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Rocky Mountain (63rd Annual) and Cordilleran (107th Annual) Joint Meeting (18–20 May 2011)

Paper No. 2
Presentation Time: 8:25 AM

THE WALLOWA BATHOLITH: NEW PB/U (SHRIMP-RG) AGES PLACE CONSTRAINTS ON ARC MAGMATISM AND CRUSTAL THICKENING IN THE BLUE MOUNTAINS PROVINCE, NE OREGON


JOHNSON, Kenneth, Department of Natural Sciences, University of Houston-Downtown, 1 Main Street, Suite N813, Houston, TX 77002, SCHWARTZ, Joshua J., Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, AL 35487, WOODEN, Joseph L., USGS-Stanford Ion Microprobe Laboratory, Stanford University, Stanford, CA 94305, O'DRISCOLL, Leland J., Earth Sciences, University of Southern California, Los Angeles, CA 90089 and JEFFCOAT, C. Ryan, Department of Earth and Atmospheric, University of Houston, 312 Science and Research Building 1, Houston, TX 77204-5007, johnsonk@uhd.edu

The composite Wallowa batholith comprises four roughly E-W trending plutons (from N to S): Pole Bridge pluton (PB), Hurricane Divide pluton (HD), Craig Mountain pluton (CM), and the Needle Point pluton (NP; Taubeneck, GSA Centennial Field Guide, 1987). Hornblende gabbro intruded the PB, but occurs as xenoliths in the other plutons. Small bodies of cordierite trondhjemite represent the last stage of Wallowa magmatism. New Pb/U (SHRIMP-RG) ages show that the Wallowa batholith was constructed over ~17 Ma. Zircons yielded the following concordant Pb/U ages: 140.2±1.4 Ma for PB, 130.8±1.5 Ma for NP, 130.2±1.0 Ma for HD, and 125.6±0.6 Ma for CM. A hornblende gabbro from the PB yielded an age of 132.9±0.6 Ma, whereas one of the trondhjemite bodies at Blue Lake (BL) gave an age of 122.9±1.7 Ma. Rocks from the PB, HD, and NP plutons are compositionally extended from diorite to granodiorite, have Sr/Y values generally <40, and La/Yb <12. Their compositions are consistent with derivation from a mantle source during the waning stages of Late Jurassic-Early Cretaceous arc activity associated with amalgamation of the Blue Mountains composite terrane province. In contrast, non-cumulate rocks from the CM are felsic (>70 wt.% SiO2), have Sr/Y values up to ~64, and La/Yb of 17-19. Similarly, rocks from the BL trondhjemite have elevated Sr/Y and La/Yb (79-138 and ~22, respectively). CM and BL compositions are consistent with partial melting of deep crustal rocks of variable composition. Their ages (~126-123 Ma) suggest they formed by melting of thickened crust following accretion of the Blue Mountains amalgamated terranes to the continental margin (128±3 Ma; Getty et al., Contrib. Mineral. Petrol., 1993). These interpretations are consistent with previous and ongoing studies in other parts of the Blue Mountains province.

Session No. 3

T7. Petrologic and Geodynamic Perspectives on Non-Arc Volcanism in the Western United States
Wednesday, 18 May 2011: 8:00 AM-12:00 PM
Juniper (Riverwoods Conference Center)
Geological Society of America Abstracts with Programs. Vol. 43, No. 4, p.5
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