A GEOCHEMICAL MODEL FOR THE ORIGIN OF THE PAN TAK GRANITE, THE GRANITE OF PRESUMIDO PEAK & THE GRANITE OF SIERRA SAN JUAN DURING THE LARAMIDE OROGENY, SOUTHERN ARIZONA & NORTHERN SONORA

Granitic plutons formed during the Laramide orogeny are exposed in the Coyote Mtns. (CM) - Sierra Pozo Verde (SPV) - Sierra San Juan (SSJ) ranges in the Sonoran Desert of southern Arizona and northern Mexico. Three granites from Sierra San Juan yield SIMS zircon crystallization ages from 59 to 51 Ma, overlapping the 58 Ma conventional TIMS age of the Pan Tak Granite, north of the Sierra Pozo Verde (Wright and Haxel, 1982). Modal and normative analyses and whole rock geochemistry illustrate similarities between the Pan Tak Granite (PT), the granite of Presumido Peak (PP), and the granite of Sierra San Juan (SJ) exposed in the CM-SPV-SSJ region and identify viable protoliths from which parental crustal partial melts were produced. Major element data show that the granites of the integrated CM-SPV-SSJ region are predominantly strongly peraluminous two-mica and garnet muscovite leucogranite with 70 to 77% SiO2, and closely resemble Neogene Himalayan granites, which serves to confirm a collisional tectonic setting. From early to late-emplaced units, alkali trace elements show depletions of 50 to 100x and REE patterns are flatter with deeper negative (and rarely, positive) Eu anomalies, implying an important role for feldspar fractionation in development of the PT-PP-SSJ rock suite. Experimental melt data give insights into the temperature and pressure conditions that initiated crustal melting and produced the parental melts that formed these granites. The PT, PP, and SJ granites may have fractionated from partial melts formed at about 5 to 10 kbars at temperatures ranging from 775 to 950 ºC via the peritectic dehydration melting of a muscovite-biotite schist or muscovite schist protolith.