Depositional age and provenance of Triassic sedimentary succession from Northwestern México: Evidence from petrography and detrital zircon U–Pb geochronology

Abstract

The sedimentary succession of the El Antimonio Group, which unconformably overlies the Permian (Guadalupian) Monos Formation, is well exposed in the Sierra del Álamo section, southwest of Caborca, Sonora, México. Here, this section is composed of the Triassic Santa Eduwiges (renamed from the previous Antimonio Formation) and Río Asunción and the Lower Jurassic Sierra de Santa Rosa formations. This study focuses on the provenance and geochronology of the Triassic formations. The sandstone of Santa Eduwiges and Rio Asuncion formations are classified from litharenite to arkose. On the QtFL diagram, sandstone samples plot in the orogenic, transitional continental and undissected arc fields. In contrast, on the QmFLt diagram, the samples are mainly concentrated in the mixed, transitional continental and quartzose recycled fields. Main zircon populations identified in this study are Triassic, Permian and Proterozoic. Proterozoic zircon grains are grouped into ∼1.8, 1.7, 1.6, 1.4 and 1.1 Ga populations whose primary sources are the crustal basement rocks of the Mojave Yavapai and Mazatzal provinces of southwestern North America, and Mesoproterozoic granitic plutons that intrude them. Permo-Triassic zircon grains may be sourced in magmatic arc rocks of this age that in southwestern North America is well presented in northwestern Sonora. The detrital zircon dated allows us to constrain maximum depositional ages for the Triassic Santa Eduwiges Formation of the El Antimonio Group: the youngest zircon grains from lowermost part (Sequence I) yielded an age of 252.64±1.98 Ma; zircon grains from its middle part (Sequence IV) yielded an age of 244.63±3.04 Ma and zircon grains from Sequence VI of the upper part of this unit yielded an age of 229.33±1.72 Ma. The El Antimonio Group was deposited in a forearc basin located south as a product of the Permo-Triassic continental magmatic arc that contributed much of the detritus to the early Triassic basin. During later stages of sedimentation in the Triassic basin, detritus was mostly sourced from the Proterozoic basement rocks.