HR: 0800h
AN: V51B-0573    [Abstracts]
TI: Chemical, Isotopic, and Ar-Geochronologic Evidence for Three-Phase Volcanic History in the Northwest Basin & Range and High Lava Plains
AU: * Coble, M A
EM: coblem@stanford.edu
AF: Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, United States
AU: Scarberry, K
EM: scarberk@science.oregonstate.edu
AF: Department of Geosciences, Oregon State University, Corvallis, OR 97331, United States
AU: Grunder, A L
EM: grundera@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, Corvallis, OR 97331, United States
AU: Wiegand, B A
EM: bwiegand@pangea.stanford.edu
AF: Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, United States
AB: The transition between active extension in the northern Basin and Range and the dextral accommodation in the High Lava Plain (HLP) of southern Oregon represents a long-lived lithospheric boundary located along the ⁸⁷Sr/⁸⁶Sr 0.704 isopleth. New element geochemistry, ⁴⁰Ar/³⁹Ar geochronology, and Sr and Nd isotopic data from 37 bimodal Eocene to late-Miocene age rocks from southern Oregon and northwest Nevada were analyzed to investigate spatial and geochemical patterns of arc, hot-spot, and extension-related volcanism as modified by this boundary. Samples were collected throughout northwest Nevada and southeastern Oregon, and represent three tectonomagmatic episodes of volcanism: 1) 38.1-20.3 Ma, low- volume, episodic, calc-alkalic magmatism (47-75.5 wt% SiO2) interpreted to have an arc signature, 2) 17.2-15.0 Ma, tholeiitic to weakly alkalic flood basalt followed by widespread peralkaline rhyolite (70.2-76.5 wt% SiO2) attributed to impingement of the Yellowstone hot-spot, and 3) 11.8-5.5 Ma, bimodal assemblage of migrating silicic magmatism (67.2-77.7 wt% SiO2) and widespread high-alumina olivine tholeiites (48.1-52.6 wt% SiO2) in the HLP accompanied by lithospheric extension. The isotopic compositions of all analyzed Eocene to late- Miocene silicic ignimbrites and lavas (⁸⁷Sr/⁸⁶Sr_i 0.7036-0.7075; ε Nd₀ -0.5 to +4.9) are more heterogeneous and differ from coeval basalts. The 38.1-20.3 Ma mafic rocks show isotopic variability which appears to be spatially correlated with the abrupt crustal transition (⁸⁷Sr/⁸⁶Sr 0.704 isopleth) that broadly follows the boundary of accreted Mesozoic terrains. Eocene to Early Miocene mafic samples east of the ⁸⁷Sr/⁸⁶Sr 0.704 line in the Basin and Range have more evolved ⁸⁷Sr/⁸⁶Sr_i and ε Nd₀ values than samples to the northwest where the isotopic compositions are closer to MORB. High K₂O/TiO₂ and low Zr/Ba differentiate 38.1-20.3 Ma mafic magmas that have an arc-signature, whereas the mid-Miocene Steens and Columbia River Basalts, and younger high-alumina olivine tholeiites, have compositions closer to MORB or OIB's.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 8109 Continental tectonics: extensional (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting