Age, duration and mineral markers of magma interactions in the deep crust: an example from the Pyrenees

Abstract

We report in-situ dating, and Hf and O-isotope studies of zircon crystals, together with electron microprobe imaging of garnet crystals from a migmatitic diorite containing noritic globules, in the Ursuya massif (French Pyrenees). Diorite zircon crystals have old inherited cores and 295 ± 2 Ma rims, while those in the norite are homogeneous, with lower U and Pb, and higher Th and Th/U ratios than in the diorite zircon rims. The 295 Ma dates represent the crystallization age of hybridised crustal melts. Norite zircon crystals have similar ages (298 ± 3 Ma) to the diorite zircon rims, interpreted as the crystallization age of the norite. Hf isotopes indicate that the noritic magmas were already crustally contaminated prior to intrusion at their present locations. The contrasting behaviours of slow- and fast-diffusing species within garnet (e.g. P vs Ca) caused sharp P and diffuse Ca core-rim transitions. Modelling of garnet Ca diffusion profiles suggests a duration of 2 ± 1 Myr for the magma interactions in the deep crust. Integrated with granite dating, these data provide insights into pulsations in the regional thermal event that concluded the Hercynian orogenic cycle.