29-3 Evaluating the roles of geochemistry, deformation, and coalescence to produce high aspect ratio garnet in Midcoast Maine

Session: Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories (Posters)

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Abstract:

Metapelitic and metavolcanic outcrops of the Yarmouth Island formation in Midcoast Maine feature garnet porphyroblasts with high aspect ratios of up to 6.4, with long axes measuring up to 1.5 cm. Curiously, these high aspect ratio garnet porphyoblasts are located within a few cm of idioblastic garnet that measure up to 1 cm in diameter. The nearby Spring Point formation contains similar high aspect ratio garnet porphyroblasts that are interpreted to have been produced through a combination of brittle deformation, ductile deformation, and the coalescence of multiple nuclei. Here, we evaluate which processes contribute to the formation of high aspect ratio garnet in the Yarmouth Island formation and compare our findings with results from the Spring Point formation.  

To assess the role of growth conditions, we investigated bulk compositional differences across six representative thin sections using energy dispersive spectroscopy (EDS) and collected point data along transects on 19 representative garnets with a range of aspect ratios to inspect core-rim chemical zoning. Most garnets have prograde zoning with some resorption near their rims, shown by sharp increases in Mn near the rim. Idioblastic and xenoblastic porphyroblasts have similar chemistries, which suggests that garnet composition does not contribute to the observed elongate garnet morphology. Absence of zoning truncation in EDS compositional transects perpendicular to foliation indicates that dissolution-precipitation creep was not a primary driver of garnet elongation. Electron backscatter diffraction (EBSD) maps reveal that some high aspect ratio porphyroblasts result from the coalescence of multiple nuclei; these multi-nuclei grains feature normal zoning across the entire porphyroblast. The lack of systematic fractures perpendicular to foliation within the porphyroblasts indicates that brittle deformation was not a primary driver in forming high aspect ratio garnets.  

High aspect ratios in garnet can be related to both deformation and growth conditions. Whereas the Yarmouth Island formation garnets have high aspect ratios because of coalescence, the nearby Spring Point formation garnets yield similar morphologies from brittle and ductile deformation. This finding emphasizes that detailed geochemistry and crystal orientation mapping are required to differentiate among these drivers.   

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