HR: 15:25h
AN: V33E-07    [Abstracts]
TI: Dating Ductile Deformation With Combined Lu-Hf and Ar-Ar Geochronology
AU: * Mulcahy, S R
EM: mulcahy@geology.ucdavis.edu
AF: Geology Department, Univ. of California, Davis, One Shields Ave, Davis, CA 95616, United States
AU: Roeske, S M
EM: roeske@geology.ucdavis.edu
AF: Geology Department, Univ. of California, Davis, One Shields Ave, Davis, CA 95616, United States
AU: McClelland, W C
EM: wmcclell@uidaho.edu
AF: Geological Sciences, Univ. of Idaho, PO Box 443022, Moscow, ID 83844, United States
AU: Jourdan, F
EM: fjourdan@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94720, United States
AU: Renne, P
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94720, United States
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: Department of Geology, Washington State Univ., PO Box 642812, Pullman, WA 99164, United States
AB: Understanding continental deformation requires accurately dating the initiation and duration of discrete deformation events. We present Lu-Hf and Ar-Ar chronology from the Duraznos shear zone, a fundamental lithologic and structural boundary between the accreted Precordillera terrane and the proto-Andean margin in the western Sierras Pampeanas of Argentina. When combined with detailed petrography and thermobarometry, the ages date the initiation and duration of deformation within the shear zone. The hanging wall consists of meta-turbidites, meta-volcanic rocks and orthogneiss. The rocks contain the assemblage Amp±Ky±St-Grt-Bt±Ep-Pl-Qtz-Ms-Rt-Ilm and experienced one metamorphic event at conditions of ~9 kb and 650° C. Garnet is pre- to syntectonic with respect to the mylonitic fabric; amphibole and muscovite grew within the fabric and amphibole does not occur within garnet. From a single sample we obtained a Lu-Hf garnet age of 469±21 Ma, an Ar-Ar amphibole isochron age of 441±8 Ma and a muscovite plateau age of 436±6 Ma. The footwall consists of mafic and ultramafic intrusive rocks and meta-volcanics of the Pie de Palo complex and preserves two separate metamorphic events. The first event is preserved as distinct core domains within garnet and amphibole. The second event resulted in a mylonitic fabric with syn- to post-tectonic garnet growth preserved as distinct rims on the earlier garnet and as fine-grained matrix garnet. P-T conditions of the younger event determined from phases within the mylonitic fabric are ~9 kb and 550° C. A Lu-Hf garnet age of 1067±14 Ma from the mylonite within the footwall dates the older of the two metamorphic events. The second event is interpreted to be synchronous with or slightly post-date the Duraznos shear zone. We interpret the observed prograde metamorphism in the footwall and the ages in the hanging wall to record initiation of the shear zone post 469 Ma and progressive cooling of the hanging wall during thrusting over the Pie de Palo complex to 436 Ma. The combination of high and intermediate-T isotopic systems with thermobarometry and petrography provides a way to constrain the duration of deformation and identify distinct packages of crustal rocks that underwent a shared deep-crustal Ordovician event.
DE: 1140 Thermochronology
DE: 3625 Petrography, microstructures, and textures
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting