Publication Date:
1993
Description:
Strongly mylonitic rocks associated with the regionally extensive Norumbega
fault zone in south-central Maine provide an excellent opportunity for testing the
effects of mylonitization on argon isotopic systems in muscovite. 40Ar/39Ar muscovite
age spectra from samples outside the zone of mylonitization are relatively undisturbed
and have well defined Early Carboniferous plateau ages. In contrast to these
nonmylonitized samples, all age spectra for muscovite from the mylonites are highly
discordant. They are characterized by young ages at low extraction temperatures, which
systematically increase to ages that equal the plateau ages for muscovite collected
outside the mylonite zone. Detailed petrographic observations suggest that these
systematic discordances reflect a mixing of argon components from older, relict,
muscovite porphyroclasts and fine-grained white mica aggregates that recrystallized
during mylonitic deformation. Total gas ages of five different grain size fractions
separated from the same mylonite sample become progressively younger with decreasing
grain size; indicating a larger component of the recrystallized grains in the finer
grain size fractions. Although the three finest grain size fractions give different
total gas ages and do not overlap in age for most of their release spectra, their
initial increments do coincide, at approximately 290 Ma. This indicates a minimal older
age contribution from the relict porphyroclasts in the initial increments and suggests
the 290 Ma age provides a good estimate for the time of mineral growth associated with
mylonitic deformation. These data, combined with kinematic analysis, reveal that the
segment of the Norumbega fault zone studied, the Sandhill Corner fault, is a Late
Carboniferous-Early Permian dextral strike-slip fault. A lack of significant offset in
regional Early Carboniferous mineral age patterns across the fault suggests that
displacement was probably less than 30 km. This study demonstrates that 40Ar/39Ar dating
methods can be used to date deformational events effectively, as long as several
important criteria are met. First and foremost, samples must be well characterized prior
to analysis. Dynamic recrystallization must have occurred at or below the closure
temperature of the mineral to be analyzed. Regional cooling patterns must also be
established through detailed thermochronology so that mineral ages and age spectra from
the deformed rocks can be compared to regional cooling ages of the same mineral.
Finally, the effects of excess argon must be negligible.
Keywords:
Fracture
;
Geol. aspects
;
Fault zone
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