U-Pb Geochronology and Hf Isotope Geochemistry of the Turtleback Complex and East Sound Group, San Juan Islands, Northwestern North American Cordillera

Abstract

Abstract This study focuses on Paleozoic rocks of the Turtleback Complex (TBC) and East Sound Group (ESG) of northwestern Washington. These assemblages record a long and complex history of magmatism, sedimentation, and volcanism as part of the Chilliwack composite terrane. This study investigates the tectonic setting and magmatic evolution of the Chilliwack terrane by analyzing U-Pb geochronologic and Hf isotopic data from zircons extracted from igneous and sedimentary rocks of the TBC and ESG. Igneous rocks of the TBC yield U-Pb ages of ~420-~370 Ma for zircon rims, which are interpreted to record pluton crystallization, and ages of ~500-370 Ma for inherited cores and antecrysts. Zircons older than 410-400 Ma yield typical igneous values of U/Th and U concentration and somewhat more evolved εHft values (+5 to +10), whereas younger grains yield values of U/Th and U concentrations that record ongoing metamorphism and have more juvenile εHft values of +10 to +15. This evolution is interpreted to have occurred in a long-lived island arc that experienced a transition at ~410-400 Ma from crustal thickening to regional crustal extension. Detrital zircon grains from the ESG yield dominant ages of ~421-~366 Ma and εHft values of +8 to +15, both of which overlap with values from igneous rocks of the underlying TBC. Strata of the ESG also contain abundant older zircon grains, with a dominant age peak of ~1860 Ma and subordinate peaks at 2673, 2570, 1485, 1113, 736, and 553 Ma. When compared with similar results from other assemblages and terranes in the northern Cordillera, our results suggest possible connections with the Yellow Aster Complex of the nearby North Cascade Range. Our results also suggest general similarities with the Alexander terrane, Wrangellia terrane, and southeast Alaska portion of the Yukon-Tanana terrane. These similarities are consistent with previous interpretations that the Chilliwack composite terrane formed in association with the Alexander, Wrangellia, and southeast Alaska portion of the Yukon-Tanana terrane along the paleo-Arctic margin of Laurentia during early Paleozoic time. Our work also illustrates the importance of recycling of older magmatic rocks over millions of years within a juvenile island arc system.