The explosive-effusive transition within the Fataga suite, Gran Canaria

Most volcanoes, across tectonic settings, can show both explosive and effusive eruptions, either as separated eruptive events or within the same eruptive episode. Such differences in eruptive style have significant implications for depositional morphology and hazards associated with the eruptions. In many cases, chemically nearly identical magmas may produce either explosive or effusive events. In contrast, we find the intercalated ignimbrites and lavas of trachytic to phonolitic compositions of the Fataga Group, Gran Canaria, differ markedly in several aspects. First, field observations revealed that the lavas pinch out toward the caldera rim and are therefore likely originated from extra-caldera sources. Second, while the explosive deposits vary in crystallinity (here referring to crystals with a long axis > 250 µm) from a few percent in crystal-poor portions to 10 – 20% in crystal-rich clasts, the lavas are almost aphyric with crystallinities < 1%. Further, while the ignimbrites have a mineral assemblage containing alkali-feldspar, biotite, pyroxene, amphibole, titanite, and Fe-Ti oxides, the lavas mostly contain alkali feldspar as a nearly unique mineral phase. Third, major elemental compositions show that while ignimbrites and lavas may overlap within the trachyte field on a TAS plot, only the lavas have compositions that extend to phonolite. Trace elemental compositions of lavas suggest extensive fractionation with compatible elements (e.g. Ba, max 227 ppm, avg. 30 ppm) depleted and incompatibles (e.g. Zr, Hf, Ce, Rb) enriched. While crystal-poor juveniles from the ignimbrites may have compositions approaching those of the lavas, crystal-rich juvenile clasts are markedly enriched in feldspar-phyric elements (e.g. Ba, max 1892 ppm, avg. 1496 ppm) suggesting involvement with a feldspar-dominated cumulate pile. Furthermore, CIPW norm calculations show that the lavas trend towards nepheline-normative compositions. The normative prediction of nepheline occurrence in the lavas is confirmed by petrographic observation of nepheline both as groundmass constituent and rarely as phenocrysts and thus suggests a somewhat different petrogenetic history for the lavas. Fourth, on a crystal scale, the feldspar phenocrysts in the lavas have relatively restricted, and low Ba contents (20 - 500 ppm), while ignimbrites have extremely Ba-enriched feldspars (up to 18,000 ppm Ba) in their crystal-rich portions.

Deposit geometry, petrography, and geochemical data lead us to the conclusion that the Fataga lavas must have followed a different petrogenetic path than their explosive counterparts. We, therefore, suggest the magmas feeding the effusive eruptions may have bypassed the main caldera system and thus provide a different window into the Miocene magmatism of Gran Canaria.