T270 Tectonic Evolution of Eastern Oregon and Western Idaho

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 200A-C

PRELIMINARY GEOCHRONOLOGY AND GEOCHEMISTRY OF THE HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON: POSSIBLE LINKS BETWEEN POLYGENETIC MESOZOIC ARC ROCKS OF THE CENTRAL CASCADES AND THE BLUE MOUNTAINS PROVINCE

Paper number: T270-9
Program: Technical Sessions
Time: 3:25 PM – 3:40 PM

Authors

Abstract

Washington State contains numerous accreted Mesozoic suspect terranes. Commonly these are correlated with terranes in northern California and southern Oregon; however, recent data has illuminated possible links between these terranes and the Blue Mountains. The Hicks Butte complex, central Cascades, Washington, consists of tonalite, diorite, and quartz diorite, with lesser hornblende gabbro, dacite, and hornblendite. Many of the rocks within this complex have been deformed to gneiss or amphibolite. Two new U-Pb zircon dates for a tonalite and a dacite from this complex were conducted by LA-ICP-MS at the Arizona LaserChron Center. 11 igneous zircons from the tonalite yield a U-Pb age of 150.0 +/- 6.8 Ma (2σ; total error = 4.5 %; MSWD = 0.2). This corroborates the published date of 153 +/- 2 Ma for this complex. This sample also contains 3 concordant Paleozoic age zircon spot analyses (317 +/- 12; 356 +/- 12; & 464 +/- 8 Ma). 28 igneous zircons from the dacite yield a U-Pb age of 144.0 +/- 2.4 Ma (2σ; total error = 1.7%; MSWD = 1.1). Initial whole rock major and trace element geochemistry for the 150-153 Ma rocks (n = 9) are: metaluminous; magnesian; calcic; and low-K. They have flat to slightly positive chondrite-normalized light rare earth element patterns and low Sr/Y ratios (≤ 36). Two 144 Ma dacites are: peraluminous; magnesian; calcic; and low-K. They have negative chondrite-normalized light rare earth elements patterns, depleted heavy rare earth elements, a high positive Eu anomaly, and high Sr/Y ratios (197 & 419).

Initial geochronology and geochemistry of the Hicks Butte complex suggests it is a polygenetic arc terrane. Paleozoic xenocrystic zircons within the complex suggest the Hicks Butte is not exotic to the North American craton. The relationship between high Sr/Y Early Cretaceous magma intruding low Sr/Y Jurassic magma is analogous to the Dixie Butte area of the Baker terrane, Blue Mountains. Further, 154 & 157 Ma arc rocks of the Indian Creek complex and Quartz Mt. stock, WA Cascades, are associated with Early Cretaceous K-Ar and Ar-Ar metamorphic hornblende ages (142 +/- 9 Ma & 143 +/- 1 Ma). This may indicate that Early Cretaceous magmatism is associated with all Late Jurassic arc rocks of the central Cascades, and may strengthen a central Cascade—Blue Mountains correlation.