Fluid-induced retrogression of granulites in the Bergen Arcs, Caledonides of W. Norway: Fluid inclusion evidence from amphibolite-facies shear zones

Abstract

At Holsnøy Island, Bergen Arcs (Caledonides of W. Norway), amphibolite-facies shear zones transect Grenvillian granulite-facies rocks. Within the shear zones, the orthopyroxene bearing mineral assemblages of the protolith are replaced by biotite- and amphibolite-bearing parageneses. The shear zones are assumed to have formed at T=600±75°C and within the kyanite field, but below the stability of omphacite. Quartz veins central in the shear zones contain saline aqueous fluid inclusions, representing a fluid phase with 2–16 wt,% NaCl and a variable Na Ca ratio. In areas with only static retrogression, aqueous fluid inclusions with variable salinity (from 5 to ca. 50 wt.% NaCl) are associated with CO 2-inclusions with minor N 2-contents ( X N2=4–6 mole%). The aqueous fluid was introduced during amphibolite-facies metamorphism, the carbonic fluid was inherited from the protolith. The composition and density properties of the aqueous fluids are compatible with introduction at the assumed amphibolite-facies conditions and suggest an uplift path (sub) parallel to their isochores. The carbonic fluid inclusions have been re-equilibrated during late stages of post-metamorphic uplift. The data presented highlight the differences between the fluid regimes in the granulite-facies (CO 2), eclogite-facies (N 2-rich fluids) and amphibolite-facies (H 2O-salt) metamorphic events at Holsnøy.