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Abstract We present230Th‐238U crystallization ages and trace element compositions for zircons spanning the late Pleistocene to Holocene rhyolite eruptive record at South Sister volcano in the central Oregon Cascade Range. Most zircon ages are between 100 and 20 ka, with very few in secular equilibrium (>350 ka). The weighted mean of zircon ages for the two oldest South Sister rhyolites, 31.5 ± 2.1 and 39.1 ± 2.4 ka, are significantly younger than the associated40Ar/39Ar ages, 47.4 ± 9.7 and 51.4 ± 9.7 ka. We propose that these40Ar/39Ar dates, performed on plagioclase separates, are compromised by a subtle amount of excess Ar and therefore the younger weighted mean zircon ages yield more reliable eruption ages. These results imply that the interval of rhyolite eruption at South Sister during the late Pleistocene was both shorter and more productive than previously thought and that eruption at South Sister initiated after Middle Sister. Compositionally, zircons from the Pleistocene rhyolites are broadly similar and show down‐temperature zircon and plagioclase crystallization trends. However, we argue that destabilized amphibole and titanite in a common mush also exert leverage on the Pleistocene zircon trace element compositions. Divergence in the Eu/Eu* ratio between the Pleistocene and Holocene lavas implies chemically distinct magma reservoirs originating from the Pleistocene rhyolite eruptive sequence and the Holocene eruptive sequence. This work suggests a higher flux of rhyolite volcanism than previously thought and characterizes magmatic storage distinctions between the Pleistocene and Holocene rhyolites, aiding in the assessment of future eruptive hazards at South Sister volcano.
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Continental rift rhyolites at the confluence of the Basin and Range and Colorado Plateau, USA: timescales of activity from the zircon and feldspar record
The Colorado Plateau, USA, is bordered by Pleistocene continental rift volcanism in New Mexico, Arizona, and Utah. While most of the eruptions have been basaltic, rhyolitic domes, tuffs, and lavas have been produced. On the western margin, where the Colorado Plateau meets the Basin and Range extensional province, the Black Rock Desert of central Utah hosts Pleistocene-Holocene bimodal basalt-rhyolite volcanic activity. The South Twin Complex consists of six rhyolites within a single basin erupted between 2.45 and 2.40 Ma, and they precede all Pleistocene basalts of the region. In this work, we share a new rhyolite eruptive stratigraphy based on high precision 40Ar/39Ar dates and examine the zircon crystal cargo from each eruptive center. The new eruption ages allow us to examine the spatial and temporal distribution of volcanism in the South Twin Complex, whereas the zircon crystal morphology, geochemistry, and U/Pb dating allow us to assess the conditions and timescales of silicic magma processes in the subvolcanic plumbing system. Our data suggest the plumbing system beneath the region experienced punctuated influxes of magma over a brief period of thousands to tens of thousands of years. Further, the timescales and patterns of silicic magma assembly and evolution of this small anorogenic region are similar to those observed within the voluminous Yellowstone province, suggesting that the volume of magmatic flux does not control magmatic evolution in intercontinental settings.
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- Award ID(s):
- 1940305
- PAR ID:
- 10401230
- Date Published:
- Journal Name:
- IAVCEI 2023
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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