U-PB AND HF ISOTOPIC ANALYSIS OF DETRITAL ZIRCONS FROM PALEOZOIC AND CRETACEOUS STRATA OF SOUTHERN VANCOUVER ISLAND, BRITISH COLUMBIA

Wrangellia, an exotic arc terrane, is interpreted to have been displaced from the Paleo-Arctic to the northern Cordilleran realm during middle-late Paleozoic time, before finally accreting to the western margin of North America during Cretaceous time. Exposures of Wrangellia occur in southern and southeastern Alaska, Haida Gwaii, and Vancouver Island. Late Devonian to Early Jurassic arc-type igneous rocks of Wrangellia are exposed throughout most of Vancouver Island; Cretaceous Nanaimo Group sedimentary strata overlie older units along the eastern coast. Utilizing the detrital zircon record of Paleozoic sedimentary rocks and Cretaceous basin fill we can provide further insight into the magmatic and depositional evolution of the Vancouver Island portion of Wrangellia. 1452 U-Pb LA-ICPMS analyses from 5 samples of the Fourth Lake Formation in the Carboniferous Buttle Lake Group were performed. 1058 U-Pb LA-ICPMS analyses from 4 samples of the Comox formation within the Cretaceous Nanaimo Group were acquired in order to provide a broader sampling of the Lower Mesozoic-Paleozoic rocks of Vancouver Island.

U-Pb analyses within the Fourth Lake Formation reveal prominent Carboniferous age peaks (317 Ma, 330 Ma, 336 Ma, 338 Ma, 342 Ma), with minor pre-400 Ma grains of Paleo-Arctic origin. Hf isotope compositions of the detrital zircons are exceptionally juvenile, with epsilon Hf_(t) values between +15 and +5. U-Pb analyses of Nanaimo Group sedimentary rocks reveal dominant age populations of 370-312 Ma, 223-183 Ma, 180-108 Ma, 101-78 Ma (corresponding peak ages of 340, 194, 166, and 86 Ma). All major populations report very juvenile epsilon Hf_(t) values in the range of +15 to +6.

The detrital zircon U-Pb geochronologic and Hf isotope data in this study suggest that sediment from the Fourth Lake Formation was derived mainly from primitive Paleozoic arc-type magmatism of the Wrangellia arc. Data from the Nanaimo sediments indicate that Triassic and Jurassic igneous rocks of Vancouver Island, and also Late Jurassic-Early Cretaceous sources in the Coast Mountains Batholith, are also highly juvenile. This new data set places Wrangellia within the Paleo-Arctic during Paleozoic time and reveals, during Cretaceous time, very locally derived detritus into sedimentary basins along the inboard margin of Wrangellia.