DATING EARLY ARC MAGMATISM IN THE WESTERN NORTH AMERICAN CORDILLERA – A STUDY FROM THE EL PASO MOUNTAINS, CALIFORNIA

The El Paso Mountains, located in the northwestern Mojave Desert, California, preserve late Permian plutons assumed to be the among the oldest recognized in the North American continental arc. Many workers have suggested that the North American continental margin transitioned from a strike-slip to a convergent boundary in Permian time, making plutons of that age important recorders of this critical tectonic transition. Previously acquired multi-grain TIMS ages from western Mojave plutons reveal magmatism occurring in the El Paso Mountain region from ca. 260 – 246 Ma. Those ages, however, are few, and many are discordant, making the timing and duration of early arc magmatism ambiguous. Nine new single-crystal zircon U-Pb ages from the El Paso Mountain region are used to constrain the timing, duration of emplacement, and spatial distribution of early arc magmas. In two cases, plutons dated as part of this study yielded ages that were older than those previously determined by TIMS, indicating that those plutons were more affected by radiogenic Pb loss than previously recognized. Pluton ages range from ca. 275 to 239 Ma, with two age clusters identified: a Late Permian cluster at ca. 259 – 252 Ma, and a Middle Triassic cluster at ca. 244 – 240 Ma. Two older plutons, with crystallization ages of 270 Ma and 274 Ma, are some of the oldest dated bodies in the western Cordilleran arc, though ca. 260 – 270 Ma plutons have been dated in Sonora, Mexico. Plutons yielding ages that overlap, but are slightly younger than those presented here, have been documented in the Transverse Ranges, southern California, suggesting spatially irregular patterns of early arc magmatism, which may have nucleated simultaneously in the northern Mojave and Sonora, Mexico, and then migrated to the south and north, respectively.