The high-grade basement of the Alboran Sea: structural and PT evolution

The Alboran basement recovered at Site 976 is characterized by a low-pressure (P) and -temperature (T) metamorphic overprint and resembles the higher Alpujarride nappes of the Betic Cordilleras. The basement, consisting mainly of Al-rich metapelites with interlayered marbles and calc-silicate rocks, records a polyphased deformation and metamorphic history. A metamorphic evolution characterized by decompression and heating can be recognized between the first (D1) and second (D2) deformation events. This evolution is documented by a decrease of spessartine and grossular toward the rim of garnet growing between D1 and D2, and the lack of staurolite during and after D2. The presence of cordierite and hercynite in the T-peak assemblage is an additional evidence of a T increase associated with a significant drop in P. Partial melting is locally revealed by cordierite-bearing leucosomes, which cut across the S2 schistosity of the high-grade gneisses. The PT conditions attained during peak metamorphism were estimated by conventional thermobarametry to be ~ 700°C and 2.8-4.4 kbar. The peak metamorphic conditions were followed by the growth of andalusite porphyroblasts and by sillimanite-bearing mylonitic bands (D3). Contemporaneous heating and decompression is consistent with crustal thinning coupled with magmatic underplating during the exhumation of the Alboran basement. The thermal anomaly induced by this geodynamic process decayed rapidly, as seen in cataclastic bands that directly cut the high-T rocks, with rare low-T plastic deformation.