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

Barker, A.J., Bennett, D.G., Boyce, A.J. and Fallick, A.E. (2000) Retrogression by deep infiltration of meteoric fluids into thrust zones during late orogenic rapid unroofing. Journal of Metamorphic Geology, 18 (3), 307-318.

Abstract

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 &dgr;18O and &dgr;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 °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 &dgr;D and &dgr;18O characteristics consistent with a metamorphic origin (&dgr;D -47 to -75; &dgr;18O+8.6 to +17.4). However, late quartz veins and associated fluids show a broad spread of &dgr;D from -42 to -148, interpreted in terms of meteoric fluid infiltration. Such negative &dgr;D values are only recorded in present-day high-latitude or high-altitude settings, and since north Norway was in an equatorial setting (10° 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.

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