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Paper No. 29-9
Presentation Time: 8:30 AM-5:00 PM
INVERTED METAMORPHISM OF THE CONDREY MOUNTAIN SCHIST (NORTHERN CALIFORNIA/SOUTHERN OREGON): A ~30 MYR RECORD OF SUBDUCTION INITIATION AND MATURATION BENEATH A COOLING UPPER PLATE
The Klamath Mountains province of northern California and southern Oregon is a series of accreted oceanic terranes produced through eastward subduction along the western margin of North America from early Paleozoic to Late Jurassic time. The relatively low grade Condrey Mountain schist (CMS) lies structurally below the high grade Rattlesnake Creek terrane (RCT) in the central Klamath Mountains. Furthermore, the CMS appears to be a composite unit consisting of structurally lower greenschist to blueschist facies metasedimentary assemblages and structurally higher greenschist to amphibolite grade metavolcanic rocks. The inverted metamorphic sequencing between the CMS and RCT and within the CMS itself is somewhat of a tectonic enigma. Our goal is to constrain the metamorphic history of the CMS in order to better understand the mechanisms behind its emplacement beneath the RCT. Garnet-bearing samples collected from the outer CMS were analyzed for major element concentrations on a JEOL 6610 LV Scanning Electron Microscope. Garnet–hornblende thermometry and garnet–hornblende–plagioclase–quartz barometry yielded values ranging from 680 to 577 ̊C and 7.8 to 6.7 kbar, respectively. Previous research suggests that the inner CMS equilibrated at comparable pressures and significantly lower temperatures of ~350 ̊C. Hence, the CMS preserves an inverted metamorphic field gradient of ~300 ̊C over ~10 km if exposed section. The magnitude of the inverted metamorphism recognized within the CMS is comparable to other Cordilleran subduction complexes (e.g., the Pelona-Orocopia-Rand and related schists). However, new U-Pb detrital zircon and metamorphic titanite geochronology indicate that inverted metamorphism in the CMS developed over a much longer (ca. 30 Myr) time interval. Collectively the P-T-t data reported here strongly suggests that the CMS represents a subduction initiation sequence emplaced beneath a progressively cooling upper plate from ca. 170 (coeval with the onset of the Siskiyou orogeny) to ca. 140 Ma, overlapping with the onset of Franciscan subduction beneath the western Klamath Mountains.