HR: 0800h
AN: T21A-0511    [Abstracts]
TI: Trace- and major-element zoning in garnets from eclogites of the Zermatt-Saas ophiolite, western Alps: LA-ICP-MS data and implications for Lu-Hf geochronology
AU: * Mahlen, N J
EM: mahlen@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Skora, S
EM: sskora@pop.unil.ch
AF: University of Lausanne, Institute of Mineralogy and Petrology BFSH2, Lausanne, CH-1015 Switzerland
AU: Johnson, C M
EM: clarkj@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Baumgartner, L P
EM: Lukas.Baumgartner@img.unil.ch
AF: University of Lausanne, Institute of Mineralogy and Petrology BFSH2, Lausanne, CH-1015 Switzerland
AU: Lapen, T J
AF: University of Wisconsin-Madison, Department of Geology and Geophysics 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Pilet, S
EM: Sebastien.Pilet@img.unil.ch
AF: University of Lausanne, Institute of Mineralogy and Petrology BFSH2, Lausanne, CH-1015 Switzerland
AU: Beard, B L
EM: beardb@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics 1215 W. Dayton Street, Madison, WI 53706 United States
AB: Trace element contents, inclusion assemblages, and element zoning patterns found in garnets from several eclogite-facies metabasalts sampled across the Zermatt-Saas ophiolite complex, western Alps, record large segments of the prograde P-T-t path to HP/UHP conditions. Ca, Fe, Mn, and Mg contents in garnet from core to rim are indicative of prograde growth zoning. Laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) on garnets cut to expose the core using 3-D x-ray tomography indicates strong HREE (Er, Yb, Lu) zoning, where the highest concentrations occur in the garnet core. The core-to-rim zoning of HREE is very similar to that of Mn determined by both electron microprobe and LA-ICP-MS along the same transect. The HREE and Mn patterns tend to be bell-shaped and have a moderate spike in concentrations toward the rim that is followed by an increase in Gd concentrations. These complex zoning patterns suggest that a reaction or multiple reactions took place that liberated HREE and possibly MREE as well during growth of garnet. Hf concentrations, estimated from Zr concentrations measured by LA-ICP-MS, are typically constant across garnet traverses. The Lu and Hf garnet zoning patterns have important implications for Lu-Hf geochronology because they indicate that the Lu-Hf ratios are highest in the core of the garnet. The high Lu-Hf ratio in garnet cores results in a measured Lu-Hf age that is strongly skewed toward the early prograde growth, if bulk garnet separates are used for geochronology for units such as the Zermatt-Saas ophiolite that were not metamorphosed to temperatures above the Lu-Hf blocking temperature. Sm-Nd and Lu-Hf geochronology using bulk garnet separates suggests that the duration of prograde metamorphism of the Zermatt-Saas ophiolite in the Lago di Cignana area, Italy, is between 50-38 Ma (Amato et al., 1999; Lapen et al., 2003). An eclogite sample collected from the Pfulwe area, Switzerland, yielded a bulk garnet-cpx-whole rock Lu-Hf age of 54 ñ 3 Ma (2-sigma). Based on our Lu and Hf zoning data, we interpret the age from the Pfulwe area as representing a stage in its P-T-t history that precedes attainment of peak metamorphic conditions. The range in ages between Cignana and Pfulwe may record the different partitioning behavior of Lu, Hf, Sm, and Nd in specific samples or the diachronous subduction of the ophiolite sheet.
DE: 8110 Continental tectonics--general (0905)
DE: 3655 Major element composition
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting