ND-SR-PB ISOTOPE COMPOSITION OF POST-COLLISIONAL VOLCANIC ROCKS IN FRONT OF AN ACCRETED TERRANE SUTURE ZONE, CANTWELL FORMATION, CENTRAL ALASKA RANGE

The Cantwell volcanics (60-55 Ma) are a subalkalic to alkalic suite north of the suture zone between the Wrangellia composite terrane and the former southern margin of Alaska. Cantwell basalts have rare earth element (REE) trends and high field strength element (HFSE) concentrations similar to those of Hawaii OIB. The basalts have enriched Nd-Sr-Pb isotope ratios with average values of (87Sr/86Sr)i = 0.7056, eNd(T)= 1.3, (206Pb/204Pb)i = 19.04, (207Pb/204Pb)i = 15.61 and (208Pb/204Pb)i = 38.52. There are increased Sr, decreased Nd and increased Pb isotope ratios from the basalts through samples with increased silica (andesites, dacites, and rhyolites). All samples show some elevation of LILE with respect to primitive mantle, but the basalts do not show strong de-coupling of the LILE from the HFSE (e.g., low Ba/Nb ratios) and they have high TiO2 (>1.5%) indicating that they did not form as typical arc volcanics over a subducting slab. We have two hypotheses for the mantle source region of Cantwell magmas: an enriched subcontinental lithospheric mantle or a more depleted mantle source (similar to that of E-MORB sources) from which parental magmas assimilated Jurassic-Cretaceous argillaceous rocks (Kahiltna assemblage) to form the enriched Cantwell basalts. The Cantwell volcanics do lie along a Nd-Sr isotope mixing trend between relatively depleted 60 Ma gabbro xenoliths of the Alaska Range and the Kahiltna rocks which supports the interpretation of a more depleted mantle reservoir. The Cantwell rhyolites have similar isotopic and trace element compositions as the 57 Ma McKinley sequence granites. This indicates that the McKinley granites (south of the Denali fault) and the Cantwell granites (north of the Denali fault) were co-magmatic and represent a cross-fault tie that constrains post-Early Eocene dextral offset on the Denali fault to about 40 km. These data show that Cantwell volcanism occurred after the final northward suturing of the Wrangellia composite terrane to southern Alaska, when regional subduction-related Alaska Range magmatism ceased, and during the transition to an episode of right-lateral slip along the McKinley fault.