IS THE ARC-ACCRETION MODEL FOR THE 1780-1650 MA CRUSTAL EVOLUTION OF SOUTHERN LAURENTIA CORRECT? LET'S LOOK AT THE ROCKS!

During the last 20 years a common model for the Paleoproterozoic evolution of southern Laurentia involves accretion of successively younger island arc terranes during the interval 1780-1650 Ma. Indeed, in recent seismic studies dipping reflectors have been identified as fossil subduction zones, formed during the accretion process. However, Paleoproterozoic rocks of the southwestern United States are dominantly bimodal metavolcanic assemblages and related granitoids. Andesite and basaltic andesite, major components of modern island arcs, are almost entirely absent, as are ophiolites, accretionary mélanges, and other arc-related features. Examples of bimodal assemblages include the Gunnison-Salida region of central Colorado, where metavolcanic rocks consist of pillowed tholeiitic basalt and high-silica rhyolite; bimodal metavolcanic gneisses in the Needle Mountains in southwestern Colorado; and both basalt-dominated and rhyolite-dominated bimodal assemblages in New Mexico. Although some intermediate volcanic rocks occur in the Paleoproterozoic of Arizona, bimodal assemblages are also dominant there. Hill and Bickford (2001) found inherited ca. 1850 and 2500 Ma zircons in ca. 1750 Ma bimodal rocks in central Colorado and proposed that they were derived from pre-existing ca. 1850 Ma (Trans-Hudson/Penokean) crust, a conclusion supported by Nd isotopic data. Inherited zircon and common Pb data indicate involvement of older crust in ca. 1750 Ma rocks of the Mojave Province of southeastern California and western Arizona and the bimodal volcanogenic rocks of the Hualapai block in west-central Arizona. Hawkins et al. (1996) determined a zircon age of 1840 +/-1 Ma for the Elves Chasm pluton in the Grand Canyon, demonstrating that rocks of Trans-Hudson/Penokean age are present in southwestern Laurentia.

These data suggest that if arc-accretion contributed to the growth of southern Laurentia, it occurred during ca. 1880-1840 Ma Trans-Hudson/Penokean orogeny, producing rocks across the southwest that are now mostly obscured by younger, ca. 1780-1650 Ma Paleoproterozoic rocks. The bimodality of these rocks across the southwest suggests that they were derived from the pre-existing Trans-Hudson/Penokean crust, most likely during extension and partial melting associated with the development of transpressional crustal-scale shear zones.