PALEOPROTEROZOIC TO MESOPROTEROZOIC EVOLUTION OF SOUTHWESTERN NORTH AMERICA: THE VIEW FROM THE LOWER CRUST

The Proterozoic evolution of North America involves the accretion of dominantly oceanic tectonic elements (island arcs, back-arc basins, micro-continents) to the southern edge of the Wyoming craton at ca. 1.8-1.6 Ga to form a wide orogenic belt that stretches from southern Wyoming to New Mexico. At ca. 1.48-1.32 Ga much of the belt was injected by a suite of dominantly granitoid rocks commonly referred to as an "anorogenic" suite. This large event is related to middle crustal deformation and distinctive high T-low P metamorphism that is observed in many exposures across the belt. Although there is much disagreement regarding the nature of this protracted magmatism, there is agreement that it must involve a major thermal perturbation and transfer of basaltic melts to the base of the crust. Seismic profiles acquired by the CD-ROM project indicate the presence of a major high-velocity (>7 km/sec) layer at the base of the crust across the belt. The origin of this layer is problematic, but it is commonly attributed to underplating of mafic magmas associated with ca. 1.4 Ga magmatism.

Lower crustal xenoliths of mafic to intermediate composition, mainly garnet granulites and minor eclogites, occur across the orogenic belt in a variety of Tertiary intrusive rocks, including kimberlites, minettes, and alkali basalts. Lower crustal xenoliths from the Archean Wyoming Province and the Paleoproterozoic orogen have been collected. Xenoliths from the latter orogen have a complex suite of zircon with mainly Paleoproterozoic U-Pb ages and Nd isotopic signatures. Minor amounts of zircon with Paleozoic and ca. 1.4 Ga ages probably grew during fluid-mediated events. Possible interpretations of these data are that we have not yet sampled the rocks responsible for the prominent reflections and high velocities or that the seismic character of the crust is the result of a protracted Paleoproterozoic history, not simply ca. 1.4 Ga underplated mafic magmas. Our continuing isotopic and thermobarometric studies of lower crustal xenoliths will place further constraints on the Proterozoic evolution of southwestern North America.