INVITED SPEAKER: PROTEROZOIC METAMORPHISM IN SOUTHWESTERN NORTH AMERICA: NEW CONSTRAINTS AND ON-GOING CHALLENGES

Proterozoic rocks of SW North America record the multi-stage southward growth of Laurentia that culminated in the assembly of Rodinia and stabilization of much of the lithosphere of the U.S. Metamorphic (T,P) data document the degree of heating, burial, and exhumation during each tectonic pulse. Two tectono-stratigraphic packages are discussed: (1) 1.75-1.70 Ga “Yavapai basement”, and (2) 1.65-1.35 Ga “Mazatzal/ 1.4 Ga cover” rocks. Yavapai basement rocks preserve evidence for ca. 1.70- 1.68 Ga pluton- enhanced regional metamorphism. Pressures range from 0.3 to 0.7 GPa, with some exhumation soon after metamorphism, but peak metamorphic isobaric sections in Grand Canyon (0.6 GPa) and central AZ (0.3 GPa) suggest stabilization of isotherms shortly following the 1.70.-1.68 Ga Yavapai orogeny. Metamorphism of the “cover” metasedimentary sequence(s) involved peak pressures near 0.3 to 0.5 GPa in NM, also with subhorizontal isobars. Variable heating led to production of multiple aluminum silicates (Ky-And-Sill) in aluminum-rich rocks. Much of this “triple point” metamorphism is known to have taken place at ca. 1.4 Ga, and the identification of metamorphosed 1.5-1.4 Ga sedimentary sequences in both Arizona and New Mexico provides a new tool to characterize the 1.4 Ga tectonism. Current questions focus on: 1) the timing(s) (~1.6 and/or ~1.4) of regional burial to mid-crustal pressures, 2) the nature and number of 1.45-1.35 Ga deformation/metamorphic events within what is now known to be a ca. 150 Ma intracratonic tectonic episode, 3) the regional tectonic setting(s) of both 1.65-1.6 and 1.45-1.35 Ga orogenesis/reactivations, and (4) the significance of the broad gap (1.6-1.48 Ga) in tectonism, magmatism, and sedimentation around the region. Continued research is needed on basement-cover contacts (e.g. Needles, N Tusas, central AZ) in areas of relatively low grade 1.4 Ga metamorphism. If depositional, then the region has undergone two or more periods of burial, exhumation, and reburial to mid-crustal conditions. If tectonic, the role of extensional juxtapositions of different crustal levels (proposed but still unproven) remains viable. Characterizing P-T-t-D loops, with absolute timing constraints that can be tied to deformational and metamorphic fabrics, is a critical challenge for continued research.