Isotopic constraints on fluid infiltration from an eclogite facies shear zone, Holsenøy, Norway

ABSTRACTGranulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d̀C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, 40Ar/36Ar, trace‐element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite‐eclogite transition.A metre‐wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet‐clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite‐granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite‐in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz‐phengite‐plagioclase.C‐N‐Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite‐eclogite transition coincides with a marked change in CO2 abundance and δ13C (<36ppm, δ13C=‐2% in the granulite; <180 ppm, δ13C=‐10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (40Ar/36Ar > 25 times 103) and pre‐existing Ar in the granulite (40Ar/36Ar < 8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post‐dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2‐rich (δ13C ∼ ‐5%) with variable N2 and Ar abundances and isotopic compositions.Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O‐rich fluid network. In contrast, a carbon‐isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2‐rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.