GEOCHEMISTRY OF THE EOCENE PAN-TAK GRANITE OF THE COYOTE MOUNTAINS, SOUTHERN ARIZONA

The peraluminous Pan-Tak Granite (SiO₂ ~73-78%) is exposed in the footwall of the Coyote Mountains metamorphic core complex and is composed of an equigranular, two-mica granite (ASI 1.05-1.11) intruded by a complex of pegmatitic and aplitic leucogranite dikes and small intrusive bodies (ASI 1.02-1.18). Both rock types are Zr poor (16-130 ppm) and contain zircons that are strongly enriched in U (avg.~6500ppm) with convolute zoning and metamict textures . New zircon LA-ICP-MS U/Pb data show linear trends between increasing age and decreasing U content indicative of lead loss. Our best age estimate for the two-mica granite is 59 +/- 3 Ma, based on a cluster of analyses that show minimal Pb-loss. Discordant zircon U-Pb LA-ICPMS analyses from the leucogranite have a concordia age of ca. 48 Ma and preliminary CA-TIMS data from the leucogranite suggests a crystallization age of ca. 55 Ma. The two-mica granite contains numerous Jurassic (160-175 Ma) and Proterozoic (1.3-1.4 Ga) inherited zircon ages. Depletion trends in Zr, Hf, Sr, Ca, and Ti suggest that the leucogranite is a highly fractionated component from the same source as the two-mica granite. Both rock units are enriched in REEs, with weak and strongly negative Eu anomalies in the two-mica granite and leucogranite respectively. The Pan-Tak granite is isotopically evolved with zircon εHf_t = -2.4 to -14.6, whole rock εNd_t = -6.2 to -10.7, and ⁸⁷Sr/⁸⁶Sr = 0.71 to 0.72 with one value at 0.74 . Quartz δ¹⁸O for both rocks is 9.38 ± 0.37‰ (n=9). Ti in zircon temperatures are 607 to 776 ⁰C in the primary granite and 546 to 812 ⁰C in the leucogranite. Zircon saturation temperatures are 730 ± 40 ⁰C. The Pan Tak Granite is interpreted have formed by crustal melting with little to no added mantle-derived mass or heat. The isotopic composition, geochemistry, and thermometry data are consistent with vapor-absent muscovite dehydration melting and/or water-deficient melting during the Laramide orogeny.