Late Pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at Crater Lake, Oregon

Earth and Planetary Science Letters
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Rhyodacite tephra and three lavas erupted ∼27 ka, interpreted to be early leaks from the climactic magma chamber of Mount Mazama, contain ubiquitous resorbed crystals (antecrysts) that were recycled from young granodiorite and related plutonic rocks of the same magmatic system. The shallow composite pluton is represented by blocks ejected in the 7.7-ka climactic eruption that formed Crater Lake caldera. Plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented Fe-oxide needles exsolved at subsolidus conditions. At least 80% of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks. Other crystals in the rhyodacite, notably zircon, also were recycled. SIMS 238U–230Th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between ∼20 ka and ≥ 300 ka with concentrations of analyses near 50–70, ∼110, and ∼200 ka that correspond to periods of dacitic volcanism dated by K–Ar. U–Th ages of zircon from a rhyodacite sample yield similar results. No analyzed zircons from the granodiorite or rhyodacite are pre-Quaternary. Zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton. Survival of zircon in zircon-undersaturated hydrous magma and of Fe-oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the ∼27-ka eruption of the rhyodacite. The ∼27-ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high-level magma chamber. In contrast, the voluminous 7.7-ka climactic rhyodacite pumice is virtually lacking in zircon, indicating dissolution of any granodioritic debris in the intervening period. Mineralogical evidence of assimilation may be destroyed in hot, vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of Mount Mazama.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Late Pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at Crater Lake, Oregon
Series title Earth and Planetary Science Letters
DOI 10.1016/j.epsl.2005.02.012
Volume 233
Issue 3-4
Year Published 2005
Language English
Contributing office(s) Volcano Hazards Program, Volcano Science Center
Description 17 p.
Larger Work Title Earth and Planetary Science Letters
First page 277
Last page 293
Country United States
State Oregon
Other Geospatial Crater Lake
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