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

Subduction of the Guadalupe-Magdalena microplate beneath Baja California en ded in the middle Miocene, and the last volcanic events in the frontal are extinguished along the present-day eastern margin of the Baja California pe ninsula. The San Luis Gonzaga area in the north-central Gulf coast contains one of the younger are-related volcanic centers in northern Baja Californi a. The volcanic succession contains three sequences. The basal sequence (Gr oup 1) is composed of stratified pyroclastic deposits, up to 500 m thick, a nd subordinate lava flows. The near-vent facies crop out in tilted fault bl ocks along the present shoreline, whereas the distal facies are exposed acr oss similar to 12 km toward the west and includes epiclastic deposits and a t least three ash flow tuffs. This sequence is internally concordant and ov erlies smooth paleosurface developed on granitic basement, and pinches out across the Gulf escarpment. The Potrero Andesite (Group 2) is a series of d acite to basaltic-andesite lava flows from a shield volcano located similar to 15 km west of today's coastline; similar source vents also occurs furth er 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 dom es 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 and esite to dacite with predominant plagioclase and pyroxene and variable amou nts of hornblende. Trace-element patterns indicate calc-alkaline to mildly alkaline magmas with high Ba and low Nb contents. incompatible-element rati os and mineralogical characteristics suggest different magma batches and/or different amount of crustal assimilation for the three sequences that prod uced contrasting eruptive styles. A pink dacitic tuff that lies in the upper portion of Group 1 yielded an Ar -40/Ar-39 age of 17.2+/-0.3 Ma. The most precise of three ages from the and esitic sheet flows of Group 2 is 15.4+/-0.5 Ma. The dacite domes of Group 3 represent the end of Miocene are activity, but they are strongly altered a nd no reliable ages were obtained. These data fall within the range of isot opic ages reported for are lavas (21 to 16 Ma) from the discontinuous serie s of isolated volcanic fields in northern Baja California. This contrasts w ith the continuous and more voluminous arc-volcanism in southern Baja Calif ornia that started earlier (25 Ma) and ended later (11 Ma). These space-tim e patterns of are volcanism reflect the geometry of the subducted Farallon slab and the history of its breakup as the Rivera triple junction migrated to the south. (C) 2000 Elsevier Science B.V. All rights reserved.