CONTROLS ON THE MECHANISMS OF FLUID INFILTRATION AND FRONT ADVECTION DURING REGIONAL METAMORPHISM - A STABLE-ISOTOPE AND TEXTURAL STUDY OF RETROGRADE DALRADIAN ROCKS OF THE SW SCOTTISH HIGHLANDS

Vein-controlled retrograde infiltration of H2O-CO2 fluids into Dalradi an epidote amphibolite facies rocks of the SW Scottish Highlands under greenschist facies conditions resulted in alteration of calcite-rich marble bands to dolomite and spatially associated O-18 enrichment of a bout 10 parts per thousand. on a scale of metres. Fluid inclusion data indicate that the retrograde fluid was an H2O-salt mixture with a low CO2 content, and that the temperature of the fluid was about 400-degr ees-C. Detailed petrographic and textural (backscattered electron imag ing) studies at one garnet-grade locality show that advection of fluid into marbles proceeded by a calcite-calcite grain edge flow mechanism , while alteration of non-carbonate wall-rock is associated with veinl ets and microcracks. Stable isotopic analysis of carbonates from marbl e bands provides evidence for advection of isotopic fronts through car bonate wall-rocks perpendicular to dolomite veins, and fluid fluxes in the range 2.4-28.6 m3/m2 have been computed from measured advection d istances. Coincidence of isotope and reaction fronts is considered to result from reaction-enhanced kinetics of isotope exchange at the reac tion front. Front advection distances are related to the proportion of calcite to quartz in each marble band, with the largest advection dis tance occurring in nearly pure calcite matrix. This relationship indic ates that fluid flow in carbonates is only possible along fluid-calcit e-calcite grain edges. However, experimental constraints on dihedral a ngles in calcite-fluid systems require that pervasive infiltration occ urred in response to calcite dissolution initiated at calcite-calcite grain junctions rather than to an open calcite pore geometry. The regi onal extent of the retrograde infiltration event has been documented f rom the high deltaO-18 of dolomite-ankerite carbonates from veins and host-rocks over an area of least 50 x 50 km in the SW Scottish Highlan ds. Isotopically exotic O-18-rich retrograde fluids have moved rapidly upwards through the crust, inducing isotopic exchange and mineral rea ction in wall-rocks only where lithology, pore geometry or mineral sol ubilities, pressure and temperature have been appropriate for pervasiv e infiltration to occur.