Cryptic variation within the Cumulate Sequence of the Oman ophiolite: magma chamber depth and petrological implications

Abstract

Summary The Cumulate Sequence of the Oman Ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic, long-lived, crustal magma chamber that underlay a late-Cretaceous spreading ridge. Regional variations in Cumulate-Sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions. Microprobe data are presented from a 600 m section of modally layered olivine gabbros. Coherent cryptic cyclic variation is shown, with both ‘normal’ and ‘reversed’ geochemical gradients present; these are explained in terms of fractionation, mixing and eruption in an open-system magma chamber. The height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m; the implications of this result are discussed. A contrast between the Fo-An co-variation between olivine and plagioclase in the Oman Cumulate Sequence and that observed for MORB volcanics is noted; this may reflect compositional stratification in spreading-ridge magma chambers or significant differences between the petrogenesis of the Oman ophiolite and MORB.