HR: 1340h
AN: MR43A-0981    [Abstracts]
TI: Microstructural evolution of garnet in a greenschist facies transpression zone
AU: * Massey, M A
EM: mamass1@uky.edu
AF: University of Kentucky, Earth & Environmental Sciences, Lexington, KY 40506-0053, United States
AU: Prior, D J
EM: davep@liverpool.ac.uk
AF: University of Liverpool, Jane Herdman Laboratories, 4 Brownlow Street, Liverpool, L69 3GP,
AU: Moecher, D P
EM: moker@uky.edu
AF: University of Kentucky, Earth & Environmental Sciences, Lexington, KY 40506-0053, United States
AB: Natural observations, laboratory experiments, and theoretical modeling support the interpretation of Grt plasticity in the lower crust and upper mantle; however, these processes are thought to be of little importance in shallow to middle crustal levels. Multiple textural varieties of Grt from the western boundary (Mt. Dumplin high strain zone) of an upper greenschist facies dextral transpression zone in southern New England, USA, display mesoscopic and microscopic evidence of syn-tectonic deformation and recrystallization. These microstructures were examined further by optical microscopy, electron probe microanalysis, orientation contrast imaging (OCI), and automated electron backscatter diffraction (EBSD) in order to understand possible low-grade deformation mechanisms and their significance. The N-S-striking shear zone dips steeply W, the mylonitic foliation is defined by aligned Ms- Chl-Rt, layers of Qtz and fine-grained Grt; Qtz-Chl-Ms and fine-grained Grt aggregates define lineations that plunge moderately SW. S-C-C¡¦ fabrics, asymmetric folds and porphyroclasts (delta- and sigma-type) are well developed on foliation-normal/lineation-parallel planes, and display sinistral kinematics; surfaces normal to foliation and normal to lineation exhibit strong asymmetries that indicate normal motion. Pre-tectonic mineral parageneses consist of St pseudomorphed by Chl-Ms-Ctd, Als pseudomorphed by Ms, and coarse-grained Grt and Ab porphyroclasts with associated asymmetric tails. Grt is manifest as three types: 1) equant Grt porphyroclasts; 2) elongate Grt aggregates consisting of 50-100 ƒİm equant Grt porphyroblasts; 3) type 1-type 2 transitional Grt morphology. Elemental x-ray mapping of Ca and Mn reveals at least two periods of growth in Grt types 1 and 3, and one period of growth in type 2 that correlates with type 1 and 3 rims; Mg is completely homogenized. Detailed mapping of type 3 Grt cores reveals ¡¥fractured¡¦ Ca-enriched cores ¡¥healed¡¦ with Ca- depleted composition. OCI of type 1 Grt shows no internal substructure. OCI of type 2 Grt also shows rare internal substructure (finer-grained equant inclusions with low angle boundaries), and EBSD shows aggregates have CPO symmetrical to tectonic fabric (parallel to lineation), high angle grain boundaries, and neighbor- neighbor grain pairs correlate with random grain pair distributions. Type 2 Grt also displays ¡¥stacking¡¦ structures where individual porphyroblasts are stacked vertically and grain boundaries are at low angle. OCI of type 3 Grt shows considerable internal substructure of three varieties: A) substructure boundaries that coincide with Ca-depleted compositions in cores; B) 50-100 ƒİm equant substructures included in rims; C) substructure boundaries in rims that ¡¥nucleate¡¦ from substructure A boundaries in cores. Detailed EBSD traverses across all substructure boundaries indicate rotation around rational crystallographic axes. Observations suggest that early amphibolite facies Grt (type 1 and 3 cores) was deformed non-penetratively by plastic deformation or sub-critical fracture (type 3 Grt, substructure A). Type 2 Grt nucleated pre- to syn-tectonically, at least partially through the consumption of type 3 Grt porphyroclasts, and was included in type 1 and 3 rims by rigid body rotations (substructure B). Substructures C in type 3 Grt rims are inherited from pre-existing crystallographic anisotropies in cores (substructure A). Additionally, type 2 Grt was deformed syn-tectonically to produce CPOs, likely as a result of flattening associated with transpression.
DE: 3699 General or miscellaneous
DE: 3902 Creep and deformation
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting