gneisses
(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) 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 feldspars. |
| Where not of granitic origin, gneisses are named for their parent rock such as diorite and amphibolite, or for the presence of minerals such as albite, biotite, biotite-plagioclase, chlorite, and garnet, hornblende-plagioclase. |
| links: 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 : late, coarse grained leucosomes in pelitic gneiss that contain large aggregates of cordierite (dark patches), and biotite-amphibole gneiss containing melt patches with euhedral orthopyroxene, both Antarctica; 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 : formations: granite sheets and diatexites with schlieric stucture (lower part) are commonly interlayered with migmatites and gneisses, and disrupted mafic dikes in a leucocratic gneiss, both Antarctica. |
