Compositional diversity in peridotites as result of a multi-process history: The Pacific-derived Santa Elena ophiolite, northwest Costa Rica

• The mantle section of the Santa Elena ophiolite is compositionally diverse.
• It is composed of abyssal and SSZ-type mantle peridotites.
• It represents a fragment of Pacific-derived, SSZ oceanic lithosphere.

The Santa Elena ophiolite (SEO) is an ultramafic nappe of more than 270 km2 overlying a tectonic serpentinite–matrix mélange in northwest Costa Rica. It is mainly composed of Cpx-rich and Cpx-poor harzburgites (~ 2.5 km-thick), with minor lherzolite, dunite and chromitite, as well as intrusive mafic sills and subvertical dikes, which coalesce into an upper Isla Negritos gabbroic sill complex. Minerals and whole-rock features of the Cpx-rich and Cpx-poor harzburgites share features of the abyssal and supra-subduction zone (SSZ) peridotites, respectively. To explain these characteristics two-stages of melting and refertilization processes are required. By means of trace element modeling, the composition of Cpx-rich harzburgites may be reproduced by up to ~ 5–10% melting of a primitive mantle source, and the composition of Cpx-poor harzburgites and dunites by ~ 15–18% melting of an already depleted mantle. Therefore, the Cpx-rich harzburgites can be interpreted as product of first-stage melting and low-degrees of melt–rock interaction in a mid-ocean ridge environment, and the Cpx-poor harzburgites and dunites as the product of second-stage melting and refertilization in a SSZ setting. The mafic sills and the Isla Negrito gabbros are genetically related and can be explained as crystallization from the liquids that were extracted from the lower SSZ mantle levels and emplaced at shallow conditions. The Murciélagos Island basalts are not directly related to the ultramafic and mafic rocks of the SEO. Their E-MORB-like composition is similar to most of the CLIP mafic lavas and suggests a common Caribbean plume-related source. The SEO represents a fragment of Pacific-derived, SSZ oceanic lithosphere emplaced onto the southern North America margin during the late Cretaceous. Because of the predominance of rollback-induced extension during its history, only a limited amount of crustal rocks were formed and preserved in the SEO.

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