Petrotectonic and paleogeographic settings of U.S. Cordilleran ophiolites

Abstract

Ophiolites are "distinctive associations of mafic to ultramafic rocks . . . which when completely developed comprise from bottom to top: (1) an ultramafic complex, consisting of variable proportions of harzburgite, lherzolite, and dunite, usually with a metamorphic tectonite fabric (more or less serpentinized); (2) a gabbroic complex, ordinarily with cumulus textures commonly containing cumulus peridotites and pyroxenites and usually less deformed than the ultramafic complex; (3) a mafic sheeted dike complex; and (4) a mafic volcanic complex, commonly pillowed. Rocks commonly associated with ophiolites include: (1) an overlying sedimentary section, which typically includes ribbon chert, thin shale interbeds, and minor limestone; (2) podiform bodies of chromite generally associated with dunite; and (3) sodic felsic intrusive and extrusive rocks" (Anonymous, 1972). It is generally thought that ophiolites represent on-land fragments of oceanic crust and upper mantle (Dewey and Bird, 1971; Gass and others, 1973; Miyashiro, 1975; Coleman, 1977; Moores, 1982). The plate tectonic settings of ophiolite pedogenesis and continent edge emplacement are the subjects of major debate. Two general ophiolite types, Cordilleran and Tethyan, were differentiated in a major ophiolite review by Moores (1982). Cordilleran ophiolites were defined as complete to partial, or dismembered, sequences lacking continental-affinity structural basement; Tethyan ophiolites are more often complete crust-upper mantle sequences and commonly rest tectonically above continent edge basement. The contrast between Cordilleran and Tethyan type ophiolites is a useful starting point for this synthesis. Cordilleran ophiolites are considered here to be representative of the circum-Pacific orogenic style.

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