Petrological and Structural Evolution of High-Grade Metamorphic Rocks from the Floor of the Alboran Sea Basin, Western Mediterranean

Abstract

The metamorphic basement beneath the Alboran Sea, a Neogene INTRODUCTION extensional basin drilled during Ocean Drilling Program (ODP) Late orogenic extension of previously thickened conLeg 161 (Site 976), consists of high-grade pelitic schist overlying tinental lithosphere is a widely documented phenomenon migmatitic pelitic gneiss. Inferred assemblages in the high-grade (e.g. Dewey, 1988; Malavieille, 1993; Ruppel, 1995), schist evolved from garnet+ staurolite+ biotite+ muscovite+ but it is unclear whether the thermal evolution of the plagioclase (Assemblage 1) to biotite + sillimanite + K-feldspar lithosphere in this situation differs significantly from that + plagioclase ± garnet (Assemblage 2) to andalusite + biotite associated with extension in non-orogenic settings. If + K-feldspar+ plagioclase (Assemblage 3). In the gneiss, which extension is triggered by removal of part of the lower shows abundant migmatitic textures, the present assemblage is biotite lithosphere (e.g. England & Houseman, 1989) there may + andalusite + sillimanite + muscovite + cordierite + Kbe a detectable rise in temperature during exhumation feldspar + plagioclase ± garnet. Phase relations and thermoof rocks (Platt & England, 1994), whereas extension barometric calculations suggest that the high-grade schist experienced caused by far-field forces is likely to produce isothermal decompression accompanied by heating from ~500°C at 10·5 kbar decompression paths followed by cooling (Ruppel et al., through 600 ± 30°C at 6–7 kbar (Assemblage 1), and thence 1988; Platt et al., 1999). Decompression accompanied by to 650–700°C at 3–4 kbar (Assemblage 2), followed by cooling heating in extensional settings has been demonstrated in through 500–600°C at 2 kbar or less (Assemblage 3). Peak the Seward Peninsula in Alaska (Patrick & Lieberman, temperature and melting occurred under low-pressure conditions. 1988), the Cordillera Darwin in the southern Andes The widespread disequilibrium and overstepping of metamorphic (Kohn et al., 1993), the Tauern window in the Alps reactions suggests that decompression was rapid. The P–T evolution (Selverstone et al., 1984), the Aegean (Buick & Holland, is consistent with metamorphism in a late orogenic extensional 1989), the High Himalaya (Inger & Harris, 1992), and basin, and suggests the existence of an external source of heat, the Betic Cordillera (Van der Wal & Vissers, 1993). An probably indicating the complete removal of lithospheric mantle interesting related question is the significance of crustal beneath the extending region. melting in extensional settings. In principle, partial melt-