Uranium-series isotope constraints on magma generation and differentiation in the central Oregon Cascades
The Cascade volcanic arc, extending for ~ 1250 km from northern California, USA, to southern British Columbia, Canada, is the surface expression of NE-directed subduction of the Juan de Fuca microplate beneath North America. Slow subduction of young, hot oceanic lithosphere creates an unusually warm subduction environment at depth. Fore-arc rotation and impingement of Basin and Range faulting create an extensional environment within much of the arc. The combination of these factors has been postulated as a cause of the wide variety of primitive basaltic compositions erupted within the arc. Within central Oregon, where intra-arc extension is highly focused, young mafic volcanism is particularly abundant, and eruption of true rhyolite is unusually common also. This dissertation seeks to address the generation and differentiation of young (< 20 ka) magmas in the central Oregon Cascades through application of uranium-series (U-series) isotope data, in conjunction with elemental and Sr-Nd isotope data. Trace element and isotopic (87Sr/86Sr, 143Nd/144Nd) data for the most primitive lavas suggest derivation by < 10% partial melting of a garnet-bearing mantle source. This source is interpreted to have experienced relatively minor modification by a slab-derived component, most likely via addition of a partial melt of subducted sediments. The 238U-230Th-226Ra U-series data are consistent with this interpretation. However, as a result of the minor (230Th/238U) disequilibrium created in the mantle wedge as a consequence of fluid addition, the data cannot resolve whether fluid modification of the source initiated melting or was an earlier (> 350 k.y. prior) event. (231Pa/235U) data for these samples, the first such data from the Cascades, are two to four times higher than any U-Pa data previously measured in lavas from any tectonic setting. The data cannot be reproduced from a secular equilibrium or U-enriched source with reasonable model parameters, implying significant pre-melting source 231Pa-enrichment. This source enrichment is interpreted to result from addition of a subducted sediment partial melt, generated in the presence of an allanite/monazite-free residue as a consequence of the hot conditions within the subducting slab. This implies melt addition to the sub-arc mantle << 150 k.y. prior to melting. The majority of the mafic lavas erupted in the study area have been modified from mantle-derived compositions by passage through the ~ 45 km thick overlying arc crust. Modification of mantle-derived magmas in the Three Sisters appears to have been dominated by interaction with a lower crustal component, likely a partial melt of mafic lower crust and/or a residual liquid from fractional crystallization of hydrous basalt. Crustal interaction at Newberry, in contrast, is dominated by assimilation of a felsic upper crustal component, similar in composition to obsidians erupted within the central caldera. Magma differentiation in the Three Sisters region leads to both decreases and increases in U-series isotope activity ratios, while at Newberry activity ratios are lowered by assimilation of felsic crust. Eruption of true rhyolite is unusually common around South Sister in comparison with the arc axis elsewhere. Newberry is a bimodal basalt-rhyolite volcanic field similar to other rear-arc volcanoes in the Cascades, with abundant true rhyolite also. Three compositionally very similar South Sister rhyolites are interpreted as 15 to 20% partial melts of lower crustal mafic amphibolite, modified slightly by AFC processes in the same shallow magma chamber prior to eruption. Two Newberry obsidians display slightly greater compositional variability, including U-series variations, and are interpreted as separate melts of upper crustal granitic rocks, which are themselves interpreted to ultimately result from lower crustal melting. A recurrent theme throughout this dissertation is the strong control exerted on magmatism in the study area by local tectonic factors. In the Three Sisters region, where intra-arc extension is highly focused and the study area lies within the High Cascades graben, primitive magmas are relatively common. Furthermore, lower crustal processes are the dominant influence on differentiation of primitive magmas as well as generation of felsic magmas, with minor upper crustal interaction. In contrast, primitive compositions are rare or absent at Newberry, where a large volcanic edifice has developed since at least 0.5 Ma. Melts from the mantle or lower crust are intercepted beneath the central caldera, where only felsic lavas generated by re-melting of shallow granitic intrusions are erupted. Evolved mafic magmas, carrying a signature of upper crustal interaction, are confined to flank eruptions along rift zones.
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