Early Miocene high-temperature metamorphism and rapid exhumation in the Betic Cordillera (Spain): evidence from U–Pb zircon ages

Well constrained concordant ion-microprobe U–Pb ages of 21.2±0.7 Ma, 21.1±1.4 Ma, and 19.3±0.3 Ma have been obtained from complex polyphase zircons separated from a high-P garnet granulite, a cordierite granite, and a sillimanite K-feldspar gneiss, respectively, from three separate locations in the Betic Cordillera of southern Spain. These ages are supported by three discordant 206Pb/238U ages in the range 19–24 Ma from the outermost growth zones of zircons from a high-grade schist, a leucogranite, and a migmatitic gneiss: these discordant ages can confidently be interpreted as maxima. The six sample sites cover a 240 km strike length along the Cordillera. Taken together with published early Miocene Ar–Ar ages on metamorphic micas from the Betic Cordillera and from ODP Site 976 in the basement of the adjacent Alboran Sea, the new data provide definitive evidence of regional early Miocene high-grade metamorphism and rapid exhumation in the Alboran Domain. After Alpine collision and crustal thickening, the Alboran Domain underwent a phase of rapid lithospheric extension, and subsided in part below sea-level by late Aquitanian to early Burdigalian time (about 20 Ma), forming the Alboran Sea. High–T metamorphism was therefore contemporaneous and directly associated with the formation of a marine extensional basin. Thermal modelling of the cooling path of the garnet granulite suggests a minimum exhumation rate of 6 km/m.y. At this rate, extension would have started at approximately 27 Ma, when the rock was at about 52 km depth; cooling below 800°C and closure of the U–Pb system in zircon probably did not take place until the rock reached about 16 km depth.