VARIETY OF OPHIOLITIC ROCKS AND THEIR EMPLACEMENT SETTINGS IN THE CALIFORNIA CORDILLERA

Researchers have applied different definitions of ophiolites and this can cause confusion when comparing literature on different orogenic belts. Whereas some ophiolite classifications have been proposed on the basis of geochemical characteristics, the emplacement environments of ophiolitic rocks along convergent plate margins can also be a useful descriptor/discriminant; the Cordillera of California serves as a good example. Most rock bodies referred to ophiolites in the Cordillera are generally large sheets with tens to hundreds of km of extent along strike and thicknesses of several km. These ophiolites apparently formed the roof (upper plate) of subduction zones and their crustal rocks display supra subduction zone (SSZ) geochemical affinity. In contrast, coherent slices of oceanic basalt and overlying pelagic and clastic sedimentary rocks are common in subduction complexes, having been offscraped from the subducting plate. These slices range up to several km in map dimension and up to a km in thickness, have been internally imbricated, and display a wide range of burial metamorphism from slight to high-pressure metamorphism. These slices have been called “ocean plate stratigraphy”. The mafic rocks commonly have MORB or OIB affinity, with less common rocks of SSZ character. Peridotite and variably serpentinized equivalents occur as major components of the large ophiolite sheets, as well as coherent slices up to several km in maximum map dimension in subduction complexes. Serpentinite matrix mélanges occur as components of some of the large ophiolite sheets, as bodies within subduction complexes, and as subunits within forearc basin deposits. Blocks-in-melange (siliciclastic as well as serpentinite matrix) in subduction complexes reach sizes of at least several hundred meters and include both volcanic and plutonic rocks, serpentinite, and pelagic sedimentary rocks, and they comprise rocks of MORB, OIB, and SSZ character.