INSIGHTS INTO REGIONAL LARAMIDE CRUSTAL MELTING: WHOLE-ROCK AND SIMS ZIRCON GEOCHEMISTRY OF LATE LARAMIDE PERALUMINOUS GRANITES IN SOUTHERN ARIZONA AND NORTHERN SONORA

Paleogene peraluminous granites in the Papago terrane of southern Arizona and north-central Sonora provide a window into lower crustal melting processes during the Laramide orogeny in southwestern North America. We present whole rock geochemistry and SIMS Pb/U ages and geochemistry of zircons from granitic rocks from the Sierra Pozo Verde and Sierra San Juan. These data provide constraints on magmatism associated with Laramide crustal thickening and new insights into contrasts with regional Late Cretaceous metaluminous magmatism. The Sierra Pozo Verde and Sierra San Juan plutons are composed of strongly peraluminous two-mica and garnet muscovite leucogranite with 70 to 77% SiO₂, compositionally similar to leucogranite complexes of the Proterozoic Black Hills and Neogene Himalaya. 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 in the Coyote Mountains, north of the Sierra Pozo Verde (Wright and Haxel, 1982). Magmatic zircons have distinctive high U and Yb and low Th/U and Gd/Yb relative to zircons from Late Cretaceous metaluminous granodiorite and granite in the same region. Premagmatic zircons in Sierra San Juan granites form a population with an age of ~166 Ma, distinct from Proterozoic inheritance in the Pan Tak Granite. These results suggest that regional peraluminous magmatism in the Papago terrane and adjacent northern Sonora occurred in Paleocene and Eocene time and records melting of Proterozoic and Jurassic lower crustal rocks associated with regional Laramide crustal thickening. Trace element studies (in progress) will better define the nature of melt-forming reactions and the contribution of entrained restitic material to granitic whole-rock compositions.