Fluid-rock interactions at high-pressure metamorphic conditions: An analysis of atoll-garnets preserved in eclogitic breccias from the Zermatt-Saas zone, Italian Alps.

In the Piemonte-Liguria ophiolites of the western Alps, the Zermatt-Saas unit is characterized by a widespread eclogite facies metamorphism. Eclogitic rocks are derived from basaltic or gabbroic protoliths. Peak metamorphic conditions are indicated by omphacite, almandine-rich garnet, rutile and Fe-poor epidote assemblages, which allow to infer temperatures between 550 and 600°C and pressures up to 2.5 GPa.
This aim of this contribution is to report the results of a petrologic analysis of an eclogitic breccia exposed in the Zermatt-Saas unit in the Monte Avic area, Aosta valley. The fragments consist of eclogitic gabbro, while the matrix is made of omphacite locally retromorphosed into blueschist to greenschist facies assemblages. Clasts are characterized by abundant atoll-shaped garnets. Two types of atoll-shaped garnets are distinguished: type I garnets consist of an almandine-rich unzoned core, a finely oscillatory zoned rim with alternating grossular-rich and almandine-rich overgrowths, and omphacite between core and rim. Type II garnets are similar to type I garnets, the only difference being titanite (+/- epidote) between core and rim. The atoll-shaped garnets result from two episodes of fluid-mineral interactions. For type I garnets, these interactions took place under high-pressure conditions (stability of omphacite), and under medium-pressure conditions (stability of titanite) for type II garnets.
Brecciation is characterized by pervasive fracturing. The two types of atoll-shaped garnets are fractured. The fractures are sealed by omphacite. This shows that brecciation took place after the formation atoll-shaped garnets. On-going investigations aim at identifying the chemical signature of the reacting fluids and estimating precise pressure and temperature conditions of fluid-mineral interactions.