A PLUTONIC BROTHER FROM ANOTHER MAGMA MOTHER: A PETROLOGIC INVESTIGATION INTO THE FORAKER-MCGONAGALL PLUTON PIERCING POINT ALONG THE DENALI FAULT, AK

The Denali Fault (DF) is a long-lived and active strike slip fault responsible for the greatest topographic relief in North America (6,190 m). Despite decades of inquiry, total DF Cenozoic displacement has remained a contentious topic with different piercing points yielding disparate reconstructions. Chief among western DF constraints is the correlation of the ca. 38 Ma Foraker and McGonagall plutons, which abut opposing sides of the fault, and were interpreted as a tectonically fragmented pluton. Reconstruction of these plutons indicates ca. 38 km of displacement since 38 Ma, corresponding to an average slip rate of 1 mm/yr: significantly lower than quaternary slip rates (~12 mm/yr to 5 mm/yr) and long term displacement to the east (~400 km since ~57 Ma). Petrography, major and trace element geochemistry, and paired U-Pb-Hf isotopic analysis were acquired to evaluate the validity of this pluton correlation.

The McGonagall pluton (north side of DF) is composed mostly of well-zoned plagioclase, biotite, quartz, matrix k-feldspar, and local hornblende whereas samples from the Foraker pluton contain greater modal K-feldspar and reddish biotite. Major element geochemistry shows minimal overlap between the two plutons with the Foraker pluton consistently having higher SiO₂ contents than the McGonagall pluton. Both plutons have an “arc” signature, plot as calcic, but the McGonagall pluton has elevated Sr and Al contents similar to crust-derived adakatitic rocks. Detrital zircon U-Pb geochronology from catchments draining each pluton yield different Eocene averages, with Foraker yielding a younger age peak (36.5 Ma) than the McGonagall Pluton (39.2 Ma). Paired U-Pb and Hf isotopic analysis of bedrock samples also display no overlap in age with the McGonagall pluton younging from west to east (42.39 ± 0.34 Ma to 38.78 ± 0.23 Ma); The Foraker pluton youngs from northeast to southwest (37.20 ± 0.30 Ma to 37.93 ± 0.42 Ma). The Foraker pluton, despite having higher SiO₂ and lower Al₂O₃, has uniformly higher ɛ_Hf(T) values inconsistent with variations between the plutons being the result of contamination, making the Foraker-McGonagall piercing point untenable. Overall each dataset supports a disparate origin for the two plutons and allows for more Cenozoic slip on the western DF than previously considered.