metamorphic rocks

Gneiss (pron. 'niece') is a common rock resulting from high-grade regional metamorphism of igneous rocks (orthogneiss) or sedimentary rocks (paragneiss).

Gneisses typically occur in gneiss belts comprising large areas within the high-grade cores of regional metamorphic belts. The high temperatures and shear stresses of high grade metamorphism are probably due to deep tectonic burial and major regional compression, so gneissic terranes can form in areas of convergent plate tectonics.

Gneissic rocks are coarsely foliated rocks with alternating light (quartz and feldspar) and dark (hornblende and biotite) bands. Micas are absent of present only in small amounts in gneissic rocks, but predominate in the often finer grained schists. Individual bands in gneisses are 1 mm to 1 cm in thickness and result from recrystalization of component minerals during subjection to formative high temperature and pressure (shear stress). Those rocks without obvious banding are termed leptites. Individual mineral grains are often flattened parallel to banding, and gneiss is defined by this texture although the term gneiss often indicates mineral composition of granitic type, dominated by quartz and feldspar.

Where not of granitic origin, gneisses are named for their parent rock such as diorite and amphibolite, or for the presence of index minerals such as albite, biotite, biotite-plagioclase, chlorite, and garnet, hornblende-plagioclase.

(above left - click to enlarge image : top, banded gneiss; center, close-up of augen gneiss showing characteristic elliptic or lenticular feldspaths (normally microcline); bottom kinked banding in gneiss)

[images, roll-over for preview : biotite gneiss : pyroxene gneiss : Passagassawakeag gneiss 1 : Passagassawakeag gneiss 2 : Passagassawakeag gneiss 3 : augen gneiss with gneissic banding : augen gneiss close-up : gneiss sample : alternating pink K-spar and black amphibole and white plagioclase layers : chloritoid gneiss 1 : chloritoid gneiss 2 : chloritoid gneiss 3 : migmatitic gneiss : transposed gneiss : diagram of banding in gneiss : gneiss boulders : Archean banded gneiss, Black River, Wi : ]

Schist (pron. shist) is a medium- to coarse-grained, often shiny, mica-laden rock. Medium-grade metamorphism causes recrystallization, rotation, and new growth of micas (predominantly muscovite, biotite, and chlorites) from fine-grained, mica-bearing rocks such as shales and slates, which results in the well-developed planar to wavy foliation (schistosity) characteristic of schists. Schists, such as garnet-biotite schists containing porphyroblasts of garnet and a schistosity dominated by biotite, are named for their assemblage of minerals.

(right - click to enlarge : top, schist bed at Corea Ck.; center, surface of garnet schist (left) and biotite-mica schist (right); bottom, photomicrograph of garnet-mica schist).

[images, roll-over for preview : gallery of rock photomicrographs : amphibolite : garnet-mica schist : mica schist : talc-tremolite schist : muscovite-foliation between quartz grains : garnet-kyonite-quartz schist : garnet-staurolite-muscovite schist : garnet-staurolite schist : kyanite schist : muscovite mica schist with crenulations : tourmaline mica schist : Connemara Schists : Schist Wave : schist landslide, NZ]

: simple animation of metamorphism :