Relationships between geochemistry and structure beneath a palaeo-spreading centre: a study of the mantle section in the Oman ophiolite

The Oman ophiolite exposes a large and well-preserved mantle section beneat h a palaeo-spreading centre. The mantle section is mainly composed of extre mely refractory harzburgites with relatively homogeneous modal and major el ement compositions. Nevertheless, our trace element data exhibit variations connected with the main mantle structures, which allow us to define three geochemical and structural domains. The main harzburgitic mantle section, m ainly constituted of strongly refractory harzburgites characterised by chon drite-normalised REE patterns that are steadily depleted from HREE to LREE. These rocks are interpreted as mantle residues after >15% melt extraction. Their REE signature can be explained by melt transport associated with par tial melting. The diapir areas (mainly the Maqsad diapir), defined by plung ing lineations. They are constituted of harzburgites with roughly the same modal composition as the main mantle section but distinct, concave-upward R EE patterns. The regions of most active upwelling (characterised by sub-ver tical lineations) are further distinguished by higher Al2O3/CaO ratios and TiO2 contents; This character is ascribed to focused partial melt upwelling . The diapirs are interpreted as local instabilities in upwelling mantle, p ossibly triggered by feedback mechanisms between deformation and melt perco lation. The Maqsad diapir is topped by a thick, dunitic, mantle-crust trans ition zone (MTZ) that displays the same trace-element signature as the diap ir. However, the dunites are distinguished by low Mg# values and Ni content s. Together with structural evidence, this allows us to interpret the MTZ d unites as diapir harzburgites that were strongly modified by olivine-formin g melt-rock reactions at high melt/rock ratios. The MTZ is thought to act a s a major collecting zone for mantle melts. The cpx-harzburgites from the b ase of the mantle section. These rocks are distinguished by high clinopyrox ene contents (>5%), low AL(2)O(3)/CaO and 'spoon-shaped' REE patterns. They were individualised from the rest of the harzburgite mantle section by a c px-forming melt-rock reaction at decreasing malt mass. This reaction probab ly occurred at near-solidus conditions along the lithosphere-asthenosphere boundary. The formation of these three domains may be integrated in a geody namic scenario involving the reactivation of an oceanic lithosphere, a proc ess that would be related to the ridge propagator identified in the Oman op hiolite. (C) 2000 Elsevier Science B.V. All rights reserved.