PETROGENESIS AND METAMORPHISM OF THE BEAR CREEK ASSEMBLAGE, COAST MOUNTAINS, YUKON, CANADA

The Bear Creek assemblage (BCA) in southwestern Yukon is an enigmatic package of Triassic metavolcanic, volcaniclastic, and clastic units originally mapped as part of the Nikolai Formation of the Wrangellia terrane. New stratigraphic interpretations from recent mapping and preliminary geochronological constraints question this correlation and open the door for new interpretations about the origin of the assemblage. Whole rock geochemical data suggest mafic metavolcanics within the BCA were originally subalkaline tholeiitic basalts. Trace element analyses indicate volcanism occurred in mid-ocean ridge (MOR) to island arc (IA) settings, compatible with volcanism in an evolving arc-backarc system off the western margin of Laurentia in the early Mesozoic. Geochemical analyses suggest possible correlation of the BCA with Triassic metavolcanic units in either the Taku terrane or Alexander terrane in southwestern Yukon and southeastern Alaska. Either correlation has major implications for the nature and magnitude of regional strike-slip faulting in the area.

Metavolcanic samples are characterized by actinolite (Act) + chlorite (Chl) + epidote (Ep) + albite (Ab) + quartz (Qtz) ± calcite (Cal) ± phengite (Ph) greenschist facies assemblages with accessory pyrite (Py), chalcopyrite (Ccp), titanite (Ttn), and magnetite (Mag) ± ilmenite (Ilm). Metapelite assemblages vary with bulk composition but are typically characterized by Chl ± biotite (Bt) + plagioclase (Pl) + Qtz + Ep or Chl + muscovite (Ms) + Pl + Qtz, with trace calcite, ilmenite, clinozoisite (Cz) and apatite (Ap).

Petrologic characterization of the BCA helps constrain the degree of burial during terrane accretion in the late Jurassic to Cretaceous. Thermodynamic modeling of observed plagioclase-free garnet (Grt) + Bt + Chl + Qtz + Ilm + Ap + Mag -bearing samples using Theriak/Domino suggests metamorphic temperatures between 520-570°C at pressures less than 6 kbar. Garnet-biotite thermometry suggests temperatures between 550-575°C, consistent with mineral composition isopleth estimates. Metamorphic conditions in the BCA reached similar temperatures but lower pressures than Late Cretaceous regional metamorphic conditions documented for the Kluane Metamorphic Assemblage (~500°C, ~7 kbar), juxtaposed to the east across the Shakwak fault.