UPLIFTED METAMORPHIC BASEMENT AS A SENSOR OF INTERCONNECTED FLUID RESERVOIRS: AN EXAMPLE FROM THE SW SCOTTISH HIGHLANDS

Metamorphic basement rocks are generally assumed to be much less permeable than overlying sedimentary basins, with fluid flow confined to fractures and faults at shallow depths beneath the sediment-basement interface. We have undertaken detailed stable isotope, fluid inclusion and textural studies, at a range of scales, of retrograde infiltration and alteration of Dalradian metamorphic basement in the SW Scottish Highlands. This study reveals much more widespread and long-lived permeability than previously recognised, which taps both deep crustal and surface-derived fluids. Early permeability created during uplift of the Caledonian metamorphic complex exploited fractures in which high temperature (>350°C), moderately saline (4-10 wt.% NaCl eq) aqueous fluids precipitated quartz and sulphides. Oxygen and sulphur isotope data indicate that fluids were derived by syn-metamorphic dehydration of rocks at depth. This fluid may also have been responsible for regional chloritisation of schists at deeper levels. The early fracture system was re-exploited by later low-temperature (<100 to ~150°C) brines which precipitated 18O-enriched dolomite-ankerite carbonates whose unusual compositions and distribution can only be explained by deeply penetrating basin fluids from overlying sedimentary basins over a large (at least 50 x 50 km) area. Similar dolomite veins cutting Carboniferous rocks in N Ireland are assigned a late Carboniferous age (Evans et al. 1998). Permeability was greatly enhanced in this event by dolomitisation of marble wall-rock and pervasive mechanical infiltration of schists and phyllites. The pervasive permeability of basement schists at moderate to low temperatures is a largely unrecognised phenomenon. Detailed CL, SEM and 18O/16O microanalysis of dolomitised marbles have identified several generations of carbonate growth, including pore-filling, low d18O calcite which indicates later (?Tertiary) influx of meteoric water. Thus, initial fracture permeability created by early retrograde metamorphic fluids has been exploited repeatedly by later downward penetrating basinal and meteoric waters, with major permeability enhancement by chemical (dolomitisation, hydration) and mechanical processes.