MINERAL ASSEMBLAGES AND DUCTILE FABRICS

Most of the high-grade schist is very biotite- and sillimanite-rich and garnet- and staurolite-poor. Graphite is a major component and is also included in mica and Al-silicate porphyroblasts. The lithologic sequence consists of garnet- and staurolite-rich layers (staurolite-bearing schist) with an Fe-rich bulk composition; layers with biotite and fibrolite (sillimanite crystals 10 µm in minimum dimensions, following Kerrick and Speer, 1988) without plagioclase (biotite-rich schist); and corundum-bearing and quartz-and-staurolite-lacking layers, with a more aluminous and silica-poor bulk composition (corundum-bearing schist). Corundum-bearing schist is usually found near calc-silicate reaction bands (e.g., Sample 161-976B-76R-1, Piece 9A, 77-80 cm; Sample 161-976B-77R-2, Piece 3A, 20-25 cm; Sample 161-976B-76R-1, Piece 9A, 77-80 cm). Quartz, plagioclase, biotite, and Al-silicates (andalusite and sillimanite) are always present as major components of the high-grade schist. Apatite, ilmenite, zircon, pyrite, and pyrrhotite (in late veins and as a transformation product of pyrite) are minor and accessory minerals. Tourmaline is a major component in some of the corundum-bearing high-grade schist. Chlorite, muscovite, and hematite are retrograde minerals. Kyanite has not been observed in any of the high-grade schist samples.

The pelitic gneiss consists of medium gray biotite-bearing gneiss, with K-feldspar, sillimanite, andalusite, and cordierite porphyroblasts. Tourmaline, apatite, zircon, magnetite, Ti-magnetite, and ilmenite are accessory components. Muscovite is locally present, particularly in veins, migmatitic segregations, and tension fractures in large andalusite porphyroblasts (up to 2-3 cm long). Small idiomorphic garnet porphyroblasts (up to 500 µm) have been found in some samples, associated with large concentrations of magnetite and Ti-magnetite (e.g., Samples 161-976B-98R-1, 38-40 cm, and 98R-1, 96-101 cm). Millimeter-long lenses of felsic material, as well as broader scale compositional layering, mineral orientation, and abundant quartz-tourmaline and felsic veins, define the main metamorphic foliation. Much of the gneiss is migmatitic, with cm-thick veins and segregations of weakly foliated or unfoliated felsic material (leucosome) containing large crystals of cordierite, as well as biotite, andalusite, fibrolite, magnetite, and tourmaline. The leucosome domains are composed of K-feldspar, plagioclase, and quartz in granitic proportions. Zoned tourmaline porphyroblasts are always present as a minor component. These mostly occur as stromatic migmatites (terminology of Mehnert, 1968; and McLellan, 1983), which implies that melting was controlled by a strong, compositional foliation before melting. In some instances, leucosome also occurs as veins crosscutting the foliation and in pull-aparts. This locally gives the gneiss a brecciated appearance (agmatitic textures; e.g., Sample 161-976B-95R-3, Piece 7, 53-63 cm).

Following are the most complete assemblages in the high-grade schist and gneiss (mineral abbreviations after Kretz, 1983):

  1. biotite-rich schist: biotite (Bt) ± garnet (Grt) + fibrolite (Fib) ± andalusite (And) + K-feldspar (Kfs) + plagioclase (Pl) + ilmenite (Ilm) + quartz (Qtz);
  2. staurolite-bearing schist: staurolite (St) + Bt + Grt + Fib + And + Kfs + Pl + Ilm + spinel (Spl) ± rutile (Rt) + Qtz;
  3. corundum-bearing schist: Bt ± Grt + Fib + And + corundum (Crn) + Kfs + Pl + Ilm; and
  4. banded pelitic gneiss: Bt ± Grt + cordierite (Crd) + Fib + And + Kfs + Pl + Ilm + magnetite (Mag) + Qtz.

There are reaction textures among the AFM (Al2O3-FeO-MgO) mineral phases, and in particular the Al-silicates, present in all the high-grade schists. Thus, these mineral assemblages are not in equilibrium assemblages. Nevertheless, the high-grade schist assemblage with St + Bt + Grt + And + Sil ± Kfs clearly reflects lower temperatures than the Bt + Crd + And + Sil + Kfs assemblage in the gneiss.

Ductile Fabrics

Two sets of ductile fabrics and structures can be distinguished in the high-grade schist, referred to as D1 and D2. The earlier fabric, S1, is characterized by compositional layering, transposed quartz veins, 0.5- to 2-mm-scale quartz-rich and biotite-rich laminae and oriented biotite. Garnet, staurolite, and early plagioclase porphyroblasts have grown over S1 and include it as graphite trails.

S1 has been intensively affected by 1- to 50-mm-scale D2 folds. These are commonly tight and asymmetric, but may also be symmetric. The compositional layering and quartz-biotite laminae that define S1 are considerably thickened in the hinges of the folds relative to the limbs. Biotite is recrystallized in the fold hinges, and tends to lie parallel to the axial surface, as do elongate mats of fibrolite; together, the two define a weak axial planar fabric, S2 (Pl. 1, fig. 1, fig. 2). In much of the high-grade schist, D2 has transposed completely the S1 fabric. D2 was followed by static crystallization of andalusite, plagioclase, a second garnet, and K-feldspar.

We have observed no clear transition between the fabrics in the high-grade schist and that in the gneiss, as there is significant faulting along the boundary between these two lithotypes (Comas and Soto, Chap. 25, this volume). Thus, relative age relationships of the gneissic foliation in relation to the deformational history of the schist is uncertain. On the basis of the textural relationships discussed below, we believe that it is a composite foliation, which may have formed initially at about the same time as D2 in the high-grade schist, but was subsequently modified by compaction or further deformation during partial melting.

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