The petrology of Poás volcano lavas: basalt-andesite relationship and their petrogenesis within the magmatic arc of Costa Rica

Abstract

Basalt, andesite and dacite from Poás volcano exhibit a distinct petrogenetic evolution. The basalts (Al 2O 3 = 16–17.4%, K 2 O = 1.43–1.61%) are hypersthene-normative tholeiites with phenocrysts of labradorite, olivine, and two pyroxenes. Orthopyroxene (En 67-56) is generally rimmed by clinopyroxene (Fs 10–18 Wo 39–48). In all basalts and in a few andesites, pyroxene phenocrysts have Al- and Ti-rich salite rims that surround cores of orthopyroxene or augite having lower values. The Al z values (% of tetrahedral site occupied by Al) for these augites are above 6%, thus defining on the Al z vs. TiO 2 diagram an intermediate trend between arc-related cumulates and rift-related tholeiites and ophiolite-layered cumulates. Moreover, the presence of olivines with coronae of wormy intergrowths of orthopyroxene and Ti-magnetite (similar to those occurring in metagabbros of the granulite facies and characteristic of Arenal gabbroic xenoliths) together with anorthite megacrysts (An 94-91) suggest a possible contamination origin for these basalts. Sr-isotope ratios for the basalts are between 0.703816 and 0.703849 whereas andesites and dacites have lower ratios (0.703676–0.703715): these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust. Pearce element-ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase.