Retrogression by deep infiltration of meteoric fluids into thrust zones during late-orogenic rapid unroofing

Hypersaline (Na-Ca-Cl) fluids are associated with late-stage quartz veining and retrogression of garnet, kyanite and other high P-T phases in the vici nity of thrusts and major lithological boundaries in the Scandian nappes of the Troms-Ofoten region, northern Norway. They record early Devonian fluid infiltration during rapid exhumation in the final stages of Caledonian oro genesis. The delta(18)O and delta D characteristics of these late fluids pr ovide compelling evidence for deep circulation of meteoric fluids. The sub- greenschist to low greenschist facies retrogression (P=2 +/- 1 kbar; T =300 -350 degrees C) suggests infiltration to depths of 7-9 km in a regime of su pra-hydrostatic fluid pressure. Peak metamorphic quartz veins and associate d fluids have delta D and delta(18)O characteristics consistent with a meta morphic origin (delta D -47 to -75 parts per thousand; delta(18)O+8.6 to +1 7.4 parts per thousand). However, late quartz veins and associated fluids s how a broad spread of delta D from -42 to -148 parts per thousand, interpre ted in terms of meteoric fluid infiltration. Such negative delta D values a re only recorded in present-day high-latitude or high-altitude settings, an d since north Norway was in an equatorial setting (10 degrees S) in the ear ly Devonian, a high-altitude origin is deduced. By calculation, and by comp arison with modern examples, the early Devonian mountains of the north Norw egian Caledonides are interpreted to have had a topography in excess of 5 k m. The deep circulation of surface waters is interpreted in terms of topogr aphically driven flow, linked with a hydrothermal system induced by elevate d geothermal gradients due to rapid uplift. Whilst the case for deep penetr ation of surface-derived fluids has been promoted for Mesozoic and younger mountain belts, this study represents one of the first documented examples for a Palaeozoic orogenic belt. It suggests that many of the fundamental pr ocesses operating during the exhumation of mountain belts are similar irres pective of age.