STABLE ISOTOPES OF HYDROGEN IN CLAY MINERALS IN FAULT GOUGE REVEALS METEORIC FLUID INFILTRATION IN THE SUBANDEAN FOLD-THRUST BELT, ARGENTINA

A study of stable isotopes of hydrogen in clay minerals collected from fault gouge was performed in order to elucidate the fluid sources and pathways in deforming upper crust. Several fault gouge samples and one folded shale sample were collected from the Argentinian Precordillera fold-thrust belt in the San Juan Province (west-central Argentina). Based on fault geometries and related sedimentary sequences, fault movement in this section of the Precordillera occurred from Miocene to recent times. Of the samples collected for analysis, two contained multiple clay phases with significant amounts of chlorite, illite and smectite, with minor kaolinite. Three samples contained only two clay phases: illite and chlorite. Detailed semi-quantitative XRD analysis of clay size fractions reveal mineralogies in gouge separates that are dominated by predominantly detrital chlorite and authigenic illite. δD_SMOW measurements on clay gouge show internal consistency in isotopic signatures among the samples, ranging from a value of -70 ± 3 ‰ to -90 ± 3 ‰. In two samples that contain smectite, δD values are clustered, but lack a discernable trend with composition. In contrast, the samples without smectite, show systematic trends in δD with composition; finer size fractions are richer in illite and are more depleted in δD than chlorite-rich, coarser-grained size fractions. From these patterns, we interpret the illite and chlorite as representing discrete mineral populations with distinct origins, with chlorite grains having a detrital origin, whereas illite grains were formed during deformation. Extrapolation of a chlorite-illite mixing line yields values for the authigenic illite component in two fault gouges of -93 ± 3 ‰ and -95 ± 3 ‰, and in folded shale of -79 ± 3 ‰. Based on relevant fractions, these results require significant, but variable involvement of meteoric fluids. The calculated δD value of fluid at 150 ± 50°C gives a range of values of -77‰ to -89‰. This meteoric signature is more negative than modern meteoric waters in the area, reflecting climatic or elevation differences, or some combination. Samples with slightly lower δD values indicate partial influence of formation and/or metamorphic fluids, but still require considerable infiltration of surface waters during regional deformation of the Precordillera of Argentina.