crystallization

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Abstract

Garnet polycrystals may form throughout the metamorphic history of a rock, starting at the earliest stages of garnet growth when closely spaced nuclei coalesce. In mica schist from Townshend Dam, VT, electron back-scattered diffraction (EBSD) analysis shows that garnet polycrystals possess two or more distinct lattice orientations separated by high-angle boundaries (28–61°). The minimum rotational displacements required to bring these lattice orientations into concordance with each other are commonly normal to the same low-energy planes that occur as crystal faces of euhedral garnet. There is no evidence for intracrystalline deformation, and the polycrystals therefore probably represent individual garnet crystals that coalesced during growth. The boundaries cross-cut growth zoning and inclusion trails of the polycrystals, indicating that early-formed polycrystals, once coalesced, behave chemically and physically as single crystals. Statistical analysis of a 3D, high-resolution X-ray computed tomographic data set of a large sample (912 cm3) of a Townshend Dam schist, combined with microprobe and EBSD analyses of garnet, are consistent with a high degree of clustering at all stages of garnet growth. The formation and prevalence of polycrystals implies that garnet nuclei impinged on each other and coalesced, and that coalescence was a common feature throughout garnet growth in the rock.

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