SOUTHWEST LAURENTIAN LOWER-MIDDLE TRIASSIC SEDIMENTARY STRATA AND THE RECORD OF EARLIEST CORDILLERAN ARC MAGMATISM

The Early to Middle Triassic Moenkopi Formation is exposed across the Colorado Plateau and into western Arizona and eastern California, where its stratigraphic equivalent is the Buckskin Formation. These Triassic strata provide evidence of Permian magmatism above the incipient subduction zone flanking the southwestern margin of Laurentia.

Detrital zircon grains age spectra from the Buckskin Formation indicate an influx of magmatism beginning at about 262 Ma. These grains, which likely derived from eruptions from or erosion of volcanoes related to Permian plutons exposed across the Mojave Desert, record late Permian initiation of subduction in this region. Geochemistry of these grains supports derivation from arc magmas that crystalized in continental crust.

Detrital zircon spectra from two samples of sandstone from the upper Moenkopi Fm near Winslow, AZ, yielded distinct arc-related signatures at ca. 265 Ma and 280 - 290 Ma. Although sparse, the ca. 265 Ma zircons have trace element contents that agree with the arc-initiation signature of the Buckskin Fm grains. The 280 – 290 Ma zircon grains are acicular and have trace element contents similar to those of the younger suite, thus suggesting that they also derive from subduction-related magmas. Early Permian ages of 280 – 290 Ma are older than known Cordilleran-arc-related rocks, although one 280 Ma volcanic cobble has been reported from Triassic strata in the Caborca block in northwest Sonora.

Paleocurrents indicate that source areas for the Moenkopi Fm were to the east and southeast of depocenters. The “East Mexico arc” has been proposed as a source for Permo-Triassic zircons, although convincing in-situ evidence for this arc is still lacking. The unabraded, acicular shape of the 280 – 290 Ma zircons indicates very minimal transport, suggesting that they originated in a Plinian eruption, were initially deposited proximal to the Moenkopi depocenter, and were reworked only slightly prior to final entrainment. A highly speculative source would be a part of Caborca block now buried under the Sierra Madre in eastern Sonora.