DECOMPRESSION AND HIGH-TEMPERATURE-LOW-PRESSURE METAMORPHISM IN THE EXHUMED FLOOR OF AN EXTENSIONAL BASIN, ALBORAN-SEA, WESTERN MEDITERRANEAN

Leg 161 of the Ocean Drilling Program (ODP) has made a major contribut ion to our understanding of the origin of the Alboran Basin by demonst rating that it is underlain by rocks of continental origin that have u ndergone high-temperature metamorphism and melting at exceptionally lo w pressure after exhumation and decompression, Basement rocks recovere d from Site 976 consist of high-grade schist and gneiss derived from a luminous sediments, and minor amounts of marble, granitic dikes, and m igmatitic segregations of granitic material, Mineral assemblages and t extural relations show that an early assemblage including biotite, gar net, staurolite, plagioclase, and rutile is overprinted by a second as semblage of biotite, sillimanite, plagioclase, potassium feldspar, and ilmenite, Both assemblages are overprinted by andalusite, potassium f eldspar, and minor garnet, Migmatitic gneiss contains relict andalusit e, overprinted by sillimanite and cordierite coexisting with granitic leucosome. Preliminary pressure-temperature estimates suggest that the metamorphic evolution followed an approximately isothermal decompress ion path from 7 to 3 kbar al temperatures in the range 580 to 630 degr ees C. After decompression, granitic melts formed at <3 kbar and >670 degrees C, after andalusite breakdown and within the sillimanite stabi lity field, The cored rocks closely resemble high-grade metamorphic ro cks in the adjacent Betic Cordillera of southern Spain, which yield ea rly Miocene radiometric dates, At ODP Site 976 they are overlain by mi ddle Miocene marine sediments, The combination of exhumation in an ext ensional tectonic environment and the evidence for high and increasing temperature during exhumation provide support for and new constraints on current models for the basin that involve the removal of lithosphe ric mantle below a zone of continental collision, accompanied or follo wed by extension.