HR: 0830h
AN: V31E-0981    [PDF]
TI: The Boring Volcanic Field of the Portland Basin: Diverse Primitive Mafic Magmas Erupted in a Frontal Arc Setting
AU: * Conrey, R M
EM: conrey@mail.wsu.edu
AF: Dept. of Geology, Washington State University, Pullman, WA 99164 United States
AU: Leeman, W P
EM: leeman@rice.edu
AF: Dept. of Earth Science, Rice University, Houston, TX 77251 United States
AU: Streck, M J
EM: streckm@pdx.edu
AF: Dept. of Geology, Portland State University, Portland, OR 97207 United States
AU: Evarts, R C
EM: revarts@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: The Boring Volcanic Field is a 0.05 - 3.0 Ma field of mafic lavas associated with the tectonically complex Portland Basin. The volcanism may be associated with the along-arc propagation of mafic magmatism and intra-arc rifting in the Cascade Range of northern Oregon from the south. The Boring lavas are intriguing because they represent eruption of hot mantle-derived magmas in the frontal arc where the subducting slab may be as shallow as 80 km. These lavas thus provide important constraints on the thermal structure of the sub-Cascade mantle wedge. Several diverse types of mafic lava have been recognized: 1) low-K tholeiite (LKT), similar to mid-ocean ridge basalt, and invariably the oldest of the Boring lavas - most of them probably erupted from vents in the Cascade Range and entered the Portland Basin via ancestral Columbia and Clackamas River paleodrainages, but some were locally erupted; 2) rare within-plate-like basalt (WIP) similar to ocean island basalt, including one erupted on the western (trench-most) margin of the field; 3) a spectrum of calc-alkaline to high-K calc-alkaline to absarokitic basalts (CAB, HKCA, ABS), typical of arc-like High Cascade basalts; and 4) primitive basaltic andesites (BA) with SiO2 in the range of 53-54 wt%. The BA, WIP, and CAB-ABS spectrum lavas erupted from many scattered small centers following the major pulse of LKT volcanism. Several shield volcanoes of more evolved basaltic andesites and minor andesites were also constructed on top of the LKT bench-forming lavas. Inverse T and P calculations were performed for suitably primitive ($>$ 8% MgO) lavas of all types. These calculations add olivine until a composition in equilibrium with olivine of Fo90 or 91 is reached, and then use paramaterized fits to experimental data to estimate the T and P of melt generation. Results suggest, perhaps counterintuitively, that BA and the CAB-ABS spectrum were generated at the shallowest mantle depths (30-50 km) from a refractory source that likely was metasomatised by earlier subduction. LKT and WIP magmas were generated at deeper, hotter levels (60-80 km) in less refractory (but still depleted) mantle. Speculatively, the hotter MORB and WIP source mantle advected northward along the arc during intra-arc rifting and reached the Portland Basin region about 3 Ma. Warming due to its encroachment resulted in remelting of arc-metasomatized lithosphere. The enigmatic location of these lavas and the results of the inversion calculations have implications for both the thermal structure of the wedge and the genesis of arc volcanism in hot subduction zones such as the Cascades.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting