THE BEAVER DAM – GOLD BUTTE TREND, UTAH AND NEVADA: A COLLISIONAL BOUNDARY OFFERING INSIGHTS INTO THE NATURE OF YAVAPAI PROVINCE CRUST

The enigmatic boundary between the Mojave Province and other Paleoproterozoic terranes of the southwestern United States is of primary importance in elucidating the Paleoproterozoic history of southern Laurentia. Accretion of juvenile arcs in the Yavapai (1.76-1.72 Ga) and Mazatzal (1.72-1.65 Ga) arc terranes has long been the preferred model for the growth of southern Laurentia. Recent workers have emphasized extension and bimodal volcanism involving older crust as possible mechanisms in the development of these terranes. The >1.84 Ga Elves Chasm Gneiss, which is sandwiched between the supracrustal rocks of the Yavapai Province and the Mojave province, may represent basement upon which the Yavapai province was built. The nature of the Mojave – Yavapai boundary may have direct bearing on this controversy. This boundary is discontinuously exposed along a trend from the Beaver Dam Mountains in SW Utah, through the Virgin mountains along the Arizona-Nevada border, and south to the Gold Butte area north of Lake Meade in Nevada. Along the Beaver Dam – Gold Butte trend, evidence exists for a major collisional boundary. This evidence includes: 1) the presence of ultramafic bodies, in the Gold Butte area, containing primary olivine, orthopyroxene and clinopyroxene (lherzolite) interpreted to be of mantle origin. 2) Kilometer scale (width) shear zones in the Virgin Mountains containing numerous mafic and ultramafic pods, and 3) the presence of similar shear zones in the Beaver Dam mountains. Here, pods of garnet-clinopyroxene rocks, retrograded eclogites and evidence for possible coesite inclusions in garnet (now pseudomorphed by quartz) suggest pressures in excess of 22 kbar, and possibly as high as 27 kbar. These features are consistent with the presence of a collisional suture, and given the extremely high pressures encountered in the Beaver Dam Mountains, collision between continental masses. Continent-continent collision between the Yavapai and Mojave Provinces suggests thickened Yavapai crust, consistent with the presence of basement material underlying Yavapai supracrustal rocks.