DATING MYLONITIC DEFORMATION BY THE AR-40-AR-39 METHOD - AN EXAMPLE FROM THE NORUMBEGA FAULT ZONE, MAINE

Strongly mylonitic rocks associated with the regionally extensive Noru mbega fault zone in south-central Maine provide an excellent opportuni ty for testing the effects of mylonitization on argon isotopic systems in muscovite. Ar-40/Ar-39 muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well de fined Early Carboniferous plateau ages. In contrast to these nonmyloni tized samples, all age spectra for muscovite from the mylonites are hi ghly discordant. They are characterized by young ages at low extractio n temperatures, which systematically increase to ages that equal the p lateau ages for muscovite collected outside the mylonite zone. Detaile d petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older, relict, muscovite po rphyroclasts and fine-grained white mica aggregates that recrystallize d during mylonitic deformation. Total gas ages of five different grain size fractions separated from the same mylonite sample become progres sively younger with decreasing grain size; indicating a larger compone nt of the recrystallized grains in the finer grain size fractions. Alt hough the three finest grain size fractions give different total gas a ges and do not overlap in age for most of their release spectra, their initial increments do coincide, at approximately 290 Ma, This indicat es a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 Ma age provides a good est imate for the time of mineral growth associated with mylonitic deforma tion. These data, combined with kinematic analysis, reveal that the se gment of the Norumbega fault zone studied, the Sandhill Corner fault, is a Late Carboniferous-Early Permian dextral strike-slip fault. A lac k of significant offset in regional Early Carboniferous mineral age pa tterns across the fault suggests that displacement was probably less t han 30 km. This study demonstrates that Ar-40/Ar-39 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 c haracterized prior to analysis. Dynamic recrystallization must have oc curred at or below the closure temperature of the mineral to be analyz ed. Regional cooling patterns must also be established through detaile d thermochronology so that mineral ages and age spectra from the defor med rocks can be compared to regional cooling ages of the same mineral . Finally, the effects of excess argon must be negligible.