MULTI-SYSTEM APPROACHES TO UNDERSTANDING HIGH GRADE METAMORPHISM

High grade metamorphic terranes provide a wide range of challenges for geochronology, including partial melting, prolonged periods at high temperature, decoupled chemical systems, and polymetamorphic textures. Modern techniques have enabled a far greater interrogation of such rocks, particularly with the development of geochronology with ever increasing precision, spatial resolution, and data integration (e.g. LASS). As a result, there is often a huge wealth of U-Pb and other age data from a wide variety of minerals collected faster and easier than ever. What has been developed more slowly are techniques to interrogate the texture of rock samples as rapidly as the ages. Pressure and temperature estimations of metamorphic rocks are typically based on large scale bulk analysis (e.g. XRF), and/or electron microprobe mineral analyses. Both techniques have their frailties in generating data from a larger number of textures over different scales, essential for more detailed investigation of larger numbers of samples.

Here we show two examples of granulite facies metamorphic rocks that have variable mineralogy and texture at the metre to millimetre scale, defining a complex relationship with regional and sample geochronological data. Mafic rocks within the Lewisian Complex of NW Scotland show varied mineralogy at the outcrop scale clearly distinguished in both hand sample and thin section, and may be related to trace element signatures in zircon that highlight Archean mafic input into the region. At the finer Scale, individual thin sections from the Rauer Islands in Antarctica show sub-mm symplectites relating to the final stages of melt crystallisation, and relate directly to the majority of zircon ages from the sample. These textures are investgated using large area mapping with quantitative EDS analysis on the SEM that can be linked directly to P-T estimates using the XMapTools software package.