Holes 976B and 976E at Site 976 penetrated into metamorphic basement of the Alboran Sea basin, where 259 m and 84 m of basement rocks, respectively, were drilled. Serravallian sediments of lithostratigraphic Unit IV directly overlie the basement. Figure 3 displays simplified lithologic columns from both holes (for detailed description see Shipboard Scientific Party, 1996).
The most abundant lithologies from Holes 976B and 976E are gneiss (including migmatite), high-grade schist, and marble; intervals of calc-silicate rocks occur in all these rocks. Dip of the main foliation ranges from 10° to 90° (maximum 10°-30°) from Hole 976B, and from 30° to 90° (maximum 30°-50°) from Hole 976E. Zones of cataclastic breccia and fault gouge are numerous and represent 18% and 29% of the total basement thickness recovered from Holes 976B and 976E, respectively.
The difference in depth (at least 11 m) to the sediment/basement boundary between Holes 976B and 976E (only 20 m apart) indicates considerable basement relief. This basement feature and the existence of breccias containing microfossils below the sediment/basement boundary suggest active faulting in a marine environment during the Serravallian. In addition, left-lateral oblique faulting was suggested based on the occurrence of striae on fault planes that developed in the basement (Shipboard Scientific Party, 1996).
Preliminary PT diagrams suggest that the metamorphic evolution of the high-grade schist and migmatitic gneiss from Site 976 followed an approximately isothermal path from 7 to 3 kbar at temperatures in the range between 580° and 630°C; granitic melts formed after decompression at <3 kbar and >670°C (Shipboard Scientific Party, 1996; Platt et al., 1996).
PT conditions for high-grade schist and gneiss from the basement recovered at Site 976 were established by Soto et al. (Chap. 19, this volume). According to these authors, the high-grade schist reached P ~ 4-5 kbar at temperatures in the range 650°-700°C at the end of the tectonic event that developed the main foliation (biotite + sillimanite + K-feldspar + quartz + plagioclase ± garnet assemblage). These PT conditions were estimated using thermobarometric studies and standard phase relations in pelitic systems. The mineral assemblage developed because of the completion of the staurolite-out and muscovite-dehydration melting reactions, determining both the final growth of biotite + sillimanite and local garnet as the main phases of the assemblage. In the gneissic rocks, melting probably occurred at P < 7 kbar and T between 700° and 750°C, with the formation of the main mineral assemblage (biotite + sillimanite + cordierite + K-feldspar + quartz + plagioclase). PT conditions for melting must be taken with caution because of considerable uncertainty in thermobarometric results and the variance of this assemblage. After decompression, accompanied by melting in the gneiss, a probable quick cooling PT path occurred up to the andalusite-stability field (P < 3 kbar, T < 600°C).