DIVERSE METAMORPHIC TRAJECTORIES, IMBRICATED OCEAN PLATE STRATIGRAPHY, AND FAULT ROCKS, YUBA RIVER AREA, FEATHER RIVER ULTRAMAFIC BELT, CALIFORNIA

The Feather River ultramafic belt (FRB), extends for more than 150 km along strike within the basement of the northern Sierra Nevada. Geologic relationships along the FRB in the Yuba River area illustrate the complexity of a suture zone and likely complexity of its tectonic evolution. In the North Yuba River area, thin slabs of ilmenite-bearing, redbrown-amphibole (high Ti,low Al) amphibolites, apparently formed by high-temperature, low to medium pressure (LP-HT) metamorphism, structurally underlie ultramafic rocks, and overlie the high-pressure, low-temperature (HP-LT) blueschist facies RAS. The RAS includes a mélange zone with blocks of rutile-bearing, green-amphibole (low Ti, high Al) rutile-bearing amphibolite, indicative of HP-HT metamorphism, that have been heavily overprinted with blueschist facies assemblages. Amphibolite grade rocks in the Forest City area, directly south of North Yuba River, occur structurally beneath serpentinite and comprise imbricates of ocean plate stratigraphy represented by repeated sheets of metabasites, metacherts, and metaclastics. Fault rocks consisting of cataclastically deformed amphibolite and possible pseudotachylites crop out, and in Alleghany is adjacent to serpentinite. The amphibolites appear to structurally overlie the RAS. In the Alleghany area, south of Forest City in the Middle Fork Yuba drainage, there appear to be two amphibolite slabs with distinct metamorphic histories, each comprising imbricated ocean plate stratigraphy and structurally overlying the RAS. In one slab, olive green amphibole, apparently representing HP-HT metamorphism (rutile cores in ilmenite) is overgrown by redbrown amphibole formed during LP-HT conditions whereas the other slab has redbrown amphibole cores mantled by olive green amphibole. One slab may record ridge subduction followed by renewed subduction (LP-HT before HP-HT) whereas the other may record subduction initiation in young oceanic lithosphere (HP-HT) followed by ridge subduction (LP-HT). Other amphibolites along strike in the FRB record only LP-HT or HP-HT. The high temperature events were followed by subduction of long enough duration to develop HP-LT metamorphism. Some HP-HT rocks were exhumed to the sea floor, shed into the trench and resubducted with the RAS.