An Upper Crustal Ophiolite Remnant within the Feather River Ultramafic Belt, California: Tectonomagmatic Origins and Implications for Its Evolution

The 150-km-long Feather River ultramafic belt (FRB) of the northern Sierra Nevada, California, consists of serpentinized ultramafic rocks, gabbroic rocks, with lesser amounts of amphibolitic rocks. The gabbroic rocks contain metagabbro and dikes within it. Based on the electron microprobe analyses, the metagabbro consists of typical igneous composition for clinopyroxene and early hornblende, indicating that they bear low grade metamorphism. The gabbroic rocks show Nb and Ta depletion in primitive-mantle normalized plots that is similar to typical island arc rocks. We present that the metagabbro within the ultramafic rocks is forming in the suprasubduction zone setting. Tectonic models are presented for the metagabbro and amphibolitic rocks in FRB. The low P burial metamorphism of the upper plate metagabbro and the high grade amphibolitic rocks are typical of relationships of upper plate ophiolites to metamorphic soles. The amphibolitic rocks record as metamorphic sole forming during subduction initiation beneath the ultramafic rocks and metagabbro. The unusual and protracted tectonic history of the FRB is similar to other ophiolite belts of the Cordillera of California but differs in the lack of crustal rocks.