Poas Volcano, Costa Rica: Geology of the Summit Region and Spatial and Temporal Variations among the Most Recent Lavas

Prosser, J.T. and Carr, M.J., 1987. Po~s volcano, Costa Rica: Geology of the summit region and spatial and temporal variations among the most recent lavas. In: S.N. Williams and M.J. Carr (Editors), Richard E. Stoiber 75th Birthday Volume. J. Volcanol. Geotherm. Res., 33: 131-146. The recent eruptive history of Po~s volcano is described here on the basis of field mapping of the summit area. This period, which may represent 10 to 20,000 years and less than 10% of the history of the volcano, began with caldera formation. Subsequent events included: composite cone construction, faulting and flank subsidence, flank fissure eruption, and multiple crater collapses. Local and regional structures have controlled the locations of summit vents and flank cinder cones. Chemical compositions of lavas of known stratigraphic position demonstrate temporal magmatic variation at Po~s. Three similar felsic to mafic magmatic sequences occurred at the summit. The first and second sequences were separated by flank and summit eruptions of a distinct magma batch, enriched in TiO., and P205. The three felsic to mafic sequences appear to be cyclical, and the repeated, similar variations are interpreted as progressive tapping of zoned magma bodies developed repeatedly from a common parent by similar crystal fractionation processes. The breaks between the three sequences correlate with shifts or major modifications of the eruptive center. The chemical composition of the most recent lava, erupted in 1954, indicates that the volcano is near the end of the present sequence. The lavas are calc-alkaline basalts and andesites that are similar to the mafic lavas that comprise the bulk of the Central American volcanic front. The samples can be divided into 3 spatial-temporal-geochemical groups. A summit group that represents the most recent activity; a TiO2-rich group that erupted primarily on the south flank about 7500 yr. B.P.; and an Al203-rich group that erupted primarily from vents north of the presently active Main Crater. Geochemical variations and projections into pseudoternary CMAS diagrams suggest a moderate pressure for the summit group, that is, a magma chamber at intracrustal to subcrustal depths. The Al203-rich group define smaller primary phase volumes for olivine and plagioclase, which suggests a deeper and or more water-rich magma chamber, The TiO2rich group appears to be a batch of mixed magma with the summit group magma as one endmember and a less siliceous magma as the other. I n t r o d u c t i o n t o v o l c a n o , o n e o f 41 h i s t o r i c a l l y a c t i v e v o l c a n oes in t h e C e n t r a l A m e r i c a n v o l c a n i c c h a i n P o d s V o l c a n o is a s t r u c t u r a l l y c o m p l e x s t r a w h i c h e x t e n d s 1100 k m f r o m G u a t e m a l a to *Present address: TexasgulfMineralsand Metals, P.O. Box C o s t a R i c a . V o l c a n i s m in t h i s c h a i n is a s s o c i 3810, Chapel Hill, NC 27515, U.S.A. a t e d w i t h s u b d u c t i o n o f t h e C o c o s P l a t e n o r t h 0377-0273/87/$03.50 © 1987 Elsevier Science Publishers B.V.