Origin of retrograde fluids in metamorphic rocks

Retrograde alteration of crystalline basement rocks is often the result of infiltration of water from sedimentary basins or the surface. At high crust al levels, fluid advects at near-hydrostatic pressure, and large fluxes can lead to mineralisation as well as retrogression. In the mid-crust (T > 300 degrees C) surface-derived fluids preserved in fluid inclusions show evide nce of overpressuring. It is proposed that fluid is pumped down into dry ba sement during faulting and causes a ductile response with recrystallisation that leads to them being sealed in and compressed. The distribution of flu id inclusion homogenisation temperatures is quite different for the two cas es, and more extensive wall rock reaction at elevated temperatures can incr ease salinity. (C) 2000 Elsevier Science B.V. All rights reserved.