Garnet of the high-grade schists occurs as 1-to 5-mm-large porphyroblasts of nearly round shape or as elongate crystals parallel to the S1. Minerals included in garnet are mainly graphite, ilmenite and titaniferous magnetite, and, to a minor extent, biotite, staurolite, sillimanite, plagioclase, and quartz.
The rounded garnets generally display cores with scarce and coarse inclusions, inclusion-rich rims, and inclusion-poor outer rims (Fig. 4A). Inclusion trails in garnet cores can be sometimes interpreted as accumulations of displaced material along crystal facies during the first garnet growth stages. Therefore, their orientation was controlled by the orientation of the garnet crystal facies. However, rare inclusions parallel to S1 are still recognizable in garnet cores (Fig. 2, Fig. 4A). Consequently, we can state that garnets started growing during or after the D1 foliation-forming event.
The subsequent crystallization of the inclusion-rich garnet inner rims took place before and during the D2 deformation. In fact, the inclusion trails bend toward parallelism with S2 in the external part of the garnet inner rims (Fig. 4A). Finally, the last stage of garnet growth is represented by an inclusion-poor outer rim, from 0.1 to 0.5 mm thick (Fig. 4A). Sometimes, biotite partially replaces the inclusion-rich garnets as atoll-like structures, especially in And-bearing schists (Fig. 4B). In the Torrox nappe of the Alpujárride complex, this has been interpreted as forming during decompression (García-Casco and Torres-Roldán, 1996).
The elongate garnets are generally inclusion-rich, but they can have inclusion-poor cores. The shape of these suggests that they grew mimetically on the S1 foliation or, sometimes, on D1 isoclinal folds (Fig. 4C).
Garnet compositional profiles across round porphyroblasts shows bell-shaped trends for grossular (Grs) and spessartine (Sps) and concave-upward trends for almandine (Alm) and pyrope (Pyr; Fig. 5). The flat, symmetrical edges of the profile in Figure 5C can indicate garnet was re-equilibrating through diffusion. However, garnet grains with strongly different diameters (from 2.5 to 0.3 mm) show similar Alm and Pyr compositional patterns (Fig. 5C, D). Therefore, we infer that growth zoning prevails over diffusional zoning, because compositional patterns are slightly correlated to grain size. The observed decrease in the Fe/(Fe+Mg) ratio toward garnet rims (Fig. 5) indicates that garnet grew under increasing T conditions.
The compositional profiles can be slightly irregular and asymmetrical, as may be observed for the Grs trend (Fig. 5A). This latter feature can be related to some garnet dissolution, which is particularly evident along the contact between two different garnet grains. Irregularities in Grt compositional profiles can be explained by diffusional equilibration close to St or Bt inclusions (Fig. 5B).
Summing up, garnets from high-grade schists display a continuous and progressive growth, before and during the D2 deformation. Garnet compositions can be separated into three slightly overlapping groups (Fig. 6). Cores are characterized by a wide variation in the Grs content, ranging from 20% to 10%, together with high Sps content (up to 14%). The inclusion-poor outer rims have very high Alm (about 85%) and relatively high Pyr (up to 8%) content, together with low Grs and Sps (less than 5%) content (Fig. 6).
Garnet is rarely developed in high-grade gneisses, in association with Crd. It shows moderate textural zoning and mimetically overgrows the S2 Sil-bearing schistosity (Fig. 4D). This evidence indicates that garnet growth postdates the D2 deformation. Garnet from high-grade gneisses has a nearly uniform composition from core to rim (Table 2). It shows low Grs% and Sps% content, together with relatively high Alm% and Pyr%.
Plagioclase from high-grade schists have inclusion-rich cores and inclusion-free rims (Fig. 4E). Locally, inclusion trails in plagioclase cores and garnet internal rim are parallel (Fig. 2). This observation suggests that plagioclase and garnet synchronously overgrew the S1 before the onset of the D2 deformation. The association inclusion-rich plagioclase and staurolite (Fig. 4D), characterized by the same orientation of inclusion trails, suggests that these two phases were in equilibrium with garnet during the first M1 metamorphism. The anorthite (An) content of plagioclase decreases toward the rim (normally from An46-63 to An42-51). Sometimes, even higher An values (up to An80) is found in inclusion-rich plagioclase cores. Rare plagioclase inclusions within garnet display a wide variation in the An content (from An63 to An34-35). The more calcic plagioclase is generally found in garnet core.
Biotite from the high-grade schists is characterized by high Al2O3, TiO2, and FeO contents (Table 2), with an average Fe/(Fe+Mg) of 0.76. It occurs in the matrix, together with fibrolitic sillimanite, and as inclusions in garnet. This latter can be in contact with prismatic sillimanite or staurolite. Therefore, we suggest that the inclusion-rich garnet rim overgrew the enclosed biotite, sillimanite, and staurolite, belonging to the M1 mineral assemblage (Fig. 7).
Matrix biotite, defining the S2 foliation, grew in equilibrium with garnet synkinematic with the D2 deformation. Therefore, the M2 assemblage includes matrix biotite + sillimanite + garnet (Fig. 7). In more detail, we should consider that matrix biotite was in equilibrium with garnet areas characterized by opposedly concave microfolds and, possibly, the inclusion-poor garnet outer rim. Generally, biotite enclosed in garnet shows a slightly lower FeO content with respect to matrix biotite (Fe/[Fe+Mg] of 0.70; Table 2). This feature, together with Fe enrichment toward garnet rims, can indicate decompression, as both biotite and garnet tend to become Fe-richer while pressure decreases (Spear, 1993; García-Casco and Torres-Roldán, 1996). In gneisses and migmatites, biotite, in equilibrium with cordierite and sillimanite or andalusite (Fig. 7), has a Fe/[Fe+Mg] content of 0.63.
Cordierite from leucosomes is compositionally zoned from core to rim, with a Fe/[Fe+Mg] ratio varying between 0.25 and 0.31 (Table 2). In contrast, Crd in equilibrium with Grt has a distinctly higher Fe/[Fe+Mg] value of 0.61 (Table 2).