U/Pb ages of Eocene and younger rocks of the eastern flank of the central Cascade Range, Washington, USA.

Abstract

Geological units located on the eastern flanks of the central Cascade Range, the Summit Conglomerate of Natapoc Mountain, Sugarloaf Peak, Basalt Peak, Horse Lake Mountain, Pole Ridge, Wenatchee Dome at the Cannon mine, and the Wenatchee Formation east of the Columbia River, have not been well dated or constrained by high precision U/Pb techniques. Zircons separated from samples were submitted and prepared for U-Pb isotopic dating; analyses were performed by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) at the Massachusetts Institute of Technology (MIT). Sixty-five dates were obtained from samples taken from igneous, sedimentary, and volcanic units from the region in and around the eastern flanks of the Cascade Range. The units sampled with results intrude crystalline basement or uncomformably overlie deformed lower Tertiary rocks. I compared those results where possible to previous work. The results reveal a robust post-Eocene tectonic history and contain evidence of late Oligocene, Miocene, and Pliocene volcanism. Specifically, the andesite of Natapoc Mountain dates to the early Pliocene at 3.981 +/- 0.053Ma. The Basalt Peak intrusion at the north intersection of the Coulter Creek and Entiat Faults dates to the late Oligocene at 27.42 +/- 0.017Ma; Sugarloaf Peak east of the Ential fault at the Eocene/Oligocene boundary at 36.0 +/- 0.43 Ma and the Wenatchee Dome intrusion through Eocene Chumstick Fm. at 44.447 +/- 0.027Ma. The results of this study represent the newest high-precision zircon U-Pb geochronology dates for the samples tested.