Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U‐Th/He, Sm‐Nd, and Lu‐Hf dating

Abstract

As one of two well‐exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu‐Hf and Sm‐Nd garnet ages, 40Ar/39Ar hornblende, mica and whole‐rock ages, and U‐Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U‐Pb ages establish the main period of arc magmatism as 202–181 Ma in the Chugach Mountains and 183–153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu‐Hf and ∼182 Ma Sm‐Nd garnet ages indicate that 25–35 km deep sections of the arc remained above ∼700°C for as much as 15 Myr. The 40Ar/39Ar hornblende ages are chiefly 194–170 Ma in the Chugach Mountains and 175–150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U‐Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from ∼180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U‐Th/He zircon ages are ∼137 to 129 Ma, indicating no Cenozoic regional heating above ∼180°C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One‐dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories.