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


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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
Contributions to Mineralogy and Petrology
Kirschner D.L., Masson H., Cosca M.A.
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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 Ar-40/Ar-39 and Rb/Sr techniques to better constrain the
timing of Alpine deformation for this region. Incremental Ar-40/Ar-39
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 Ar-40/Ar-39 total gas dates obtained for the same
samples. The Rb/ Sr dates were calculated using initial Sr-87/Sr-86
ratios obtained from the carbonate-dominated host rocks, which are
higher than normal Mesozoic carbonate values due to exchange with fluids
of higher Sr-87/Sr-86 ratios (and lower O-18/O-16 ratios). Model dates
calculated using Sr-87/Sr-86 values typical of Mesozoic marine
carbonates more closely approximate the Ar-40/Ar-39 total gas dates for
most of the samples. The similarities of Rb/Sr and Ar-40/Ar-39 total gas
dates are consistent with limited amounts of detrital mica in the
samples. The delta(18)O values range from 24-15%. (VSMOW) for 2-6 mum
micas and 27-16parts 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 degreesC, 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 Ar-40/Ar-39 steps above 20 Ma, and 2) most
samples from the deepest Helvetic thrusts have steps (often accounting
for more than 80% of Ar-39 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.
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07/12/2012 14:51
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20/08/2019 12:21
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