The upper zone of the Ivrea-Verbano layered complex (Italian Western Alps)

The Ivrea-Verbano Complex is generally considered as a deep seated multiple intrusion. A layered series occurs at the base of the Complcx, which has been subdivided in a basal, an intermediate and an upper zone (BZ. IZ andUZ). Thc three zones, possibly, derive by fractionation from a unique magma.The upper zone is mainly constituted by a gabbroic layered series, dominated in the lower part by melanocratic terms and in the upper part by leucocratic ones, up to anorthosite. Olivine gabbros occur al various levels in this series. Texture is mainly cumulitic and it becomes coronitic by subsolidus reactions.The fractionation pattern presents plagioclase, olivine and pyroxenes as phases of the liquidus and is similar to that of the pure system MgO-Al2O3-CaO-SiO2 at low pressure. Formation of coronitic textures is determined by instability of mafic minerals in presence of plagioclase. This instability is controlled by the composition of the reacting phase and is attributed to re-equilibration by slow cooling and not to a metamorphic event.