Petrological and structural evolution of high-grade metamorphic rocks from the floor of the Alboran Sea basin, western Mediterranean

The metamorphic basement beneath the Alboran Sea, a Neogene extensional basin drilled during Ocean Drilling Program (ODP) Leg 161 (Site 976), consists of high-grade pelitic schist overlying migmatitic pelitic gneiss. Inferred assemblages in the high-grade schist evolved from garnet + staurolite + biotite + muscovite + plagioclase (Assemblage 1) to biotite + sillimanite + K-feldspar + plagioclase +/- garnet (Assemblage 2) to andalusite + biotite + K-feldspar + plagioclase (Assemblage 3). In the gneiss, which shows abundant migmatitic textures, the present assemblage is biotite + andalusite + sillimanite + muscovite + cordierite + K-feldspar + plagioclase +/- garnet. Phase relations and thermobarometric calculations suggest that the high-grade schist experienced decompression accompanied by heating from similar to 500 degrees C at 10.5 kbar through 600 +/- 30 degrees C at 6-7 kbar (Assemblage 1), and thence to 650-700 degrees C at 3-4 kbar (Assemblage 2), followed by cooling through 500-600 degrees C at 2 kbar or less (Assemblage 3). Peak temperature and melting occurred under low-pressure conditions. The widespread disequilibrium and overstepping of metamorphic reactions suggests that decompression was rapid. The P-T evolution is consistent with metamorphism in a late orogenic extensional basin, and suggests the existence of an external source of heat, probably indicating the complete removal of lithospheric mantle beneath the extending region.