HR: 15:45h
AN: U53A-08    [Abstracts]
TI: Protolith and metamorphic ages of gneiss hosting eclogite in the Chuacús complex, Central Guatemala
AU: * Martens, U
EM: umartens@pangea.stanford.edu
AF: Dept. Geological and Env. Science, 450 Serra Mall, Bldg 320, Stanford, CA 94305, United States
AU: Mattinson, C G
EM: cgm@pangea.stanford.edu
AF: Dept. Geological and Env. Science, 450 Serra Mall, Bldg 320, Stanford, CA 94305, United States
AU: Wooden, J
EM: jwooden@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield road, Menlo Park, CA 94025, United States
AU: Liou, J G
EM: liou@pangea.stanford.edu
AF: Dept. Geological and Env. Science, 450 Serra Mall, Bldg 320, Stanford, CA 94305, United States
AB: Epidote-amphibolite-facies gneisses in the Sierra de Chuacús, Central Guatemala, contain mafic domains with relics of eclogite-facies mineral parageneses. Field relations and petrography suggest that the protolith of orthogneiss was intruded by mafic dikes, subsequently subjected to eclogite-facies conditions, and finally strongly overprinted at epidote-amphibolite conditions. The host orthogneiss for an eclogitic band ca. 1m thick in the Agua Caliente River, and a quartz-pyroxene-garnet-zoisite gneiss from Los Altos were sampled for zircon SHRIMP-RG U/Pb dating and REE analyses. Cathodoluminescence images, Th/U, U/Pb and REE patterns of Agua Caliente gneiss zircon cores indicate magmatic crystallization between 217-229 Ma. Magmatic zircon cores of the Los Altos sample have similar texture, Th/U and REE, and yield 220-245 Ma ages. Coeval migmatite formation has been reported near Huehuetenango, Western Guatemala, suggesting extensive Triassic crustal melting and plutonism at the southern edge of the Maya block. A second population of Los Altos zircon cores show metamorphic textures with ages between 840-900Ma, suggesting a connection to Neoproterozoic orthogneisses in the northern flank of the Sierra de Chuacús. Bright CL metamorphic zircon rims from Chuacús gneiss contain low U (<60 ppm), and yield a weighted mean U²³⁸/Pb²⁰⁶ age of 75+/- 3Ma (n=7). Ages in the 217-80 Ma interval do not form clusters and are compatible with partial recrystallization of magmatic zircon or mixed analysis. Zircon rims of Los Altos gneiss show lower REE than magmatic cores, and steep heavy REE patterns. Zircon rims of Agua Caliente orthogneiss show less steep heavy REE patterns suggestive of growth in equilibrium with garnet, and are enriched in light REE. Eu anomalies in REE patterns indicate that cores and rims grew stable with plagioclase at crustal depth. This implies the ca.75 Ma metamorphic age represents the epidote-amphibolite overprint, and constrains the high-pressure event to have occurred after the Triassic and before the Late Cretaceous.
DE: 1100 GEOCHRONOLOGY
DE: 3660 Metamorphic petrology
DE: 9350 North America
SC: Union [U]
MN: 2007 Joint Assembly