Crystallization and ‘cooling’ ages for igneous and high-grade metamorphic rocks are routinely determined from plateau in 40
Ar apparent age spectra. In lower-grade sedimentary and metasedimentary rocks, for which peak temperatures are not sufficient to overprint detritally-inherited thermal histories, 40
Ar apparent age spectra are often considered too complex to resolve useful geochronologic data. However, previous studies of the classic low-grade Barrovian (Viete et al. 2011) and Buchan (Stenhouse et al. 2014) metasedimentary rocks in the Grampian Terrane, Scotland have demonstrated the feasibility of the method of asymptotes and limits (Forster & Lister 2004) for extracting pre-metamorphic detrital age populations in white micas. Zircon—being an exceptionally refractory mineral, with a high closure temperature—is well-suited for detrital studies examining large-scale tectonothermal events related to orogenic processes. Conversely, white mica is less physically resilient and thermal ages can be more easily ‘reset’ or partially reset by subsequent tectonothermal events; detrital white mica 40
Ar may offer the opportunity to perform spatially-resolved analysis of tectonothermal events on a more local scale than in more detrital zircon U/Pb approaches.
We investigate the utility of detrital white mica 40Ar/39Ar geochronology as a tool for resolving similarities and differences in tectonothermal histories along the Appalachian–Caledonian orogenic belt. We have collected chlorite-grade pelitic rocks from Iapetan rift–drift facies from Tennessee, Virginia, Vermont, Quebec and Scotland. Electron probe analyses of major-element mineral chemistry in these rocks has revealed distinct white mica populations that may be linked to distinct age populations in published 40Ar/39Ar step-heating spectra from Scotland. This work will seed a more thorough assessment of Iapetan rifting history.
Forster, MA & Lister, GS, 2004. J Struct Geol 26, 287–305.
Stenhouse, IR, Forster, MA & Lister, GS, 2014. J Geol Soc 171, 343–352.
Viete, DR, Forster, MA & Lister, GS, 2011. J Geol Soc 168, 133–146.