AGE OF THE PERMIAN MONOS FORMATION, NORTHERN SONORA, MEXICO, AND IMPLICATIONS FOR INITIATION OF THE CORDILLERAN MAGMATIC ARC

The timing of subduction initiation along southwestern Laurentia in latest Paleozoic time is uncertain, together with a complete understanding of how transcurrent motion transitioned to convergence. The Permian Monos Formation, exposed near Caborca, Sonora, is proximal to some of the oldest dated plutonic rock along the margin. Siltstone, sandstone, and limestone were deposited in the forearc to the growing magmatic system. Detrital zircons from a silty limestone in the upper, likely shallow-marine part of the Monos Fm provide initial insight into the early evolution of the arc. Based on the heterogeneity of detrital zircon ages and Th/U ratios, we speculate that the earliest arc morphology was highly irregular, and detrital sediments from the continent had easy access to the forearc.

The Monos Fm yielded 103 concordant grains that are Permian (~20%), other Paleozoic (300-600 Ma, ~30%), Grenville (1250-1000 Ma, ~ 20%), Paleoproterozoic (~1800-1500 Ma, ~20%), and Archaean (5%). A group of 21 Permian grains yields a TuffZirc age of 273 Ma, which provides a maximum age for the unit and indicates a significant influx of magmatic grains, likely related to the incipient arc. The spread of Paleozoic and Proterozoic ages indicates a broad range of source areas, effectively requiring a nearby continental landmass.

The zircon ages are complemented by Th/U ratios of the grains, which reflect crustal sources. Th/U ratios in Permian zircons from the Los Monos Fm average ~0.6. Zircons in similar-age volcanic clasts in the overlying Antimonio Formation have ratios ~ 0.08 – 0.3, and those from the ~270 Ma Los Tanques granite are ~ 0.1 and 0.6, approximately spanning the two. These ratios contrast with the high Th content of Mojave province Proterozoic crust, suggesting that the source magmatic rocks did not originate in that crust. The plutonic rocks may have been the source of zircons in the Monos Fm rather than coeval volcanic rocks.

The presence of a significant continental suite of zircons is enigmatic. The arc may have been offshore, and we speculate that in its earliest phases it was not a topographic barrier. A shallow-marine setting may have included redistributed sediments that originated in the backarc or in streams draining the continent. Synchronous transcurrent faulting may have brought plutonic material to the surface to be eroded.