10-1 A magmatic vs tectonic paradigm for arc granitoids – an Alaska Range perspective

Session: How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

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Felsic magmatism within arc settings is essential to the development, differentiation, and preservation of continental crust. However, the degree to which felsic plutonic rocks preserve interactions with background tectonic processes is contentious. Geochronology has shown that granitoids are emplaced incrementally, and numerical modeling has demonstrated that magmatic flux will control the thermal evolution and melt content of a growing pluton. Therefore, textural and compositional evolution of felsic plutons should systematically vary with magmatic flux. Seven plutons along a ~400 km segment of the Denali fault display varying internal preservation of tectonic interactions. The Foraker (FP; 435 km²) and Black Rapids glacier (BRGP; 220 km²) plutons are in identical kinematic settings along the southern edge of the Denali fault, bounded on the south by reverse fault splays. The FP is texturally and compositionally homogenous with minimal evidence for incremental emplacement, whereas the BRGP preserves abundant igneous flow foliations and cross-cutting relationships indicative of a composite origin. High-precision CA ID-TIMS U-Pb geochronology demonstrates that the Foraker pluton youngs toward the center of the intrusion (36.967 ± 0.044 Ma to 36.280 ± 0.052 Ma) with areal addition rate of >300 km²/my, whereas the BRGP preserves areal addition rates of <50 km²/my decreasing in age toward bounding thrust fault splays at a linear rate of 2 km/my (38.458 ± 0.072 Ma to 34.749 ± 0.043 Ma). These data corroborate evidence from numerical modelling that indicate high flux systems should support melt-rich reservoirs and sustained high temperatures that promote chemical and textural homogenization, fractionation, and cooling from the margins inward. In contrast, low flux systems have transient thermal histories, which preserve textural heterogeneity and primary intrusive relationships compatible with background tectonic processes. A synthesis of all data from the remaining Eocene intrusions provide further validation that plutonic rocks preserve varying interactions with background tectonic processes controlled by the melt addition rate. A positive correlation between εHf(t) of zircon and areal addition rate suggests that magmatic flux through the column is linked to the flux of juvenile mafic rocks to the base of the crust, and does not correlate with slip rate along the Denali fault.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7644

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