Influence of Triassic and Jurassic arc magmatism in Lower Jurassic Sierra de Santa Rosa Formation, northwestern México: Constraints from geochemistry and U-Pb geochronology

Abstract

The Permo-Triassic and Lower Jurassic sedimentary succession of the El Antimonio Group in northern Sonora, Mexico is inferred to be deposited in a forearc basin and its age has been constrained by fossil assemblages and detrital zircon geochronology. In this work, petrography, geochemistry and detrital zircon U-Pb geochronology were undertaken on the Sierra de Santa Rosa Formation, the younger unit of the El Antimonio Group in the locality of the Sierra del Álamo, to constrain its tectonic setting, age, source area paleoweathering, and provenance. Sandstone of Sierra de Santa Rosa Formation is petrographically classified as arkose. The elemental ratios, REE patterns and Eu anomaly, as well as bivariate and ternary plots suggest igneous felsic sources subjected to weak to moderated chemical weathering under arid climatic conditions for detritus of the Sierra de Santa Rosa Formation. Detrital zircon U-Pb geochronology of seven sandstone samples of this unit record main populations of Proterozoic age (58% of the total grains) with age peaks at 1.68. 1.39 and 1.14 Ga and Triassic age (31% of total grains) with age peak at 219 Ma, and subordinate groups of Paleozoic, Jurassic and Archean ages. Main sources for the zircon grains are the regional nearby igneous-metamorphic Proterozoic basement rocks, recycled detritus from the Proterozoic and Paleozoic sedimentary cover, and the Permo-Triassic and Jurassic continental magmatic arcs of southwestern North America. The detrital zircon U-Pb geochronology of the dated samples allows to establish Sinemurian to early Toarcian maximum depositional ages for the Sierra de Santa Rosa Formation in the locality of the Sierra del Álamo.