Petrology

Meyer-Abich (1956) describes several lavas from Boquerón as olivine basalts and one lava from the western slope of Picacho as a porphyritic andesite, rich in plagioclase phenocrysts. Fairbrothers et al.(1978) did an extensive study of Boquerón lavas but did not consider lavas from the old edifice, identified by Picacho and Jabali.

Fairbrothers et al. (1978) describes Boquerón lavas as "grey to black, porphyritic, pilotaxitic basalts, basaltic andesites, and andesites that have phenocrysts of plagioclase, augite, olivine and titaniferous magnetite. Orthopyroxene is present in some samples; amphibole is absent. The groundmass consists of plagioclase, augite, magnetite and glass in varying proportions. Many samples are moderately vesicular."

Chemical analysis of lavas from the walls of the Boquerón crater and from historical lava flows on the northwest flank of the volcano were done by Fairbrothers et al.(1978) and were classified as calc-alkaline basalts and andesites to tholeiitic andesites.

Cox-Bell-Pank (1979) diagram showing the rock types for several samples from Boqueron.

Temporal variation in the chemical composition of the Boquerón lavas shows three different components. First, there is a cyclical alternation of andesites and basalts. Second, these cycles show gradual increase in the silica content, and finally, in the middle of the Boquerón crater stratigraphic section there is an abrupt change from Al-rich/Fe-poor lavas to Fe-rich/Al-poor lavas.

Processes which can explain these results are: an increased proportion of plagioclase fractionation with respect to the proportion of augite fractionation. The cyclical changes in silica content can be best explained by a combination of crystal fractionation, which increases silica content, followed by influx of basaltic magma that mixes with residual magma, driving the silica content down.