SHALLOW OPEN-SYSTEM EVOLUTION OF BASALTIC MAGMA BENEATH A SUBDUCTION ZONE VOLCANO - THE MASAYA CALDERA COMPLEX, NICARAGUA

Lavas and pyroclastic material from the Masaya Caldera Complex have a number of distinctive geochemical features: relative compositional hom ogeneity, low Al2O3 and high FeO contents, a tholeitic differentiation trend, and elevated, large-ion-lithophile (LIL)-element concentration s (e.g., Ba approximately 800 ppm). On CMAS projections their composit ions always fall on or near low-pressure cotectics. In addition, the b asalts of Masaya have unusually high Sr-87/Sr-86 and Be-10. Masaya has exhibited medium-term compositional cycles, best exhibited by the saw toothed changes in TiO2 and FeO/MgO. There are also a number of longe r-term compositional changes which are abrupt and generally coincide w ith caldera formation. Many of the geochemical characteristics of Masa ya, coupled with a number of volcanological observations, indicate Mas aya is underlain by a large, shallow, open-system magma chamber, perha ps on the order of 10 km3 in size. Although fractional crystallization is a significant magmatic process in Masaya's open-system chamber, ma gma mixing/contamination is equally important. Magma mixing is necessa ry to explain the discontinuous stratigraphic changes in magma composi tion observed at Masaya, and crustal contamination is necessary to exp lain their generally elevated Sr-87/Sr-86 and LIL-element concentratio ns. Two components, therefore, have been admixed into the magma chambe r of Masaya: a LIL-poor basaltic component such has been erupted from the nearby Nejapa and Granada cinder cones; and a LIL-rich acidic comp onent such has been erupted from the nearby calderas of Apoyo and Apoy eque. Admixtures of the former have dominated. Admixtures of the latte r ended with caldera formation. Ironically, open-system behavior has e xerted fundamental control on the maintenance of relative compositiona l homogeneity.