Abstract
Four volcanoes form Terceira, one of the islands of the Azores group; three contain both basaltic and peralkaline and one only peralkaline rocks. A recently active basaltic fissure zone trends NW-SE across the island.
The rocks fall into the alkaline olivine basalt suite although some young basalts are of transitional affinity. The geochemistry shows two general basaltic series: 1) undersaturated, found in lavas of the oldest volcano and in some recent fissure zone basalts and hawaiites; 2) saturated, found in the younger basaltic lavas.
Since the emergence of Terceira there has been a contemporaneity of basalt and salic peralkaline lavas. The younger rocks show a bimodal composition distribution, the most voluminous compositions being alkali olivine basalt and comendite with negligible volume in the benmoreite-trachyte range. Two processes appear viable for the derivation of voluminous oversaturated peralkaline rocks: 1) partial melting of upper mantle material giving small magma batches of contrasting composition or 2) fractionation from a transitional basaltic parental magma.
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Now at Department of Geology, Victoria University of Wellington, New Zealand.
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Self, S., Gunn, B.M. Petrology, volume and age relations of alkaline and saturated peralkaline volcanics from Terceira, Azores. Contrib. Mineral. Petrol. 54, 293–313 (1976). https://doi.org/10.1007/BF00389409
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DOI: https://doi.org/10.1007/BF00389409