BEYOND SIMPLE MODELS OF OROGENIC METAMORPHISM: HP/HT, LP/HT, AND HP/LT METAMORPHISM IN THE FEATHER RIVER ULTRAMAFIC BELT, NORTH YUBA RIVER CANYON, CALIFORNIA

The Feather River ultramafic belt in the North Yuba River canyon, California may be repeated by a regional-scale fold whose limbs crop out in two distinct belts of serpentinized ultramafic rocks. Between them, occur a variety of metamorphic rocks that formed in very different pressure (P)-temperature (T) regimes. A 100-meter-thick selvage of amphibolite ("western amphibolites") crops out directly east of the western ultramafic exposures. Lenses of amphibolite ("eastern amphibolites"), up to a few hundred meters thick, are found directly west of the eastern ultramafic exposures. The western amphibolites comprise mostly metabasites composed mainly of brown hornblende that probably coexisted with plagioclase; the latter has been replaced by fine-grained mats secondary minerals. Minor metasedimentary rocks occur. These rocks have abundant quartz and garnet, plus what appears to have been micas that have been altered to chlorite and clay minerals. In both the metabasites and metasediments ilmenite occurs, rimmed by titanite. The eastern amphibolites contain mineral assemblages similar to the metabasites of the western amphibolites. Some clinopyroxene is also present and it is not clear whether this clinopyroxene represents igneous relics from a gabbroic protolith or whether it is metamorphic. Qualitatively the ilmenite-bearing amphibolites appear to be LP-HT metamorphic rocks similar those analyses in another recent study of amphibolites from the Feather River belt. Between the two ultramafic exposures and flanking amphibolites is the blueschist facies Red Ant schist. We have not found a metamorphic gradient in the Red Ant schist where it abuts the bordering amphibolites or ultramafic rocks. Cutting the Red Ant schist is a 50-m-thick shear zone that has amphibolites blocks. These blocks contain relics of hornblende, with abundant glaucophane and lawsonite in addition to late clay mineral growth. Rutile occurs and it is rimmed by ilmenite that is in turn rimmed by titanite. These rocks initial HP-HT metamorphism and overprinting with HP-LT bluechist facies metamorphism and the Ti-phase growth may reflect two separate metamorphic events (possibly separated by exhumation of the amphibolite) rather than anticlockwise PT evolution at the inception of subduction.