PROVENANCE AND DEPOSITIONAL SETTING OF THE PINAL SCHIST, SOUTHWESTERN ARIZONA AND SONORA

The Pinal schist, which forms the basement of much of southern Arizona, is composed mostly of quartz-sericite schists, meta-wackes, metavolcanic rocks, and associated granitic rocks. It was named for exposures in the Pinal Mountains near Globe, AZ. The age of the Pinal Schist is not well constrained, but it is associated with local granites and rhyolites of the Mazatzal Province whose U-Pb ages are mostly in the 1650-1700 Ma range. Among the various tectonic settings proposed for the depositional environment of the Pinal Schist protolith are a continental rift (Condie et al.,1985), a continental rift that evolved into a passive margin (Keep, 1996), a northwest-trending failed arm (aulacogen) of a triple junction, and a backarc or intra-arc basin (Copeland and Condie, 1986; Anderson 1989). More recently, Meijer (2014) proposed that the Pinal terrane formed as a forearc-subduction complex that experienced a ridge subduction event at ca. 1.65 Ga. To further understand the origin and tectonic setting of this enigmatic unit we have analyzed detrital zircons from five Pinal Schist samples, three from the type locality near Globe and two from the Whetstone Mountains. The resulting U-Pb ages cluster closely about 1700 Ma, suggesting derivation from the granitoid rocks that are common in the southwestern Mazatzal Province. However, there are also grains that are ca. 1850, 2100, 2500, 3000, and 3300 Ma, indicating some input from much older sources. The youngest detrital zircons in our samples are ca. 1680 Ma, constraining deposition to a time younger than this. We believe the available data are most compatible with deposition in a continental back-arc setting, although it is striking that mafic to andesitic rocks are sparse, quartz-rich sediments are abundant, and feldspars were evidently mostly weathered away. Hf isotopic analyses of the zircons yielded data giving ε_Hf (1700) ranging from -5 to +15, but centered on +5. Model ages are mostly in the range 1850-1900 Ma. These data suggest that there is a significant older component in the source of the dominant ca. 1700 Ma zircons. Possibilities include derivation from melting of a uniform cryptic older crust or mixing of a mantle source with older crust. Whole-rock Sm-Nd, Lu-Hf, and Pb-Pb trace element and isotopic analyses are in progress.