Fluid‐driven transformation of blueschist to vein eclogite during the Early Eocene in a subducted sliver of continental crust (Monte Emilius, Italian Western Alps)

Abstract

AbstractIn the Penninic nappe stack of the Western Alps, high‐ to ultrahigh‐pressure metamorphic ophiolites of the Zermatt‐Saas Zone are associated with slivers of continental crust. In one of these slivers, Monte Emilius, the overprinting of pre‐Alpine granulite‐facies rocks by subduction‐related, Alpine eclogite‐facies metamorphism can be studied. Mafic granulites were initially transformed into blueschists. In a second step, shear zones were developed in which the blueschists recrystallized to fine‐grained, foliated glaucophane eclogites, and eclogite veins. The combination of petrographic and field observations as well as whole‐rock compositions suggests that the eclogite assemblage formed only in shear zones where Ca‐metasomatism induced a change in major element composition. These substantial differences in bulk rock composition demonstrate how spatially limited eclogitization may be controlled by chemical redistribution, the degree of fabric development, and associated metamorphic reactions along fluid pathways. Thermodynamic modelling of selected bulk rock compositions yielded only slightly different conditions of 1.8 ± 0.1 GPa/550 ± 50°C for blueschist and 1.9–2.3 GPa/550 ± 50°C for eclogite, constraining Ca‐rich fluid infiltration and transformation to a depth of ~60–70 km. Eclogitization occurred in the Early Eocene at 52.96 ± 0.91 Ma, as indicated by a well‐defined Lu–Hf garnet isochron.