ZIRCON AGES AND COMPOSITIONS CONSTRAIN CHANGES IN MELT SOURCES DURING AND AFTER PROGRESSIVE ACCRETION OF THE WRANGELLIA COMPOSITE TERRANE TO THE SOUTHERN ALASKA MARGIN (Invited Presentation)

Most plutonic rocks in the western Alaska Range were emplaced during and after accretion of the (mostly Mesozoic) oceanic Wrangellia Composite Terrane (WCT) to the (mostly Paleozoic) continental margin of southern Alaska (locally, Farewell terrane, FT). Calcalkaline diorite to granite plutons were emplaced during progressive basin closure and terrane accretion (~95-76 Ma). These plutons were emplaced in WCT basement or proximal to the WCT–FT margin. After 76 Ma, plutons were organized linearly and crossed the suture zone, suggesting fault-controlled emplacement during crustal shortening and deformation. These Latest Cretaceous plutons are gabbro to granodiorite and have arc to collisional geochemical affinity, some with adakite-like compositions, possibly due to crustal thickening associated with WCT collision. In contrast, Paleocene plutons are fractionated granites with a widespread, scattered geographic distribution. Hf isotopes, trace element concentrations, and U/Pb ages were measured in zircons from ~110 to ~30 Ma igneous rocks by LA-ICPMS using the split-stream configuration. Maximum eHf decreases gradually but systematically over time (+15 to +10), suggesting either a progressively more enriched mantle or an increasing role of crustal components in the melt source and/or during magma ascent and emplacement. However, most Late Cretaceous, and a subset of Paleocene, plutons have anomalously low eHf (+6 to –3). Paleocene granite eHf correlates with location and basement type; plutons in Paleozoic basement have lower eHf compared with those in Mesozoic basement. This pattern, most pronounced in the Paleocene plutons, is also seen in other age groups where coeval rocks were emplaced in both domains, suggesting strong basement control on Hf isotopic compositions. The contrast in isotopes among coeval plutons across terrane boundaries demonstrates a vital role for preexisting basement contributions in the formation of nascent continental crust. Zircon trace element compositions from ~100 Ma to ~50 Ma are consistent with less oxidized crystallization conditions over time. This observation indicates a shift to decreasingly subduction-fluxed primitive magmas, a progressively dehydrated crustal column, and/or a lower ratio of juvenile to recycled components.