HR: 0800h
AN: V51C-0706    [Abstracts]
TI: Zircons From the Aucanquilcha Volcanic Cluster: Volcanic Sampling of an Evolving Batholith
AU: * Walker, B A
EM: barry.walker@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-5506, United States
AU: Grunder, A L
EM: anita.grunder@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-5506, United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: US Geological Survey, Green Earth Sci. 95, Stanford, CA 94305-2220, United States
AB: The Aucanquilcha Volcanic Cluster (AVC), northern Chile, is a complex of about 19 volcanoes which has experienced episodic volcanism from ~11 Ma to present. Its spatial and chemical evolution through time suggests that the AVC is the result of repeated sampling of a growing batholith at depth. Though compositionally diverse in its beginning stages, spanning from basaltic andesite to rhyodacite, protracted AVC magmatism rapidly gave way to a relatively voluminous and homogeneous (dacite to rhyodacite) output after about 6 m.y. of activity. Four main pulses of volcanism have been identified: Alconcha (11-8 Ma), Gordo (6-5 Ma), Polán (4.5-2 Ma), and Aucanquilcha (1- 0.2 Ma). New zircon U/Pb ages and trace element data (SHRIMP) from six lavas from the AVC further constrain the chemical and temporal evolution of its magmatic underpinnings during progressive stages of development. We analyzed between 16 and 24 spots from each sample. Samples from different stages of AVC volcanism show varying crystal residence times as indicated by multiple peaks in the U/Pb age spectra. Only small populations of zircon ages coincide with the ⁴⁰Ar³⁹Ar eruption age of each sample, and three samples have dominant age populations of zircon grains that are a few hundred thousand years older than the ⁴⁰Ar³⁹Ar age. The greatest age spectrum observed (~2.5 my) was in the oldest sample analyzed (8.49 ± 0.09 Ma, error is 2σ hereafter), from the Alconcha group. Minor populations (8.97 ± 0.32, 9.44 ± 0.44, and 11.09 ± 0.62, all Ma) demonstrate widespread inheritance of presumably antecrystic grains. Four samples are from the Polán group (with ages of 3.86 ± 0.08, 3.53 ± 0.25, 2.92 ± 0.14, and 2.56 ± 0.10, all Ma), which corresponds to the sudden increase in AVC output. These samples show more restriced age spectra within samples (300 to 500 Ma), indicating that the time of homogenization and increased volumetric output rate corresponds to a time of substantial resetting of the zircon ages, presumably through pervasive dissolution and recrystallization of new populations. Because the Polán samples come from a time of peak AVC volcanism, it is plausible that they reflect a series of hot magma chambers whose crystal cargo had largely been equilibrated with more recent magmatic events. Preliminary trace element data (relatively low Yb/Gd relative to Th/U) from zircons support the interpretation that a more limited spectrum of age populations reflects a hotter differentiation history. A sample from Volcán Aucanquilcha (recent AVC activity when volumetric output rates were again low) yielded a dominant age population at 1.22 ± 0.05 Ma, but with a complex age spectrum (~2 my) ripe with antecrystic grains. Three Mesozoic grains and one early Tertiary grain from this sample are interpreted to be xenocrystic. The age data suggest that crystal entrainment was common in the AVC magmatic system and that most entrainment involved recycling of magmatic phases only a few hundred thousand years older than the eruption age. The diversity of recycled material appears greatest at times of high thermal contrast, that is during beginning and waning phases of magmatism.
DE: 1020 Composition of the continental crust
DE: 1115 Radioisotope geochronology
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting