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

• Published in: Journal of Metamorphic Geology
• Main Authors: Barker, A.J., Bennett, D.G., Boyce, A.J., Fallick, A.E.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2000
• Subjects: QE Geology; Norway; Ofoten; North Norway; Northern Norway; Troms
• Online Access: https://eprints.gla.ac.uk/871/; https://doi.org/10.1046/j.1525-1314.2000.00257.x

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 vicinity 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 orogenesis. The delta(18)O and delta D characteristics of these late fluids provide 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 supra-hydrostatic fluid pressure. Peak metamorphic quartz veins and associated fluids have delta D and delta(18)O characteristics consistent with a metamorphic origin (delta D -47 to -75 parts per thousand; delta(18)O+8.6 to +17.4 parts per thousand). However, late quartz veins and associated fluids show a broad spread of delta D from -42 to -148 parts per thousand, interpreted in terms of meteoric fluid infiltration. Such negative delta D values are only recorded in present-day high-latitude or high-altitude settings, and since north Norway was in an equatorial setting (10 degrees S) in the early Devonian, a high-altitude origin is deduced. By calculation, and by comparison with modern examples, the early Devonian mountains of the north Norwegian Caledonides are interpreted to have had a topography in excess of 5 km. The deep circulation of surface waters is interpreted in terms of topographically driven flow, linked with a hydrothermal system induced by elevated geothermal gradients due to rapid uplift. Whilst the case for deep penetration 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 processes operating during the exhumation of mountain belts are similar irrespective of age.