NEOPROTEROZOIC–PALEOZOIC ROCKS OF THE PORCUPINE SHEAR ZONE, YUKON AND ALASKA, AND THEIR SIGNIFICANCE IN THE EVOLUTION OF NORTHERN LAURENTIA

The origin and displacement history of terranes emplaced along the northern margin of North America remain contentious. One of these terranes is the North Slope subterrane of Arctic Alaska, which is separated from the northwestern margin of Laurentia (Yukon block) by the Porcupine shear zone (PSZ). Here, we present new field observations, detrital zircon U-Pb geochronology, and stable isotope geochemistry to elucidate the stratigraphic architecture of rocks exposed along the PSZ in Alaska and Yukon. The oldest strata in the study area consist of brown quartz arenite and shale overlain by white cross-stratified quartz arenite. Major detrital zircon populations from these strata are ca. 1050-1250, 1350-1450, 1600-1650, and 2500-2800 Ma (n = 1100). This clastic succession is overlain by chert-bearing doloboundstone and black limestone with molar-tooth structures. δ¹³C_carb values from these strata range from ca. -3‰ to +3‰, with an average ⁸⁷Sr/⁸⁶Sr value of 0.7064 (n = 4), indicating a late Tonian age. The provenance of the Tonian clastic succession closely resembles that of age-equivalent strata deposited in the Tatonduk inlier, Amundsen, and Mackenzie basins of Yukon, but differs from the Mount Weller Group of the North Slope subterrane. Tonian strata along the PSZ are intruded by Late Devonian plutons and unconformably overlain by Devonian(?) conglomerate with detrital zircon populations ranging from ca. 420–620 Ma. In addition, overlying Lower Mississippian(?) sandstone and conglomerate contain substantial ca. 375 Ma zircon populations with εHf_(t) values of -12.4 to -7.8, suggesting the local Old Crow plutonic suite as a probable source region. This magmatic history and middle Paleozoic provenance signature distinguish the Porcupine succession from the Yukon block. These combined results indicate that the strata exposed along the Porcupine shear zone most likely represent a peri-Laurentian crustal fragment that differs from the North Slope subterrane but has experienced a similar translation history on the Porcupine shear zone.