MIXING AND HYBRIDIZATION OF MELTS AND CUMULATES IN THE RATTLESNAKE TUFF MAGMA SYSTEM REVEALED BY GLASS CHEMISTRY AND ZIRCON TEXTURES

Intracontinental magmas are stored at shallow crustal depths and commonly produce compositionally variable deposits when erupted. A core objective of igneous petrology is to understand the processes by which these magmas are generated and differentiated. There are two end member hypotheses of how compositional variability develops in magma bodies; one proposes long term storage and crystal fractionation of a mafic or intermediate parent magma, while the other proposes a transient system resulting from anatexis, high magma flux rates and magma mixing.

The Rattlesnake Tuff (RST) in the High Lava Plains volcanic province of central Oregon is a rhyolitic ashflow tuff that provides a case study of compositional variability characterized by five clustered pumice compositions. To explain these clusters, Streck and Grunder proposed a petrogenetic model in which each less evolved composition derives an increasingly evolved magma via nonmodal roof downward fractionation [1]. New in situ LA-ICPMS glass analysis reveals broader variations that expand these compositional groups and define trends between them. The variations between pumice compositions, and in some cases within a single pumice, are too large to be explained by fractional crystallization alone. Mixing arrays between groups suggest that disparate magmas communicated and partially hybridized prior to eruption. Steep Ba and REE enrichments in some glasses suggest that part of the RST magma network was fed by partial melts of remobilized feldspar-rich cumulate residues.

The zircon crystal record of the RST magma, illustrated by complex disequilibrium crystallization textures and intracrystal thermochemical zoning profiles, offers further evidence for open system recharge and magma mixing. Tandem LA-ICPMS and CA-IDTIMS U-Pb geochronology demonstrates zircon growth over ≤10⁴ year timescales, precluding protracted closed system differentiation as a mechanism for development of a compositionally zoned pre-eruptive RST magma body. Rather, the thermochemical gradient was imparted primarily by recharge and hybridization. High magma flux rates resulted in incomplete pre-eruptive mixing and preservation of distinct magma compositions that were mingled upon eruption at 7.266 ± 0.010 Ma.

[1] Streck & Grunder (1997) Journal of Petrology 38, 133-163.