The Lesvos mafic–ultramafic complex, Greece: Ophiolite or incipient rift?

Abstract

On the island of Lesvos in the north-eastern Aegean Sea, thrust sheets of mantle peridotite override a tectonic mélange containing metasediments, metabasalts and a few metagabbros. Previous workers have collectively called the above rock types the “Lesvos ophiolite” and suggested it represents Neotethyan (Jurassic) oceanic crust of N-MORB and IAT affinities. However, the complete absence of a sheeted dyke complex and of a layered cumulate sequence, in turn overlain by isotropic gabbros does not support the ophiolite description. The basalts of the mélange define two distinct groups on the basis of Nb/Y values: the Vatera (low Nb/Y) and the Mélange (high Nb/Y) groups. The Vatera basalts show multi-element profiles and REE patterns lying well within the field of N-MORB with the exception of Y and HREE abundances that are lower than those of N-MORB and match those of E-MORB. They originated by ∼ 10% melting of depleted MORB mantle that carried a small amount (∼ 3%) of an enriched (OIB-like) component, corroborated by moderate initial εNd values (+ 4.6–+ 6.2). The Mélange basalts, in contrast, have multi-element profiles and REE patterns typical of within-plate compositions, transitional between E-MORB and OIB. They originated by < 5% melting of spinel (and occasionally garnet) lherzolite that carried a small amount (∼ 3%) of recycled crustal component, as supported by low initial εNd values (+ 3.8–+ 5.1). The Lesvos peridotites comprise both spinel lherzolites and harzburgites. Using mineral compositions and calculations of temperature and oxygen fugacity, the lherzolites plot in the fields of alpine-type and continental peridotites. Trace element modelling suggests they have experienced anhydrous melting in the stability field of spinel but also garnet lherzolite; calculated degrees of melting are generally small and never exceed 14%. There is no evidence for refertilisation or hydrous melting in a supra-subduction zone environment for the lherzolites. The harzburgites, despite being considerably more altered than the lherzolites, also display evidence of anhydrous melting in the stability field of spinel. However, they also display spoon- and dish-shaped REE patterns that can be produced by contamination of the harzburgites by a percolating OIB-like component with an inherent recycled sedimentary component, which is necessary to explain their isotopic shift towards EM2 compositions. SHRIMP dating of gabbro zircons revealed an Upper Permian (253 Ma) crystallisation age and a Middle Triassic (236 Ma) resetting of the zircon rims, compatible with the coeval and widespread rift-related intrusion of A-type granites in the eastern Mediterranean. Gabbro zircon ages of 777, 539 and 338 Ma strongly suggest inheritance from the basement into which the gabbros were intruded and correspond to ages of distinct terranes recently recognized in the Hellenides. We propose that the Lesvos mélange and ultramafic rocks best represent an incipient continental rift setting of Triassic age, similar to what has been described for the Italian Ligurides, that led to the subsequent formation of a branch of Neotethys, and that the term “Lesvos ophiolite” should be used with due caution or abandoned altogether.