PRELIMINARY AGES FOR REGIONAL METAMORPHISM IN THE MT. WADDINGTON AREA OF THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA, CANADA

Continental magmatic arcs produce and recycle large volumes of continental crust, particularly in high-flux events (HFE) in which voluminous quantities of magma are emplaced during short time spans; however, the causes for HFE remain poorly understood. We present Sm-Nd garnet ages for metamorphism in order to evaluate connections between crustal shortening/thickening, metamorphism, and HFE.

The Coast Mountains batholith (CMB) in British Columbia is composed of Jurassic to Tertiary age plutons and metamorphic rock pendants or screens. This study focuses on metamorphic rocks between Bute and Knight inlets east to the eastern Waddington fold-thrust belt. In the Mount Waddington area two garnet Sm-Nd ages were obtained from garnet-biotite schist: a 3-point garnet core isochron with a 73.5±1.2 Ma age and a 4-point garnet rim isochron with a 71.5±0.8 Ma age. These ages overlap with 92-70 Ma monazite ages from ca. 350 km north in the Central Gneiss Complex along Douglas Channel, but differ from peak metamorphic ages of 82-84 Ma from ca. 25 km west in the eastern Waddington fold-thrust belt. Additional Sm-Nd ages of 99.3±2.3 (3-point isochron) and 90.3±2.0 Ma (4-point isochron) were obtained from peritectic garnet in undeformed veins from a leucocratic orthogneiss. The leucocratic orthogneiss has a 110±1.6 Ma igneous age and is in the lowest exposed structural unit near Mount Raleigh, ca. 45 km SE of Mount Waddington. These new garnet ages indicate that deformation occurred between 110 and 99 Ma with garnet growth from 99 to 90 Ma. Metamorphism in higher thrust sheets was interpreted to be 83-85 Ma (Woodsworth et al., 2000), synchronous with NE-directed thrusting in the eastern Waddington thrust belt. The new ages delineate a previously unrecognized older metamorphism and deformation in or below the thrust belt.

The new garnet Sm-Nd ages presented here are interpreted to date regional metamorphism during contractional deformation. These ages show that the timing of deformation aired significantly over short distances and by structural level in this part of the CMB. Future work will determine the timing of HFE in this part of the CMB for comparison with ages for metamorphism.