LITHOLOGICAL AND STRUCTURAL CONTROLS ON REGIONAL 3-D FLUID-FLOW PATTERNS DURING GREENSCHIST FACIES METAMORPHISM OF THE DALRADIAN OF THE SW SCOTTISH HIGHLANDS

Greenschist facies metabasite sills enclosed within phyllites and psam mites of the Dalradian Supergroup of the SW Scottish Highlands have be en used as quantitative sensors of fluid flow, front which 'fossil' fl uid flow-paths have been mapped. Infiltration of H2O-CO2 fluid caused carbonation of the metabasite assemblage. Consequently, metabasite sil ls have developed a characteristic asymmetric zonation of reacted (car bonate-bearing) margins and unreacted (carbonate-absent) interiors. Re action front advection theory has been used to constrain the time-inte grated fluid flux and flow direction in one dimension, for each sill, from the widths of the reacted margins. Three-dimensional fluxes and f low directions have been constrained from one-dimensional fluxes by ge ometrical computation, and regional fluid flow patterns have been dedu ced. On a regional scale, fluid flow was channelled through phyllites and focused towards the axial zone of major antiformal fold structures . Fluid flow was channelled through the 6 km thick Ardrishaig Phyllite s, within which fluid fluxes were found to increase exponentially with proximity to the axial surface of the Ardrishaig Anticline. Fluid flo w through the psammites was limited to narrow Zones of intense deforma tion associated with metabasic or phyllitic intercalations. Although t ime-integrated fluid fluxes were locally high (4 x 10(2) m(3)/m(2)), a verage fluxes (100 m(3)/m(2)) are compatible with availability of flui d from underlying devolatilizing racks. Complete isotopic homogenizati on of phyllites, psammites and metabasites within the axial region of the Ardrishaig Anticline is Predicted by these high measured fluxes.