New Mexico Geological Society Annual Spring Meeting — Abstracts

New 40Ar/39Ar dates show previously unrecognized Mogollon-Datil age (~35 Ma) plutonic activity in southwestern New Mexico (Table) that was related to the early phases of mid-Tertiary subduction-to-extension transition along the western coast of North America. Within the time framework of the large-volume silicic magmatism in New Mexico and Colorado (Boot Heel, Mogollon-Datil, Central Colorado/San Juan volcanic fields), all of these plutons are in the same age range as the older, 37-32 Ma, “subduction-related” pulse of caldera activity (Steins, Muir, Juniper, Animas Peak, Tullous, Geronimo Trail, Socorro, Twin Sisters, Emory, Organ, Schoolhouse Mountain), and none fall within the later 29-24 Ma “subduction-to-extension transition” caldera pulse. All but two of the plutons (Organ, Animas) are not associated with calderas. The Organ pluton is 3 m.y. younger than caldera collapse, which does not fit classic caldera development models, where intracaldera sequences are intruded within a few hundred thousand year after collapse. The most mafic of the plutons is a diorite from the Prospect Hills, the other plutons are granodiorite to granite to quartz monzonite. The granites from the western Burro Mountains and Little Hatchet Mountains have epsilon-Nd (at 0 Ma) values of -7.3 and -6.5, respectively. They thus contain slightly more radiogenic Nd than the Precambrian crustal source component. This is similar to epsilon-Nd (at 0 Ma) values of -7.3 of ignimbrites that erupted from the calderas in Hidalgo County (Bryan, 1995). Tungsten-molybdenum skarns are found adjacent to the Victorio, Granite Gap, Granite Pass, and Eureka stocks and tungsten and molybdenum have been reported from near the Organ and Tres Hermanas plutons. Collectively, the geochemical and isotopic data, and metal association suggest that source magmas were mixtures of mantle-derived mafic magmas and Proterozoic crust. Compositional differences between the various rhyolitic and granitic rocks are probably a result of fractional crystallization.