GEOCHEMICAL AND ND-PB ISOTOPIC EVOLUTION OF METABASITES FROM ATTENUATED CONTINENTAL LITHOSPHERE, NOME GROUP, SEWARD PENINSULA, ALASKA

The Late Proterozoic to Devonian Nome Group consists of metasedimentary and metavolcanic rocks metamorphosed to blueschist facies during the Jurassic. Two lithologic subunits in the Nome Group (Mixed unit, Casadepaga Schist) contain metabasite bodies thought to have been sills, dikes, and flows. The Mixed unit hosts stratiform massive sulfide deposits and other base-metal occurrences. Compositional characterization of the metabasites could help constrain regional crustal evolution and metallogenesis. New geochemical data and recent results (Werdon et al. 2005, DGGS Rep. Inv. 2005-1, Dept. Nat. Res., State of Alaska) show two groups of subalkalic to mildly alkalic (Nb/Y < 1.5) basaltic rocks: continental tholeiites with weakly developed arc signatures (e.g., slight Nb depletion in spidergrams) reflecting crustal contamination, and another group exhibiting features associated with enriched mantle (E-MORB) and alkaline intercontinental rifts (no Nb depletion, small positive Ti anomalies in spidergrams). Both chemical groups occur in the Mixed unit and Casadepaga Schist. Nd-Pb isotopic data reveal a trend in which both the continental tholeiites and the incipient rift-related metabasites have overlapping signatures and include mixtures of mantle-derived and crustal sources. Values of ¹⁴³Nd/¹⁴⁴Nd range from 0.512529 to 0.512814 (εNd at 300 Ma = -1.4 to +7.0). Crustal residence ages (TDM = 0.8 to 1.6 Ga) suggest contributions as old as Middle to Early Proterozoic basement (crustal, Pb-rich source). Whole rock Pb isotopic variations trend toward radiogenic values (²⁰⁶Pb/²⁰⁴Pb = 18.079-21.411; ²⁰⁷Pb/²⁰⁴Pb = 15.569-15.898; ²⁰⁸Pb/²⁰⁴Pb = 37.929-38.702), plot at lower μ than the average crustal curve, and are consistent with derivation predominantly from an enriched mantle-type source. Values of ²⁰⁸Pb/²⁰⁴Pb are not enriched relative to ²⁰⁶Pb/²⁰⁴Pb, implying that the crustal component was relatively young and uncratonized (<1.6 Ga?). Metabasites in the Nome Group are the products of Early Paleozoic mafic volcanism involving enriched mantle and Proterozoic crust. The production of alkaline to transitional tholeiitic magmas indicates a tectonic setting related to the early stages of continental, rift-related magmatism.