GARNET TECTONICS: INTEGRATED COMPOSITIONAL AND 3-D TEXTURAL DATA FROM A MICASCHIST, TOWNSHEND DAM, VERMONT

Three-dimensional imaging of a garnet schist provides spatial information that can be integrated with chemical (composition, zoning) and physical data (distribution of garnets with respect to microstructures and compositional layering) to understand processes of metamorphic crystallization, including the influence of deformation on garnet nucleation and growth. A 900-cm³ block of garnet schist from Townshend Dam, Vermont, was analyzed with high-resolution X-ray computed tomography (HRXCT) to reveal the distribution, shape, and size of garnets. Garnets in selected sections were analyzed by microprobe for composition/zoning and by EBSD for crystallographic orientation. The schist is comprised of alternating 1-10 mm thick layers that vary in modal amounts of Ms, Pg, Bt, Chl, Qz, Pl, and Grt. Deformation of the layers was late syn-kinematic with respect to Grt crystallization, and resulted in development of Qz pressure shadows on Grt; these can be seen as pits on the sides of garnets in the HRXCT images. The garnets, which are Fe-rich and growth zoned (core: Alm66 Sps10 Prp5 Grs19; rim: Alm79 Sps1 Prp10 Grs9), are typically 5 mm in diameter, but vary from 1 mm to 2.2 cm. Inclusion trails of elongate Qz, Ep, Ilm, Ap, and Pl are oriented oblique to the matrix foliation. Garnets in Ms+Pg-rich layers are typically euhedral and as wide as the layers, in some cases impinging on adjacent Bt-rich layers. Garnets in Bt-rich layers range are typically subhedral to anhedral.

Garnets in the sample are commonly comprised of 2-3 crystals whose grain boundaries are at high-angles, crosscut growth zoning and inclusion trails, and, although irregular, typically involve segments of the dodecahedral crystal face. 35% of the 815 garnets in the sample are in clusters of 2 or more garnets. Integration of textural and composition information indicates that clustering occurred at different stages of metamorphism, ranging from very early (clusters that share a high Mn core and are only detected by EBSD) to later (morphologically obvious clusters with distinct Mn cores). Future analysis of the sample will evaluate whether there are patterns to the distribution of clustered garnets with respect to compositional layering or other microstructural features, and assess the significance of clustering for metamorphic crystallization processes.