OLIGOCENE AND MIOCENE IGNIMBRITES IN THE CONCEPCIÓN PENINSULA, GULF OF CALIFORNIA RIFT, BAJA CALIFORNIA SUR: BAJA BASINS NSF INTERNATIONAL RESEARCH EXPERIENCE FOR STUDENTS1

The Concepción Peninsula is a 40 by 14 km, east-tilted horst block that exposes Cretaceous granitic basement overlain by (1) Oligocene forearc and Miocene arc strata related to subduction of the Farallon plate, and (2) Late Miocene rift strata related to the opening of the Gulf of California. We identify four widespread ignimbrites that can be used for regional-scale correlation, and also for structural markers within the Concepción Peninsula. We present the following data: (1) new digital geologic mapping, showing the distribution of the ignimbrites; (2) petrographic analysis; (3) whole rock major- and trace-element geochemistry; and (4) element analysis of minerals using SEM and EDS.

The Oligocene ignimbrites ­lie in the Salto Formation, which is dominated by forearc eolian sandstones and lesser fluvial sandstones, but also contains ignimbrites erupted from the arc, which lay to the east in what is now the Sierra Madre Occidental of mainland Mexico. Three of these ignimbrites can be correlated along the length of the Concepción Peninsula, and all three occur at all localities: the lower white rhyolite welded ignimbrite (dated at 28 Ma), the upper white rhyolite welded ignimbrite, and the pink dacite welded ignimbrite. All three have biotite, and although they are altered, they form a tight cluster on immobile element plots. A widespread Miocene ignimbrite, herein named the Tuff of San Lino is an unaltered, up to 85 m thick clinopyroxene trachydacite, containing distinctive orthopyroxene with celadonite rims. It lies in a section of less extensive clinopyroxene trachydacite ignimbrites, as well as alkalic mafic to intermediate lavas. All of these ignimbrites, including the Tuff of San Lino, are higher in Nb and Zr and lower in Th than the Oligocene ignimbrites. The Tuff of San Lino is welded from base to top, but its upper surface is eroded and overlain by alkalic lavas. The Miocene trachydacite ignimbrites and the interstratified mafic to intermediate alkalic lavas may record the onset of continental rifting.