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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 [sup.40]Ar/[sup.39]Ar and Rb/Sr techniques to better constrain the timing of Alpine deformation for this region. Incremental [sup.40]Ar/[sup.39]Ar 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 [sup.40]Ar/[sup.39]Ar total gas dates obtained for the same samples. The Rb/ Sr dates were calculated using initial [sup.87]Sr/[sup.86]Sr ratios obtained from the carbonate-dominated host rocks, which are higher than normal Mesozoic carbonate values due to exchange with fluids of higher [sup.87]Sr/[sup.86]Sr ratios (and lower [sup.18]O/[sup.16]O ratios). Model dates calculated using [sup.87]Sr/[sup.86]Sr values typical of Mesozoic marine carbonates more closely approximate the [sup.40]Ar/[sup.39]Ar total gas dates for most of the samples. The similarities of Rb/Sr and [sup.40]Ar/[sup.39]Ar total gas dates are consistent with limited amounts of detrital mica in the samples. The [[delta].sup.18]O values range from 24-15 [per thousand] (VSMOW) for 2-6[micro]m micas and 27-16[per thousand] 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[degrees]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 [sup.40]Ar/[sup.39]Ar steps above 20 Ma, and 2) most samples from the deepest Helvetic thrusts have steps (often accounting for more than 80% of [sup.39]Ar release) between 15 and 25 Ma. These dates are consistent with the order of thrusting in the foreland-imbricating system and increase proportions of neoformed to detrital mica in the more metamorphosed hinterland and deeply buried portions of the nappe pile. Individual thrusts accommodated the majority of their displacement during their initial incorporation into the forelandimbricating system, and some thrusts remained active or were reactivated down to 15 Ma.

Staircase 40Ar/39Ar spectra of 2 to 6 [[micro]meter] white micas in ductile carbonate mylonites (Morcles-Doldenhorn nappe, Switzerland) record ages between 32 and 13 Ma. These ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold-thrust nappe. We interpret these data to record protracted white mica neocrystallization and growth during low-temperature ductile deformation, such that the differences between the maximum and minimum ages in the 40Ar/39Ar spectra record the time span over which mica formed and grew. We have modeled the argon concentrations in these micas, taking into account the thermal history of the nappe, mica growth, and diffusional loss of argon. Our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite, and that the retentivity of argon in white mica is greater than generally thought.

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 δ18O values range from 24–15‰ (VSMOW) for 2–6 μm micas and 27–16‰ 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 °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 above 20 Ma, and 2) most samples from the deepest Helvetic thrusts have steps (often accounting for more than 80% of 39Ar release) between 15 and 25 Ma. These dates are consistent with the order of thrusting in the foreland-imbricating system and increase proportions of neoformed to detrital mica in the more metamorphosed hinterland and deeply buried portions of the nappe pile. Individual thrusts accommodated the majority of their displacement during their initial incorporation into the foreland-imbricating system, and some thrusts remained active or were reactivated down to 15 Ma. [ABSTRACT FROM AUTHOR]