Fluid circulations in response to mantle exhumation at the passive margin setting in the north Pyrenean zone, France

Abstract

Sub-continental lithospheric mantle rocks are exhumed in the distal part of magma-poor passive margins. Remnants of the North Iberian paleo-passive margin are now exposed in the North–Pyrenean Zone (NPZ) and offers a field analogue to study the processes of continental crust thinning, subcontinental mantle exhumation and associated fluid circulations. The Saraille Massif which belongs to the ‘Chainons Bearnais’ range (Western Pyrenees), displays field, petrographic and stable isotopic evidence of syn-kinematic fluid circulations. Using electron probe micro-analyses on minerals, O, C, Sr isotopes compositions and micro thermometry/Raman spectrometry of fluid inclusions, we investigate the history of fluid circulations along and in the surroundings of the Saraille detachment fault. The tectonic interface between the pre-rift Mesozoic sedimentary cover and the mantle rocks is marked by a metasomatic talc-chlorite layer. This layer formed through the infiltration of a fluid enriched in chemical elements like Cr leached from the exhuming serpentinized mantle rocks. In the overlying sediments (dolomitic and calcitic marbles of Jurassic to Aptian age), a network of calcitic veins, locally with quartz, formed as a consequence of the infiltration of aqueous saline fluids (salinities up to 34 wt% NaCl are recorded in quartz-hosted fluid inclusions) at moderate temperatures (~ 220 °C). These brines likely derived from the dissolution of the local Triassic evaporites. In the upper part of the metasomatic system, upward movement of fluids is limited by the Albian metasediments, which likely acted as an impermeable layer. The model of fluid circulation in the Saraille Massif sheds light onto other synchronous metasomatic systems in the Pyrenean realm.