MESO-PROTEROZOIC LOWER-CRUSTAL GARNET GRANULITE, GRANULITE FACIES METASEDIMENTS, AND CHARNOCKITE, FROM THE PINAL TERRAIN OF SOUTHERN ARIZONA: IMPLICATIONS FOR THE ORIGIN OF MESO-PROTEROZOIC GRANITOIDS AND LARAMIDE COPPER PORPHYRIES

Meso-Proterozoic lower-crustal garnet granulite, kyanite-bearing plagiogranite, and granulite facies meta-sediments were recently identified in outcrops in the Sierra Estrella mountain range just south of Phoenix, Arizona. Zircon dating (LA-ICP-MS) of the garnet granulite and plagiogranite yielded ages (n=35) of 1.43 and 1.41 Ga, respectively. A kyanite-bearing metasediment from the northern Sierra Estrella yielded a zircon age spectrum (n=108) with a dominant peak at 1.4 Ga, a broad lesser peak (~20 grains) between 1.48 and 1.58 Ga, and a few older grains (<1.75 Ga). It appears this metasediment was deposited at the surface at the time of the 1.4x Ga magmatic event or shortly thereafter. Presumably, there were volcanic centers at the surface at this time contributing zircons to sediments. A sillimanite-bearing metasediment from the central Sierra Estrella yielded a spectrum (n=105) with a dominant peak at about 1.69 Ga, a broad peak (45 grains) between 1.52 and 1.64 Ga, and a few younger grains (>1.4 Ga). It appears this metasediment was also deposited during or shortly after the 1.4x Ga magmatic event but had a somewhat different provenance. Both these samples were buried to mid to lower crustal depths after deposition. A sample of lower-crustal charnockite from the western end of South Mountain (S. Phoenix, AZ) was found to have a more complex spectrum of zircon ages (31 grains) between 1.35 and 1.65 Ga. Notably, all three of these samples contain zircons with ages (1.48-1.58 Ga) not usually found in rocks from southern Arizona although similar to “exotic” zircon ages reported by Doe et al (2013) for samples from White Ledges, Arizona. These new age and mineralogical data suggest the lower crust of southern Arizona may contain a sizable Meso-Proterozoic component that includes mafic rocks. The mineralogy and Hf, Nd, Pb, and Sr isotopic compositions of these samples will likely offer new insight into processes that produced the 1.4x Ga “anorogenic” suite of granitoids that are ubiquitous in southern Arizona and possibly those processes that produced some Mesozoic and Tertiary magmas (e.g., copper porphyries) known to contain abundant inherited 1.4x Ga zircons.