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 polyphas e zircons separated from a high-P garnet granulite, a cordierite granite, a nd a sillimanite K-feldspar gneiss, respectively, from three separate locat ions in the Betic Cordillera of southern Spain. These ages are supported by three discordant Pb-206/U-238 ages in the range 19-24 Ma from the outermos t growth zones of zircons from a high-grade schist, a leucogranite, and a m igmatitic gneiss: these discordant ages can confidently be interpreted as m axima. The six sample sites cover a 240 km strike length along the Cordille ra. Taken together with published early Miocene Ar-Ar ages on metamorphic m icas 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 u nderwent a phase of rapid lithospheric extension, and subsided in part belo w sea-level by late Aquitanian to early Burdigalian time (about 20 Ma), for ming the Alboran Sea. High-T metamorphism was therefore contemporaneous and directly associated with the formation of a marine extensional basin. Ther mal modelling of the cooling path of the garnet granulite suggests a minimu m exhumation rate of 6 km/m.y. At this rate, extension would have started a t approximately 27 Ma, when the rock was at about 52 km depth; cooling belo w 800 degrees C and closure of the U-Pb system in zircon probably did not t ake place until the rock reached about 16 km depth. (C) 1999 Elsevier Scien ce B.V. All rights reserved.