Petrology of the Ejected Plutonic Blocks of the Soufriere Volcano, St. Vincent, West Indies

The ejected blocks of the Soufrière volcano consist ofthe mineral phases anorthite (An₉₆-An₈₉; average An₉₃), olivine(Fo₇₉-Fo₆₇; most frequent interval Fo_74-71), salite con taining5–6 per cent Al₂, O₃, hastingsitic amphibole, and magnetitecontaining 6 per cent A1₂O₃, 4 per cent MgO, 7 per cent TiO₂.The minerals occur in various proportions and textures. Theyare virtually unzoned and represent material which has beenejected at, and quenched from, a high temperature. The interstitialscoria present among the mineral grains in the blocks has thecomposition of a saturated sub-alkaline aluminous basalt, andis believed to represent the liquid phase with which the mineralswere in equilibrium at depth. The high-temperature mineralogy,the textures and structures of the rocks support the view thatthe blocks represent crystal cumulates which have crystallizedunder high water-vapor pressures from a fractionating basaltmagma at a depth approximating 6 km. The bulk composition ofthe blocks is such that the resultant liquid fractions are enrichedin silica. Fractional crystallization may be an important factorin the evolution of some calc-alkaline suites.