PARSING THE LONG-LIVED OROGEN: USING MICROSTRUCTURAL ANALYSIS AND MICROPROBE MONAZITE DATING TO CLARIFY THE PROTEROZOIC HISTORY OF SOUTHWESTERN NORTH AMERICA

The CDROM seismic image of the southwestern Laurentian crust is a composite image, superimposing the effects of a number of tectonic events, including: (1) early arc development and related tectonism (ca. 1.78-1.72 Ga); (2) accretion of arcs, crustal shortening, and tectonic burial during the Yavapai and Mazatzal orogenies (1.7-1.65 Ga); (3) crustal shortening, metamorphism, and widespread plutonism (1.45-1.35 Ga); and (4) localized thermal, deformational, and plutonic events during the Grenville orogeny (ca 1.3-1.0 Ga). The geometry of the crust was further modified by a series of largely brittle events throughout the Neoproterozoic and Phanerozoic. In many parts of the orogen, localized younger deformation modified older structures, and reactivated older fabrics. One of the great challenges for interpreting the CDROM image and for interpreting surfacial geology is distinguishing the effects of 1.7 Ga accretion and 1.4 Ga reactivation events, which together shaped the fundamental crustal structure. Microprobe monazite geochronology has proven to be extremely valuable, providing age information in direct textural context. In Arizona, rocks are dominated by 1.65-1.7 Ga structures and fabrics with little 1.4 Ga overprint. Monazite ages in garnet preclude older metamorphism , even near older plutons such as the Elves Chasm gneiss. In the Homestake shear zone, Colo., older high-T, 1.65-1.7 Ga gneissic fabrics have been distinguished from localized 1.4 and post-1.4 Ga shear zone fabrics with distinct styles and kinematics. In the Wet Mountains and Blue Ridge areas, Colo. early metamorphism and intense deformational fabrics are constrained to stages within a prolonged ca. 1.4 Ga event, perhaps more strongly developed here than in other parts of the region. In New Mexico, 1.4 Ga deformation and metamorphism is intense, but in situ geochronology has illuminated a gradient in the 1.4 Ga event, including identification of cryptic 1.65 Ga fabrics and several 1.4 Ga shear and fault zones. Even for widely separated events such as these (100+ m.y. resolution), we have found that microprobe monazite geochronology must be based on high-resolution compositional mapping and careful scan-based background analysis, and ultimately is most powerful in combination with other geochronologic techniques