An 40Ar/39Ar, Rb/Sr, and stable isotope study of micas in low-grade fold-and-thrust belt: An example from the Swiss Helvetic Alps

White micas in carbonate-rich tectonites and a few other rock types of large thrusts in the Swiss Helvetic fold-and-thrust belt have been analyzed by 40Ar/ 39Ar and Rb/Sr techniques to better constrain the timing of Alpine deformation for this region. Incremental 40Ar/ 39Ar heating experiments of 25 weakly metamorphosed (anchizone to low greenschist) samples yield plateau and staircase spectra. We interpret most of the staircase release spectra result from variable mixtures of syntectonic (neoformed) and detrital micas. The range in dates obtained within individual spectra depends primarily on the duration of mica nucleation and growth, and relative proportions of neoformed and detrital mica. Rb/Sr analyses of 12 samples yield dates of ca. 10-39 Ma (excluding one anomalously young sample). These dates are slightly younger than the 40Ar/ 39Ar total gas dates obtained for the same samples. The Rb/Sr dates were calculated using initial 87Sr/ 86Sr ratios obtained from the carbonate-dominated host rocks, which are higher than normal Mesozoic carbonate values due to exchange with fluids of higher 87Sr/ 86Sr ratios (and lower 18O/ 16O ratios). Model dates calculated using 87Sr/ 86Sr values typical of Mesozoic marine carbonates more closely approximate the 40Ar/ 39Ar total gas dates for most of the samples. The similarities of Rb/Sr and 40Ar/ 39Ar total gas dates are consistent with limited amounts of detrital mica in the samples. The d 18O values range from 24-15% (VSMOW) for 2-6 micro m micas and 27-16 per mil for the carbonate host rocks. The carbonate values are significantly lower than their protolith values due to localized fluid-rock interaction and fluid flow along most thrust surfaces. Although most calcite-mica pairs are not in oxygen isotope equilibrium at temperatures of ca. 200-400 deg C, their isotopic fractionations are indicative of either 1) partial exchange between the minerals and a common external fluid, or 2) growth or isotopic exchange of the mica with the carbonate after the carbonate had isotopically exchanged with an external fluid. The geological significance of these results is not easily or uniquely determined, and exemplifies the difficulties inherent in dating very fine-grained micas of highly deformed tectonites in low-grade metamorphic terranes. Two generalizations can be made regarding the dates obtained from the Helvetic thrusts: 1) samples from the two highest thrusts (Mt. Gond and Sublage) have all of their 40Ar/ 39Ar steps abov