HR: 16:50h
AN: T24A-04    [Abstracts]
TI: Age of eclogite facies metamorphism of metabasic boudins within schist and gneiss of the Monte Rosa nappe, Val di Gressoney, Western Alps, Italy
AU: * Lapen, T J
EM: lapen@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St, Madison, WI 53704 United States
AU: Johnson, C M
EM: clarkj@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St, Madison, WI 53704 United States
AU: Baumgartner, L P
EM: lukas.baumgartner@img.unil.ch
AF: Institute of Mineralogy and Petrology, University of Lausanne BFSH2, Lausanne, VD 1015 Switzerland
AU: Beard, B L
EM: beardb@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St, Madison, WI 53704 United States
AB: High precision 238U-206Pb geochronology of rutile from quartz-carbonate-rutile tension veins in eclogite boudins indicate that eclogite-facies metamorphism may have lasted at least from 52 to 46 Ma. The eclogite boudins occur within quartzo-feldspathic schist and gneiss of the Monte Rosa nappe near the Indren glacier, Val di Gressoney, Western Alps, Italy. The association of a few eclogite boudins with serpentenite and gabbro suggests that some of the boudins may be tectonic slices of the Zermatt-Saas ophiolite, which lies structurally above the Monte Rosa nappe. The sampled tension veins, which typically consist of quartz, carbonate, and rutile, are usually oriented at high angles to the long axis of the boudins and are likely synchronous with boudinage of more coherent tracts of metabasite. The sampled tension veins cross-cut eclogite facies minerals but there is little or no associated retrogression of the eclogite-facies minerals adjacent to the veins, suggesting that vein formation occurred under eclogite-facies conditions. The age data suggests that rutile growth was episodic, perhaps associated with fluid pulses, and likely represents a possible age range of eclogite facies conditions recorded in these boudins. The eclogite boudins are locally isoclinally folded, possibly under lower eclogite to greenschist facies conditions based on the mineral assemblages in the fold hinges. This indicates that the eclogites, if they are exotic to the Monte Rosa nappe, were juxtaposed with schist and gneiss at least prior to this folding event. Regardless of the exotic or in-situ origin of the eclogites, if the boudinage of eclogites occurred within the Monte Rosa nappe, the age of high pressure metamorphism in the Monte Rosa may also be between 46 and 52 Ma. The geodynamic implications are that the Monte Rosa nappe experienced high pressure conditions synchronously with the Zermatt-Saas ophiolite and that they were in contact during exhumation (e.g. Reddy et al., 1999), raising the possibility that buoyancy-driven processes in part aided in the exhumation of the Zermatt-Saas ophiolite complex.
DE: 8102 Continental contractional orogenic belts
DE: 5475 Tectonics (8149)
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting