Microstructural and Metamorphic Constraints on the Thermal Evolution of the Southern Region of the Lewisian Gneiss Complex, NW Scotland

Abstract

Felsic metagranitoids form a major part of the crust, but the metamorphic story they record is difficult to decipher because of a lack of index minerals. The microstructures and metamorphic assemblages of felsic gneisses and metadolerite dykes from the Lewisian Gneiss complex, NW Scotland, have been examined to estimate the pressure^temperature^time (P^T^t) history of the region. Characteristic geometries and compositions of zoned epidote and plagioclase from the gneisses and amphibole from the dykes provide key information. Bulk-rock compositions are modelled to constrain the likely metamorphic conditions experienced by the rocks. P^T^t paths are refined using a novel model for fractionation during grain-recycling of the plagioclase. In the gneisses, plagioclase grains have relatively albitic cores (An10^12) grading to more anorthitic rims (An20^30). The equant grain shapes of the plagioclase and asymmetry of the zoning across grain boundaries are consistent with the zoning having formed during coarsening, or grain-recycling, following deformation.The increase in anorthite content is due to the breakdown of epidote to release Ca and Al. Sharp boundaries between Fe-poor cores and Fe-rich rims in epidote result from the resorption of epidote whilst the plagioclase is growing followed by later regrowth.The possible P^Tconditions for the end of deformation and start of grain-recycling are restricted to those that occur along the plagioclase isopleth with the same value as the core compositions. These starting conditions are explored along with P^T^t path orientations over a range of values. The results are compared with the observed compositions and grain sizes to determine the best-fit P^T^t path. Most of the best-fit paths are dominated by decompression rather than heating (both of which result in epidote breakdown). Starting conditions are probably between 9 and 10·2 kbar at around 5808C along a quasi-linear path ending at around 7·5^8 kbar at 600^6208C.The timescale of decompression is poorly constrained owing to the uncertainty in the grain-recycling parameters. Rates of exhumation are between 0·14 and 2 mm a, which are reasonable within the range of present-day processes. Scourie dyke assemblages and mineral zoning broadly corroborate this P^T^t path. The path is similar to those recorded in Phanerozoic orogenic cycles but the significance of this work lies in our new methods for elucidating the metamorphic histories of metagranitoids.