Comparative volcanology and petrology of the atlantic island-arcs

Abstract

Comparisons are made between the Lesser Antilles and the South Sandwich Islands, the recent volcanic island chains at the eastern margins of the Caribbean and Scotia arcs. Although situated in similar geological and structural environments there are differences in the type of volcanic activity which prevails in these two arcs and in the petrography and chemistry of the lavas emitted. There is good evidence that the South Sandwich Islands are in general appreciably younger than the islands of the Lesser Antilles. Basaltic rocks predominate in the South Sandwich Islands whereas andesite is the dominant rock-type of the Lesser Antilles. Many of the lavas of the South Sandwich Islands, including the andesites and dacites are aphyric whereas those of the Lesser Antilles are almost invariably porphyritic. The basalts of the South Sandwich Islands are of tholeiitic type and the series shows more pronounced iron enrichment than does that of the Lesser Antilles. Basalts of the South Sandwich Islands have a lower Fe₂O₃/FeO ratio, contain lower concentrations of K, Sr and Ba and higher Cr, Co and Ni than the basalts of the West Indies. It is thought that the South Sandwich Islands may represent a volcanic island-arc in the early stages of development and the Lesser Antilles a later stage.