ENRICHED MANTLE SOURCE AND ADAKITE LAVAS AT TURRIALBA VOLCANO, SOUTHERN COSTA RICA

Much previous work reveals a change along the Central American Volcanic Arc (CAVA) from a depleted mantle source beneath Nicaragua to an enriched mantle source farther south beneath southern Costa Rica. In addition, volcanoes in Nicaragua have the strongest geochemical signature of slab flux (high Ba/La) whereas volcanoes in southern Costa Rica show less slab influence (low Ba/La). This study is focused on Turrialba volcano in southern Costa Rica and provides details on magma source variations within a single Quaternary volcano at the southeastern end of the CAVA. Turrialba lavas include a basalt-basaltic andesite-dacite suite with adakites present at different stratigraphic levels of the volcano. The basalts exhibit REE patterns similar to ocean island basalts with La/Yb of 17 to 20, ~100 times chondrite, indicating an enriched mantle source. The non-adakite intermediate and felsic rocks have relatively low ratios of Ba/La (17.7-26.3) and Pb/Ce (0.05-0.08) and show a progressive enrichment of La/Yb from the basalts through the dacites. These data are consistent with the regional trends along the CAVA and also indicate magma evolution by AFC processes. The adakites exhibit high Sr concentrations of 877 to 1160 ppm, Sr/Y ranging from 53.57 to 127.47, and have the highest ratios of Ba/La (22-53) and Pb/Ce (0.07-0.23) among the Turrialba samples. These Ba/La ratios are higher than was previously known for this portion of the CAVA indicating a larger slab flux than would be predicted by the regional CAVA trends. The adakites were not co-magmatic with the enriched mafic magmas but represent a separate magmatic phase. Adakite genesis is most likely related to partial melting of the young, buoyant Cocos Ridge that is currently being subducted beneath southern Costa Rica and northern Panama. The Turrialba adakites may therefore represent a continuum of adakite magmatism that began during Plio-Pleistocene time above the subducted Cocos Ridge in the adjacent Cordillera de Talamanca region. These results require a model to explain the co-existence of adakites and magmas derived from an enriched mantle source.