Structure and petrography of the Middle Feather River ultramafic body, Plumas County, California
Standlee, Larry A.
Lallemant, Hans G. Avé
Master of Arts
The ultramafic body on the Middle Fork of the Feather River is a typical Alpine-type peridotite. It has fabrics and mineral assemblages indicating formation in the upper mantle. It is surrounded by Paleozoic(?) metasedimentary and metavolcanic rocks of eugeosynclinal origin. The ultramafic rocks and accompanying amphibolite body have undergone two phases of metamorphism, while the country rocks exhibit one low-grade greenschist metamorphism. Three deformational phases are recognized in the ultramafic body. The earliest recognizable foliation probably formed in the upper mantle, with later structures associated with emplacement into the crust. Petrofabric analyses of olivine and enstatite reveal preferred orientations related to the first, deep seated deformation. The ultramafic body is believed to have formed as a rising diapir in a marginal basin. The closing of this basin in Permo-Triassic time caused the ultramafic rocks to be thrust westward into an island arc that was moving eastward with the closing of the basin.