Duality of magmatism in the plutonic sequence of the Sumail Nappe, Oman

Abstract

Detailed field mapping of the crustal sequence in several areas of the Sumail Ophiolite Nappe, mainly in the Fizh, the Salahi and the Haylayn blocks, and comparisons with other well-studied areas, have shown that the crustal sequence was constructed by two distinct, although nearly contemporaneous, magmatic series: The first, a gabbroic series, was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit, the high-level isotropic gabbro unit and their by-products, the sheeted dyke complex and the lower extrusives. Fine-grained homogeneous, planar-laminated gabbros commonly occur at the top of the layered gabbro sequence. In some specific areas (Wadi Haymiliyah and Wadi Rajmi), these are strongly differentiated Fe-Ti-rich gabbronorites and norites, which coincide with large-scale discontinuities within the plutonic sequence, interpreted as axial discontinuities of the spreading magma chambers. The second series, an ultramafic wehrlitic series, is locally rooted in the upper mantle (i.e., the impregnated dunitic transition zone). This has intruded the whole crustal sequence up to the pillow lavas. Countless discordant and concordant wehrlitic bodies intruded the plutonic sequence, making up 20–40% of it in volume. We describe the various settings and forms of these intrusions, which were largely underestimated in previous studies. We also describe their mineralogy and textures, and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems, independently from each other. We infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion. At the base of the crust however, indications of immiscibility between gabbros and wehrlites are common, suggesting that both magmatic series were quite contemporaneous. The wehrlites are commonly isotropic and may exhibit an internal layering of the dunite-wehrlite-clinopyroxenite type, sometimes appearing at the upper or lower margins of the intrusions. In this case, the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros. These plagioclase-free ultramafic layered cumulates, therefore, are not interbedded within the gabbro cumulates, but actually belong to the wehrlitic intrusive series. Picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives, which are for the main part cogenetic with the wehrlites. The time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre. We believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust.