AMPHIBOLITE AND BLUESCHIST FACIES METAMORPHISM, FEATHER RIVER ULTRAMAFIC BELT, YUBA RIVER DRAINAGE: A RECORD OF SUBDUCTION INITIATION, RIDGE SUBDUCTION, AND CONTINUED SUBDUCTION?

The Feather River ultramafic belt (FRB) of the northern Sierra Nevada, California has a complex spatial distribution of metamorphic grade. In the North Yuba River area, thin slabs of ilmenite-bearing, redbrown-amphibole amphibolites structurally underlie ultramafic rocks, and overlie the blueschist facies Red Ant schist. The Red Ant schist includes a mélange zone with blocks of rutile-bearing, green-amphibole amphibolite that have been heavily overprinted with sodic amphibole and lawsonite. In the Alleghany and Forest City areas, directly south of the North Yuba River, Red Ant schist, amphibolites, and serpentinites are interleaved as a result of isoclinal folding. The amphibolite units consist primarily of metabasite, with metacherts and metagraywackes, and contain felsic segregations apparently related to partial melting. Common metabasite assemblages include brownish green to redbrown amphibole + plagioclase (pervasively altered) ± epidote ± biotite ± diopside. Ilmenite appears to be associated with redbrown-amphibole amphibolites whereas titanite, commonly with rutile cores appears in amphibolites with brownish-green amphibole. Garnets are present in metasedimentary rocks but not metabasites with the exception of one sample consisting of garnet, diopside, clinozoisite, plagioclase (altered), quartz and ilmenite. Garnet-clinopyroxene temperatures from this sample using more recent calibrations appear anomalously low (480-690°C), much higher with the Ellis and Green calibration (700-920°C), a reflection of the very high grossular component of the garnet (Xgrs 0.71-0.78) Redbrown amphibole compositions from a diopside-bearing amphibolite in the Forest City area suggest T=910-980°C at 0.4-0.6 GPa. Rutile occurs as cores within ilmenite. This suggests earlier HP-HT metamorphism similar to that observed in the amphibolite blocks-in-mélange noted above whose calcic amphibole compositions suggest T=710°C and P=1.4 GPa. These relationships suggest early HP-HT metamorphism in a metamorphic sole environment at subduction initiation, followed by lower P-HT metamorphism possibly associated with ridge subduction. Exhumation of some HP amphibolites resulted in shedding of blocks into the mature trench where they were resubducted to blueschist facies conditions.