Late Pleistocene granodiorite beneath Crater Lake caldera, Oregon, dated by ion microprobe

Geology
By: , and 

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Abstract

Variably melted granodiorite blocks ejected during the Holocene caldera-forming eruption of Mount Mazama were plucked from the walls of the climactic magma chamber at ∼5 km depth. Ion-microprobe U-Pb dating of zircons from two unmelted granodiorite blocks with SHRIMP RG (sensitive high-resolution ion microprobe–reverse geometry) gives a nominal 238U/206Pb age of

100+7880100−80+78

ka, or

174+89115174−115+89

ka when adjusted for an initial 230Th deficit. SHRIMP RG U-Th measurements on a subset of the zircons yield a 230Th/238U isochron age of 112 ± 24 ka, considered to be the best estimate of the time of solidification of the pluton. These results suggest that the granodiorite is related to andesite and dacite of Mount Mazama and not to magmas of the climactic eruption. The unexposed granodiorite has an area of at least 28 km2. This young, shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and powered violent explosive eruptions ∼7700 yr ago, resulting in collapse of Crater Lake caldera.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Late Pleistocene granodiorite beneath Crater Lake caldera, Oregon, dated by ion microprobe
Series title Geology
DOI 10.1130/0091-7613(2000)28<467:LPGBCL>2.0.CO;2
Volume 28
Issue 5
Year Published 2000
Language English
Contributing office(s) Volcano Science Center
Description 4 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Geology
First page 467
Last page 470
Country United States
State Oregon
Other Geospatial Crater Lake