Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 6
Presentation Time: 8:00 AM-5:00 PM


MACDONALD Jr, James H.1, HARPER, Gregory D.1, MILLER, Jonathan2 and ZHU, Bin1, (1)DEAS, Univ at Albany, ES 351, 1400 washington ave, Albany, NY 12222, (2)Department of Geology, San Jose State Univ, San Jose, CA 95192-0102,

The Middle to Late Jurassic Galice Fm of the Klamath Mts., OR-CA, overlies the ~162 Ma Josephine ophiolite (JO) and the slightly younger Rogue-Chetco volcanoplutonic arc complex. The Galice Fm (sensu lato) overlying the JO consists of a siliceous hemipelagic sequence that grades upward into a thick flysch sequence. Bedded hemiplegics and rare graywacke, however, also occur within the JO pillow basalts at several localities. Detritial modes of sandstones from the flysch of the Galice Fm indicate an arc source (predominantly undissected arc) as well as a dominantly chert-argillite source with minor metamorphic rocks. Some chert pebbles from the flysch have Triassic radiolaria. Heavy minerals from basal flysch graywackes include clinopyroxene (up to 5%), hornblende, and rare blue amphibole, while Cr-spinel and zircon are ubiquitous in all graywackes. Ion microprobe dating of detrital zircons from a flysch graywacke gives a bimodal age distribution with peaks at 153 Ma, and ~227 Ma, and a minor Proterozoic component. A graywacke located ~20 m below the top of the JO has detritial modes and heavy mineral suites similar to the flysch graywackes; however, zircon has not yet been found. Detrital Cr-spinel compositions from the flysch and intra-pillow graywackes are also similar, indicating SSZ mantle peridotite and volcanic sources, including ocean island basalt (OIB) affinities. The flysch sample has spinels of MORB and boninitic affinities. The detritial sandstone modes of the graywackes suggest mixing of arc and accreted terrane(s) sources, with the source of the Triassic chert pebbles most likely originating from older Klamath terranes. Cr-spinels compositions suggest source areas for the Galice were predominately within the Klamath Mts., with the OIB-MORB affinity mélange of the Rattlesnake Creek terrane and the SSZ Paleozoic Trinity terrane being possible Cr-spinel sources. This data also suggest that the source areas for the Galice Fm were already established by ~162 Ma, probably during a Middle Jurassic orogeny that predated formation of the Josephine Basin. The 153 Ma detritial zircon age of the flysch is @ to its ages of deposition, and is apparently originating from an active arc. The ~227 Ma age zircons, however, is apparently not from the Klamath Mts., because nothing of that age is known.