Earth and Oceanographic Science

Story posted May 18, 2011

Formation of elongated garnets in the Spring Point amphibolite, Harpswell, Maine

Numerous garnet porphyroblasts (Alm_55-70Grs_20-25Prp_3-6Sps_6-12) in the Spring Point Formation have an anomalous aspect ratio ranging from 3:1 to 8:1. This is well beyond the 1:1 ratio typical for isometric garnets. These elongated garnets have the potential to expand our understanding of garnet growth and deformation mechanisms. The Spring Point Formation, part of the Ordovician Casco Bay Group, originated as backarc basin volcanics that were subsequently metamorphosed into garnet-bearing amphibolite gneiss during the Acadian Orogeny. Garnet-hornblende-plagioclase thermobarometry yields near-peak metamorphic conditions of 500-550°C and 4-5 kbar. Most garnets do not have pronounced major-element zoning; a few garnets show minor Ca and Mn concentric zoning.

Electron backscatter diffraction (EBSD) maps reveal complex patterns of crystal lattice orientations for the elongated garnets. High-angle grain boundaries are common, and suggest that many – but not all – garnets are polycrystalline. Most polycrystal domains, as well as single crystal garnets, show evidence of low-angle subgrain boundaries, and gradual lattice orientation shifts about a single axis. Field observations and backscattered electron images show brittle cracks, spiraled inclusion trails and deformed pressure shadows indicating dominant pre- to syn-kinematic growth of garnet.

The elongated garnets appear to have formed through a combination of growth and deformation mechanisms. Some polycrystals seemed to have formed early in the growth history given the observed concentric major-element zoning and high-angle boundaries. Dislocation creep may explain the observed gradual lattice misorientations about a common axis and the observed subgrains. Inclusion-poor terminations formed after deformation are likely the result of dissolution-reprecipitation processes. Late, post-growth, brittle deformation further elongated the garnet.