ZIRCON U-PB CONSTRAINTS ON THE AGES AND OBSCURED BOUNDARIES OF TERRANES IN THE WESTERN NORTH AMERICAN CORDILLERA: CENTRAL GNEISS COMPLEX, BRITISH COLUMBIA

Much of the western margin of the North American Cordillera is composed of tectonostratigraphic terranes, many of oceanic affinity, which were originally bounded by suture zones. However, major Mesozoic and early Cenozoic tectonism and plutonism have precluded placement of some regions into a regional tectonic framework and definition of terrane-bounding structures. One such region is the Central Gneiss Complex of western British Columbia, which preserves middle and lower crustal rocks from the roots of a continental magmatic arc, but whose protolith and early deformational history are poorly known due to overprinting magmatism and high-grade metamorphism. To decipher the tectonic affinities of rocks within the Central Gneiss Complex and constrain the location of the major terrane-bounding fault between the western Insular and eastern Intermontane terranes, we conducted a coupled field-based and U-Pb zircon geochronological study near Terrace and Prince Rupert. At structurally high localities, we interpret protoliths of the Central Gneiss Complex to consist of a sequence of intermediate to felsic volcanic rocks with minor interbedded limestones and fine-grained clastic sedimentary rocks. Detrital U-Pb zircon analysis from rocks at this locality obtained by laser-ablation ICPMS yield unimodal ages of ~190 Ma, suggesting a correlation with the western Stikine terrane. At a structurally deeper locality, heterogeneous, quartzofeldspathic gneiss interpreted to have a volcanic protolith yields an age of ~314 Ma, which is an excellent age and lithologic match to basement of the Stikine terrane. These results suggest a major suture adjacent to or northeast of the Coast shear zone, located ~30 km to the southwest. Deformation within the Central Gneiss Complex consists of cm- to km-scale, recumbent, isoclinal folds with axes that plunge parallel to stretching and mineral lineations. Crosscutting relationships with a syn-metamorphic pluton require that major deformation and burial occurred prior to pluton intrusion at 80 Ma. Metamorphic zircon growth suggests that metamorphism occurred from >80 Ma to ~50 Ma, culminating with major extensional collapse and associated plutonism. These results indicate that U-Pb zircon dating can be a valuable tool for deciphering tectonism in high-grade orogenic cores.