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TitleInfluence of deformation and fluids on Ar retention in white mica: dating the Dover Fault, Newfoundland Appalachians
AuthorKellett, D AORCID logo; Warren, C; Larson, K P; Zwingmann, H; van Staal, C R; Rogers, N
SourceLithos vol. 254-255, 2016 p. 1-17, Open Access logo Open Access
Alt SeriesGeological Survey of Canada, Contribution Series 20150445
Alt SeriesEarth Sciences Sector, Contribution Series 20150445
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
ProvinceNew Brunswick; Newfoundland and Labrador; Quebec; Prince Edward Island
NTS1M/03; 1M/04; 1M/05; 1M/06; 1M/07; 1M/10; 1M/11; 1M/12; 1M/13; 1M/14; 1M/15; 1M/16; 2C/04; 2C/05; 2C/11; 2C/12; 2C/13; 2C/14; 2D/01; 2D/02; 2D/03; 2D/04; 2D/05; 2D/06; 2D/07; 2D/08; 2D/09; 2D/10; 2D/11; 2D/15; 2D/16; 2E/01; 2E/02; 2E/08; 2E/09; 2F/02; 2F/03; 2F/04; 2F/05; 2F/06; 2F/12; 11P/08; 11P/09; 11P/16
AreaFortune Bay; Bonavista Bay; Appalachians
Lat/Long WENS -56.4167 -52.8750 49.6250 47.0000
Subjectsstructural geology; igneous and metamorphic petrology; geochronology; tectonic interpretations; geological history; terranes; tectonic evolution; thermal history; deformation; temperature; emplacement; radiometric dating; argon argon dating; uranium lead dating; mica; crystallization; recrystallization; structural features; faults, strike-slip; shear zones; fluid flow; igneous rocks; intrusive rocks; granites; illite; dykes; Appalachian Orogen; Avalonia; Ganderia; Dover Fault; Hermitage Bay Fault; Ackley Granite; Wing Pond Shear Zone; Acadian Orogeny; Salinic Orogeny; Harbour Breton Granite; Phanerozoic; Paleozoic; Carboniferous; Mississippian; Devonian; Silurian
Illustrationsgeological sketch maps; photographs; schematic cross-sections; tables; photomicrographs; equal-area stereonet projections; Concordia diagrams; plots; spectra
ProgramTargeted Geoscience Initiative (TGI-4) Intrusion/Porphyry Ore Systems
AbstractWhite mica 40Ar/39Ar 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 40Ar/39Ar dates. White mica 40Ar/39Ar 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. 40Ar/39Ar 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 40Ar/39Ar is a widely applied method in orogenic settings, this dataset shows that relatively flat step-heat 40Ar/39Ar spectra are in contradiction with wide spreads in in-situ 40Ar/39Ar 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 40Ar/39Ar analysis preserves the spatial resolution of 40Ar 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 and 422 Ma (U-Pb zircon). High strain deformation along the Wing Pond Shear Zone occurred between ca. 422-405 Ma (U-Pb monazite and 40Ar/39Ar). Subsequent patchy Ar loss in white mica occurred locally during low T shear (b400 C), and via post-deformation fluid interactions. Low-temperature reactivation of the Dover Fault, a narrow segment of the Wing Pond shear zone, occurred at ca. 385 Ma (40Ar/39Ar). K-Ar dating of authigenic illite in fault gouge from the broadly co-linear brittle Hermitage Bay Fault indicates that slip along the terrane boundary persisted until at least the Mississippian.
Summary(Plain Language Summary, not published)
The mineral 'white mica' contains potassium. The natural decay of potassium to argon gas in white mica can be used to date events in geological history. In this contribution, we apply two different techniques to date white mica that was deformed in a major shear zone in Newfoundland. We argue that in different samples, argon gas in the white mica was variably liberated by thermal diffusion, deformation processes, and fluid infiltration. The resulting dates are used to reconstruct the timing of major events in the formation of the Appalachian mountain belt, including the accretion of Ganderia, Avalonia and Meguma crustal blocks to the western margin of the North American continent.

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