Zahn, R., Comas, M.C., and Klaus, A. (Eds.), 1999 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 161 251 18. METAMORPHISM OF CALC-SILICATE ROCKS FROM THE ALBORAN BASEMENT 1 Vicente López Sánchez-Vizcaíno 2 and Juan I. Soto 3 ABSTRACT Calc-silicate rocks are found in the high-grade metapelitic sequence of the Alboran Sea basement, drilled at Site 976 during Ocean Drilling Program Leg 161. These rocks occur as reaction zones, millimeters to several centimeters thick, along the con- tact between the marble and metapelite layers, and are interpreted as having been formed by diffusion processes between the two lithologies. Two different groups of calc-silicate rocks are distinguished according to the presence of garnet or scapolite. The garnet-bearing rocks display several reaction zones from the marble (Zone VII: calcite + quartz + plagioclase) to the metapelite (Zone I: biotite + corundum + plagioclase + K-feldspar + spinel; and Zone II: biotite + almandine-rich garnet + pla- gioclase + K-feldspar + quartz). The calc-silicate reaction zones are characterized by the appearance or disappearance of key mineral assemblages: appearance of pargasitic amphibole + biotite in Zone III , disappearance of biotite in Zone IV, appearance of clinopyroxene in Zone V, and appearance of grossular-rich garnet in Zone VI. Garnet and clinopyroxene break down to anorthite and actinolitic amphibole, respectively, which defines a secondary assemblage. Phase-relationship analysis suggests P-X CO2 peak conditions of 7.4-8.2 kbar and X CO2 = 0.11-0.16 for an assumed temperature of 650°C. The secondary growth of amphibole and plagioclase probably occurred under decreasing pressure and increasing X CO2 conditions. Scapolite-bearing assemblages in calc-silicate rocks indicate minimum temperatures of 550°–600°C. INTRODUCTION The Alboran Sea, in the western Mediterranean, is one of several extensional basins of late Tertiary age that formed in close associa- tion with the compressional orogens comprising the Alpine system. The Alboran Sea is partly surrounded by an arcuate mountain chain formed by the Betic (southern Spain) and the Maghrebian Chains (Rif and Tell, in Morocco; Fig. 1). Within these mountain chains, contrac- tional deformation continued during basin formation. The Alboran Sea in fact appears to have formed on the extended and thinned rem- nants of a late Mesozoic–Paleogene orogen (known as the Alboran Domain) that forms the Internal Zones of the surrounding mountain chains (Fig. 1). During the Neogene evolution of the basin, contrac- tion continued in the External Betic and Rif belts to form the present arcuate structure (Platt and Vissers, 1989; Comas et al., 1992; García- Dueñas et al., 1992). One of the objectives of Ocean Drilling Program (ODP) Leg 161 was to investigate the basement of the Alboran Sea basin, as its nature, structural history, and thermal evolution contain evidence useful for constructing satisfactory rifting models for the region, and to test vari- ous tectonic hypotheses (see Comas, Zahn, Klaus, et al., 1996). To allow substantial penetration of the acoustic basement, Site 976 was lo- cated on a basement high (Fig. 1), covered by a thin, discontinuous sed- imentary sequence, middle Miocene through Pliocene–Pleistocene in age (Shipboard Scientific Party, 1996). This basement high is ~50 km long and 15 km wide, and was partially drilled during Leg 13 at Deep Sea Drilling Project (DSDP) Site 121 (Shipboard Scientific Party, 1973). The orientation of this basement high changes southward from northeast–southwest at Site 976, to north-northwest–south-southeast. Interpretation of multichannel commercial seismic profiles indicates that this basement elevation corresponds to a horst limited by north– south and northeast-southwest–trending normal faults, which bound a major depocenter with up to 8 km of lower Miocene–Pleistocene sedi- ments toward the west (Comas et al., 1992; Watts et al., 1993; de la Linde et al., 1996; Soto et al., 1996). During Leg 161, the JOIDES Resolution drilled into the acoustic basement at Site 976 (Holes 976B and 976E), penetrating and sam- pling 258.97 m (Hole 976B, 1108.0 m water depth, from 669.73 to 928.70 mbsf) and 84.22 m (Hole 976E, 1107.6 m water depth, from 652.08 to 736.30 mbsf) of metamorphic rocks. These rocks include 1 Zahn, R., Comas, M.C., and Klaus, A. (Eds.), 1999. Proc. ODP, Sci. Results, 161: College Station, TX (Ocean Drilling Program). 2 Departamento de Geología, Universidad de Jaén, Escuela Universitaria Politécnica de Linares, C/Alfonso X el Sabio 28, 23700 Linares, Spain. vlopez@ujaen.es 3 Instituto Andaluz de Ciencias de la Tierra and Departamento de Geodinámica, C.S.I.C.-Universidad de Granada, 18071 Granada, Spain. 1000 200 1000 ODP Site 976 DSDP 121 Site 979 Málaga Granada AI Al-Hoseima Gibraltar Tangier Tétouan West Alboran Basin Rif Alborán Sea Atlantic Ocean Cordillera Bética IBERIAN FORELAND Tell South Iberian and Maghrebian Cover Flysch Trough units Alboran Domain Alpujarride Complex, peridotite (p) p Malaguide Complex and Dorsal units 6°W 5°W 4°W 3°W 37°N 36°N 35°N 6°W 5°W 4°W 3°W 37°N 35°N 36°N 50 km Figure 1. Location of Sites 976 and 121 in the western Alboran Sea basin and simplified geologic map of the surrounding Betic and Rif mountain belts. Bathymetric contours are every 200 m; each latitude grade equals 111 km (from Comas et al., 1993).