TECTONIC VS MAGMATIC CONTROLS DURING EOCENE ARC GRANITOID PLUTON CONSTRUCTION, ALASKA RANGE, AK

Granitoid plutons are emplaced incrementally. However, the degree and scale with which plutonic rocks preserve composite features indicative of incremental growth vary. Magma addition rates exert the main control on the thermal evolution of an intrusive system and thus regulate magmatic processes capable of overprinting primary intrusive relationships. Consequently, the interplay of tectonic processes on the physical mechanisms responsible for pluton growth is difficult to constrain, complicated by magma addition rates that commonly exceed background tectonic rates. We focus on a suite of calc-alkaline Eocene granitoids emplaced along a 400 km segment of the dextral Denali fault. The Foraker and Black Rapids plutons were both emplaced on the south side of the Denali fault into the hanging wall of thrust fault splays of the main strand. Despite an identical structural setting, they preserve contrasting internal intrusive relationships.

The Foraker pluton is a large (436 km²) homogenous pluton composed entirely of biotite granodiorite. High-precision zircon geochronology preserves a systematic decrease in age toward the center of the intrusion from 36.967±0.016 Ma to 36.280±0.033 Ma. In contrast, the Black Rapids pluton (217 km²) preserves internal intrusive relationships consistent with magmatic crack-seal processes during contractional uplift on the Meteor Peak and Valdez Creek faults. Ongoing geochronology indicates a more protracted emplacement history for the Black Rapids pluton. We propose that varying magma addition rates controlled the degree to which tectonic processes influenced pluton emplacement, such that Foraker’s higher rates catalyzed thermally activated processes capable of overprinting internal intrusive relationships. Existing Hf in zircon data for the entire suite show a strong positive correlation between areal addition rates and ε_Hf values, which we interpret to reflect flux of mantle-derived mafic melts to the base of the crust as a catalyst for elevated magma addition rates in granitic systems higher in the crustal column. These data suggest that arc granitoid systems likely fall on a spectrum from slow growth with strong tectonic controls to fast growth with minimal preserved tectonic controls.