EARLY MESOZOIC CORDILLERAN ARC MAGMATISM: THE DETRITAL ZIRCON RECORD OF THE BUCKSKIN FORMATION, WEST-CENTRAL ARIZONA AND SOUTHEASTERN CALIFORNIA

A shift in the depositional systems and tectonic regime along the western margin of Laurentia marked the end of the Paleozoic Era. The record of the transition and the inception and development of the early Mesozoic Cordilleran magmatic arc is preserved in plutonic rocks in southwestern North America in successions in the distal backarc region, such as the Chinle Fm on the Colorado Plateau (CP), and in the proximal backarc region. The region between arc and backarc has been dissected by Cenozoic extension, but the metasedimentary Triassic Buckskin Fm is exposed in several ranges within the Colorado Extensional Corridor. The Buckskin Fm has been correlated to the Triassic formations of the CP based on stratigraphic facies and position. U/Pb geochronology of zircons of the Buckskin Fm reveals the significant and complex influence the developing magmatic arc had on the evolution of early Mesozoic proximal backarc depositional centers.

New U/Pb detrital zircon ages from the four members of the Buckskin Fm in three sample locations confirm the correlation of this succession to the Moenkopi and Chinle Fms on the CP. The lowest stratigraphic member of the Buckskin has zircon age distributions that comprise ~60% Phanerozoic ages, with the largest cluster of ages around 252 Ma likely derived from the arc. The overlying marker unit lacks any significant age populations and only contains approximately four arc-related grains. The Phyllite member has a significant proportion of Phanerozoic ages (~55%), with the largest age peak occurring at 219 Ma. Few arc-related grains are present in the Upper Member; ~90% of the grains are Proterozoic with significant age peaks from 1800 to 1400 Ma.

The Lower and Phyllite members show very strong age overlap with grains derived from Cordilleran arc plutons (i.e. San Bernardino and San Gabriel suites), which likely reflects derivation from the arc. The ages suggest that during the time of Buckskin deposition, the basin received two significant fluxes of arc detritus at ~252 and 219 Ma. The lack of arc-derived detritus in the marker and Upper members of the Buckskin Fm could suggest (1) lulls in or migration of magmatic activity between 252 Ma and 219 Ma in segments of the arc or (2) uplift of paleo-divide(s) that prevented the influx of arc detritus into the backarc region.