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Department of Earth Sciences, University of Waikato, Private Bag, Hamilton, New Zealand
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
Department of Geology, University of Texas at Arlington, UTA Box No 19049, Arlington, TX 76019, USA
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Surrey TW20 0EX, UK
The island of Sao Miguel in the Azores exhibits bimodal volcanism: eruptives from Agua de Pau volcano and the ‘Waist’ consist almost entirely of basalt or trachyte with rare intermediate products. Although some intermediate lavas are fractionates from basalt, the majority are mixed-magma hybrids anomalously enriched in europium, barium and potassium. These features clearly indicate that they have also accumulated alkali feldspar. The source of the alkali feldspar contaminant must be the least evolved trachyte in the Agua de Pau system, because barium, europium and strontium are rapidly depleted in liquids and crystals with differentiation to more evolved trachytes. Many less evolved trachytes are also accumulative in alkali feldspar, suggesting a genetic association with the hybrid lavas. Geochemical modelling indicates that the hybrid lavas are the result of contamination of basalt by accumulative trachyte containing up to 75% alkali feldspar.
Field and petrological evidence points to the episodic injection and ponding of basalt beneath trachytic magma at Agua de Pau volcano. One trachytic lava, with alkali feldspar-contaminated basaltic inclusions, represents a sample of this interface. Density calculations indicate that highly accumulative trachyte can have a neutral or negative buoyancy with respect to basalt and could descend through the interface. Resorption of alkali feldspar will lower the bulk density of the contaminated basalt, with the effect of favouring even greater contamination by overlying accumulative trachyte. The most attractive mechanism for achieving the accumulative trachyte compositions is the slumping of side-wall cumulates and the formation of magmatic density currents which flowed along the compositional interface between trachyte and basalt. Flowage differentiation and crystal settling would result in local ‘parcels’ of denser highly accumulative trachytic magma. The process is illustrated with results from a simple laboratory experiment.
