AGE RELATIONSHIPS WITHIN THE TALKEETNA ARC OF SOUTH-CENTRAL ALASKA

The Talkeetna arc is thought to be a nearly complete section of an intra-oceanic arc, from its ultramafic and mafic mantle and lower-crustal root up through the volcanic and volcaniclastic sequences. It is also generally accepted that the arc is a true intra-oceanic arc, with the attendant geochemical and isotopic signatures. However, some have interpreted the “Talkeetna arc” as a series of separate accreted terrane slivers. We will assess the origins of the Talkeetna arc by evaluating the full available data sets of pressures, temperatures, and ages of emplacement, as well as geochemistry, across the arc.

The data suggest that the Talkeetna arc represents a cohesive section within the Chugach and southeastern Talkeetna Mountains consistent with intraoceanic origin. Initial arc magmatism is recorded within the plutonic and volcanic section in the Chugach Mountains (201.5-180.4 Ma). The geographic distribution and progression of U-Pb zircon ages indicate a northwest shift of the arc magmatic axis between 181.4 and 177.4 Ma from the Chugach Mountains to the southeastern Talkeetna Mountains (177.4-168.9 Ma). Older plutons (~190-194 Ma) in the northwestern Talkeetna Mountains display evolved isotopic ratios and may represent a previously unidentified terrane sliver. The intrusion of a ~153 Ma trondhjemite pluton in the Talkeetna Mountains links juvenile intraoceanic arc plutons in the southeastern Talkeetna Mountain with the isotopically evolved plutons in the northeastern Talkeetna Mountain, indicating the Talkeetna arc had assimilated with the Wrangellia terrane by at least the late Jurassic. Continued assessment of geochemical and age relationships will also help to further clarify the geological history of the emplacement and accretion of the Talkeetna arc.

Intraoceanic island arc studies are usually limited to analysis of xenoliths, geochemical trends in modern volcanic arcs, and geophysical data. Understanding and comparing the origin and emplacement history of Talkeetna arc will yield important information about crust-building processes at convergent margins.