The Patrimonial Value of the Betic Ophiolites: Rocks from the Jurassic Ocean Floor of the Tethys

Abstract

The Betic ophiolites consist of numerous tectonic slices of eclogitized mafic and ultramafic rocks together with oceanic metasediments. They form a tectonic Unit of the Mulhacén Complex in the Betic Cordillera (SE Spain), intercalated between two crustal units named Caldera (below) and Sabinas (above). A comprehensive review of the petrological, geochemical and geochronological characteristics of the Betic ophiolites shows notable similarities with other MORB-type ophiolites from the Alps–Apennines system and the different lithologies forming the Atlantic Ridge. This suggests that they represent relics of an oceanic lithosphere deriving from the Western Tethys. Consistent magmatic ages of around 185 Ma were determined by U–Pb dating with SHRIMP (sensitive high resolution ion micro probe) of zircon grains selected by cathodoluminescence images of several eclogitized metagabbros from the main outcrops (Lugros, Cobdar and Algarrobo) and a metadolerite dyke intruded in ultramafic rocks (Almirez outcrop). This age marks the beginning of Betic oceanic magmatism, with T-MORB to N-MORB (Transitional to Normal Mid Oceanic Ridge Basalt) geochemical affinities, corresponding to ultra-slow mid-ocean ridge spreading from the Lower Jurassic to the Cretaceous, following the Pangaea break-up between the Iberian and African plates. The Betic oceanic-floor segment represents the earliest oceanic accretion process in the Western Tethys Ocean. Subsequently, this ocean floor continued to develop from the Betic oceanic basin northeastward to the Ligurian and Alpine Tethys domains, mainly from 165 to 140 Ma, and to the western rift zone up to contact with the Central Atlantic Ocean. According to radiometric datings of the Betic Ophiolites and palaeogeographic reconstructions for the Pliensbachian, these ophiolites are the only preserved relics of the westernmost end of the former Mesozoic Tethys Ocean. They are, therefore, of considerable scientific value for their potential in the palaeogeographic, petrogenetic and geodynamic reconstructions of the Betic Cordillera. Moreover, because of their big hardness and density some outcrops of these eclogitized ophiolites (Caniles de Baza) have been quarried from prehistoric times in the making of hammers and axes, which can be found in many human settlements in the area. Consequently, the Betic Ophiolites have a number of extremely interesting scientific, educational and cultural characteristics giving their outcrops a great patrimonial value, despite which most of their outcrops have been partly destroyed, or are at risk of destruction for use as ballast in industry.