ON THE ORIGIN AND TECTONIC ENVIRONMENT OF THE YAVAPAI ARC OF SOUTHERN ARIZONA AND ITS BEARING ON THE START OF PHANEROZOIC-STYLE SUBDUCTION

The Paleoproterozoic Yavapai “Province” of Arizona has been considered as a greenstone belt, a rift basin, and an arc terrain. Although it has been studied for over 120 years, its origin and tectonic setting are still debated. A new analysis of compositional and geospatial data for granitoid rocks in the belt has revealed close similarities to analogous data reported for granitoids in the Mesozoic Sierra Nevada Batholith. For example, K2O concentrations in the Yavapai granitoids increase from southeast to northwest in a manner analogous to the west to east increase in K2O found in Sierra Nevada granitoids. Given that the Sierra Nevada granitoids are believed to have formed in a magmatic arc associated with an eastward dipping subduction zone, it is reasonable to conclude the granitoids of the Yavapai “Province” formed in a magmatic arc that was associated with a northwest dipping subduction zone. The K2O-SiO2 dataset also indicates the Yavapai “Province” does not extend (e.g., as basement) into the Mazatzal “Province”. Thus, the Mazatzal “Province” must be a separate terrane. New zircon age data suggest the Yavapai arc was initiated at ca. 1.78 Ga and was active until approximately 1.66 Ga. This duration of magmatism (i.e., 120 m.y.) is similar to the duration of granitoid magmatism determined for the Sierra Nevada batholith (i.e., 140 m.y.). Together, the new age and chemical data suggest the existence of Phanerozoic-style subduction at 1.78 Ga. A distinctive aspect of the Yavapai arc compared to the Sierra Nevada batholith/arc is the abundance of juvenile granitoids in the former. Modern oceanic arcs are largely juvenile, but they are generally not known to contain large volumes of granitoid rock. Phanerozoic arcs built on continental margins contain abundant granitoids, but these generally contain significant fractions of non-juvenile components. The great abundance of juvenile granitoid rocks in the Yavapai arc suggests a Paleoproterozoic thermal and crustal thickness regime that was distinct from that found in most modern arcs.