FIELD ANALOG FOR THE NORTH AMERICAN 7.XX KM/S LAYER IN THE CANADIAN SHIELD

G. Randy Keller’s “dynamic Moho hypothesis” for the southern Rocky Mountain region of North America implies dramatic changes in crustal thickness and composition over billion year timescales. The physical mechanisms for this variability at the crust-mantle interface can be difficult to resolve from the xenolith record and geophysical images of the 7.XX km/s layer. Accurate interpretations of this layer require uplifted exposures of rocks that equilibrated near the continental Moho.

The Upper Deck of northern Saskatchewan is >400 km² of migmatitic paragneisses, felsic granulites, mafic granulites, and eclogite. The felsic gneisses host abundant sills and transposed layers of mafic granulite that range in thickness from <1 cm to >1 km-thick and up to 10s of km in strike-length. Eclogite occurs as m-scale boudins and as a >10 m-thick, >10 km-long horizon near the domain’s structural base. Field relationships, phase equilibria modelling, and geochronology are consistent with the Upper Deck representing an exhumed intraplate of basaltic magma that facilitated widespread melting near the base of thickened continental crust at T>950C and P>1.4 GPa in the Neoarchean.

New seismic velocity calculations are reported for common rock types near the base of the Upper Deck: Opx + Grt felsic granulite, Ky + Grt felsic granulite, Cpx + Grt + Pl + Qtz mafic granulite, and Ky-bearing eclogite. Phase equilibria modelling and calculations using the Voigt-Reuss-Hill averaging scheme were performed with Perple_X 6.7.2. Bulk compositions were modelled in the NCKFMASHTO (felsic) and NCFMASTO (mafic) systems with the updated Holland and Powell (2002) thermodynamic database, augmented with shear moduli data from Connolly and Kerrick (2002). Densities and P-wave velocities for the inferred equilibrium mineral assemblages are: a) 2.8-2.9 kg/m³ and 6.5-6.7 km/s for Opx-felsic granulites, b) 3.0 kg/m³ and 6.7-7.0 km/s for Ky-felsic granulites, c) 3.4-3.5 kg/m³ and 7.2-7.8 km/s for mafic granulites, and d) >3.4 kg/m³ and >8.2 km/s for eclogite. The Upper Deck represents a viable field analog for the Rocky Mountain 7.XX km/s layer. Our results emphasize that the bulk composition of continental lower crust need not be mafic, and significant portions imaged seismically may correspond to residual Grt-rich felsic and intermediate compositions.