Mineralogical, K-Ar, Rb-Sr and stable isotope analyses have been carried
out on K-white micas from Helvetic Malm limestones in order to examine
their evolution during very low- to low-grade Alpine metamorphism,
associated with intense ductile deformation. Metamorphic temperatures
were estimated al approximately 300-degrees-C from stable isotopes
(quartz-calcite thermometry), occurrence of chloritoid, and `'epizonal''
illite crystallinity index. K-white micas consist of variable mixtures
of 2M, phengite and muscovite, as revealed by detailed X-ray diffraction
analyses using peak decomposition of the (060, 331) spectra. K-Ar
apparent ages display a strong grain-size dependence in which mainly
fine-grained size fractions (< 2 mum) record Alpine ages (37-15 Ma).
However, these ages provide a relative rather than an absolute
chronology of the diachronous Alpine metamorphic evolution of the
Helvetic nappes. The resetting of the K-Ar isotopic system of K-white
micas to Alpine metamorphic conditions reflects an apparent combination
of crystallization/recrystallization and radiogenic Ar-40 diffusion
loss. The oxygen isotope compositions of micas (+ 15 to + 22 parts per
thousand) are intermediate between detrital and O-18-enriched values
expected for micas neoformed within an abundant marine carbonate matrix.
No isotopic equilibrium has been reached between calcite and micas. The
variable depletion of hydrogen isotope compositions (- 126 to - 82 parts
per thousand) is influenced by the interaction with organic matter under
closed-system conditions. Organic matter, if not removed, may also
represent a serious source of error in K-Ar age determination, by
introducing radiogenic Ar-40 contamination. Sr-87/Sr-86 isotope ratios
of micas range from 0.70879 to 0.70902 with one outlier at 0.71794. The
low values reflect Sr exchange with calcite occurring during
crystallization/recrystallization of micas under closed-system
conditions.