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 psammites of the Dalradian Supergroup of the SW Scottish Highlands have been used as quantitative sensors of fluid flow, from which ‘fossil’ fluid flow-paths have been mapped. Infiltration of H2O–CO2 fluid caused carbonation of the metabasite assemblage. Consequently, metabasite sills have developed a characteristic asymmetric zonation of reacted (carbonate-bearing) margins and unreacted (carbonate-absent) interiors. Reaction front advection theory has been used to constrain the time-integrated fluid flux and flow direction in one dimension, for each sill, from the widths of the reacted margins. Three-dimensional fluxes and flow directions have been constrained from one-dimensional fluxes by geometrical computation, and regional fluid flow patterns have been deduced. 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 Phyllites, within which fluid fluxes were found to increase exponentially with proximity to the axial surface of the Ardrishaig Anticline. Fluid flow through the psammites was limited to narrow zones of intense deformation associated with metabasic or phyllitic intercalations. Although time-integrated fluid fluxes were locally high (4 × 102 m3/m2), average fluxes (100 m1/m2) are compatible with availability of fluid from underlying devolatilizing rocks. Complete isotopic homogenization of phyllites, psammites and metabasites within the axial region of the Ardrishaig Anticline is predicted by these high measured fluxes.