PROTEROZOIC ULTRAMAFIC ROCKS IN SOUTHWESTERN NORTH AMERICA: DISTINGUISHING OPHIOLITES FROM ARC CUMULATES

Small ultramafic bodies occur throughout Proterozoic orogens of southwestern North America. The possibility that these rocks are parts of ophiolites has prompted the suggestion that they might be used to mark crustal sutures; however, styles and inferred tectonic significance of the peridotites varies considerably. In the Grand Canyon, ultramafic bodies are spatially associated with the Briad suture zone, bounding the Mojave province to the northeast and the Yavapai province to the southwest. All identified ultramafic bodies in the Grand Canyon are in tectonic contact with surrounding supracrustal rocks, are ultramafic throughout (no interlayering of intermediate or felsic rocks), have primary mineral assemblages including olivine, opx, magnetite, and/or cpx, and are interpreted to be ophiolite fragments. In contrast, many of the ultramafic bodies in the Park Range of northern Colorado, where the Archean Cheyenne belt/Proterozoic suture and the suture between the Proterozoic Green Mountain and Rawah blocks have been proposed, are intimately associated with intermediate to felsic rocks, and appear to be cumulate layers in mafic and felsic layered intrusions. The gabbro of Elkhorn Mountain is mostly porphyritic gabbro with large pyroxene phenocrysts. The peridotites that occur within it are cumulate layers in the gabbro, in some cases associated with tonalitic to granodioritic material. Intermingled gabbros and Seven Lakes granodiorite yield similar zircon ages in a complex that extends south to the Lester/Farwell Mountains suture zones (Foster et al., 1999; Tyson et al., in press), which juxtaposes the Green Mountain and Rawah Paleoproterozoic arcs. In the Three Island Lake region, near Mica Lake, and near Fish Creek Reservoir, ultramafic rocks are cumulate layers in a layered bimodal plutonic complex. The bimodal complexes may represent magmatism in an extensional setting, in some cases within arc complexes. An ultramafic exposure north of Republic Creek that is in close proximity to exposures of marble, chert, and deposits of copper and molybdenum could represent a mélange terrane marking the Green Mountain/Rawah arc suture. The ability to distinguish ophiolites from arc cumulates using combined field context, lithologic associations, and geochemical data, provides a powerful tool for understanding the accretionary history of the Southwest.