HR: 1400h
AN: V23A-02    [Abstracts]
TI: Holocene and Late Pleistocene Tephra Stratigraphy of the Mono Craters
AU: * Meyn, C R
EM: crmeyn@buffalo.edu
AF: University of Buffalo, Department of Geology, Buffalo, NY 14260, United States
AU: Bursik, M I
EM: mib@buffalo.edu
AF: University of Buffalo, Department of Geology, Buffalo, NY 14260, United States
AB: The Mono Craters consists of ~30 high-silica rhyolitic domes and flows and one rhyodacite dome. The exposed domes were emplaced during eruptions from the late Pleistocene to the most recent eruption 600 y BP. The ages of the eruptions with exposed domes were constrained by obsidian hydration rind dating by previous workers to two distinct phases; between 20,000 and 13,000 y BP and from 6,000 y BP to 600 y BP. The first eruptive phase consists of a biotite bearing assemblage while the more recent suite of eruptions began with alternating orthopyroxene and fayalite bearing rhyolites, which gave way to sparsely porphyritic and most recently aphyric rhyolites. By studying the Holocene tephra stratigraphy of the Mono Craters, this work aims to confirm the apparent gap in late Pleistocene and Holocene activity from 13,000 to 6,000 y BP and better constrain the local volcanic chronology. Excavation of a number of sites of tephra deposition from the Mono Craters were conducted and the various tephras were analyzed for petrology geochemistry and radiocarbon. The tephras include the full range of petrologic assemblages of the Mono Craters and range from the most recent eruptions to the biotite bearing eruptions of the late Pleistocene. Results of radiometric carbon dating are consistent with the cessation of the emplacement of a biotite bearing high silica porphyritic suite of eruptions prior to the 13,300 y BP eruption of the Black Point basalt, and the resumption of activity with alternating orthopyroxene and fayalite bearing assemblages. Between a pair of proximal radiocarbon dated layers (1672 +/- 36 and 1688 +/- 47 ¹⁴C y BP) are four volcanic units, representing three of the petrological assemblages (aphyric, sparsely porphyritic, and porphyritic orthopyroxene-bearing); indicating that the different assemblages were, at least in this eruptive sequence, erupted contemporaneously. This indicates that the chamber beneath the Mono Chain is likely weakly zoned. Major and trace element chemistry of the younger volcanics from this and previous work indicate that the orthopyroxene and fayalite bearing rhyolites are less evolved than their biotite bearing predecessors. The tephras contain abundant mafic xenoliths, which are particularly pronounced in some of the orthopyroxene bearing units. Fayalites in the fayalite bearing assemblage show varying degrees of skeletal texture. These data point toward a more mafic magma interacting with the Mono source at depth.
DE: 8404 Volcanoclastic deposits
DE: 8415 Intra-plate processes (1033, 3615)
DE: 8428 Explosive volcanism
DE: 8455 Tephrochronology (1145)
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly