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

Nd, Sr, and Pb isotopic data were obtained for fourteen fine- to coars e-grained samples of accreted flysch of the Late Cretaceous and early Tertiary Valdez and Orca Groups in southern Alaska to determine the fl ysch provenance. Argillites and greywackes from the Orca Group, as wel l as compositionally similar but higher metamorphic grade rocks from t he 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 de position (approximately 50 Ma). Pb isotopic compositions also vary ove r a narrow range (Pb-206/Pb-204 = 19.138-19.395, Pb-207/Pb-204 = 15.59 3-15.703, Pb-208/Pb-204 = 38.677-39.209), and in the Orca Group the sa mples generally become more radiogenic with decreasing epsilon(Nd) and increasing Sr-87/Sr-86. All samples have similar trace element compos itions characterized by moderate light rare earth element enrichments, and low ratios of high field strength elements to large ion lithophil e 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 ro cks with epsilon(Nd) values between 0 and -5. The latter conclusion is supported by the epsilon(Nd) values of a tonalite clast and a rhyodac ite clast in the Orca Group (epsilon(Nd) = -4.9 and -0.9, respectively ). However, trondjemitic clasts in the Orca Group have significantly l ower epsilon(Nd)(approximately -10) and require a derivation of a port ion 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 north ern portion of the Late Cretaceous to early Tertiary Coast Mountains P lutonic Complex in western British Columbia and equivalent rocks in so utheastern Alaska. The isotopic data support previous conclusions base d on geologic studies which suggest that the flysch was shed from this portion of the batholith, and from overlying continental margin arc-r elated volcanic rocks, following its rapid uplift in the Late Cretaceo us and early Tertiary. The Precambrian crustal material present in the flysch may have been derived from Late Proterozoic or older metasedim entary and metaigneous rocks now exposed along the western margin of t he Coast Mountains Plutonic Complex.