ISOTOPIC AND FLUID INCLUSION STUDIES OF FLUID MOVEMENT ALONG THE GAVARNIE THRUST, CENTRAL PYRENEES - REACTION FRONTS IN CARBONATE MYLONITES

Geochemical fronts for Sr isotopes have been identified within carbona te mylonites along the Gavarnie Thrust in the central Pyrenees. Over 2 00 samples have been analyzed for Sr-87/Sr-86, including samples from seven measured sections through the thrust zone. A geochemical profile has been drawn through the maximum Sr-87/Sr-86 values at each sample site, and two separate geochemical fronts identified. The data show th at Sr was transported southward along a 2m thick mylonite zone for at least 1.5 km. Fluid inclusion crush-leach analyses show that a Sr-rich hypersaline brine with a high Sr-87/Sr-86 ratio was trapped within Tr iassic redbeds beneath the Thrust; this is a likely candidate for the fluid that passed through the mylonites, increasing their Sr-87/Sr-86 ratios from depositional values close to that of Cretaceous seawater. Comparison of the Sr content of fluid inclusions and calcite in the sa me vein systems allows the solid-fluid partition coefficient, K(v-Sr) to be estimated. Values range from 0.5 in the northern part of the thr ust system to 1.9 farther south, where fluids are more dilute. These v alues can be combined with minimum transport distances for the geochem ical fronts to give minimum time-integrated fluxes of between 1800 and 3000 m3/m2. The estimates of K(v-Sr) are supported by partial-derivat iveO-18 data on carbonates, which show a good correlation with Sr-87/S r-86 in the southern part of the thrust system but a wide range in val ues at high Sr-87/Sr-86 in the north. This is expected if K(v-Sr) was similar to K(v-O) in the south but lower in the north. Veins in the ca rbonates almost invariably have more radiogenic Sr than the adjacent m atrix. This suggests a highly organized flow pattern, with unidirectio nal flow parallel to the mylonitic banding. Veins in hangingwall phyll ites have similar Sr-87/Sr-86 ratios to the mylonitized Cretaceous lim estones and are less radiogenic than the adjacent matrix, suggesting l imited escape of fluid upward out of the carbonates into hangingwall v ein systems. Although the precise shapes of geochemical profiles canno t be constrained, Damkohler numbers were probably in the range 20 to 1 00, indicating relatively rapid rates of fluid-rock equilibration comp ared with fluid flow velocity. Comparison of flux estimates suggests t hat time-averaged permeabilities were three orders of magnitude higher in the mylonites than in undeformed rocks and varied by a factor of a bout two within the mylonites. Overall, fluid flow can be explained by a fault-valve model, with slow pervasive flow through the mylonite du ring plastic deformation punctuated by rapid flow into veins during se ismic events.