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TECTONICS, VOL. 25, TC4003, doi:10.1029/2005TC001912, 2006

Multistage exhumation and juxtaposition of lower continental crust in the western Canadian Shield: Linking high-resolution U-Pb and ⁴⁰Ar/³⁹Ar thermochronometry with pressure-temperature-deformation paths

R. M. Flowers

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

K. H. Mahan

Department of Geosciences, University of Massachusetts, Amherst, Massachusetts, USA

S. A. Bowring

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

M. L. Williams

Department of Geosciences, University of Massachusetts, Amherst, Massachusetts, USA

M. S. Pringle

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

K. V. Hodges

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Abstract

New U-Pb (titanite, apatite, rutile) and ⁴⁰Ar/³⁹Ar (hornblende, muscovite, biotite) data are linked with existing pressure-temperature-deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive (>20,000 km²) region of exhumed lower crust in the East Lake Athabasca region of the western Canadian Shield. In the ∼200 m.y. between circa 1.9 Ga high-pressure granulite facies metamorphism and circa 1.7 Ga unconformable deposition of Athabasca basin sediments on the exhumed rocks, our analysis reveals at least three distinct phases of unroofing at different rates. Specifically, we distinguish (1) an early phase of extensional unroofing in the Chipman domain at rates of 1.5–2.0 km/m.y. from ∼1.0 GPa to ∼0.8 GPa associated with mafic magmatism and metamorphism, (2) an episode of regional contractional uplift and associated unroofing along the Legs Lake shear zone at circa 1850 Ma from 0.7–0.8 GPa to 0.4–0.5 GPa, and (3) a final period of extensional unroofing at rates of 0.2–0.3 km/m.y. from 0.4 to 0.5 GPa that culminated in transport of current exposures to near-surface conditions. The cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases. The apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1.89–1.88 Ga implies their juxtaposition at 0.7–0.8 GPa conditions. Regional east directed thrusting of the deep crustal domains as a coherent unit at 1.85 Ga juxtaposed the granulites with middle crustal Hearne domain rocks at 0.4–0.5 GPa conditions. The detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of Laurentia. The temporal and spatial heterogeneity of exhumation patterns in the East Lake Athabasca region may be a common feature of the unroofing histories of lower crustal rocks.

Received 11 October 2005; accepted 30 March 2006; published 7 July 2006.

Keywords: lower crust; thermochronometry; exhumation.

Index Terms: 1020 Geochemistry: Composition of the continental crust; 1115 Geochronology: Radioisotope geochronology; 1140 Geochronology: Thermochronology.

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Citation: Flowers, R. M., K. H. Mahan, S. A. Bowring, M. L. Williams, M. S. Pringle, and K. V. Hodges (2006), Multistage exhumation and juxtaposition of lower continental crust in the western Canadian Shield: Linking high-resolution U-Pb and ⁴⁰Ar/³⁹Ar thermochronometry with pressure-temperature-deformation paths, Tectonics, 25, TC4003, doi:10.1029/2005TC001912.

Copyright 2006 by the American Geophysical Union.