Fluid history during deep burial and exhumation of an oil-bearing volcanics: Hercynian belt of southern Brittany, France

In southern Brittany (France), the Porphyroid nappe, a Variscan tectonic unit made of volcanic pyroclastic deposits that was involved in a low-temperature/high-pressure prograde metamorphic path followed by exhumation and extension, displays evidence for very large and distributed strains achieved through dissolution/crystallization processes. Such distributed strains at low temperature are rather unusual in felsic lithologies. Recent studies have documented that these rocks deformed within a system closed to fluids. This paper presents results of detailed analyses of fluid inclusions and stable isotopes within the deformed metavolcanics and their undeformed counterparts in order to document the fluid history of the tectonic unit. Results show that its initial thermal evolution, likely occurring in a context of basin inversion, involved important open-system alteration, with K-metasomatism (adularization), oil impregnation and hydration. During the subsequent tectonic evolution, the fluids maintained a constant composition in the C-H-O-N system within the Porphyroid nappe and its para-autochthon, being water-rich and containing hydrocarbon components likely related to oil pyrolysis/oxidation. Atoll-like fluid inclusion morphology and rare undisturbed fluid inclusions with steep isochore slopes are consistent (> 750 MPa) with pressures estimated for the peak of metamorphism using mineral solid-solution thermodynamics. Fluid inclusions indicate a sub-isothermal retrograde path (between 420 and 300°C) followed by a high thermal gradient at the end of the retrograde path. Within the underlying para-autochthonous unit, only the late, high-temperature gradient is recorded by fluid inclusions. The sub-isothermal low-grade path suggests fast exhumation of the unit, without thermal relaxation. The high geotherm observed at the end of the P-T path is consistent with the post-thickening extensional collapse and associated intrusion of granites documented in the area. Hydration and heterogeneous oil/water distribution achieved in the early metasomatic evolution of the porous pyroclastic deposits are responsible for subsequent fluid confining in the Porphyroid nappe and for its strongly distributed strain pattern.