FLUID HISTORY DURING DEEP BURIAL AND EXHUMATION OF OIL-BEARING VOLCANICS, HERCYNIAN BELT OF SOUTHERN BRITTANY, FRANCE

摘要

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 unde-formed 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 (adulariza-tion), 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℃) 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.