EBSD AND HRXCT ANALYSIS OF ELONGATED GARNETS OF THE SPRING POINT FORMATION, CASCO BAY, ME

Unusually elongated garnet porphyroblasts in amphibolite-facies gneiss have aspect ratios ranging from 1:1 to 1:5 and long axes of 0.5 cm to 1.5 cm. The garnets are found in the meta-igneous Ordovician Spring Point Formation exposed in Harpswell, Maine. The formation is part of the 170 km long Casco Bay Group consisting primarily of metamorphosed proto-Atlantic sedimentary rocks. Garnet size and distribution vary across the Harpswell exposure, but are consistent along foliation-parallel compositional bands. Garnets are surrounded by an amphibole-, plagioclase-, and quartz-rich matrix, and have abundant inclusions of quartz, ilmentite, and plagioclase, as well as pressure shadows of biotite and occasional calcite. Garnet compositions are approximately Py5Alm68Sps8Grs19, as determined through Energy Dispersive Spectrometry (EDS). Garnet-biotite thermometry yields peak metamorphic temperatures of 550-600 °C; regional metamorphic pressures have been previously reported at 3-4 kbar. The amphibolite-facies metamorphism occurred during orogenic compression with additional heat supplied by the underlying granitic plutons.

Three dimensional imaging of the garnets, obtained with high-resolution X-ray computed tomography (HRXCT) at University of Texas at Austin, showed that they are irregularly ellipsoidal with primary and secondary axes parallel to foliation. Electron backscatter diffraction (EBSD) analyses, at Bowdoin College, of 18 garnets reveals 3-25° shifts in crystallographic orientation across garnet grains. Low-angle subgrain boundaries of <10° in larger porphyroblasts are observed with consistent 1-3° gradual shifts in orientation across subgrains. These garnet microstructures suggest ductile metamorphic deformation or atypical dynamic crystallization. Previous researchers have shown that low to intermediate metamorphic grades of <700 °C cannot account for ductile deformation of garnet. Other processes responsible for the uncommon low-temperature garnet elongation could include fluid-facilitated porphyroblast growth or retrograde plagioclase replacement.