Peralkaline ignimbrites of middle Miocene age in central Sonora: review of the stratigraphy and significance of Tertiary volcanism
A peculiar volcanic episode represented by peralkaline ignimbrites and rhyolitic domes, has been recognized among the Tertiary volcanic sequences of central Sonora. Field studies, 40Ar/39Ar geochronology, mineral chemistry and geochemistry (major and trace elements), contribute to establishing the volcanic stratigraphy and the characteristics of these lavas. 40Ar/39Ar dates give very similar ages of ~12 Ma, showing that all the peralkaline rocks from central Sonora belong to the same volcanic episode. Peralkaline rocks have a typical anhydrous mineral association (sanidine+fayalite+Fe-rich, green clinopyroxene), very distinctive from that of the calc-alkaline silicic rocks in the region. These high-silica magmas have relatively high iron, and low alumina contents that classify them as comendites. The REE patterns are characterized by a slight enrichment in LREE, a strong negative anomaly in Eu, and flat HREE pattern. The low concentrations in Ti, P, Sr and Ba are in accordance with the high degree of differentiation of these liquids. The existence of such a middle Miocene peralkaline episode, with a wide geographic extension, constitutes a clear geological marker in the region. The intimate association of these lavas with clastic sedimentary deposits and the eruption, shortly after (~11 Ma), of differentiated tholeiitic magmas (islandites), show that the middle Miocene volcanism was clearly related to extensional tectonism and lithospheric thinning processes which precede the opening of the Gulf of California.
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