AGES FROM PALEOZOIC INTRAOCEANIC VOLCANIC ARCS AND OVERLAPPING DEVONIAN CONTINENTAL CLASTIC ROCKS: IMPLICATIONS FOR THE TECTONIC HISTORY OF THE SOUTHERN ALEXANDER TERRANE, ALASKA

Age data constrain tectonic settings for parts of the Alexander terrane, SE Alaska. Published U-Pb zircon ages for Paleozoic intrusive rocks on south Prince of Wales (POW) and adjacent islands range from 476 to 410 Ma, decreasing in age to the NE. These rocks intrude the Neoproterozoic Wales Group and unconformably overlying Ordovician-Silurian Moira Assemblage, and follow regional metamorphism dated by 475-484 Ma K-Ar ages, known as the Wales orogeny. Detrital zircons (DZ) in Moira rocks range in age from 480 to 600 Ma. All rocks have high positive Ɛ_Nd and low ⁸⁷Sr/⁸⁶Sr values inferred to indicate a juvenile intraoceanic arc. NE-younging ages suggest this Paleozoic Moira arc on the Wales Group is SW-facing.

On north POW, new unpublished ages constrain the age of the Descon arc. U-Pb zircon ages of ca. 476 and 441 Ma for granitic rocks that intrude Descon Formation and Luck Creek volcanics are located NE of a NW-trending belt of Silurian igneous rocks, including diorite with a hornblende ⁴⁰Ar/³⁹Ar age of ca. 426 Ma and volcanic rocks interbedded with Silurian limestone. The Silurian igneous rocks are NE of diorite with a hornblende ⁴⁰Ar/³⁹Ar age of ca. 410 Ma. Farther SW, a NW-trending belt of granitic rocks that have U-Pb zircon ages of ca. 380, 367, 363, and 362 Ma intrude Silurian and Devonian sedimentary and volcanic rocks. DZ ages for pre-Devonian Descon rocks range from 420 to 490 Ma. Descon arc rocks also have high positive Ɛ_Nd and low ⁸⁷Sr/⁸⁶Sr values, and trace element geochemistry indicating an intraoceanic arc setting. Decreasing ages to the SW suggest 1) a NE-facing Descon arc coeval with the SW-facing Moira arc and 2) bimodal volcanics in the Moira Assemblage may be intra-arc rift deposits.

The magmatic belts in the Descon arc are parallel to NW-trending structures in the Moira arc dated by white mica with Early Devonian cooling ages, which are coeval with molasse of the Karheen Formation that denotes the Klakas orogeny. The Early Devonian Karheen rocks unconformably overlie both Descon and Moira arc rocks and contain DZ as old as 2.9 Ga, indicating a continental source not recognized in the basement of either arc. Paleocurrent indicators of a southern source and the shortening orientation suggested by the NW fabric support Early Devonian juxtaposition with a continental source that contributed clastic material from the SW.