Apparent isobaric cooling paths from granulites: Two counterexamples from British Columbia and New Hampshire

Textural evidence for the reaction histories of pelitic granulites from British Columbia and New Hampshire shows that pressure-temperature paths obtained from thermobarometry on these samples are an artifact of the calculation technique. Samples from both localities show breakdown of garnet to form cordierite, a reaction that occurs during decompression. Additional reaction textures and fluid-inclusion data also imply decompression at high temperatures. For both localities, however, core-rim thermobarometry yields nearly isobaric cooling paths. Similar cooling paths have been used by other authors to argue for a magmatic underplating model for regional mafic granulites. We show that mineral homogenization in pelitic granulites can lead to erroneous isobaric cooling paths and hence to incorrect conclusions concerning tectonic similarities between different types of granulites.