PRELIMINARY CHARACTERIZATION OF THE DENALI FAULT EOCENE ARC PLUTONS

The Alaska-Yukon Denali Fault (DF) is a ~2000 km long strike-slip fault that has accommodated ~480 kms of margin parallel translation since 52 Ma. The DF of interior Alaska hosts a suite of calc-alkaline Eocene plutons ranging from 6 to 700 km² of exposed surface area. Constraining the assembly histories and geometries of these plutons is an important step for assessing the passive and active roles lithospheric-scale strike-slip faults play during the transport and emplacement of arc magmas. The DF Eocene Pluton Suite contains five main plutonic bodies: Foraker (south of DF), McGonagall (north of DF), Panorama-Schist Creek-Nenana (north of DF), East Susitna (south of DF), and Pyramid (north of DF). Samples from each pluton were analyzed using petrography, major and trace element geochemistry, magnetic susceptibility, and paired U-Pb-Hf zircon analysis with these fundamental objectives: 1) evaluate spatial-temporal geochemistry-isotopic variations of each plutonic body, 2) establish a broad baseline dataset for the entire suite of arc rocks emplaced along the DF, and 3) explore the compositional diversification of each body with respect to (1). The Foraker and Panorama- Schist Creek-Nenana composite plutons contain coarse potassium feldspar and biotite in a matrix of quartz, potassium and plagioclase feldspars; and were both emplaced 38-37 Ma. They contain a strong Eu depletion and both bodies young from west to east, and may share a genetic origin. The McGonagall pluton contains well-zoned plagioclase feldspar with texturally early hornblende. It contains a positive Sr anomaly, no Eu depletion, and youngs from east to west 42-38 Ma. The East Susitna batholith ranges from hornblende-biotite diorite to biotite granite containing a variable positive Sr anomaly, a range in Eu concentrations, and was emplaced over 8 m.y. between 42-34 Ma. These compositional and temporal variations indicate that these high apparent-flux plutonic systems (Foraker and Panorama- Schist Creek-Nenana) tend to be more compositionally evolved than the systems with low apparent flux (McGonagall and East Susitna Batholith) suggesting in-situ crystal-liquid fractionation due to a larger thermal budget. Together, these plutonic bodies represent a large arc system emplaced along an active DF strike-slip fault from 42-34 Ma.