U-Pb and Hf isotope analysis of detrital zircons from the Banks Island assemblage (coastal British Columbia) and southern Alexander terrane (southeast Alaska)

Abstract

The Banks Island assemblage consists of regionally metamorphosed and deformed metasedimentary rocks that occur in a narrow belt between the Alexander and Wrangellia terranes in coastal British Columbia. In an effort to evaluate potential correlations with adjacent terranes, we conducted U-Pb and Hf isotope analyses on detrital zircons from quartz-rich metasedimentary rocks of the Banks Island assemblage and from Paleozoic strata of the southern Alexander terrane. U-Pb data from the Banks Island assemblage provide maximum depositional ages ranging from Ordovician to Permian; complementary Hf data demonstrate proximity to a continental landmass during accumulation of most strata. These ages and Hf isotope compositions are quite different from the more juvenile signatures of arc-type rocks that characterize the southern Alexander terrane, but they strongly resemble values previously reported from the northern Alexander terrane. We accordingly suggest that the Banks Island assemblage formed as part of the northern Alexander terrane and was offset southward by ∼1000 km to now reside adjacent to the southern portion of the terrane. At least some of this motion was accommodated along the Early Cretaceous Kitkatla shear zone, which now forms the inboard margin of the Banks Island assemblage. Deformation and metamorphism of the Banks Island assemblage occurred prior to this sinistral motion, however, as crosscutting dikes yield U-Pb ages as old as ca. 156 Ma. Our data provide support for a circum-Arctic origin of the Alexander terrane, as suggested by many previous workers. U-Pb and Hf data for Ordovician and older rocks of the Banks Island assemblage and the northern Alexander terrane are similar to values from the Timanides, whereas Silurian–Lower Devonian rocks yield values shared with Caledonian rocks in Baltica and northern Greenland. Banks Island assemblage strata of suspected late Paleozoic age are more juvenile, perhaps recording motion of the Alexander terrane from the circum-Arctic into the paleo-Pacific realm.