COMPOSITIONAL DIVERSIFICATION IN EOCENE ARC PLUTONIC ROCKS ALONG THE DENALI FAULT: IMPLICATIONS FOR ARC MAGMA SOURCES AND EVOLUTION

Our current understanding of the compositional diversification of granitoids relies on well-established processes, notably, fractional crystallization. However, the growing consensus that granitoid systems are constructed incrementally over 10⁴-10⁶ yrs limits the role that system-scale fractional crystallization can play during crustal magmatism. Here we integrate observations from exhumed lower continental crust with petrography, geochronology, and geochemistry to explain the compositional diversity preserved in a suite of Eocene arc magmas exposed along the Denali Fault, with possible implications for calc-alkaline magmatic suites in general.

Four composite granitoid plutonic systems are exposed along the Denali Fault (central Alaska Range) consist of two-feldspar biotite granitoids ± hornblende, and were constructed over varying durations (1-4 Ma). Plutonic systems emplaced over longer durations have lower ε_Hf(Tc) values and preserve higher Sr and Eu concentrations and lower SiO₂ than the two plutons emplaced over shorter durations. Compositional trajectories show a clear divergence at approximately at 70% SiO₂. We interpret these granitoids to have formed from partial melting of juvenile hydrous gabbroic rocks via hornblende-dehydration melting reactions that produced almandine-rich garnet as a residue and a tonalitic liquid. Plutons with a higher magmatic flux preserve a record of fractionation to greater than 75% SiO₂ with corresponding enrichment of incompatible elements. The lower-flux systems contain significant hornblende, corresponding to calc-alkaline trends with lower overall SiO₂ concentrations. These new data corroborate assertions that high-flux plutonic systems may fractionate to higher SiO₂ contents owing to a larger partial melt content. In contrast, lower-flux systems may be dominated by contamination from hornblende at or near the site of melting and driven toward intermediate compositions, while still preserving a juvenile isotopic signature. Our integrated interpretations reconcile the widely observed Fe-depletion in calc-alkaline granitoids, and account for chemical disequilibrium of mafic phases within host granitoids; the mafic phases may be derived from the melt protolith rather than fractionated crystals from a single liquid line of descent.