INVITED SPEAKER: RIFTING VS. ARC ACCRETION AS MECHANISMS FOR PALEOPROTEROZOIC CRUSTAL GROWTH IN SOUTHWESTERN LAURENTIA: POSSIBLE RESOLUTION FROM THE MOJAVE-YAVAPAI BOUNDARY ZONE, NW ARIZONA

Two outstanding questions regarding Paleoproterozoic crustal growth and assembly in southwestern Laurentia are (1) the mechanisms of crustal growth (rifting of older crust coupled with juvenile magmatism vs. accretion of juvenile arcs) and (2) the nature and significance of pre-Yavapai orogeny deformation. Structural, metamorphic, and geochronological studies in the Mojave-Yavapai boundary zone of NW Arizona have yielded insights to these problems that may be broadly applicable to other parts of the Paleoproterozoic orogen. Supracrustal successions from four ranges in NW Arizona are dominated by >1730 Ma bimodal metavolcanic sequences and compositionally immature siliciclastic metasedimentary rocks, with minor ultramafic rocks, chert, and carbonate. This lithologic assemblage requires juvenile mafic volcanism in a basin that also received abundant quartz-rich (i.e., continental) detritus. We suggest that a pre-D₁, backarc-rift setting for NW Arizona best explains the observed supracrustal assemblage. Plutonic rocks that intrude the supracrustal rocks are generally 5-15 m.y. younger and span the entire compositional range of granitoids; i.e., they are not plutonic equivalents of the metavolcanic rocks. In our model, supracrustal rocks were deposited in a backarc rift basin above an east-dipping subduction zone. Shallowing of the slab resulted in (1) eastward migration of the arc causing arc magmatism within the former backarc basin, (2) closure of the basin, (3) thrusting of Yavapai province rocks over the basin (D₁), (4) burial of supracrustal rock to >20 km and ultimately, (5) gravitational collapse of the orogen. These events predated the 1.70-1.68 Ga Yavapai orogeny and occurred within a time interval of 10-20 m.y. Similar time intervals for “tectonic switching” have been documented from the Lachlan orogen of eastern Australia. We view both rifting and associated magmatism and arc accretion as viable, and not mutually exclusive, mechanisms for crustal growth.