The textural observations for the two rock types encountered are described for every individual phase in relation to the different ductile fabrics.
Sillimanite occurs as elongated fibrolitic mats, and in places as larger prismatic porphyroblasts. Both textural types of sillimanite define the S2 planar fabric. Fibrolite forms in the matrix aggregates with submicroscopic intergrowths of K-feldspar and plagioclase. In many cases, these fibrolite aggregates are intergrown with biotite (Pl. 1, fig. 5). In these aggregates, needle-shaped crystals of fibrolite are parallel to the external foliation (harmonious fibrolite-type; Vernon and Flood, 1977), although there are abundant small fibrolite crystals in the matrix, which are largely oblique to the S2 foliation (disharmonious fibrolite-type; Pl. 1, fig. 5). Andalusite forms large randomly oriented porphyroblasts (up to 2-3 cm long) that grew postkinematically with respect to S2. It commonly contains oriented inclusions of ilmenite, biotite, and fibrolite parallel to S2 (Pl. 2, fig. 6), and corroded staurolite porphyroblasts partially transformed to ilmenite and minor spinel (Pl. 1, fig. 5). Andalusite commonly shows irregular outlines interfingering with plagioclase and K-feldspar, and locally occurs as rounded blebs included in these minerals. Andalusite is also locally altered to randomly oriented white mica.
We have clearly identified (see also Shipboard Scientific Party, 1996) two families of garnet porphyroblasts on the basis of textural relationships with S2. Garnet I usually has graphitic and opaque mineral (ilmenite) inclusion trails defining an internal foliation (S1) oblique to the external S2 foliation. This internal foliation is straight in the central part of the crystals, suggesting interkinematic garnet growth between D1 and D2 (Pl. 2, fig. 1, fig. 3). Garnet I porphyroblasts can be partially corroded to andalusite, plagioclase, and biotite, and in some cases to a fine-grained aggregate of chlorite-muscovite-Na-rich plagioclase-quartz (e.g., Sample 161-976E-14R-1, 98-102 cm; Pl. 2, fig. 4). Many garnet I porphyroblasts have an inclusion-poor outer zone with idiomorphic outlines (garnet II). This outermost rim usually has fibrolite inclusions that are continuous with the matrix (Pl. 2, fig. 3). In some grains, the boundary between the two zones is irregular and shows evidence of resorption (e.g., Sample 161-976E-15R-1, 02-07 cm; Pl. 2, fig. 2). Garnet II grains without garnet I cores also occur in the matrix as large (up to 1 cm long), idiomorphic porphyroblasts with abundant inclusions parallel to the external S2 foliation. Garnet II includes fibrolite, biotite, plagioclase, K-feldspar, tourmaline (Pl. 2, fig. 3), and in places staurolite and corundum (e.g., Sample 161-976B-76R-1, 45-49 cm).
Plagioclase occurs as inclusions in garnet I and garnet II porphyroblasts, and as large porphyroblasts in the matrix (up to 5-7 mm long). In the matrix, plagioclase has two main growth stages. The first, plagioclase I, is represented by cores crowded with linear inclusion trails of graphite, opaque minerals, biotite, and tourmaline (Pl. 1, fig. 3, fig. 4). The straight internal foliation indicates interkinematic growth between D1 and D2, although locally plagioclase I grows over D2 microfolds (early syn-D2 growth). Plagioclase II occurs as inclusion-free rims of porphyroblasts with local inclusions of K-feldspar and biotite. It rarely contains inclusions of fibrolite, parallel to S2, indicating growth at a late stage (post-D2). Plagioclase together with K-feldspar occurs also in the matrix including resorbed blebs of andalusite.
K-feldspar occurs in different textures: finely intergrown within fibrolite mats, interstitially with quartz in late veins cutting across the S2 foliation, as large irregular poikiloblasts, and as thin rims surrounding zoned plagioclase porphyroblasts. K-feldspar occurs in some places as intergrowths with plagioclase within garnet II porphyroblasts.
Dark brown biotite porphyroblasts occur in very different textures: oriented parallel to S1 and to S2 in the hinge areas of D2 folds (Pl. 1, fig. 1, fig. 2), as straight inclusion trails in plagioclase II and garnet II porphyroblasts (Pl. 2, fig. 3), and as reaction products of garnet I and staurolite (Pl. 2, fig. 4). Biotite porphyroblasts defining the S2 are closely related to fibrolite and commonly occur as intergrowths (Pl. 1, fig. 5). Mimetic growth of chlorite at the expense of biotite has also been observed (e.g., Sample 161-976B-76R-1, 03-07 cm). Large porphyroblasts of biotite with pyrite inclusions partially transformed pyrrhotite are common in the corundum-bearing schist (e.g., Sample 161-976B-77R-2, 24-26 cm). All of these textural relationships indicate that biotite was stable during the entire metamorphic evolution of these rocks, and biotite with K-feldspar from late D2 onward.
