Spatial coupling between spilitization and carbonation of basaltic sills in SW Scottish Highlands: evidence of a mineralogical control of metamorphic fluid flow

摘要

In a geochemical and petrological analysis of overprinting episodes of fluid–rock interaction in a well-studiednmetabasaltic sill in the SW Scottish Highlands, we show that syn-deformational access of metamorphic fluids andnconsequent fluid–rock interaction is at least in part controlled by preexisting mineralogical variations. Lithologicalnand structural channelling of metamorphic fluids along the axis of the Ardrishaig Anticline, SW Scottish Highlands,ncaused carbonation of metabasaltic sills hosted by metasedimentary rocks of the Argyll Group in the DalradiannSupergroup. Analysis of chemical and mineralogical variability across a metabasaltic sill at Port Cill Maluaig showsnthat carbonation at greenschist to epidote–amphibolites facies conditions caused by infiltration of H2O-CO2 fluidsnwas controlled by mineralogical variations, which were present before carbonation occurred. This variability probablynreflects chemical and mineralogical changes imparted on the sill during premetamorphic spilitization. Calculationnof precarbonation mineral modes reveals heterogeneous spatial distributions of epidote, amphibole, chloritenand epidote. This reflects both premetamorphic spilitization and prograde greenschist facies metamorphism priornto fluid flow. Spilitization caused albitization of primary plagioclase and spatially heterogeneous growth of epidoten± calcic amphibole ± chlorite ± quartz ± calcite. Greenschist facies metamorphism caused breakdown of primarynpyroxene and continued, but spatially more homogeneous, growth of amphibole + chlorite ± quartz. Thesenprocesses formed diffuse epidote-rich patches or semi-continuous layers. These might represent precursors of epidotensegregations, which are better developed elsewhere in the SW Scottish Highlands. Chemical and field analysesnof epidote reveal the evidence of local volume fluctuations associated with these concentrations of epidote.nTransient permeability enhancement associated with these changes may have permitted higher fluid fluxes andntherefore more extensive carbonation. This deflected metamorphic fluid such that its flow direction became morenlayer parallel, limiting propagation of the reaction front into the sill interior.