AN UPPER CRUSTAL OPHIOLITE REMNANT WITHIN THE FEATHER RIVER ULTRAMAFIC BELT, NORTHERN SIERRA NEVADA, CALIFORNIA: UNSUBDUCTED, BUT AFFECTED BY RIDGE SUBDUCTION?

The 150-km-long Feather River ultramafic belt (FRB), of the northern Sierra Nevada, California, consists of serpentinized ultramafic rocks, with lesser amounts of amphibolite-grade metabasites. The amphibolites are completely recrystallized, with a strong tectonite fabric, lack relict igneous minerals, and record a range of metamorphic conditions from high-pressure, high-temperature (HP-HT) to low-pressure, high-temperature (LP-HT), with some rocks showing overprinting of one high temperature event on another. The amphibolites are intepreted to represent partially subducted oceanic crust recording subduction initiation and ridge subduction events (possibly multiple events of each). Unsubducted "upper plate" ophiolitic rocks are rare in the FRB, but such rocks crop out along La Porte Road in the southern Feather River region. These rocks include gabbroic, diabasic, and possibly basaltic rocks. Preliminary geochemical results suggest a supra subduction zone origin for these rocks. In contrast to the amphibolites, these rocks preserve igneous textures and minerals and lack a penetrative fabric although local shear zones are present. The gabbroic rocks display LP-HT metamorphism, however, recorded by red-brown amphibole that rims actinolite that rims relict igneous clinopyroxene. Diabasic or basaltic rocks have metamorphic actinolite and pumpellyite and record much lower grade metamorphism. This suggests either an exceptionally steep metamorphic gradient in this ophiolite remnant or, more likely, significant post metamorphic faults within the ophiolite. The metamorphism of this ophiolite remnant may reflect ridge subduction beneath it that preferentially metamorphosed the lower crustal levels of the upper plate, or it may record metamorphism associated with oceanic core complex formation near a spreading center. The latter, however, would not be expected to result in prograde metamorphism recorded by hornblende rimming actinolite. The juxtaposition of these upper plate ophiolitic rocks with the amphibolites reflects the complex nature of tectonic processes associated with the development of the FRB suture zone.