CRETACEOUS PROTOLITHS OF CENTRAL GNEISS COMPLEX ROCKS, BRITISH COLUMBIA, CANADA

The Central Gneiss Complex in western British Columbia consists of mid-crustal rocks exhumed along a regional-scale, low-angle ductile shear zone. Much of the Central Gneiss Complex consists of quartzo-feldspathic, garnet-biotite, and calc-silicate gneiss, as well as amphibolite, attesting to both sedimentary and igneous protoliths. Previous work indicates voluminous Late Cretaceous to early Cenozoic magmatism synchronous with high-grade metamorphism and rapid exhumation in the footwall of the detachment. However, the protolith ages of metamorphic rocks are poorly constrained. New U-Pb results from oscillatory-zoned zircon cores from paragneiss collected from the deepest structural levels of the Central Gneiss Complex contain multiple U-Pb age populations, the youngest constraining a Late Cretaceous maximum depositional age. Distinct high U/Th overgrowths on zircon rims suggest metamorphic zircon growth during Paleocene time. In contrast, widespread gray-weathering orthogneiss metamorphosed at lower grade and exposed at higher structural levels yielded distinctly older Cretaceous ages. Taken together, these results suggest that deeper metasedimentary rocks were buried at a high rate and structurally juxtaposed beneath older, mainly Cretaceous arc and metamorphic rocks during Late Cretaceous time. The timing of burial and metamorphism within an active magmatic arc, followed by rapid exhumation in Paleocene-Eocene time, is a remarkably similar scenario to that proposed for the Pelona-Orocopia-Rand-Sierra de Salinas schists in southern California and the Swakane Gneiss of central Washington. The California localities have been interpreted to reflect tectonic underplating above a flattened Laramide slab. The current results suggest that 1) underplating is a common mechanism for the redistribution of crust and/or 2) a similar tectonic setting characterized a broad region of North America in Late Cretaceous time and has previously been underappreciated.