Francisco A Paz Moreno

The Quaternary Moctezuma volcanic field: A tholeiitic to alkali basaltic episode in the central Sonoran Basin and Range Province, Mexico

The Quaternary Moctezuma volcanic fi eld at the foothills of the Sierra Madre Occidental, Mexico, is characterized by a close association of tholeiitic and alkaline magmas. The vents of the tholeiitic magmatism (1.7 Ma), which forms the overall mesa morphology of the volcanic fi eld, lie along the major faults that defi ne the eastern margin of the Moctezuma basin. This half-graben formed during the early Miocene as indicated by the emplacement of basaltic fl ows (22.3 Ma), which were intercalated with fanconglomerates of the Baucarit Formation. The youngest volcanics are alkaline lavas (0.53 Ma), which erupted from scoria cones located in the center of the basin. The main mineralogical difference between the Quaternary tholeiitic and alkaline lavas is the composition of the clinopyroxenes: they are Ca-rich in the alkaline lavas, whereas they are subcalcic with orthopyroxene or pigeonite in the tholeiitic lavas. The geochemical data show a gradual change in the characteristics of the lavas. The Sr and Nd isotopic compositions, and the shape of the mid-oceanic-ridge basalt (MORB)-normalized pat- terns, are similar to oceanic island basalt suites, suggesting that the source of the Moct- ezuma volcanic fi eld lavas was dominated by asthenospheric mantle. The absence of mantle or granulite xenoliths in the Moctezuma volcanic fi eld lavas, compared to the neighboring Geronimo volcanic fi eld, refl ects slower ascent rates. The Southern Cordillera Basaltic Andesite (SCORBA)-type geochemical sig- nature of the early Miocene synextensional basalts of Moctezuma compared to the Quaternary Moctezuma volcanic fi eld lavas can be correlated with a progressive thin- ning of the mantle lithosphere during the Neogene.

Insights into the tectonomagmatic evolution of NW Mexico: Geochronology and geochemistry of the Miocene volcanic rocks from the Pinacate area, Sonora

Miocene volcanic rocks in the Pinacate area, Sonora, record a progressive change in the source of magmatism induced by asthenospheric upwelling and lithospheric thinning. 40 Ar/ 39 Ar age data, mineral chemistry, and major- and trace-element contents allow the identifi cation of two volcanic sequences: an oldest basaltic episode (ca. 20 Ma), and a middle Miocene (12‐15.5 Ma) sequence that consists of mesa basalts with transitional alkali character, calc-alkaline dacites, and high-silica rhyolites evolving toward peralkaline liquids. Sr, Nd, and Pb isotope ratios reveal different sources for the Miocene basalts. The easternmost basalts have signatures indicating a Precambrian lithospheric mantle source, while the westernmost tholeiitic to transitional basalts are related to mixing of lithospheric and asthenospheric mantle. Rhyolites are the result of fractional crystallization of transitional basalt magmas with slight contamination by Precambrian crust. Chemical modeling shows that peralkaline rhyolites are related to slightly higher assimilation during their residence in the upper crust but also to a change in the mantle source of the parent basalt. The evolution of the isotopic signatures in space and time indicates that: (1) the volcanic activity is located over a major lithospheric boundary, i.e., the western limit of the North American Craton; (2) the lithosphere was progressively thinned so that huge volumes of alkalic basalts could access the surface during the Quaternary, building the Pinacate Volcanic Field. Correlation between geochemical signatures and the tectonic evolution of the western margin of the North American Craton shows that a progressive change in the source of magmatism can be related to the development of a slab window during the Miocene.

The Recent Isla San Luis volcanic centre: petrology of a rift-related volcanic suite in the northern Gulf of California, Mexico

Abstract Isla San Luis, one of the most recent eruptive centres in the Gulf of California, presents a complete magma evolution trend from basaltic andesites to rhyolites. The less-evolved lavas are palagonite tuffs, related to Surtseyan-type activity which characterized the emergent stage of the island. Subaerial lava flows and later high-energy hydromagmatic eruptions are dacites which make up the tuff rings of the southeastern corner of the island. Rhyolites are the latest erupted products and the development of the younger dome in the centre of the island was preceded by ash and pumice fallout containing quenched bombs with an obsidian crust and a pumiceous core. Basaltic andesites contain olivine (Fo87–80), calcium-rich plagioclase and sparse clinopyroxene microphenocrysts; the typical mineralogy of dacitic and rhyolitic lavas is plagioclase+two-pyroxenes. Major and trace elements vary regularly with MgO taken as a differentiation index. Isla San Luis lavas are enriched in LILE and depleted in Nb, Ta and Ti. Their characteristics are intermediate between tholeiitic and calc-alkaline lavas. They are also LREE enriched and the [(La/Yb)n] ratios steepen slightly with differentiation. Such an evolution is better explained by combined fractional crystallization and assimilation processes.