The eclogite-facies Allalin gabbro of the Zermatt-Saas ophiolite, Western Alps, a record of subduction zone hydration

The Allalin gabbro is a 2km large block of layered olivine-gabbro and troctolite included in the Zermatt-Saas ophiolite of the Western Alps. Comprehensive texture, mineral and rock composition data together with a thermodynamic analysis of the complex phase associations permit a detailed reconstruction of the igneous and metamorphic reaction history recorded by the gabbroic rocks. Based on bulk composition and primary plagioclase-olivine compositions, the Allalin gabbro represents part of a Middle Jurassic underplate of mafic magma intruded at the base of the continental Apulian Plate (Dent Blanche - Sesia Lanzo system).
Granulite facies recrystallization during cooling at ~825°C/1.0GPa involving formation of orthopyroxene-garnet coronas between olivine-plagioclase and replacement of olivine by orthopyroxene-diopside took place during crustal thickening. Eocene subduction of the Tethys oceanic lithosphere under the Apulian Plate detached the gabbro from the base of the continent and incorporated it into the ophiolite. First partial hydration of the gabbro by water derived from epidote amphibolite/blueschist to eclogite facies transformation of Tethys ophiolite is indicated by the formation of chlorite that represents the latest stage of corona mineral assemblage. Increasing pressure in the descending slab had little effect on the gabbro, which still locally contains unaltered igneous olivine, augite and plagioclase and well preserved magmatic textures. With increasing subduction depth an increasing amount of aqueous fluid accessed the gabbro mainly by brittle deformation of plagioclase and facilitated by expansion cracks created by earlier chlorite formation, transformed plagioclase to zoisite-jadeite-kyanite-quartz. When the gabbro reached about 2.5GPa (93km) at ca.610°C, a dramatic hydration process resulting from the breakdown of antigorite in host ophiolite serpentinite converted most of the rocks (>90 vol%) into a fully hydrated eclogite-facies assemblage of omphacite-zoisite-talc-chloritoid±garnet±kyanite-rutile. After detachment from the downgoing slab, i.e. along the ascent path, glaucophane, paragonite, and margarite formed as additional hydrates in the eclogite as they became successively stabilised with decreasing pressure during uplift. This last phase of hydration desiccated the metagabbro at a depth of ca. 78km and from then on the rocks were essentially devoid of a free fluid phase. The Allalin gabbro confirms that the fundamental high-pressure transformation of mafic rocks is, gabbro + H2O = eclogite. The overall hydration reaction together with a density increase from 3.1g/cm3 (gabbro) to 3.4g/cm3 (eclogite) shows that the gabbro represented a passive player in the subduction process, requiring externally derived water to hydrate at the return point and an external buoyancy force to ascend with the host ophiolite along the subduction surface.