HR: 0800h
AN: V51D-1526    [Abstracts]
TI: Progressive assimilation and recycling of mafic arc lower crust beneath North Sister Volcano, Central Oregon Cascade Range
AU: * Schmidt, M E
EM: schmidtm@geo.oregonstate.edu
AF: Department of Geosciences, 104 Wilkinson Hall Oregon State University, Corvallis, OR 97331
AU: Grunder, A L
EM: mariekschmidt@hotmail.com
AF: Department of Geosciences, 104 Wilkinson Hall Oregon State University, Corvallis, OR 97331
AB: North Sister, a composite volcano in the central Oregon Cascade Range has produced monotonous basaltic andesite (52.5-54.0 % SiO2) depleted in incompatible elements since ~300 ka. Changes in eruptive style, unconformable contacts, ⁴⁰Ar/³⁹Ar dating, and geochemical correlations divide North Sister's eruptive history into four stages: (1) the early shield (320 ± 66 ka), (2) the subglacial stage (191.2 ± 28.7 ka to 105.2 ± 38.6 ka), (3) the upper shield stage, and (4) the stratocone stage (71.7 ± 39.6 ka to 57.2 ± 36.8 ka). These stages roughly correspond to four compositional groups (1, 2a, 2b, and 3-4) that record a general decrease in compatible elements such as MgO (5.9 to 4.0%) and Ni (107 to 35 ppm), while incompatible elements remain the same or generally decrease (e.g. Ba 302 to 247 ppm) through time. Isotopic variations at North Sister are small, but systematic with time and with decreasing MgO such that ⁸⁷Sr/⁸⁶Sr decreases (0.70356 to 0.70369), ¹⁴⁴Nd/¹⁴³Nd increases (0.51285 to 0.51292) and Pb isotopic ratios decrease. δ¹⁸O increases (5.5 to 6 permil) with decreasing MgO. These isotopic and trace element variations constrain petrologic models that require assimilation of a mafic amphibole-bearing lower crust. The contaminant must be depleted in incompatible elements and progressively acquire Sr, Nd, and Pb isotopic signatures like those of regional primitive basalts. North Sister's ¹⁸⁷Os/¹⁸⁸Os are higher than regional primitive basalts (0.1858 vs. 0.1413), but do not appear to vary systematically. Os isotopes constrain the age of the contaminant by calculating the time it takes for regional basalt to decay to North Sister's ¹⁸⁷Os/¹⁸⁸Os and assuming start ¹⁸⁷Re/¹⁸⁸Os similar to modern arc lower crustal xenoliths elsewhere. The resulting age range of 5 to 30 Ma is younger than the Siletz Terrane and the initiation of Cascade volcanism. North Sister's trace element and isotopic variations through time thus record progressive assimilation and recycling of mafic arc underplate and its homogenization toward more mantle-like Sr and Nd isotopic compositions.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005