HR: 1340h
AN: V13B-1470    [Abstracts]
TI: Geochemical Evolution of Pre-caldera Magmas at Caviahue Caldera, Neuquen Province, Argentina
AU: * Todd, E
EM: et8@dana.ucc.nau.edu
AF: Northern Arizona University, PO Box 4099, Flagstaff, AZ 86001 United States
AU: Ort, M
EM: michael.ort@NAU.EDU
AF: Northern Arizona University, PO Box 4099, Flagstaff, AZ 86001 United States
AB: Caldera subsidence and glacial erosion at Caviahue, an upper Miocene to Pliocene volcanic center located in the Andean Southern Volcanic Zone (SVZ) at 37ø50'S, has exposed a detailed cross-section of pre-caldera volcanic activity from the upper Miocene to the Pliocene. Caldera walls expose 500 to 800 m of ignimbrites, cinder cones, volcanic breccias, and lava flows, which range from 1 to nearly 100 m in thickness. Lavas erupted from the monogenetic pre-caldera volcanic field have compositions ranging from evolved basaltic andesites (4% MgO, 10% FeO) to trachytes. Strong Ni-depletion signatures and high Fe/Mg ratios indicate extensive geochemical modification of Caviahue lavas. Petrologic and geochemical analyses of major and trace element abundances in Caviahue lavas indicate cyclic fractionation and recharge in an upper-crustal magma chamber during pre-caldera volcanism. Compatible and incompatible element abundances (especially Ni, MgO, K, and Zr), plotted in stratigraphic succession, show at least six distinct fractionation trends occurred between emplacement of the oldest exposed lava flows and the eruption of the ignimbrite associated with caldera formation. Each fractionation trend is punctuated by the infusion of a volume of new, more primitive magma. Modeling of recharge events indicates that these introduced from less than half to several times the volume of the existing magma body of new, more primitive (but still evolved) magma to the chamber. Geochemical analyses of lavas deposited between intermittent periods of magma residence and volcanic eruptions show strong patterns of plagioclase, olivine, clinopyroxene, and oxide fractionation. Deposits recognized on the caldera floor thought to be associated with caldera collapse are correlated with extra-caldera trachytic ignimbrite deposits dated at 2.02 Ma, providing a late Pliocene age for caldera collapse. Post-caldera volcanism has been active until present, but has shifted to smaller polygenetic volcanic centers on the periphery of the Caviahue Caldera with the majority of volcanic activity at the historically active Volc n Copahue, located on the western rim of the caldera.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting