BRIEF HISTORY OF COOLING FOLLOWING GRANULITE-FACIES METAMORPHISM, BARROVIAN ZONES, SCOTLAND

Cooling rates of granulite-facies metapelitic gneisses in the northeastern Grampian Highlands of Scotland were investigated by modeling multicomponent diffusion between garnet and biotite. Peak-T conditions were attained ~470 Ma following crustal thickening, and were likely driven by proximal synmetamorphic gabbros (e.g., Insch Gabbro, Morven Cabrach Gabbro). The sillimanite−K-feldspar-bearing gneisses of this study are located near the Morven Cabrach Gabbro in Glen Muick. The gneisses reached ~0.9 GPa and ~800 °C, the highest temperatures recorded by exposed metamorphic rocks in the Barrovian zones.

Cooling rates were estimated by forward modeling the time required for diffusion to transform assumed initially flat garnet compositional profiles to the measured retrograde profiles (diffusion coefficients from Chakraborty and Ganguly, 1992; Carlson, 2006). Garnet grains in contact with biotite were analyzed for major elements using the JEOL JXA8530F electron microprobe at Yale University. Chemical profiles were acquired across garnets starting at the biotite contact and traversing inward. Mg/Fe is generally constant across the interiors of the garnet grains and then decreases toward the rims, consistent with retrograde cooling. The diffusion profiles penetrate 150 to 300 μm into garnet rims.

Results indicate a simple and brief history of cooling following peak conditions. The measurements can be fit by a two-stage cooling model comprising an initially faster rate of cooling (~30-35 °C/Myr) for approximately 3-4.5 Myr, followed by a rate of ~10-15 °C/Myr down to 450-500 °C. These rates are consistent with previous studies which have used other lines of evidence (e.g., radiometric dating, thermobarometry) to estimate rapid rates of erosion (~3 to ~7 mm/yr) and of cooling (~14 to ~35 °C/Myr) (e.g., Friedrich et al., 1999; Oliver et al., 2008). The results are also consistent with the preservation of evidence for short timescales of peak heating recorded in nearby lower-grade samples, as relatively rapid cooling would limit the length of time the rocks were exposed to high-T conditions during exhumation. This type of modeling can determine continuous retrograde T-t paths, thus providing valuable information about cooling histories beyond that which can be obtained by conventional thermochronology.