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 trachyte eruption s of Agua de Pao volcano, Sao Miguel, Azores, represent bulk liquid co mpositions resembling the more evolved Fogo A trachytes. Alteration du e to the influx of a hydrous fluid has depleted the nodules in P2O5, R b, U, Ba, and Sr, and increased the SiO2 content. The nodules can be b roadly divided into three textural groups: (I) fresh, friable syenite with open miarolitic cavities and homogeneous sanidine; (III) dense sy enite with exsolved alkali feldspar and no void space; (II) intermedia te between (I) and (III), with small amounts of void space. U-series m odel ages range from < 10000 years to greater-than-or-equal-to years, suggesting that trachytic magmatism has occurred in Agua de Pao volcan o for > 200 ka. The youngest nodules are from the textural group I and are cogenetic with the Fogo A trachytes, whereas those from groups II and III are xenoliths and possibly correlate with older volcanic even ts. A strong correlation of age with texture shows that with increasin g age the nodules become cooler, lose their void space, and undergo gr anulation and recrystallization. Considerations of the fluid dynamic r egimes in the magma chamber during crystallization of the nodules sugg est that the bulk liquid composition nodules formed at the roof and/or upper walls of the magma chamber, where crystallization rates exceed the convection rate of residual liquid. We envision an onion-skin arra ngement, with older syenite on the outside and young, cogenetic syenit e on the inside in contact with the magma. Long cooling histories, con tinuous flux with hydrous fluids, and successive explosive eruptions c ause the older syenite to evolve in composition and texture. Fragmenta tion of the roof and upper sidewalls during eruption can explain the o ccurrence of a range of nodule ages.