DETRITAL ZIRCON GEOCHRONOLOGY OF THE CONDREY MOUNTAIN SCHIST (NORTHERN CALIFORNIA/SOUTHERN OREGON) REVEALS A ~30 MILLION YEAR RECORD OF SUBDUCTION-RELATED UNDERPLATING

The Condrey Mountain schist (CMS), a tectonic composite of marginal greenschist grade metavolcanic and internal blueschist grade metasedimentary units, crops out in the central Klamath Mountains as a ~30 km wide structural window. The upsection increase in metamorphic grade within the CMS and its structural position beneath the older and relatively high-grade Rattlesnake Creek terrane were most likely produced through subduction-related underplating. However, the magnitude and duration of tectonic displacements accompanying CMS assembly are poorly constrained. This study aims to clarify the age and provenance of CMS protoliths via a U-Pb detrital zircon geochronologic transect from low to high structural levels. Upper Jurassic inter-arc basin fill assemblages of the Galice Formation and the South Fork Mountain Schist of the Franciscan Eastern Belt were also sampled to evaluate potential correlation with the CMS. Tonalitic stocks that intrude metavolcanic rocks at the highest structural level of the marginal unit yield a U-Pb zircon age of ca. 172 Ma. Maximum depositional ages in metasedimentary rocks of the CMS yield ages from ca. 162 Ma to ca. 137 Ma moving downward structurally from the marginal to internal unit. These relations suggest that 1) CMS protoliths formed over a >30 Myr time interval and 2) underplating of the CMS beneath the Rattlesnake Creek terrane initiated in middle Jurassic time and additional slices were subcreted into Early Cretaceous time, spanning Siskiyou and Nevadan orogenic events. Two samples of the Galice Formation yielded maximum depositional ages of ca. 152 Ma, implying that the marginal unit of the CMS is not equivalent to the Galice Formation but may instead correlate with structurally lower mafic metavolcanic and metasedimentary complexes of the same inter-arc basin (e.g. the China Peak and Preston Peak complexes). Petrographic data as well as detrital zircon ages permit a tentative correlation between the CMS internal unit and the South Fork Mountain Schist. If, as we suggest, assembly of the CMS began with intra-arc basin deposits and culminated with Franciscan trench assemblages, then the CMS contains a ~30 Myr record of tectonic underplating of progressively more allochthonous tectonic slices.