Waning Miocene subduction and arc volcanism in Baja California: the San Luis Gonzaga volcanic field

Abstract

Subduction of the Guadalupe–Magdalena microplate beneath Baja California ended in the middle Miocene, and the last volcanic events in the frontal arc extinguished along the present-day eastern margin of the Baja California peninsula. The San Luis Gonzaga area in the north-central Gulf coast contains one of the younger arc-related volcanic centers in northern Baja California. The volcanic succession contains three sequences. The basal sequence (Group 1) is composed of stratified pyroclastic deposits, up to 500 m thick, and subordinate lava flows. The near-vent facies crop out in tilted fault blocks along the present shoreline, whereas the distal facies are exposed across ∼12 km toward the west and includes epiclastic deposits and at least three ash flow tuffs. This sequence is internally concordant and overlies smooth paleosurface developed on granitic basement, and pinches out across the Gulf escarpment. The Potrero Andesite (Group 2) is a series of dacite to basaltic-andesite lava flows from a shield volcano located ∼15 km west of today's coastline; similar source vents also occurs further south of the San Luis Gonzaga area. A sequence of dacite domes (Group 3) intrudes the near-vent facies of Group 1 and contains subordinate volcanic breccia and minor lava flows that overlie Group 1 sequence. Cross-cutting relationships and the abundance of volcanic breccia associated with the domes suggest that these domes were emplaced as semi-rigid intrusions (spines) with low explosive activity. The San Luis Gonzaga volcanic suite ranges in composition from basaltic andesite to dacite with predominant plagioclase and pyroxene and variable amounts of hornblende. Trace-element patterns indicate calc-alkaline to mildly alkaline magmas with high Ba and low Nb contents. Incompatible-element ratios and mineralogical characteristics suggest different magma batches and/or different amount of crustal assimilation for the three sequences that produced contrasting eruptive styles. A pink dacitic tuff that lies in the upper portion of Group 1 yielded an 40Ar/ 39Ar age of 17.2±0.3 Ma. The most precise of three ages from the andesitic sheet flows of Group 2 is 15.4±0.5 Ma. The dacite domes of Group 3 represent the end of Miocene arc activity, but they are strongly altered and no reliable ages were obtained. These data fall within the range of isotopic ages reported for arc lavas (21 to 16 Ma) from the discontinuous series of isolated volcanic fields in northern Baja California. This contrasts with the continuous and more voluminous arc-volcanism in southern Baja California that started earlier (25 Ma) and ended later (11 Ma). These space–time patterns of arc volcanism reflect the geometry of the subducted Farallon slab and the history of its breakup as the Rivera triple junction migrated to the south.