THERMOCHRONOLOGY OF THE TALKEETNA INTRA-OCEANIC ARC OF ALASKA: 40Ar/39Ar, U-TH/HE, SM-ND, AND LU-HF DATING

As one of two well-exposed intra-oceanic arcs on Earth, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs, providing information complementary to that gained by studying active 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 Mtns, the Talkeetna Mtns and the Alaska Peninsula, that—in conjunction with existing geochronology—provide considerable insight into the thermal history of the arc section. Zircon ages establish the main period of arc magmatism as 202–181 Ma in the Chugach Mtns and western Talkeetna Mtns, and 183–153 Ma in the eastern Talkeetna Mtns and Alaska Peninsula. ~184 Ma Lu-Hf and ~182 Ma Sm-Nd ages of igneous and metamorphic garnet indicate that deeper (25–35 km depths) sections of the arc remained above ~700°C for as much as 15 Myr. ⁴⁰Ar/³⁹Ar hornblende ages are chiefly 194–170 Ma in the Chugach Mtns and 175–150 Ma in the Talkeetna Mtns and Alaska Peninsula, but differ from zircon ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially varying thermal history. Mica ages are even more variable, from ~180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U-Th/He zircon ages are ~137 to 129 Ma. Although the signal is likely complicated by Cretaceous and Oligocene post-arc magmatism, the aggregate record indicates that the thermal history of the Talkeetna arc was not simple, but spatially variable. Simple one-dimensional finite-difference thermal models in which the arc was built by repeated injection of subvolcanic intrusions show this kind of spatial variability; models in which the arc was built by repeated injection of Moho-level intrusions show simpler, slow cooling histories.