THE SIGNIFICANCE OF PALEOGENE FAULTS IN THE RECONSTRUCTION OF METAMORPHIC HISTORIES IN THE CANADIAN CORDILLERA: AN EXAMPLE FROM THE NORTHWESTERN THOR–ODIN DOME OF THE MONASHEE COMPLEX, SOUTHEASTERN BRITISH COLUMBIA

Differences in U-Pb metamorphic monazite ages in the northwestern Thor–Odin dome of the Monashee Complex, southern Canadian Cordillera, are explained in the context of the NNW-trending subvertical Paleogene Victor Creek Fault (VCF). Similar dextral and/or normal faults are present throughout the Canadian Cordillera and we demonstrate their potential importance in the interpretation of the history of Cordilleran deformation and metamorphism.

A pervasive transposition foliation (S_T) is present throughout the Thor-Odin dome as a result of Cordilleran deformation. A pre-S_T (or early-S_T?) foliation is preserved in porphyroblasts such as garnet and kyanite. We attempted to date pre- and syn-S_T fabrics and associated metamorphism by U-(Th-)Pb monazite geochronology. Monazite in a pelitic gneiss and a pelitic schist west of the VCF, and a quartzite and a pelitic schist east of the VCF were analyzed by U-Pb isotope dilution thermal ionization mass spectrometry methods. Monazite in the two pelitic schists were subsequently analyzed by sensitive high resolution ion microprobe, and the textural relationships of monazite were unraveled using QEMSCAN and backscattered electron (BSE) imaging.

Our pelitic rocks display two monazite age populations per sample, which we correlate with pre-S_T monazite growth in garnet and kyanite porphyroblasts, and syn-S_T monazite that is embayed by sillimanite and aligned with the transposition foliation. Both populations decrease in age to the east. The general younging of monazite ages from west to east is consistent with a previously recognized younging of ages of deformation and metamorphism towards deeper structural levels in the Monashee Complex. However, we believe an age domain boundary exists across the VCF, because east of the VCF pre- and syn-S_T monazite crystals are Eocene, while west of the VCF they are latest Cretaceous to earliest Paleocene, and late Paleocene to earliest Eocene. Displacement along the VCF may therefore have juxtaposed two domains with different metamorphic histories. The Three Valley – Joss Pass Fault between the northwestern Thor–Odin dome and the adjacent Joss Mountain area to the west, and the Ratchford Creek – Perry River Fault in the Frenchman Cap dome to the north (and other faults in the Canadian Cordillera?) may be of equal importance.