Petrography of the Samoan Islands
Tutuila Islands consists of five overlapping shield volcanoes. The lavas of all are predominantly olivine basalts, but picritic basalts, olivine-poor basalts, and olivine-free basalts also are present. Two flows of oligoclase andesite are found in the easternmost (Olomoana) volcano; both andesine and oligoclase andesites are present in the westernmost (Taputapu) volcano. The pre-caldera lavas of Pago Volcano are predominantly olivine basalts, with less abundant olivine-poor basalts, and picritic basalts rich in olivine phenocrysts. The same types are present in the caldera-filling lavas, but the olivine-poor basalts are more abundant; andesites and olivine-free basalts also are present. The post-caldera lavas include olivine basalts, olivine-poor basalts, picritic basalts, andesine andesites, and oligoclase andesites. The picritic basalts differ from those in the earlier groups in containing numerous augite phenocrysts in addition to those of olivine. During the period of eruption of the caldera-filling and post-caldera lavas, there were extruded several masses of quartz trachyte. The last eruptions on Tutuila were of limburgite-basalt. Specimens from Ofu include olivine basalt, basaltic andesite, and picritic basalt with many phenocrysts of augite. Similar olivine basalts and a picritic basalt were collected on Tau. The oldest (Pliocene?) lavas of Upolu are preponderantly olivine basalt, with smaller amounts of olivine-poor basalt, picritic basalt, and basaltic andesite. At least one mass of trachyte is present. The picritic basalts contain phenocrysts of both olivine and augite. With one possible exception, all the Pleistocene lavas of Upolu are olivine basalts. The exception is a specimen of basaltic andesite which may belong with the older lavas. The Recent lava flows also are olivine basalts. The lavas of Savaii are predominantly olivine basalts, but picritic basalts containing augite and olivine phenocrysts also are known. All available chemical analyses of Samoan rocks are listed. The suite is alkalic, near the boundary of the alkali-calcic group, with an alkali-lime index of 50.5. Differentiation was probably largely by crystal settling.