Dating high-grade metamorphism - constraints from rare-earth elements in zircon and garnet

We integrate petrography and SIMS REE analyses of garnet and polyphase zircon from a pelitic granulite adjacent to the Ronda peridotite, Betic Cordillera, southern Spain to constrain the significance of zircon U-Pb geochronology. Sillimanite inclusions in garnet rims suggest that they grew during decompression, and Ca enrichment in their rims records initiation of partial melting. Chondrite-normalised REE profiles of zircon cores are typically magmatic (positive La to Lu slope and Cc anomaly), whereas overgrowths have flat or negatively sloping heavy-REE profiles (Gd-Lu). The presence of rimmed zircon grains only in the garnet rims and the matrix suggests that this zircon phase grew after garnet had already sequestered heavy REEs, a process documented here by progressive depletion of heavy REE in the garnets from centre to rim. Combined with the textural evidence, we suggest that the U-Pb age of 21.3 +/- 0.3 Ma obtained from the zircon rims dates a point on this decompression path rather than the peak metamorphic pressure.