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