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Coherent French Range blueschist: Subduction to exhumation in <2.5 m.y.?

M.G. Mihalynuk^,1, P. Erdmer^,2, E.D. Ghent^,3, F. Cordey^,4, D.A. Archibald^,5, R.M. Friedman^,6 and G.G. Johannson^,7

¹ British Columbia Geological Survey Branch, P.O. Box 9320, Stn Prov Govt, Victoria, British Columbia V8W 9N3, Canada
² Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
³ Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada
⁴ Sciences de la Terre, CNRS UMR 5125 Paléoenvironnements et Paléobiosphère, Université Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, France
⁵ Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada
⁶ Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
⁷ Canadian Discovery Limited, Suite 1100, 603–7th Avenue southwest, Calgary, Alberta T2P 2T5, Canada

Coherent oceanic strata in the French Range belong to the exotic Cache Creek terrane of the Canadian Cordillera. They were metamorphosed to blueschist grade, tectonically extruded, eroded, and intruded by plutons—perhaps in <2.5 m.y. Sodic amphibole overprint chert as young as late Pliensbachian to Toarcian age (ca. 191 to ca. 177 Ma). Blueschist mineral assemblages define the early metamorphic fabric along with phengite dated by ⁴⁰Ar/³⁹Ar as 173.7 ± 0.8 Ma. Lack of evidence for phengite recrystallization, secondary muscovite growth, local plutonism, or overprinting of regional prehnite-pumpellyite metamorphic facies limits the possibility of Ar loss subsequent to formation of the blueschist in a subduction zone that had formed >150 m.y. earlier. Southwest-verging folds and northeast-dipping thrust faults along which the blueschist was emplaced affected the width of the northern Cache Creek terrane and the adjacent Whitehorse Trough. Exhumation is recorded by eclogite clasts and by early Bajocian chert granule deposition in Whitehorse Trough before ca. 171 Ma. Oldest postkinematic plutons require that emplacement-related regional deformation, pluton crystallization, and cooling was complete by ca. 172 Ma.

Exhumation may have accelerated upon rupture of Cache Creek oceanic crust as it subducted between two conjoined arc segments (Stikine and Quesnel) that were rotating into parallelism. Paleogeographic changes are recorded by dramatic arc uplift, paleoflow reversal, and changes in ammonite zonation as the last vestiges of Cache Creek ocean crust became isolated. Blueschist was tectonically extruded over the colliding Stikine arc segment from beneath a backstop of Quesnel arc, which had been accreted to the continental margin by ca. 180 Ma.

Key Words: blueschist • Cache Creek • geochronology • structure • radiolarians • microfossils • obduction • exhumation • tectonics

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