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