CRUSTAL EVOLUTION OF SOUTHERN LAURENTIA DURING THE MAZATZAL OROGENY: NEW EVIDENCE FROM HF ISOTOPIC COMPOSITIONS OF PLUTONIC AND DETRITAL ZIRCONS AND SM-ND COMPOSITIONS OF THE PINAL SCHIST (Invited Presentation)

The Pinal terrane, which is exposed over a large region in southern Arizona and northern Sonora, is underlain by the Pinal schist, which comprises quartz-sericite schists, meta-wackes, and metavolcanic rocks, and is intruded by numerous ca. 1.65-1.70 Ga granitoid plutons. Several tectonic settings for the Pinal basin have been proposed, but its relationship to the Mazatzal arc is crucial. Detrital zircons from the Pinal schist yield almost unimodal U-Pb ages of ca. 1.70 Ga, but with a sprinkling of older grains, suggesting principal derivation from granitic rocks of this age in the Mazatzal arc and minor input from older sources. Meijer (2016) presented the U-Pb ages of zircons from 18 plutons that intrude the Pinal schist, arguing that their age progression from south to north is consistent with a fore-arc basin model incorporating a spreading ridge subduction event (SRSE; Meijer 2014). Most plutons yielded ages of ca. 1.65 Ga, but those in the northern part of the basin yielded ca. 1.70-1.75 Ga. Recently determined Hf isotopic compositions of zircons from these plutons, and also from the Pinal schist detrital suite, yield εHf values that are positive, but less than the DM at either 1.65 or 1.70 Ga. Single-stage depleted mantle model ages (T_DM) range from 1.74 to 1.91 Ga, whereas two-stage crustal model ages (T_Cr) are from 1.85-2.00 Ga. Coupled with initial εNd(t) values of about 1.22-1.94 for metasedimentary samples of the Pinal schist, these isotopic data indicate that the ca. 1.65-1.70 Ga plutons, and the ca. 1.70 Ga source rocks of the Pinal metasediments, have a component of older crust, either from partial melting of cryptic ca. 1.85-2.00 Ga crust or contamination of mantle melts with even older crust. These considerations suggest that older continental crust extended well to the south within the current Yavapai and Mazatzal arcs; the 1.84 Ga Elves Chasm terrane (exposed in the Grand Canyon) may be an exposure of this older crust. Moreover, at least the Mazatzal was likely a continental arc and the Pinal basin, therefore, was likely either a fore-arc or intra-arc basin.