Segmentation at a fossil spreading axis: The plutonic sequence of the Wadi Haymiliyah area (Haylayn Block, Sumail Nappe, Oman)

Abstract Wadi Haymiliyah, in the central part of the Haylayn Block, offers a splendid continuous section across the crustal sequence of the Sumail Ophiolite. The plutonic sequence of this area presents several specific features of great interest for the understanding of the structure and evolution of this ophiolite. The petrological Moho and the whole plutonic sequence display a subvertical attitude, parallel to the mean dyke-complex orientation. High-temperature folding occurred in the mantle sequence and in the lower crust, whereas the upper crust remained unfolded and behaved as a rigid slab during this deformation, resulting in a tectonic decoupling between lower and upper crust. In the gabbro cumulate sequence, a remarkable large-scale curvature of the magmatic structures (layering and lamination planes), whose axis is totally discordant with the regional folds axes, is interpreted as a primary magmatic feature indicating the southern ending of a magma chamber. The lower crust, about 2 km thick, consists of a primary layered gabbroic sequence, consisting of rhythmic mineral-graded layers from several metres to several hundred metres thick. Syndepositional structures such as oblique stratification, microslumps, eroding mini-channels and small normal faults are widespread. The mineralogical composition, modal variations and cryptic evolution of the whole layered sequence are described, both at the megascale and on the simple cyclic scale. The data support a model of crystallization of the layered sequence by open-system fractionation at the floor and sides of a periodically replenished, steady-state magma chamber. A unique feature of this cumulate sequence is the ubiquitous presence of orthopyroxene throughout the whole cumulate pile. This is very unusual in the Sumail crust, with the exception of the Wadi Rajmi section in the Fizh block. Two huge plagioclase-free dunite-wehrlite laccoliths, about 300 m thick, and numerous ultramafic sills and mushroom-like bodies intrude the base of the layered sequence. These ultramafic intrusions exhibit in places an internal layering and dunitic differentiation products. They have sharp, sinuous and lobate contacts with the gabbros, which were still hot and probably not entirely consolidated at the moment of the intrusion. The mineral compositions of these intrusions are much more primitive than those of the adjacent gabbros. Higher in the plutonic section, the layered gabbros are overlain by 1 km of fine-grained, non-layered planar-laminated gabbronorites, characterized by the abundance of pleochroic cumulus hypersthene, the disappearance of olivine, and the development of Fe-Ti oxides and amphibole as post-cumulus phases. Numerous occurrences of concordant, coarse-grained layered gabbros occur in this unit, which is characterized also by a dense net of highly residual dioritic to tonalitic veins, resulting in a huge magmatic breccia in the uppermost few hundred metres where the gabbronorites are brecciated by the acidic rocks. Hydrothermal veins are also extremely abundant in this unit, which coincides with the core of the large-scale curvature. This unit, comprising unusually fractionated rocks, is interpreted as resulting from the evolution of the Haymiliyah magma chamber towards a closed-system fractionation, due to the cutting off of source supply. Our interpretation of both structural and petrological data may be summarized by the following major points: 1. (1) The Haymailiyah magma chamber was located at the southern tip of a spreading cell, and was cut off from the magma supply at the end of its evolution either vertically, when accretion slowed down, or horizontally, when it became isolated from the central part of the spreading segment. We suggest that orthopyroxene-bearing cumulates should be looked for all along the Sumail Nappe and used as markers of segmentation. 2. (2) Early, high-temperature compressional deformation took place in both the mantle and crust before the intraoceanic detachment of the ophiolite. The development of this compression regime activated filter-pressing processes, and in particular helped with the extraction of wehrlitic magmas from the upper mantle, and their ascent to the uppermost levels of the crust.