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 homogeneity, low Al2O3 and high FeO contents, a tholeitic differentiation trend, and elevated, large-ion-lithophile (LIL)-element concentrations (e.g., Ba approximately 800 ppm). On CMAS projections their compositions always fall on or near low-pressure cotectics. In addition, the basalts of Masaya have unusually high Sr-87/Sr-86 and Be-10. Masaya has exhibited medium-term compositional cycles, best exhibited by the sawtoothed changes in TiO2 and FeO*/MgO. There are also a number of longer-term compositional changes which are abrupt and generally coincide with caldera formation. Many of the geochemical characteristics of Masaya, coupled with a number of volcanological observations, indicate Masaya is underlain by a large, shallow, open-system magma chamber, perhaps on the order of 10 km3 in size. Although fractional crystallization is a significant magmatic process in Masaya's open-system chamber, magma mixing/contamination is equally important. Magma mixing is necessary to explain the discontinuous stratigraphic changes in magma composition observed at Masaya, and crustal contamination is necessary to explain their generally elevated Sr-87/Sr-86 and LIL-element concentrations. Two components, therefore, have been admixed into the magma chamber of Masaya: a LIL-poor basaltic component such has been erupted from the nearby Nejapa and Granada cinder cones; and a LIL-rich acidic component such has been erupted from the nearby calderas of Apoyo and Apoyeque. Admixtures of the former have dominated. Admixtures of the latter ended with caldera formation. Ironically, open-system behavior has exerted fundamental control on the maintenance of relative compositional homogeneity.