High-grade gneisses and migmatites have been recovered below the high-grade schists in Hole 976B (Fig. 1), associated with leucogranite sheets (Comas, Zahn, Klaus, et al., 1996). They are characterized by the Crd-Kfs-Sil-And-Bt-Qtz-Plg mineral association, with fibrolitic Sil occurring before or after And along the main foliation or in isolated shear zones. The gneisses range from pelitic to graywacke composition, with an Al content comparable to or lower than the PAAS (Spadea and Prosser, Chap. 28, this volume).
The contact between high-grade schists and high-grade gneisses is tectonic, because they are separated by a carbonate-cemented fault breccia. As previously stated in Comas, Zahn, Klaus, et al. (1996), the correlation of the mineral assemblages and the foliation-forming events between these two rock units is not straightforward. The gneisses lack inclusion trails related to the D1 deformation, and a mineral assemblage comparable with the M1 of the schists. We tried to correlate the main foliation observed in the gneisses with the S2 of the schists, because both pre-date the growth of And porphyroblasts.
The first metamorphic assemblage of the high-grade gneisses, composed of Bt-Sil-Pl-Kfs-Qtz (Table 1), is likely correlated with the M2 assemblage of the schists, because it is synkinematic with the S2 foliation. Sometimes, optically determined ilmenite (Ilm) and rutile (Rt) crystals occur in association with fibrolitic sillimanite, or overgrown by later-stage Grt or And.
The S2 foliation has been later cut by Kfs-Pl-Qtz-Crd-Bt ± Ms ± And leucosomes, related to partial melting of crustal rocks (Table 1). The composition of the leucosomes, characterized by an A/CNK value of 2.2 (interval 161-976B-98R-2, 48-50 cm; Spadea and Prosser, Chap. 28, this volume), indicates that Crd probably represents restitic material, because A/CNK should not exceed 1.4-1.5 in peraluminous granites (Clarke, 1992). Sometimes, crystallization of corundum and tourmaline took place in the gneisses, close to the contact with leucosome pockets.
Further evolution of the gneisses and migmatites occurred during decreasing pressure, as indicated by the growth of andalusite, K-feldspar, cordierite-andalusite intergrowths, and garnet. This assemblage grew after the D2 phase, because garnet and cordierite statically overgrow the S2 Sil-bearing foliation. Therefore, it can be correlated with the M3 assemblage found in the high-grade schists.
The M3 metamorphism was followed by the development of Sil-bearing shear zones (S3), which wrap around andalusite and cordierite porphyroblasts and overprint the leucosomes (Fig. 3D). Ksp crystals from the leucosome are partially replaced by myrmekite when they are in contact with the S3 shear zones. For this reason the development of myrmekites is probably strain-induced (Simpson and Wintsch, 1989).
The crystallization relationship between sillimanite of the S3 shear zones and andalusite can be conflicting. Typically, andalusite porphyroblasts mimetically overgrow the S2 foliation, which is later crosscut by S3 Sil-bearing shear zones. On the other hand, andalusite crystals can form after microfolds related to the S3 shear zones (Fig. 3E). Therefore, the development of the S3 probably happened close to the sillimanite/andalusite phase boundary. The final retrograde evolution is similar to the high-grade schists, with abundant muscovite growth at the expense of andalusite, sillimanite, and cordierite.