NEW U-PB ZIRCON ANALYSES RECORD PALEOCENE AND MID EOCENE-EARLY OLIGOCENE MELTING OF LATE CRETACEOUS, JURASSIC, AND PROTEROZOIC CRUST IN LOWER PLATE OF THE MAGDALENA-MADERA METAMORPHIC CORE COMPLEX, SONORA, MEXICO

We report ICP-MS and SHRIMP-RG ²⁰⁶Pb/²³⁸U analyses of zircon from the footwall of the Magdalena-Madera core complex that complement previously published ID-TIMS data from the area; e.g. 78±3 Ma Sierra Guacomea granodiorite (Anderson et al., 1980) and 174 Ma Sierra El Pinito rhyolite porphyry (Anderson and Silver, 2005). CL images distinguish magmatic grains and inherited cores from dark, high-U overgrowths attributed to Tertiary crustal melting. Strongly foliated biotite granite porphyry of northern Sierra Guacomea has escaped the Tertiary thermal effects observed farther south; 29 magmatic zircons yield an age of 171.4±1.1 Ma. In Sierra Madera, two-mica granite of Cerro Mezquital intrudes biotite granite gneiss. The Mezquital granite is dominated by inherited Late Cretaceous (72.1±1.3 Ma) and Jurassic (165.3±5.6 Ma) cores. High-U zircon overgrowths (59.8±2.2 Ma) record a crustal melting event interpreted to be the age of this pluton. The gneiss contains inherited Jurassic cores (176 to 145 Ma; N=9) and a second zircon population (N=11) that constrains a magmatic age of 61.2±1.8 Ma. Most zircons in the gneiss display dark rims (47-31 Ma) interpreted to have grown during intrusion of peraluminous granite / pegmatite that is abundant farther south. The youngest coherent age group records magmatic culmination at 32.5±1.3 Ma. In Canada Honda of Sierra Magdalena, magmatic zircons in biotite granodiorite record an age of 65.9±2.9 Ma, with 41.7±0.5 Ma overgrowths related to pervasive leucogranite intrusions. Only 3 of 145 total analyses were inherited Proterozoic grains (1677, 1645, and 1370 Ma). The one known Proterozoic outcrop is a pendant of biotite granite augen gneiss within mylonitized biotite granodiorite and leucogranite of Arroyo El Salto. A weighted mean age of 7 oscillatory-zoned zircon interiors analyzed on the SHRIMP is 1037±10 Ma, with 3 concordant grains at 1043.2±4.0 Ma. Near-concordant ICP-MS analyses from the same outcrop yield a magmatic age of 1070.3±6.8 Ma (N=18), with inheritance at 1375±32 Ma (N=9). Collectively, our data record separate granite pulses during mid Paleocene and mid Eocene-early Oligocene time, produced by anatexis of Late Cretaceous, Jurassic, and Proterozoic crust. Footwall rocks were sheared and exhumed along the normal-slip Magdalena-Madera detachment fault at ~25-20 Ma.