Syenite Nodules as a Long-Term Record of Magmatic Activity in Agua de Pao Volcano, Sao Miguel, Azores

Syenite nodules ejected during the Fogo A and Fogo C trachyteeruptions of Agua de Pao volcano, Sao Miguel, Azores, representbulk liquid compositions resembling the more evolved Fogo Atrachytes. Alteration due to the influx of a hydrous fluid hasdepleted the nodules in P₂O₅, Rb, U, Ba, and Sr, and increasedthe SiO₂ content. The nodules can be broadly divided into threetextural groups: (I) fresh, friable syenite with open miaroliticcavities and homogeneous sanidine; (III) dense syenite withexsolved alkali feldspar and no void space; (II) intermediatebetween (I) and (III), with small amounts of void space. U-seriesmodel ages range from <10000 years to 200000 years, suggestingthat trachytic magmatism has occurred in Agua de Pao volcanofor >200 ka. The youngest nodules are from the textural groupI and are cogenetic with the Fogo A trachytes, whereas thosefrom groups II and III are xenoliths and possibly correlatewith older volcanic events. A strong correlation of age withtexture shows that with increasing age the nodules become cooler,lose their void space, and undergo granulation and recrystallization.Considerations of the fluid dynamic regimes in the magma chamberduring crystallization of the nodules suggest that the bulkliquid composition nodules formed at the roof and/or upper wallsof the magma chamber, where crystallization rates exceed theconvection rate of residual liquid. We envision an onion-skinarrangement, with older syenite on the outside and young, cogeneticsyenite on the inside in contact with the magma. Long coolinghistories, continuous flux with hydrous fluids, and successiveexplosive eruptions cause the older syenite to evolve in compositionand texture. Fragmentation of the roof and upper sidewalls duringeruption can explain the occurrence of a range of nodule ages. ^* Present address: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road N.W., Washington, DC, 20015