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

Abstract

The Oman ophiolite exposes a large and well-preserved mantle section beneath a palaeo-spreading centre. The mantle section is mainly composed of extremely refractory harzburgites with relatively homogeneous modal and major element 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, mainly constituted of strongly refractory harzburgites characterised by chondrite-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 partial melting. The diapir areas (mainly the Maqsad diapir), defined by plunging lineations. They are constituted of harzburgites with roughly the same modal composition as the main mantle section but distinct, concave-upward REE patterns. The regions of most active upwelling (characterised by sub-vertical lineations) are further distinguished by higher Al 2O 3/CaO ratios and TiO 2 contents. This character is ascribed to focused partial melt upwelling. The diapirs are interpreted as local instabilities in upwelling mantle, possibly triggered by feedback mechanisms between deformation and melt percolation. The Maqsad diapir is topped by a thick, dunitic, mantle–crust transition zone (MTZ) that displays the same trace-element signature as the diapir. However, the dunites are distinguished by low Mg# values and Ni contents. Together with structural evidence, this allows us to interpret the MTZ dunites as diapir harzburgites that were strongly modified by olivine-forming melt–rock reactions at high melt/rock ratios. The MTZ is thought to act as a major collecting zone for mantle melts. The cpx-harzburgites from the base of the mantle section. These rocks are distinguished by high clinopyroxene contents (>5%), low AL 2O 3/CaO and ‘spoon-shaped’ REE patterns. They were individualised from the rest of the harzburgite mantle section by a cpx-forming melt–rock reaction at decreasing malt mass. This reaction probably occurred at near-solidus conditions along the lithosphere–asthenosphere boundary. The formation of these three domains may be integrated in a geodynamic scenario involving the reactivation of an oceanic lithosphere, a process that would be related to the ridge propagator identified in the Oman ophiolite.