MORPHOLOGICAL EVIDENCE FOR KINETIC CONTROLS ON GARNET GROWTH DURING REGIONAL METAMORPHISM, ORANGE-MILFORD BELT, CONNECTICUT, USA

Morphological variations in garnets from two localities within the Orange-Milford Belt suggest garnet nucleation and growth relatively far from chemical equilibrium, i.e., overstepped growth. Regional metamorphism of the Belt occurred during the Acadian Orogeny. Garnet porphyroblasts within graphite-rich garnet to kyanite zone rocks in the Wepawaug Schist exhibit well-defined textural sector zoning. Anhedral or star-shaped graphite-free interiors abruptly give way to heavily graphitic but euhedral exteriors. Agglomerations of graphite on the edges of the interior structure, as well as numerous inclusions marking the sector boundaries are observed. Grossular-uvarovite garnet porphyroblasts from carbonated serpentinite in the greenschist facies Derby Hills Schist, which structurally underlies the Wepawaug Schist, exhibit a distinctive petal-like morphology. The garnet porphyroblasts emanate from central nucleation points or within fine-grained garnet clots, expand in a rounded bulbous form and have curved lineations along the crystal surface.

Kinetic controls on garnet growth were assessed using qualitative crystal morphology diagrams, which relate crystal morphologies and growth mechanisms to growth rate and the driving force for crystallization (Dm/kT). The internal morphology of the Wepawaug garnets as well as the external morphology of the Derby Hills garnets indicates nucleation and growth under significantly overstepped conditions. The two dissimilar morphologies may also represent different degrees of overstepping and speed of growth with the internal morphology of the Wepawaug garnet suggestive of a greater degree of overstepping. The change from an irregular or star-shaped interior to a euhedral exterior in the Wepawaug garnets could have resulted from a rough to smooth interface transition related to a decrease in driving force for crystallization (Dm/kT) over time. We speculate that the regional extent of overstepping is related to: 1) widespread metamorphic fluid infiltration focused into fractures and permeable layers, and/or 2) relatively rapid heating.