THE MIDDLE-UPPER PERMIAN MONOS FORMATION: DETRITAL ZIRCON AND STRATIGRAPHIC EVIDENCE FOR LAURENTIAN SUBDUCTION INITIATION, NORTHWESTERN SONORA, MEXICO

The southwestern margin of Laurentia transitioned from a passive to active margin during Permian time. However, due to the paucity of late Paleozoic and early Mesozoic rocks, structural and temporal reconstructions of this transition remain unclear. The Permian siliciclastic and bioclastic Monos Formation is exposed in the Cerros Monos (Sierra del Álamo, Sonora, Mexico). Its proximity to the oldest felsic-intermediate plutons related to the incipient arc suggests it may record the initiation of subduction. Detrital zircon and stratigraphic analysis of the Monos Formation captures the timing and sedimentary nature of this critical tectonic period of western Laurentia.

A sample of calcareous fine-grained sandstone part of the Monos Formation yielded 83 concordant grains that are ~75% Permian (258–291 Ma), ~5% other Paleozoic (300–450 Ma), ~15% Neo-Mesoproterozoic (908–1522 Ma), and ~5% Paleoproterozoic-Neoarchean (1771–2718 Ma). The high percentage of Permian grains in the sample suggests that the incipient arc was a large contributor to Monos sedimentation. However, the presence of Paleozoic through Archean grains implies sources on the Laurentian continent. These data are augmented by Hf, U, Y, and Yb trace element compositions that indicate these zircons were derived from continental crust. Thus, zircon age and trace element data support Laurentian subduction initiation in Early Permian time on an east-vergent collisional zone.

The Monos Formation is composed of a lower 300-m-thick section of red siltstone, fine-grained sandstone, and clay-rich limestone, and a 200-m-thick upper section of thin- to thick-bedded fossiliferous limestone. The basal portion of the formation consists of 60 m of medium- to coarse-grained, angular, fining-upward sandstone interbedded with chert. The clastic rocks fine upsection into red siltstone interbedded with lensoidal to bedded micritic marlstone. Fossiliferous limestone then caps the siliciclastic unit of the Monos Formation. These stratigraphic relations may represent a deepening-to-shoaling-upward sequence, which is commonly associated with sedimentary rocks recording incipient subduction processes.