HR: 11:20h
AN: V11I-05    [PDF]
TI: $^{40}$Ar/$^{39}$Ar Geochronology of the Pleistocene to Historic Puyehue-Cordon Caulle Volcanic Complex, Andean Southern Volcanic Zone, Chile
AU: * Harper, M A
EM: mharper@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton St., Madison, WI 53706 United States
AU: Singer, B S
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton St., Madison, WI 53706 United States
AU: Moreno Roa, H A
EM: hmoreno@surnet.cl
AF: Observatorio Volcanologico de los Andes del Sur, Casilla 23-D, Temuco,, IX Region Chile
AB: Linking magma reservoir processes beneath arc volcanoes to their surficial expression as eruptive events is hampered, in part, by an incomplete knowledge of long-term eruptive flux. To address this problem, we have begun to quantify the eruptive history of the 105 km$^{3}$ Puyehue-Cordon Caulle Volcanic Complex at $40.5\deg$S in the Andean Southern Volcanic Zone (SVZ) using a combination of $^{40}$Ar/$^{39}$Ar dating and geologic mapping. Puyehue-Cordon Caulle has erupted low-medium K basaltic to rhyolitic magma that spans the largest range of major element compositions in the southern SVZ. Low radiogenic argon contents necessitate the incremental heating of multiple aliquots (200-800 mg) of groundmass from each lava using a low-blank resistance furnace. New $^{40}$Ar/$^{39}$Ar plateau ages from 26 lavas suggest that volcanism began ca. 250 ka. Numerous Mid- to Late Pleistocene vents were active between 250 and 33 ka and produced at least 85 km$^{3}$ of basaltic to andesitic lava, tephra, and ignimbrites. More recently, the large Puyehue stratovolcano, built between 45 and 2.7 ka, erupted 18-21 km$^{3}$ of basaltic to rhyolitic lava and tephra. Puyehue began to grow after 45 ka with the eruption of 8-10 km$^{3}$ of dacitic and rhyodacitic lava followed by several km$^{3}$ of basalt and basaltic andesite that erupted between 15.1 $\pm$ 2.6 ka and 10.0 $\pm$ 1.1 ka. The final stage of cone growth, comprising 1-2 km$^{3}$ of rhyolitic and rhyodacitic lava and tephra, began by 10 ka and culminated at 2.66 $\pm$ 0.19 ka ($^{14}$C date) with the plinian eruption of several km$^{3}$ of airfall deposits zoned from dacite to basaltic andesite. The average eruptive rate for Puyehue volcano is 0.4-0.5 km$^{3}$/k.y., twice that of Tatara-San Pedro volcano located $\sim$ 500 km to the north. Glaciers eroded extensive parts of the complex between 200 and 15 ka, making our growth rate a minimum estimate. The Cordon Caulle fissural zone, which extends for 15 km to the NW of the modern Puyehue edifice, has erupted 2 km$^{3}$ of rhyodacitic to rhyolitic lavas, tephras, domes, and pyroclastic cones during the past 1 kyr, with explosive and effusive historic eruptions occurring in 1921-22 and 1960. In contrast to adjacent Puyehue, historic Cordon Caulle rhyolite and rhyodacite erupted at a much faster rate of 6.7 km$^{3}$/k.y.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting