MESOPROTEROZOIC FORMATION OF THE SOUTHERN MARGIN OF NORTH AMERICA VIEWED FROM THE PERSPECTIVE OF DETRITAL AND MINERAL GROWTH AGES (Invited Presentation)

The accepted model for Paleoproterozoic accretionary orogens along the southern margin of Laurentia proposes sequential suturing between ~1800 and ~1700 Ma (Yavapai orogeny), ~1700 and 1600 Ma (Mazatzal orogeny), followed by intracontinental tectonism between ~1600 and ~1300 Ma. However, there is considerable uncertainty regarding the nature of Mesoproterozoic tectonism.

Detrital zircon geochronology has provided a powerful tool for determining protolith ages of metasedimentary rocks. In the southwestern USA, significant Mesoproterozoic basins have been identified that were interpreted as Paleoproterozoic. These rocks are complexly deformed, sharing the same deformation history as older rocks on which they were deposited. The Mesoproterozoic sequences include the Yankee Joe and Black Jack Formations of central Arizona, the Defiance Quartzite of northeastern Arizona, and the Trampas group of northern New Mexico. These sedimentary successions define east-northeast trending belt formed between ~1500 and ~1460 Ma.

In central Arizona, contact metamorphism around the 1.43 Ga Ruin granite, produced peak amphibolite facies metamorphism within the Yankee Joe and Black Jack formations, coeval with map scale folding. The Defiance quartzite records greenschist facies metamorphic conditions. Rocks in New Mexico record regional amphibolite facies metamorphism and superposed deformation. Metamorphic studies in New Mexico suggest pressure variation of about 500 MPa across the exposed sequence, indicating metamorphism of the entire middle crust.

Lu/Hf garnet geochronology provides an additional tool to investigate the timing of metamorphism. Samples collected from the andalusite, sillimanite, and kyanite zones exposed in New Mexico indicate metamorphism between ~1470 and ~1400 Ma. A critical observation is that garnet ages come from rocks with both Paleoproterozoic and Mesoproterozoic protoliths, and in regions far removed from plutons. The combined stratigraphic relationships, detrital zircon ages, garnet growth ages and structures indicate a contractional orogen active between ~1460 and ~1360 Ma, from northern New Mexico to central Arizona. It remains unclear what the extent of the orogen is, or if it is collisional or Andean in style, providing a fruitful topic for future research.