The most common rock types of the Alboran basement are schists characterized with Grt-Sil-Bt-K-feldspar (Kfs)-plagioclase (Pl)-quartz (Qtz) peak mineral assemblage (Table 1). These rocks sometimes contain relics of staurolite (St) and late-stage andalusite (And) porphyroblasts. In the same sample sillimanite can be either prismatic or fibrolitic, the latter occurring along the main foliation. Muscovite (Ms) sometimes is present as inclusion in garnet, but more frequently occurs as a retrograde mineral replacing andalusite.
The protolith of the high-grade schists is a shale (Spadea and Prosser, Chap. 28, this volume) whose composition is slightly Ca- and Al-richer than the post-archean average shale (PAAS) of Taylor and McLellan (1985). The Fe enrichment (FeO vs. FeO+MgO ranging between 0.71 to 0.81) and abundant graphite are features comparable with the middle to lower Paleozoic metapelites of the Alpujárride complex (Torres-Roldán, 1981).
The first mineral assemblage (M1), consisting of Grt-Bt-staurolite (St)-Pl-Sil-Qtz ± Ms (Table 1), statically overgrows a first foliation (S1), recorded by graphite, ilmenite, and titaniferous magnetite inclusions within staurolite, plagioclase, garnet, and by alternating Bt-rich and Pl-Qtz-rich bands (Fig. 2). In some cases it may be seen that the S1 foliation developed parallel to the axial plane of D1 isoclinal folds and crenulations, preserved within minerals of the M1 assemblage (Fig. 3A). Therefore, a previous foliation (Sx) preceded the S1 schistosity.
The second D2 deformation event developed the main schistosity (S2) in the axial plane of tight to isoclinal folds, which refold the D1 crenulations preserved within Pl, St, and Grt porphyroblasts and the S1 foliation. The S2 foliation is characterized by folia composed of fibrolitic sillimanite and biotite intergrowths (M-domains; Bell, 1981; Vernon, 1987) intercalated between layers rich in quartz and feldspar (Q-domains). In areas where the S banding is preserved, the S2 is still recognizable because of the preferred orientation of biotite and plagioclase crystals.
The mineral assemblage (M2) synkinematic with the D2 deformation is composed of the Grt-Bt-Pl-Kfs-Sil-Qtz assemblage (Table 1). Millipede structures, displaying types 2 and 3 opposedly concave microfolds (Johnson and Bell, 1996), demonstrate the synkinematic growth of garnet inner rims (Fig. 3B).
In schists from Hole 976E, characterized by centimeter-thick calc-silicate intercalations (Table 1), the M2 mineral assemblage consists of the association Bt-Pl-Kfs-Qtz-hercynite (Hc) ± corundum (Crn). Textural evidence suggests that Crn belongs to the M2 assemblage, because it shows synkinematic relationships with the D2 deformation. Optically determined rutile inclusions can be observed in some Crn crystals. Sometimes schists with calc-silicate intercalations show indications for incipient partial melting, because aggregates of inclusion-free Qtz, Pl, and Kfs around Hc (Fig. 3C) can derive from the crystallization of melt pockets.
After the M2 metamorphism, the S2 foliation has been statically overgrown by andalusite and abundant K-feldspar porphyroblasts, belonging to the third (M3) mineral assemblage (Table 1). In Qtz-poor and Al-rich lithologies, Hc is present in And cores, together with St relics. We interpret this texture to be indicative of the final breakdown of St, to give Hc and And.
Sometimes, late fibrolitic sillimanite can overgrow the andalusite porphyroblasts. This can imply that the M3 crystallization occurred close to the sillimanite/andalusite phase boundary. Alternatively, a slight variation of the PT conditions occurred during or after the M3 metamorphism. The late retrograde evolution is characterized by the replacement of andalusite and corundum crystals by abundant retrograde muscovite.