TIMING OF DUCTILE DEFORMATION IN THE ALBION-RAFT RIVER-GROUSE CREEK (ARG) METAMORPHIC CORE COMPLEX: PRELIMINARY U-PB SHRIMP RESULTS

Deep crustal rocks in the ARG metamorphic core complex were exhumed during Basin and Range extension and preserve evidence for multiple phases of deformation and metamorphism that have been difficult to date because of this superposition of events. The core of the range exposes latest Archean gneissic basement that is unconformably overlain by a series of Neoproterozoic to Paleozoic quartzites, schists, and calc-silicate rocks. An older, variably preserved, amphibolite-facies fabric (S₁) is overprinted by the dominant foliation in these rocks: a greenschist-facies, penetrative, sub-horizontal foliation with NW-SE to E-W extensional lineation (S₂). In the contact aureoles of Eocene-Oligocene plutons, S₂ fabrics are amphibolite-facies, and in the Albion Mountains the crosscutting relationships of these granites can be used to determine the timing of D₂. A deformed granite that contains ductile, extensional, D₂ fabrics yielded a U-Pb SHRIMP zircon age of 29.8 ± 0.5 Ma. Zircons from an aplitic pegmatite that is boudinaged but internally undeformed yielded a U-Pb age of ~31 Ma, whereas the undeformed Almo pluton is 30.5 ± 1 Ma. The equivalence of these ages is interpreted to indicate that ductile deformation and magmatism were largely coeval. All of these granites contained a significant component of inherited late Archean zircons (cores), and are therefore incorporated pre-existing Precambrian crust. In the Basin Creek area of the northern Grouse Creek Mountains, the deformed Vipoint pluton has a U-Pb zircon age of 29 ± 0.3 Ma, but detrital zircons from country rock schist exhibit metamorphic rims that yield an Early Cretaceous age (ca. 120-140 Ma). This is the first Mesozoic U-Pb age reported for the ARG, and may represent the age of S₁ fabrics, but additional data is needed. In the southern Grouse Creek Mountains and northern Albion Mountains, sphene and zircon locally preserve U-Pb ages of 2.5 Ga (age of the basement) and 1.77 Ga (timing of accretion along the southern edge of the Archean craton). This indicates that, although the ~30 Ma igneous and deformational event represents a major influx of heat into the crust, some regions of the core complex were not as affected and preserve a record of older deformation.