WAS THERE A MAZATZAL OROGENY? A TEST OF CONFLICTING MODELS FOR ASSEMBLY OF LAURENTIA

The prevailing model for continental growth of southern Laurentia has involved incremental additions of juvenile terranes along a long-lived (1.8-1.0 Ga) convergent margin. Recent detrital zircon data from NM and AZ indicate that at least the upper parts of some supracrustal sequences are Meso-, rather than Paleoproterozoic and that some, perhaps most, of the tectonism thought to be related to the 1650-1600 Mazatzal orogeny might be Mesoproterozoic. We test two incompatible hypotheses: 1) the traditional model wherein a 1.65-1.60 Ga Mazatzal accretionary orogeny was followed by 1.45-1.35 Ga intracrationic magmatism and tectonism; 2) an alternative in which much of southern Laurentia was built during a 1.48-1.35 Ga accretionary event. A summary of Mazatzal “tectonic elements” that need to be accommodated by any model include: extensive 1.66-1.60 granites, some with 1.65 Ga Hf and Nd model ages; 1.65-1.60 Ga likely syntectonic basins; widespread localities of 1.65-1.60 deformation that extend from Wyoming to Mexico; and metamorphism that is generally low grade, but higher near pluton margins. Similarly, 1.45-1.35 Ga tectonic features include: deposition (1.48-1.45) of newly discovered Yankee Joe-Defiance- Pilar 1480-1450 basins; major shear zones, over 100 Ma of A-type magmatism, and variable grade metamorphism. New Ar-Ar dating of coarse muscovites that cut penetrative fabrics and U-Th/Pb electron microprobe dating of monazite in NM and central Arizona confirm the presence of 1.65-1.60 Ga penetrative fabrics (preserved in blocks that were relatively cold at ~ 1.4 Ga) but also shows overprinting and transposition of fabrics at 1.45-1.35 Ga in hot blocks. Additional work is needed to separate 1.65-1.60 from 1.49-1.45 Ga successions and to better understand partitioning of 1.65-1.60 and 1.45-1.35 Ga components of finite strain fabrics. Nd and Hf data do not support the model that AZ/NM Proterozoic tectonics involved extensive 1.5-1.4 Ga terrane accretion, but additional data are needed in the midcontinent. Overall, we recognize evidence for a significant 1.65-1.6 Ga Mazatzal orogeny, as distinct from the subsequent 1.6-1.5 tectonic “gap”, and support models for a 100 Ma period of partitioned intracratonic 1.45-1.35 Ga tectonism in AZ/NM that may have been related to outboard accretionary processes.