INSIGHTS INTO THE EXHUMATION HISTORY OF THE SHUSWAP COMPLEX AT JOSS MOUNTAIN, SOUTHEASTERN CANADIAN CORDILLERA

We present new structural, microstructural, petrologic, and absolute age data from the Joss Mountain area of the Shuswap complex, southeastern Canadian Cordillera. The Shuswap complex is part of the Omineca belt within the North American craton involved in the Cordilleran convergence, in which deep continental crust core complexes are exposed. One of these is the Thor-Odin dome (TOD) that lies structurally below the Selkirk Allochthon, which includes Joss Mountain (JM) rocks. The Greenbush Lake shear band zone is a late down-to-W, W-dipping movement zone separating the western Thor-Odin dome from JM rocks. These rocks mainly consist of marble, calcsilicate gneiss, and orthogneiss. All these rocks record a non-coaxial deformation that produced a penetrative transposition foliation (S_T). Minor quartzites and metapelites form S_T-parallel layers up to a few meters thick. During transposition, immature F₁ and F₂ and mature F₃ folds developed.

We reconstructed the PTdt path for Wm- and Bt-bearing schists by combining multiscale structural data with petrology and U-Pb ID-TIMS data to contribute to the reconstruction of the geodynamic evolution of the Shuswap complex. The age of an igneous protolith was constrained at ~ 360 Ma and is consistent with late Devonian magmatism, interpreted as related to a continental arc along the western margin of the paleo-North American plate. PT estimates on equilibrium mineral assemblages from metapelites indicate that the transposition fabric developedat 20 - 30 km depth, under conditions approaching those of a crust thermally relaxed after orogenic thickening. Age data suggest that transposition was probably ongoing at 93 Ma, based on a U-Pb monazite age in schist, and ended by 70 Ma, based on the age of an S_T-crosscutting pegmatite. Subsequent near-isothermal decompression resulted in high geothermal gradients consistent with lithospheric thinning, but lower than those of the late LP-HT imprint recorded in the adjacent northwestern TOD. This suggests that the JM rocks were further away from the axis of lithospheric thinning than the northwestern TOD during late thermal relaxation. Possibly they were displaced along the Greenbush Lake shear band zone that contributed to Eocene unroofing of the Shuswap complex.