Staurolite is present in two different textures that are both observed in the same rock sample (e.g., Sample 161-976B-76R-1, 45- cm). Small staurolite crystals (<200 µm) are preserved as corroded inclusions in garnet II and andalusite porphyroblasts. In andalusite, staurolite is in places transformed into an Fe-rich spinel (hercynite; Pl. 2, fig. 5). In some cases, it is observed as nontransformed crystals the matrix with an internal foliation characterized by straight inclusions of graphite and ilmenite (Pl. 1, fig. 6). Both textural positions suggest that staurolite grew interkinematically between D1 and D2 was involved in some of the garnet-producing reactions.
Corundum is present as elliptical porphyroblasts (up to several long) in quartz-poor assemblages. Corundum crystals have inclusion trails of graphite and ilmenite that are generally continuous S2 (e.g., Sample 161-976B-77R-2, 24-26 cm).
Ilmenite is a major constituent in all the different types of highgrade schist. It is present in the matrix, parallel to S2, or as inclusions garnets I and II, plagioclase, andalusite, corundum, staurolite, and feldspar porphyroblasts. Wormlike crystals of ilmenite in a radial pattern are common inside andalusite. Rutile is only present as needle-shaped crystals preserved in large ilmenite grains, suggesting that rutile was transformed into ilmenite before D2. Another minor Ferich oxide present is hercynite, which commonly occurs as small grains in poikiloblastic andalusite and plagioclase.
Fibrolite and coarse, prismatic sillimanite are common fabric-forming elements in the gneiss. They form dense, monomineral aggregates, which are particularly well developed on the margins of leucosome segregations (harmonious fibrolite-type; Vernon and Flood, 1977; Pl. 3, fig. 1). In leucosome veins, minor fibrolite needles also present without preferential orientation, growing over Kfeldspar and plagioclase (disharmonious fibrolite-type; Pl. 3, fig. 5; Pl. 4, fig. 3). Andalusite shows contradictory textural relationships the main foliation in the gneiss. Andalusite porphyroblasts contain oriented inclusion trails of ilmenite, usually oblique to the external foliation, or in some cases, they include oriented fibrolite crystals parallel to the external foliation (see also Shipboard Scientific Party, 1996). This can be interpreted as growth of andalusite after sillimanite, but that deformation continued also after andalusite growth. Other textural evidence in leucosome veins supports the view that andalusite coexisted with a melt phase. This evidence includes andalusite present as small and partially resorbed fragments of single crystals, surrounded by a granitic matrix formed by K-feldspar, quartz, and plagioclase (Pl. 4, fig. 3); in places andalusite is partially replaced by cordierite (Pl. 4, fig. 6); and subhedral andalusite, together with quartz, is sometimes included in large porphyroblasts of Kfeldspar (Pl. 4, fig. 4). These textures could be interpreted as magmatic andalusite in a low-T crystallizing granitic melt (e.g., Vernon, 1986).
Garnet is a minor component of the migmatite gneiss, occurring in two textures: either as corroded relics largely transformed to plagioclase, K-feldspar, pyrite, magnetite, and quartz (garnet I; Pl. 3, fig. 3), or as small euhedral porphyroblasts partially surrounded by magnetite (garnet II; Pl. 3, fig. 4). Some of the gneissic rocks with cordierite contain corroded garnet porphyroblasts. The disappearance of garnet in those samples with cordierite suggests that garnet is consumed in the cordierite-forming reactions.
Both feldspars occur as idiomorphic grains in the matrix of leucosome domains. Plagioclase porphyroblasts with zoning patterns similar to those described in the high-grade schist are preserved in the pelitic domains. The two textural positions of plagioclase could be interpreted as relic "metamorphic" crystals in pelitic (restitic) domains, and new "granitic" plagioclase coexisting with K-feldspar in leucosome domains. K-feldspar also occurs as subhedral phenocrysts with inclusion-rich cores (Pl. 4, fig. 4).
This mineral occurs in two textures, with both optical and mineral assemblage differences. In pelitic domains of the gneiss, biotite occurs as dark brown porphyroblasts, oriented parallel to the foliation, and closely related to oriented fibrolite mats. In leucosome domains, in contrast, biotite occurs as light green porphyroblasts forming decussate clusters with large magnetite and Ti-magnetite cubic grains. These biotite crystals usually have cleavage-parallel oriented inclusions of spinel transformed to ilmenite and rutile (Pl. 4, fig. 2).
Cordierite is present in the leucosome domains as large, irregular porphyroblasts of up to 1-2 cm long (Pl. 4, fig. 1). It mostly appears to postdate the main foliation (Pl. 3, fig. 6). Cordierite includes surrounding porphyroblasts from the K-feldspar, plagioclase, biotite, quartz (Pl. 4, fig. 5), and corroded andalusite matrix (Pl. 4, fig. 6). It is commonly corroded and altered to pinnite and also to coarsegrained muscovite. Cordierite is more abundant in those domains with scarce to absent biotite, indicating that biotite, together with garnet (see above comment), is partially involved in the cordierite-forming reactions.
Idiomorphic cubes of magnetite and Ti-magnetite are the most abundant oxides in the pelitic and migmatite gneiss. They usually occur as large porphyroblasts included in fibrolite and biotite aggregates (Pl. 3, fig. 2), andalusite, and relic garnet I. Ilmenite is a minor component of the gneiss, and it appears included in garnet I and biotite porphyroblasts. Some of these ilmenite crystals probably have submicroscopic intergrowths with rutile.