A COAST MOUNTAINS PROVENANCE FOR THE VALDEZ AND ORCA GROUPS, SOUTHERN ALASKA, BASED ON ND, SR, AND PB ISOTOPIC EVIDENCE

Nd, Sr, and Pb isotopic data were obtained for fourteen fine- to coarse-grained samples of accreted flysch of the Late Cretaceous and early Tertiary Valdez and Orca Groups in southern Alaska to determine the flysch provenance. Argillites and greywackes from the Orca Group, as well as compositionally similar but higher metamorphic grade rocks from the Valdez Group, show a restricted range of correlated epsilon(Nd) (-0.6 to -3.8) and Sr-87/Sr-86 (0.7060-0.7080) at the time of sediment deposition (approximately 50 Ma). Pb isotopic compositions also vary over a narrow range (Pb-206/Pb-204 = 19.138-19.395, Pb-207/Pb-204 = 15.593-15.703, Pb-208/Pb-204 = 38.677-39.209), and in the Orca Group the samples generally become more radiogenic with decreasing epsilon(Nd) and increasing Sr-87/Sr-86. All samples have similar trace element compositions characterized by moderate light rare earth element enrichments, and low ratios of high field strength elements to large ion lithophile elements. Based on petrographic, geochemical, and isotopic data the sedimentary rocks are interpreted to have been derived largely from a Phanerozoic continental margin arc complex characterized by igneous rocks with epsilon(Nd) values between 0 and -5. The latter conclusion is supported by the epsilon(Nd) values of a tonalite clast and a rhyodacite clast in the Orca Group (epsilon(Nd) = -4.9 and -0.9, respectively). However, trondjemitic clasts in the Orca Group have significantly lower epsilon(Nd)(approximately -10) and require a derivation of a portion of the flysch from Precambrian crustal sources. The Nd, Sr, and Pb isotopic compositions of both the Valdez and Orca Groups overlap the values determined for intrusive igneous rocks exposed within the northern portion of the Late Cretaceous to early Tertiary Coast Mountains Plutonic Complex in western British Columbia and equivalent rocks in southeastern Alaska. The isotopic data support previous conclusions based on geologic studies which suggest that the flysch was shed from this portion of the batholith, and from overlying continental margin arc-related volcanic rocks, following its rapid uplift in the Late Cretaceous and early Tertiary. The Precambrian crustal material present in the flysch may have been derived from Late Proterozoic or older metasedimentary and metaigneous rocks now exposed along the western margin of the Coast Mountains Plutonic Complex.