U-Pb and Hf Isotopic Analysis of Detrital Zircons From Paleozoic and Cretaceous Strata of Southern Vancouver Island, British Columbia

Abstract

Wrangellia, an exotic arc terrane to North America, is interpreted to have been constructed near the margin of the Paleo-Arctic and Paleo-Pacific during middle-late Paleozoic time, before finally accreting to the western margin of North America during Late Jurassic to Early Cretaceous time. 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 southern Wrangellia. 1422 U-Pb LA-ICPMS analyses from five samples of the Fourth Lake Formation in the Carboniferous Buttle Lake Group were performed. 1055 U-Pb LA-ICPMS analyses from four 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 (344, 339, 336, 331, and 317 Ma), with minor pre-400 Ma grains from adjacent terranes of Paleo-Arctic origin. Paleozoic detrital zircons exhibit juvenile, with ƐHf (t) values between +15 and +5. U-Pb analyses of Nanaimo Group sedimentary rocks reveal dominant peak ages at 341, 195, 167, and 86 Ma. All major populations yield juvenile epsilon ƐHf (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 bimodal magmatism within the Paleozoic southern Wrangellia arc system as well as minor 6 contributions of recycled detritus from the adjacent Alexander Terrane. Hf isotope data from the Comox Formation indicate that Triassic and Jurassic igneous rocks of the Bonanza Arc, and Late Jurassic-Early Cretaceous sources from the central Coast Mountains Batholith (CMB), are highly juvenile. This new geochronologic and geochemical data set contributes to a new tectonic model for the Paleozoic Southern Wrangellia Arc system from Late Devonian to Early Permian time and reveals, during Cretaceous time, very locally derived detritus was deposited in sedimentary basins along the inboard margin of Wrangellia.

U-Th-Pb Geochronology and Lu-Hf Isotope Geochemistry of Detrital Zircons in Metasedimentary Rocks of the Southern Coast Mountains Batholith

“…In contrast to these geologic differences, our U-Pb ages and Hf isotopic results are very similar to the results available from Vancouver Island, which include U-Pb ages on Paleozoic igneous rocks [14] and U-Pb/Lu-Hf data from detrital zircons extracted from Mississippian-Pennsylvanian sandstone layers of the Fourth Lake and Thelwood Formations and from Upper Cretaceous sandstone layers of the overlying Comox Formation of the Nanaimo Group [34]. As part of this project, we attempted to complement these results by analyzing detrital zircons from strata of the Karmutsen, Quatsino, Parsons Bay, and Harbledown Formations (Figure 2) but were unable to identify quartz-rich clastic strata in any of these units.…”

“…Two samples of the coarsest-available graywacke horizons of the Harbledown Formation failed to yield zircons of sufficient size for analysis. Data from Ruks [14] and Alberts et al [34] are summarized in Figure 9, with the age distributions of igneous and detrital zircons in the lower panel and εHf t values of detrital zircons in the upper panel.…”

“…U-Pb ages and εHf t values of detrital zircons from Bute-Knight samples and Wrangellia[14,34], as well as detrital zircon U-Pb ages from strata of the Stikine terrane [fromFigure 4of Evenchick et al[41]]. (b) lists peak ages for each age distribution and also shows periods of widespread magmatic activity within the Stikine terrane (with stars;[35][36][37][38]40]).…”