GROWTH AND EVOLUTION OF THE PENINSULAR TERRANE, SOUTHERN ALASKA

We report new U-Pb zircon ages and whole-rock geochemical data from the Peninsular terrane in southern Alaska, which document the evolution of juvenile magmatism throughout the terrane and place new constraints on the timing of Mesozoic crustal growth along the North American margin. The Peninsular terrane is defined by the accreted Talkeetna arc and overlying sedimentary sequences and includes major plutonic and volcanic sections from the Chugach Mtns, the Talkeetna Mtns, Kodiak Island and the Alaska Peninsula.

High-precision chemical abrasion-TIMS U-Pb zircon ages from the Chugach Mtns indicate that initial arc magmatism lasted from 201.5–181.4 Ma. This was followed by a northward shift in the arc magmatic axis and generation of the extensive plutonic suites in the Talkeetna Mtns (177.4–ca. 156 Ma) and on the Alaska Peninsula (183.3–163.9 Ma). The youngest arc plutons are exposed along the northern margin of the terrane in the Talkeetna Mtns and include evidence for inheritance of Permian to Carboniferous zircons. Additional Early Jurassic plutons (ca. 193–191 Ma) from the terrane margin may represent either Wrangellian crust or older Talkeetna arc magmatism.

Radiogenic-isotope data illuminate the composition of the source and the role of crustal contamination in Peninsular terrane magmas. Age-corrected ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr from the Chugach Mtns define a restricted array of isotopic ratios (ε_Nd = 6.0 to 6.9 and ε_Sr = –14.2 to –4.7), similar to modern intraoceanic arcs. Samples from the eastern Talkeetna Mtns (ε_Nd = 5.6–7.2 and ε_Sr = –17.6 to –9.4) are indistinguishable from the Chugach data, whereas samples from the terrane margin have lower ¹⁴³Nd/¹⁴⁴Nd and elevated ⁸⁷Sr/⁸⁶Sr (ε_Nd = 4.0 to 5.5 and ε_Sr = –9.3 to 27.4) indicating an evolved source or contamination by an enriched/older crustal component. Data from the Alaska Peninsula have intermediate values of ε_Nd = 5.2 to 6.6 and ε_Sr = –11.0 to –6.3.

Taken together, the U-Pb zircon ages and radiogenic isotope data indicate that the Peninsular terrane formed as a primitive arc with limited involvement of evolved older crust. Discordant U-Pb zircon ages and evolved isotopic ratios along the northern margin of the terrane are consistent with assimilation of adjacent Wrangellia terrane crust and suggest that the two terranes were amalgamated by 156 Ma.