T186 Future Directions in Tectonics and Lithospheric Processes: A Session for Early Career Researchers

Tuesday, 11 October 2022
1:30 PM - 5:30 PM
407

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

Paper number: T186-11
Program: Technical Sessions
Time: 3:50 PM – 4:05 PM

Authors

Abstract

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 εHf(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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379153

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