Mixing characteristics and compositional differences in mantle‐derived melts beneath spreading axes: Evidence from cyclically layered rocks in the ophiolite of North Oman

Abstract

Within the ophiolite of north Oman, interlayered gabbros and peridotites form a unit up to 3.5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites. Spatial and chemical relationships within this sequence require the existence of a large reservoir (20 km wide, 3.5 km high) of tholeiitic magma that was periodically replenished by mantle‐derived picritic to tholeiitic melts during the spreading process. Cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks. Within these cyclic units, basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases. This abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs. By virtue of its greater density than tholeiitic magma (Sparks et al., 1979) any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid‐liquid interface. Cooling of this basal layer by contact with the cooler, overlying magma would result in the precipitation of olivine + chrome spinel ± pyroxene from the picritic liquid. This process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir. The precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle. Variable thicknesses of the cyclic units, different liquid lines of descent, and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume, primary composition, and the extent and nature of prefractionation in the mantle. Liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm. The existence of a large magma reservoir supports earlier interpretations based on age (Tilton et al., 1981) and geochemical data (Pearce, 1980) that the Oman ophiolite is an analogue for fast spreading ridges.