THE NATURE OF THE MAZATZAL OROGENY IN SOUTHWESTERN NORTH AMERICA: NEW DATA AND PERSISTENT QUESTIONS

The (1.7-1.6 Ga) Mazatzal Province and the associated Mazatzal orogeny have been central components of virtually all tectonic models involving the Proterozoic accretionary history of SW Laurentia. The province occurs spatially and temporally between the largely juvenile Yavapai Province (north) and the ca.1.45 Ga granite-rhyolite province (south). Recent recognition that some sedimentary sequences and some major structures are Mesoproterozoic rather than Paleoproterozoic has led to new questions about the nature, even the existence, of the Mazatzal orogeny and about the exotic vs. in-situ provenance of the Mazatzal Province. Rocks in the Four Peaks area of southern AZ and in the Manzano Mountains of central NM provide key constraints. 1.7 Ga-1.65 Ga sedimentary and volcanic rocks (in AZ) and 1.7-1.6 Ga rocks (in NM) were deformed and metamorphosed twice. The first is particularly apparent in aureoles of ca. 1.65 Ga plutons, but a range of methods, including pluton/aureole fabric relations and new in situ monazite dating confirm a regional contraction event synchronous with magmatism over a timespan from 1.68 to 1.60 Ga. Hf-isotopic data suggest the involvement of older, non-juvenile crust. The second event involved large scale folding, thrusting, and mid-crustal metamorphism at ca. 1.5-1.45 Ga, with structures cut by 1.45 Ga A-type plutons. Monazite data document thrusting that juxtaposes “hot” and “cold” blocks that each preserve different aspects of the accretionary history. The data support a model whereby a Mazatzal continental arc was built in-situ across Yavapai basement, as well as juvenile 1.65 Ga crust. Paleoproterozoic (1.7-1.6 Ga) sedimentary (quartzite) basins, and the subsequent 1.5-1.45 Ga basins, reflect extension in the upper plates due to northward subduction. Mazatzal deformation is interpreted to represent the closing of these basins and possibly the building of an orogenic plateau that persisted through the subsequent 100 m.y. tectonic gap.