Fluid mixing and recycling during Pyrenean thrusting: evidence from fluid inclusion halogen ratios

Abstract

Syntectonic fluids have been sampled through fluid inclusion microthermometry and crush-leach analyses (cations and halogens) from a 50 km N-S transect through the central-southern Pyrenees. The fluid inclusions are contained in syntectonic quartz veins in Triassic redbeds, Cretaceous carbonates and Hercynian basement rocks, with some calcite and dolomite data from limestones and evaporites in more external parts of the belt. The main datasets come from (1) Alpine shear zones cutting the Néouvielle granodiorite in the Hercynian Axial Zone at the north end of the transect; (2) An imbricate zone beneath the Alpine Gavarnie Thrust at the Pic de Port Vieux; (3) Several localities in the footwall and hangingwall of the Gavarnie Thrust on the southern margin of the Axial Zone. The inclusion fluids generally decrease in salinity from 27–35% at the northern end of the transect to 7–22% on the southern margin of the Axial Zone. The majority of the inclusions have Cl/Br ratios lower than seawater and are interpreted as relict fluids after seawater evaporation and halite precipitation in the upper Trias. This interpretation is supported by Cl-Br-Na systematics, which are consistent with a change from halite to halite + sylvite precipitation with progressive evaporation. Fluids in the basement shear zones are interpreted to have essentially the same evaporitic origin as those still contained in sedimentary formations, although it is possible that final concentration of brines in the Néouvielle Massif involved retrograde hydration reactions with removal of water by precipitation of hydrous minerals. The fluids are also very similar in salinity and halogen chemistry to those found in veins associated with Mesozoic Pb-Zn-F deposits which predate the thrusting. The lower salinities seen at the southern margin of the Axial Zone are interpreted to reflect mixing of the brines with a higher level fluid (connate or meteoric water) circulating within the Mesozoic carbonates of the higher thrust sheets. At one locality where Triassic evaporites are still present, high Cl/Br ratios at relatively low salinities are present in inclusions within the underlying Triassic redbeds, but low Cl/Br ratios at higher salinities are seen lower in the sequence. This is consistent with dissolution of halite by a dilute fluid, but with limited penetration downwards. We suggest that the fluid history of the Pyrenees evolved through a series of stages: (1) Upper Triassic evaporite formation with sinking of brines into underlying redbeds and fractured basement rocks; (2) Circulation of brines with formation of Pb-Zn deposits along faults at some time between the Triassic and the Upper Cretaceous; (3) Renewed extension with erosion of Triassic rocks in many areas and further drawing down of Triassic brines into the basement; (4) Deposition of U. Cretaceous and Palaeocene carbonates containing connate waters of marine origin; (5) Formation of the Pyrenean thrust belt with overpressuring and expulsion of the brines along shear zones and faults; (6) Creation of topography with a high-level circulation system in the Mesozoic thrust sheets driven largely by topography. At the southern margin of the Axial Zone there was limited mixing of the deeper, overpressured brines with these more dilute, hydrostatically pressured fluids. An important point is that because of their density, hypersaline brines are difficult to expel from the upper crust, and may be involved in a succession of alteration and mineralisation events in the same general area over hundreds of millions of years.