INCREMENTAL EMPLACEMENT OF THE MCGONAGALL PLUTON ALONG AN ACTIVE DENALI FAULT, ALASKA – STRUCTURAL CONTROLS ON THE GROWTH OF AN ARC MAGMATIC SYSTEM

The role of lithospheric-scale strike-slip faults during the transport and emplacement of plutonic systems is widely debated and poorly understood. The ca. 43-38 Ma McGonagall pluton is a structurally and compositionally complex, calc-alkaline pluton emplaced along the 2000 km long strike-slip Denali fault in interior Alaska. It has an aerial extent of 290 km² and displays a geometry similar to dextral shear zones with blocks seemingly rotated between major faults. We present detailed mapping and accompanying geochemical results from 2022 field work focused on three transects across the McGonagall pluton. The pluton is predominantly composed of two main units: an equigranular biotite-hornblende-plagioclase granite to quartz diorite (EqGD) and a texturally heterogenous porphyritic hornblende-plagioclase granodiorite (PoGD). EqGD is crosscut by PoGD with clearly defined boundaries at outcrop scale and asymmetrical and gradational boundaries at map scale. These boundaries are generally oriented NE-SW. EqGD contains 63-77 wt % SiO₂ and has locally abundant alkali feldspar. It occurs as massive outcrops near the Denali fault and smaller dikes throughout the rest of the pluton. Samples with silica contents above 74 wt % display an Eu depletion consistent with some plagioclase fractionation. Unit PoGD has a more restricted SiO₂ range (64-70 wt %), no Eu depletion and tightly clustered major and trace element compositions (Eu 0.8-1.1 ppm, Sr 430-580 ppm, Rb 28-80 ppm, U 1.0-2.0 ppm) within the wider range of EqGD results (Eu 0.1-1.3 ppm, Sr 2-800 ppm, Rb 28-130 ppm, U 0.5-3.7 ppm), perhaps the consequence of relatively rapid cooling and resultant inability for fractionation to drive compositional diversification. This unit outcrops predominantly on the north side of the pluton. Extrusive equivalents of the McGonagall pluton may be present nearby as the Mt. Galen volcanics. Given the ubiquitous evidence for incremental assembly paired with asymmetric distribution of the two main units about the Denali fault and second order faults, the McGonagall pluton shows clear evidence for emplacement during right lateral slip on the Denali fault. The McGonagall pluton is an example of a structurally controlled pluton complex, which may be related to its relatively long duration history.