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

Syntectonic fluids have been sampled through fluid inclusion microthermomet ry and crush-leach analyses (cations and halogens) from a 50 km N-S transec t 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 limesto nes and evaporites in more external parts of the belt. The main datasets co me from (1) Alpine shear zones cutting the Neouvielle granodiorite in the H ercynian 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 l ocalities in the footwall and hangingwall of the Gavarnie Thrust on the sou thern margin of the Axial Zone. The inclusion fluids generally decrease in salinity from 27-35% at the nort hern 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 a re interpreted as relict fluids after seawater evaporation and halite preci pitation in the upper Trias. This interpretation is supported by Cl-Br-Na s ystematics, which are consistent with a change from halite to halite + sylv ite precipitation with progressive evaporation. Fluids in the basement shea r zones are interpreted to have essentially the same evaporitic origin as t hose still contained in sedimentary formations, although it is possible tha t final concentration of brines in the Neouvielle Massif involved retrograd e hydration reactions with removal of water by precipitation of hydrous min erals. The fluids are also very similar in salinity and halogen chemistry t o those found in veins associated with Mesozoic Pb-Zn-F deposits which pred ate 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 le vel fluid (connate or meteoric water) circulating within the Mesozoic carbo nates of the higher thrust sheets. At one locality where Triassic evaporite s are still present, high Cl/Br ratios at relatively low salinities are pre sent in inclusions within the underlying Triassic redbeds, but low Cl/Br ra tios at higher salinities are seen lower in the sequence. This is consisten t with dissolution of halite by a dilute fluid, but with limited penetratio n 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 in to underlying redbeds and fractured basement rocks; (2) Circulation of brin es with formation of Pb-Zn deposits along faults at some time between the T riassic and the Upper Cretaceous; (3) Renewed extension with erosion of Tri assic rocks in many areas and further drawing down of Triassic brines into the basement; (4) Deposition of U. Cretaceous and Palaeocene carbonates con taining connate waters of marine origin: (5) Formation of the Pyrenean thru st belt with overpressuring and expulsion of the brines along shear zones a nd 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, overpress ured brines with these more dilute, hydrostatically pressured fluids. An im portant point is that because of their density, hypersaline brines are diff icult to expel from the upper crust, and may be involved in a succession of alteration and mineralisation events in the same general area over hundred s of millions of years. Copyright (C) 2000 Elsevier Science Ltd.