NEOBLASTIC MINERAL PARAGENESES IN FRANCISCAN METAGRAYWACKES, NORTHWESTERN CALIFORNIA

We studied 90 metasandstone samples from road cuts transecting the Cape Mendocino-Garberville-Covelo 30 x 60' quadrangles. The Franciscan geology consists of five imbricate, east-rooting allochthons, from east to west, and generally from structurally higher to lower: Eastern Belt—massive, coherent metagraywackes ± interstratified dark metashale; Central Belt mélange—clay matrix mélange enclosing arkosic to lithic metagraywacke layers and boudins; Yager Terrane—massive, coherent arkosic metasandstone, metasiltstone and laminated shale; Coastal Terrane—feebly recrystallized broken formation and minor mélange, comprising arkosic to lithic metasandstone and shale; and King Range Terrane—well-bedded, highly folded, locally broken, sheared recrystallized arkosic graywacke and dark shale.

Petrography allowed textural classification: 1 = no flattening or recrystallization; 1.5 = minor matrix recrystallization, no flattening; 2 = nonparallel anastomosing shearing, modest matrix recrystallization; 2.5 = planer shearing, intense recrystallization. Sparsely zeolitized King Range and Coastal terrane rocks show clear detrital features and are textural zone 1; Yager terrane rocks are textural zone 1, but may contain minor neoblastic pumpellyite (?). Central Belt pumpellyite ± lawsonite ± aragonite-bearing metasandstones are modestly deformed/reconstituted, and principally textural zone 1.5. Investigated Eastern Belt metagraywackes are intensely contorted lawsonite + jadeitic pyroxene ± aragonite ± glaucophane-bearing textural zone 2.5 lithologies. Microprobed neoblastic phases in 23 rocks document mineral parageneses for the studied allochthons to better constrain the tectonic accretion and metamorphic P-T evolution of this part of the Franciscan Complex. Eastern Belt metasedimentary rocks were subducted to depths approaching 25-30 km, whereas structurally lower Central Belt mélanges returned surfaceward from depths of 10-20 km. Metagraywacke phase assemblages in the Coastal Belt suggest inferred burial depths less than 5-10 km. The Eastern and Central belts evidently were sequentially offloaded from one (or two) downgoing oceanic plate(s) and ascended into the accretionary margin, whereas outboard Coastal Belt rocks were later stranded along the western margin without undergoing appreciable subduction.