USING MAJOR AND ACCESSORY MINERAL REACTION TEXTURES TO CONSTRAIN THE P-T-T EVOLUTION OF UHT GRANULITES FROM THE GRUF COMPLEX, EUROPEAN CENTRAL ALPS

Estimating peak and retrograde P-T conditions of UHT granulites is challenging because UHT rocks commonly undergo some retrogression or overprinting, and melt loss changes bulk composition. Therefore, bulk mineral assemblages and compositions do not always represent equilibria. However, local reaction microtextures preserve snapshots of the P-T conditions the rock experienced. Detailed studies of these textures can constrain the P-T-t evolution of high-grade rocks. We present P-T estimates of reaction textures involving major and accessory minerals to elucidate the thermal history of the sapphirine-bearing granulites from the Gruf Complex, which underwent UHT metamorphism followed by an upper amphibolite to granulite facies overprint.

The P-T conditions of major mineral reactions were estimated with equilibrium assemblage diagrams calculated by Theriak-Domino for discrete textural domains at sub-thin section scale. Sapphirine + orthopyroxene ± cordierite ± rutile symplecties and coronae formed during UHT garnet breakdown (900–960°C at 8–10 kbar). Rutile within these textures, from other UHT assemblages, and exsolved from orthopyroxene preserve Zr-in-rutile temperatures above 900°C. Some rutile grains within UHT assemblages record temperatures of ca. 750°C; these grains are mostly in contact with zircon. In sub-peak, recrystallized assemblages, rutile records lower temperatures (746 ± 66°C). Metamorphic zircon rims have Ti-in-zircon temperatures of 650–750°C and an age of 32.5 ± 0.5 Ma; older domains were resorbed prior to growth of the rims.

We interpret that rutile can lose Zr during cooling/retrogression only if zircon can nucleate or is already present along grain boundaries. Without zircon, grain boundary diffusion of Zr in rutile is sluggish and thus peak temperatures are preserved. Furthermore, zircon growth was inhibited during UHT metamorphism possibly because rutile sequestered much of the Zr at peak conditions. This implies that zircon ages correspond to the timing of cooling, retrogression, and melt crystallization (not of UHT metamorphism). The occurrence of UHT symplectites, coranae, and exsolutions and the preservation of UHT rutile indicate that both the UHT and lower T events were short-lived and cooled rapidly, consistent with geochronological results.