The eclogite-facies Allalin gabbro of the Zermatt-Saas ophiolite, Western alps: A record of subduction zone hydration

Abstract

The Allalin gabbro is a 2 km × 0.5 km block of layered olivine-gabbro and troctolite included in the Zermatt-Saas ophiolite nappe 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 rock and mineral composition data, the Allalin gabbro represents part of a Middle Jurassic underplate of mafic magma at the base of the continental crust of the Apulian plate (Dent Blanche-Sesia Lanzo system). Granulite-facies recrystallization during cooling at ∼825°C and 1.0 GPa, involving the formation of Opx-Grt coronas between Ol-Pl, can be related to crustal thickening. Eocene subduction of the Tethys oceanic lithosphere under the Apulian plate detached the gabbro from the base of the continental crust and incorporated it into the ophiolite. Increasing pressure in the descending slab had little effect on parts of the gabbro, which still locally contain unaltered igneous Ol, Aug and Pl and well-preserved magmatic textures. With increasing subduction depth an increasing amount of aqueous fluid accessed the gabbro and transformed Pl to Zo-Jd-Ky-Qtz. At about 2.5 GPa (93 km) at c. 610°C, a dramatic hydration process converted most of the rocks (>90 vol. %) into a fully hydrated eclogite-facies assemblage of Omp + Zo + Tlc + Cld ± Grt ± Ky + Rt. The full hydration under water-present conditions occurred at the greatest depth reached by the gabbro. After detachment from the downgoing slab (i.e. along the ascent path), Gln, Pg and Mrg formed as additional hydrates. This last phase of hydration desiccated the metagabbro at a depth of c. 78 km and from then on the rocks were essentially devoid of a free fluid phase. The Allalin gabbro provides strong evidence that the fundamental high-pressure transformation of mafic rocks involves the hydration of gabbro to form eclogite. © The Author 2009. Published by Oxford University Press. All rights reserved.