Exhumed lower crust in NW Ireland, and a model for crustal conductivity

Abstract

Granulite-facies gneisses within the Ox Mountains inlier, NW Ireland, are interpreted as an exhumed segment of sub-Dalradian lower crust, formed at 605 ± 37 Ma and later wedged upwards by crustal imbrication along the N-dipping Highland Boundary Fault-Clew Bay terrane boundary. A lower crustal provenance for the gneisses is suggested by evidence for eclogite decompression and for the later replacement of sillimanite by kyanite in response to slow cooling at depth. Granulite-facies rocks are believed to be present throughout much of the lower continental crust. Granulites are electrically resistant in contrast to the lower crust, which is usually electrically conducting. This paradox may be resolved by inferring that the granulite-facies rocks form dry, kilometre-scale lenses (mega-augen) wrapped around by anastomosing, retrograded, brine-soaked, electrically-conducting shear zones. Retrogressive hydration is believed (a) to seal the margins of the dry mega-augen with expanded reaction products, and (b) to concentrate the residual brine. Amphibolite-facies rocks associated with the lower crustal shear zones (above about 350°C) would equilibrate with strong, unsaturated brine and become electrically conducting. In the upper crust, similar rocks would concentrate the brine to the point of saturation, whereupon halite would precipitate, all water be consumed, and the rock would become an insulator.