Structural and geochronological constraints on the role of partial meltingduring the formation of the Shuswap metamorphic core complex at the latitude of the Thor-Odin dome, British Columbia

At the latitude of the Thor-Odin dome, the Shuswap metamorphic care complex exposes a similar to 15 km thick structural section composed of an upper u nit that preserved Mesozoic metamorphism, structures, and cooling ages, sep arated from the underlying high-grade rocks by low-angle detachment zones. Below the detachments, the core of the complex consists of an amphibolite-f acies middle unit overlying a migmatitic lower unit exposed in the core of the Thor-Odin dome. Combined structural and super high resolution ion micro probe (SHRIMP) U-Pb geochronology studies indicate that the pervasive shall owly dipping foliation and east-west lineation developed in the presence of melt during Paleocene time. SHRIMP analyses of complexly zoned zircon grai ns suggest that the migmatites of the lower unit crystallized at similar to 56 Ma, and a syntectonic leucogranite at similar to 60 Ma. We suggest that leucogranite migrated upward from the migmatites through an array od dikes ;and sills that permeated the middle unit and ponded to form laccoliths spa tially related to the detachment zones. The similarity in ages of inherited zircon cores in the two migmatite and the leucogranite samples suggests a genetic link consistent with the structural analysis. Following the crystal lization of migmatites, the terrane cooled rapidly, as indicated by argon t hermochronology. We propose that exhumation of the core of the Canadian Cor dillera during the formation of the Shuswap metamorphic core complex occurr ed from similar to 60 to 56 Ma at a time when the crust was significantly p artially molten. These structural and temporal relationships suggest a gene tic link between mechanical weakening of the crust by partial melting, late -orogenic collapse, and exhumation of high-grade rocks in the hinterland of a thermally mature orogenic belt.