HR: 1340h
AN: T33B-1390    [Abstracts]
TI: Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador
AU: Bryant, J
EM: jbryant@geol.sc.edu
AF: Univ. of South Carolina, Dept. of Geological Sciences, Columbia, SC 29208 United States
AU: * Yogodzinski, G
EM: gyogodzin@geol.sc.edu
AF: Univ. of South Carolina, Dept. of Geological Sciences, Columbia, SC 29208 United States
AU: Hall, M
EM: volcan_pete@yahoo.com
AF: Instituto Geofisico, Escuela Politecnica Nacional, P.O. Box 17-01-2759, Quito, 123456 Ecuador
AU: Lewicki, J
AF: Lawrence Berkeley National Lab, Earth Sciences Division, Berkeley, CA 94720 United States
AU: Bailey, D
EM: gyogodzin@geol.sc.edu
AF: Hamilton College, Dept. of Geology, Clinton, NY 13323 United States
AB: Whole-rock geochemical data on basaltic-to-rhyolitic samples from 12 Late Pleistocene-Holocene volcanic centers are used to constrain the role of continental crust in the genesis of melts formed beneath the anomalously wide arc in Ecuador. Primitive and relatively homogeneous isotopic compositions observed across the arc imply that Ecuador lavas were produced largely within the subduction zone and not by extensive melting of crustal rocks similar to those upon-which the volcanoes were built. Mixing calculations limit the quantity of assimilated crust to less than approximately 10 percent. Cross-arc geochemical variation (e.g., in 143/144Nd, Ba/Nb, La/Yb) indicates that assimilation of crustal rock is most active in the center-arc area, around the Inter-Andean Graben, at distances of 330-360 km from the trench. Characteristically `adakitic' features (low Y, Yb, high La/Yb), which are observed in virtually all andesites and dacites in Ecuador, appear to be strongly influenced by crystal fractionation (e.g., as indicated by compatible element behavior for Y) and are most clearly developed in lavas that, based on isotopic compositions, appear to have assimilated the most crust. A subset of andesites, which display an unusual combination of high Sr ($>$900 ppm) and non-radiogenic isotopes (epsilonNd $>$4.1, delta7/4Pb $<$6.0), appear to be regionally distinctive in Ecuador (i.e., not observed among modern lavas in the Andean central and southern volcanic zones or in Colombia). Only these lavas, which are well represented at Imbabura Volcano, appear to have geochemical features that may be consistent with a slab-melt interpretation associated with the subduction of relatively young, over-thickened oceanic crust of the subducting Carnegie Ridge.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting