T80 Modeling Grain-Scale Processes in Metamorphic Rocks

Monday, 8 November 2004
8:00 AM - 12:00 PM

GARNET MISORIENTATIONS

Paper number: T80-6
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

Application of electron back-scattered diffraction (EBSD) analysis to garnets in several recent studies reveals misoriented regions within petrographically coherent garnet crystals. In this study, we show that misoriented zones extend from rim to rim across garnets and do not correspond to chemical or textural features (zoning, inclusion patterns), leading to questions about the origin of the misorientations and the mechanisms of garnet growth. We have used EBSD to document multiple (2-3) crystallographic orientations within petrographically coherent garnet crystals in mica schist from Townshend Dam, Vermont. Three lattice orientations were detected in a 1 cm long garnet with slightly curving inclusion trails of quartz, plagioclase, and ilmenite observed in a lineation-parallel section. The inclusion trails crosscut the roughly subparallel lattice boundaries. In a lineation-perpendicular section, we documented a garnet with 2 lattice orientations, and a misorientation zone that bisects the composite garnet and its inclusions. The misorientation angles between adjacent lattices are high (30 to 60 degrees).

Major element zoning in the garnets is overall concentric in garnets with and without misorientations. Mn decreases from the petrographic center of the garnets to the rim. Although the overall zoning pattern crosscuts lattice boundaries, there are very minor changes in Mn across the boundaries, with slightly lower Mn contents (< 2.5 mol% Sps) along the boundary. The distribution of Mn corresponds spatially to the location of quartz and plagioclase inclusions (lower Mn near inclusions), and does not outline distinct garnet lattice orientations. Also, in a garnet with 3 different orientations, one lattice zone corresponds to a low-Mn garnet rim region.

Our data rule out post-crystallization deformation as a cause of the misorientations. However, because there is no obvious relationship between Mn-zoning and lattice misorientations, the presence of Mn-zoning across the misorientation boundaries is not sufficient evidence to prove that such misorientations are the result of coalescencing high-Mn nuclei. The garnets may have formed from several nuclei that formed at different times in the metamorphic history of the rock, but the mechanism of attachment and continued growth remains a question.