HR: 0800h
AN: V51D-1515    [Abstracts]
TI: Eocene Magmas as Probes of Wrangellia Composite Terrane Crust in the Talkeetna Mountains, Alaska
AU: * Oswald, P
EM: oswa8070@uidaho.edu
AF: University of Idaho, Dept. of Geological Sciences, Moscow, ID 83844 United States
AU: Vervoort, J
EM: vervoort@WSU.edu
AF: Washington State University, Dept. of Geology, Pullman, WA 99164
AU: Geist, D
EM: dgeist@uidaho.edu
AF: University of Idaho, Dept. of Geological Sciences, Moscow, ID 83844 United States
AB: Eocene volcanic rocks in the southern Talkeetna Mountains are part of a 90 km-long northwest trending volcanic field that is oriented perpendicular to the trend of the Aleutian arc. The magmatic rocks intrude and overlie the Wrangellia-Peninsular terrane boundary and range in composition from basalt through rhyolite, with most being mafic to intermediate in composition. Basalts from across the volcanic field are characterized by flat REE patterns and low Ba/Nb, whereas intermediate to felsic samples show elevated Ba/Nb and positive correlations between La/Sm and SiO₂. Two distinct REE vs. SiO₂ trends are present, with samples from northern part of the volcanic field having higher La/Yb and La/Sm relative to SiO₂ than samples from the southern part of the field. Mixing between a mafic magma and two different crustal sources is also evident in the isotopic composition of the felsic end members: rhyolite from the southern part has a mantle-like isotopic signature (⁸⁷Sr/⁸⁶Sr(i)= 0.70384; ¹⁴³Nd/¹⁴⁴Nd(i) = 0.513001), whereas rhyolite from the north has a more evolved isotopic signature (⁸⁷Sr/⁸⁶Sr(i) = 0.70639; ¹⁴³Nd/¹⁴⁴Nd(i) = 0.512798). Intermediate rocks from both the northern and southern end of the volcanic field have isotopic compositions intermediate between the basalt and the rhyolite samples from those areas. We interpret the difference in isotopic composition as resulting from anatexis and AFC processes involving crust of varying age and composition across the Wrangellia Composite terrane. Mantle-derived magmas interacted with juvenile crustal rocks associated with the Jurassic arc of the Peninsular terrane to the south, and a more evolved crustal source associated with Wrangellia to the north. The volcanic field is located in the hinge area of the southern Alaska orocline and was active during a lull in regional arc magmatism. Normal faults and dikes parallel to the trend of the field suggest volcanism was associated with extension, which may have been related to right lateral displacements along major regional strike-slip faults or translation into a preexisting oroclinal hinge area.
DE: 1020 Composition of the continental crust
DE: 1025 Composition of the mantle
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005