HR: 1340h
AN: V13B-1476    [Abstracts]
TI: Tertiary volcanic activity at Sonora Pass, CA: arc and non-arc magmatism in the central Sierra Nevada
AU: * Roelofs, A
EM: roelofs@email.unc.edu
AF: Dept. of Geological Sciences, UNC Chapel Hill, Chapel Hill, NC 27599 United States
AU: Glazner, A F
EM: afg@unc.edu
AF: Dept. of Geological Sciences, UNC Chapel Hill, Chapel Hill, NC 27599 United States
AU: Farmer, G L
EM: farmer@ceres.colorado.edu
AF: Dept. of Geological Sciences, UC Boulder, Boulder, CO 80309 United States
AB: The volume and composition of Tertiary volcanic rocks in the Sierra Nevada of California changes dramatically near Sonora Pass (latitude $38\deg$ N). North of Sonora Pass is a large volume of volcanic rocks petrographically and chemically linked to subduction in the Cascade arc. South of Sonora Pass these calc-alkaline rocks are lacking and the only preserved volcanic rocks are small-volume mafic to intermediate potassic lavas that may have been generated by Pliocene lithospheric delamination (e.g. Farmer et al 2002). We have undertaken geologic mapping and geochemical and isotopic analysis of rocks near Sonora Pass at the boundary between these two magmatic provinces. At Sonora Pass, the 16-10 m.y.-old Relief Peak Formation and its hypabyssal equivalents are dominated by hornblende-phyric andesite lava flows and mudflow breccias (a stratovolcano assemblage) with marked high field-strength element (HFSE) depletions relative to large-ion lithophile elements (LILE), high Sri ($\sim$ 0.7056), and low $\epsilon$Nd (-1.4 $>$ $\epsilon$Nd $>$ -2.5). The overlying Stanislaus Group ($\sim$10-8 my old) has elevated HFSE and LILE relative to the Relief Peak Formation, anhydrous mineralogy, and similar isotope ratios (Sri $\sim$ 0.7056, -1.9 $>$ $\epsilon$Nd $>$ -3.4). The overlying Disaster Peak Formation is petrographically similar to the Relief Peak Formation. Lavas of the Relief Peak Formation may have been derived from the hydrated, LILE-rich and HFSE-poor mantle wedge above the subducting Juan de Fuca plate as part of the ancestral Cascade arc. The eruption of the dry, HFSE- and LILE-richer Stanislaus Group from vents near those of the Relief Peak Formation and to the east during an apparent pause in Relief Peak-type activity may represent a temporary shift to a dry, more fertile, isotopically enriched source in the mantle. The shift coincides with the arrival of the subducted Mendocino Fracture Zone (MFZ) beneath Sonora Pass, and the change in plate stress as the MFZ traversed the region may have influenced this shift. The Disaster Peak Formation probably represents the final episode of Cascade arc magmatism near Sonora Pass, ending about 5 Ma. No Pliocene or younger rocks derived from the low $\epsilon$Nd, high-K source tapped in the 3.5 Ma lithospheric delamination event to the south were found.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting