STRUCTURAL AND U-PB DETRITAL ZIRCON GEOCHRONOLOGIC CONSTRAINTS ON THE ORIGIN OF THE CONDREY MOUNTAIN SCHIST, CALIFORNIA - OREGON

During high-flux events in magmatic arcs, supracrustal material is incorporated into arc root zones. However, the process(es) by which supracrustal rocks are emplaced beneath continental arcs and how these rocks interact with mantle-derived melts are poorly understood due to a lack of exposed arc roots. The Condrey Mountain Schist (CMS) of the Klamath Mountains (northern California and southern Oregon) represent low grade oceanic and terrigenous sediments thrust beneath higher grade ophiolitic and oceanic volcanic rocks of the Rattlesnake Creek Terrane (RCT). We present new U-Pb detrital zircon dates from the CMS, as well as from the possibly correlative inter-arc basin rocks of the Galice Formation and/or accretionary rocks of the South Fork Mountain Schist. The Galice Formation yields a maximum depositional age of ca. 152 Ma with prominent age distributions of ca. 150 – 200 Ma (43% of analyzed grains), 200 – 1200 Ma (6%), 1200 – 2000 Ma (34%), and 2000 - 3200 Ma (17%). The South Fork Mountain schist yields similar detrital zircon age spectra, with a younger maximum depositional age of 135 Ma, and prominent age distributions of ca. 120 – 200 Ma (29% of analyzed grains), 200 – 800 Ma (30%), 800 – 1200 (22%), 1200 – 2000 (14%), and 2000 – 2850 Ma (5%). An interior unit of the CMS contains detrital zircon age distributions quite similar to the found in the South Fork Mountain Schist, with a maximum depositional age of ca. 136 Ma and prominent age distributions of ca. 130 - 200 Ma (25% of analyzed grains), 200 – 350 Ma (12%), 350 – 700 Ma (27%), 700 – 1600 Ma (30%), and 1600 – 2300 Ma (6%). Structural, petrographic, and detrital zircon geochronologic similarities between the CMS and South Fork Mountain schist suggest a common provenance. Furthermore, kinematic indicators within the Condrey Mountain shear zone, a shallowly dipping ductile structure separating the CMS from the RCT, suggest that the CMS was transported eastward during prograde metamorphism. An exterior metavolcanic unit of the CMS yields an inferred protolith eruption age of ca. 172 Ma. These relations indicate that the exterior CMS erupted at least 35 Myr prior to deposition of the interior CMS and that the CMS as a whole was assembled beneath the RCT as a series of nappes during a protracted phase of east-directed underplating.