WHAT CAN POST-COLLISION PLUTONIC EVENTS TELL US ABOUT THE TECTONIC HISTORY OF THE ALASKA RANGE SUTURE ZONE?

The Alaska Range suture zone south of the Denali Fault formed between the Yukon-Tanana terrane and the Wrangellia composite terrane. The variable age distribution of plutons along the suture zone is poorly understood. Moving from west to east south of the Denali Fault there are not well-defined suites of granitoids and the little we know is based on minimal U-Pb zircon ages. Paleozoic and Mesozoic granitoids in Wrangellia, such as those found in the Skolai and Chisana arcs, record subduction zone events. However, there are many granitoids with post 90 Ma ages in the suture zone that are recording igneous processes more than 300 km inboard of the subduction zone and do not clearly relate to subduction processes. The first step in classifying this distribution of plutonic rocks is to petrographically describe them and to group them by age and composition. Preliminary results from new U-Pb zircon ages suggest there are three distinct populations centered around 38, 56, and 70 Ma. Thin section analysis has shown that there is primary muscovite in the youngest suite, suggesting a peraluminous source. In addition, plots of muscovite, biotite, and hornblende cooling ages are closer together in samples from the younger suite. This could be due to being emplaced shallowly or exhumed quickly. Additional U-Pb zircon dates and petrographic descriptions will build on these preliminary findings and provide a basis for more advanced classification of these suture zone granitoids.