Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

• Published in: Lithos
• Main Authors: Kellett, Dawn A., Warren, Clare, Larson, Kyle, Zwingmann, Horst, van Staal, Cees R., Rogers, Neil
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2016
• Subjects: Canada; Dover; Newfoundland
• Online Access: https://oro.open.ac.uk/45705/; https://oro.open.ac.uk/45705/1/Influence%20of%20deformation.pdf; https://doi.org/10.1016/j.lithos.2016.03.003

White mica 40 Ar/ 39 Ar analyses may provide useful constraints on the timing of tectonic processes, but complex geological and thermal histories can perturb Ar systematics in a variety of ways. Ductile shear zones represent excellent case studies for exploring the link(s) between dynamic re-/neo-crystallization of white mica and coeval enhanced fluid flow, and their effect on 40 Ar/ 39 Ar dates. White mica 40 Ar/ 39 Ar dates were collected from compositionally similar granites that record different episodes of deformation with proximity to the Dover Fault, a terrane-bounding strike-slip shear zone in the Appalachian orogen, Newfoundland, Canada. 40 Ar/ 39 Ar data were collected in situ by laser ablation and by step heating single crystals. Results were compared to each other and against complementary U-Pb zircon and monazite, and K-Ar fault gouge analysis. Although step-heat 40 Ar/ 39 Ar is a widely applied method in orogenic settings, this dataset shows that relatively flat step-heat 40 Ar/ 39 Ar spectra are in contradiction with wide spreads in in-situ 40 Ar/ 39 Ar dates from the same samples, and that plateau dates in some cases yielded mixed dates of equivocal geological significance. This result indicates that the step-wise release of Ar from white mica likely homogenizes and obscures spatially-controlled Ar isotope reservoirs in white mica from sheared rocks. In contrast, in situ laser ablation 40 Ar/ 39 Ar analysis preserves the spatial resolution of 40 Ar reservoirs that have been variably reset by deformation and fluid interaction. This study therefore suggests that laser ablation is the best method for dating the timing of deformation recorded by white mica. Final interpretation of results should be guided by microstructural analysis, estimation of deformation temperature, chemical characterization of white mica, and complementary chronometers. Overall the dataset shows that granitic protoliths were emplaced between 430-422 Ma (U-Pb zircon). High strain deformation along the Wing Pond Shear Zone ...