Eocene to Quaternary mafic-intermediate volcanism in San Luis Potosí, central Mexico: The transition from Farallon plate subduction to intra-plate continental magmatism
Résumé
The San Luis Potosí Volcanic Field (SLPVF) of central Mexico includes volcanic sequences of felsic, intermediate and basic compositions that were erupted as discrete episodes from the Eocene to the Pleistocene. Volcanism was dominated by widespread and voluminous rhyolitic ignimbrites of the mid-Tertiary Ignimbrite Flare-up. However, the complete volcanic history must consider basaltic and andesitic Eocene-Pleistocene volcanic successions that provide key evidence for understanding the geochemical evolution of the volcanism in the SLPVF during this time span. Five sequences are recognized according to their geochemical characteristics, each comprising a volcano-tectonic episode. The first episode comprises basaltic andesites and andesites erupted during three intervals, 45-42 Ma, 36-31 Ma, and 31-30 Ma. The oldest was derived from subduction magmatism, whereas the youngest has an intra-plate magmatic signature and this represents the transition from the end of a long lasting subduction regime of the Farallon plate to the initiation of intra-plate continental extension in the North American plate. The second episode, at 29.5-28 Ma, comprises a bimodal succession of high-silica rhyolites and alkaline basalts (hawaiites) that are interpreted as magmatism generated in an intra-plate continental extension regime during the Basin and Range faulting. The third episode, at 21 Ma, is characterized by trachybasalts and trachyandesites that represent mantle basaltic melts that were contaminated through assimilation of the lower crust during advanced stage of intra-plate extension that started at Oligocene. The fourth episode includes 12 Ma alkaline basalts and andesites that were erupted from fissures. These mantle derived magmas evolved to andesites by crustal anatexis and crystal fractionation within a continued, extensional, intra-plate regime. Lastly, the fifth episode comprises 5.0 to 0.6 Ma alkaline basalts (basanites) containing mantle xenoliths, that were erupted from maars and tuff cones, which are the youngest manifestations of mantle-derived intra-plate extensional events. Based upon this volcanic record, the last subduction manifestations of the extinct Farallon plate occurred at about 42 Ma, this was followed by a transition to intra-plate magmatism between 42 and 31 Ma, and an extensional, intra-plate tectonic setting from 31 Ma to almost Present.