FLUID SOURCES AND TIMING OF FLUID INFILTRATION IN UPPER CRUSTAL ROCKS; FINGERPRINTING CLAYS FROM FOLDS IN THE MEXICAN FOLD-THRUST BELT

A well-exposed train of mesoscopic, asymmetrical folds in a sequence of Cretaceous bentonitic shale interbedded with limestone in the Mexican fold-thrust belt that formed in the anchizone was targeted to explore the potential of clay to determine timing and source(s) of fluids interacting during deformation. Using fluid-inclusion microthermometry, illite-crystallinity, H isotope analyses and Ar dating of neoformed illite, we test the hypothesis that syn-folding vein minerals and clay minerals preserve the same fluid source(s) by comparing the δ²H composition of inclusion fluids in calcite and quartz from veins, and illite in folded shale layers.

Five clay size-fractions (<0.05, 0.05-0.2, 0.2-1,1-2 and <2µm) were separated from eight shale samples, in which illite, smectite, calcite, kaolinite, smectite, chlorite and minor quartz were identified by XRD analysis. Most samples show different proportions of various clay minerals, except for the finer fractions in two of the samples were illite is the only clay phase present. The timing of neomineralization is tested by dating four fractions from two samples. These size fractions produce a reliable Ar age range of 74-76 Ma, which is younger than the age of deposition and falls within the stratigraphic age for deformation in the Zimapán Basin. Samples containing chlorite and smectite show very low values in δ²H (-75.9 to -53.9 ‰), while samples containing illite and kaolinite or pure illite show relatively high δ²H values (-33.1 to -50.1 ‰). The latter fall within the δ²H range (-39 to -49 ‰) determined in water from fluid inclusions of syn-folding veins, indicating isotopic equilibrium between water, vein filling and illitic clay, according to fractionation factors at relevant temperature (200-250°C). The δ²H values, supported by fluid inclusion salinities, show that water active during folding was partly marine and partly meteoric. Thus, dating and isotopic analysis show that illite in shale layers precipitated during flexural folding and involved significant infiltration of surface fluids into the system at ~5km depth. We conclude that clay minerals have excellent potential to trace fluid sources and the timing of fluid infiltration in shallow-crustal rocks.