THE CERRO RAJÓN FORMATION: A CAMBRIAN RIFT-RELATED VOLCANO-SEDIMENTARY SUCCESSION FROM NORTHWEST SONORA, MEXICO

The Cerro Rajón Formation, formerly Unit 1 of the Puerto Blanco Formation, is a volcano-sedimentary succession of tuffaceous conglomerate, metabasalt, mafic tuff, mafic lapillistone, mafic agglomerate and quartzite with minor siltstone, limestone, dolostone and sedimentary conglomerate that is well exposed near Caborca in northwest Sonora, Mexico. Trace fossils, body fossils, chemostratigraphy, and young detrital zircon populations in the unit constrain its deposition to the Fortunian (Terreneuvian), and are internally consistent with constraints from the underlying La Ciénega Formation and overlying Unit 2 of the Puerto Blanco Formation. Metabasalts and ultramafic rocks in the succession exhibit anorogenic intraplate volcanic characteristics, and interfinger with strata deposited in coastal continental and intermittently emergent shallow marine settings. Petrographically, the volcanic rocks are actinolite-sphene-calcite granofels consistent with a greenschist facies metamorphism. These volcanics are characterized by low silica values (SiO_2N = 4.32–48.21%) and high TiO₂ concentrations (3.63–7.52%). Hydrothermal contact metamorphism overprints these signatures, causing mobilization in the major oxides, while preserving immobile trace elements. This metamorphism is related to the emplacement of Laramidic plutons close to the study area. The volcanics in the unit record rift related volcanic events in what is traditionally considered a passive margin. Immobile trace element analyses show alkaline within-plate basalts characteristics and a magmatism produced from an enriched mantle source. They correlate with similar rift-related alkaline mafic and ultramafic volcanics intercalated in Cambrian successions in the western United States and Canada, suggesting that the Caborca volcanism may represent the southern tip of a Neoproterozoic-Cambrian rift system developed along the west margin of North America.