Mesozoic formation of pyroxenites and gabbros in the Ronda area (southern Spain), followed by Early Miocene subduction metamorphism and emplacement into the middle crust: U–Pb sensitive high-resolution ion microprobe dating of zircon

Abstract

A combination of cathodoluminescence studies and U–Pb sensitive high-resolution ion microprobe dating of distinct magmatic and metamorphic zircon domains constitutes a powerful tool to decipher the geological history of complex metamorphic terranes. This combination of techniques has been applied to the internal zone of the Betic chain (southern Spain), where several orogenic lherzolite massifs — the Ronda peridotites — occur as a slice of mantle rocks, containing ∼5% of pyroxenite layers. The Ronda peridotites are emplaced within metamorphic country rocks that consist mainly of granulites and eclogite-bearing migmatites. In order to determine both the protolith ages of the pyroxenites and the age of the high-pressure (HP) metamorphism in the country rocks, mafic layers within the peridotites and eclogite relics within the country rocks have been studied. Anatectic granites (formed by partial melting of the metasedimentary country rocks during hot emplacement of the peridotites) were also studied, in order to determine the age of tectonic emplacement of the ultramafic rocks into the continental crust. Early Jurassic and Late Jurassic–Early Cretaceous protolith ages have been obtained from magmatic oscillatory-zoned domains of the zircons separated from three HP/ultra-HP garnet pyroxenites (178±6 Ma, 143±16 Ma and 131±3 Ma). Magmatic zircons of an eclogite yielded an Early Jurassic age (183±3 Ma). These ages are interpreted to reflect extensional/transtensional stages in the area of the Alboran Sea, related to Pangea's break-up and the opening of the Atlantic Ocean. HP/high-temperature (HT) metamorphism has been recorded in unzoned rims of the zircons of the eclogite at 19.9±1.7 Ma, which is interpreted to represent the T-peak of the metamorphism (790±15°C), immediately following the P-peak (17±1 kbar). Thus, Miocene subduction-zone metamorphism could be deduced geochronologically for the first time in the Betic chain. Crystallisation of anatectic granites crosscutting both peridotites and country rocks has been dated at 18.8±4.9 Ma. Our data suggest very high exhumation (>31 mm/y) and cooling rates (200–340°C/Ma) for the area of Ronda.