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Augen gneiss, from the German: Augen, meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains (porphyroclasts), normally feldspar, surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this ...
Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thus there is a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology . Protoliths are non-metamorphic rocks and have no protoliths themselves.
Archean TTG rocks appear to be strongly deformed grey gneiss, showing banding, lineation, and other metamorphic structures, whose protoliths were intrusive rocks. [4] TTG rock is one of the major rock types in Archean cratons .
Hall found that this produced a material strongly resembling marble, rather than the usual quicklime produced by heating of chalk in the open air. French geologists subsequently added metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism. However, metamorphism can take place ...
Metamorphic rocks form one of the three great divisions of rock types. They are distinguished from igneous rocks, which form from molten magma, and sedimentary rocks, which form from sediments eroded from existing rock or precipitated chemically from bodies of water.
Geological map of the Hebridean Terrane showing distribution of rocks of the Lewisian complex Undeformed Scourie dyke cutting Lewisian Gneiss, about 1.6 km west of Scourie Scourie dykes (now foliated amphibolites) cutting grey gneiss of the Scourie complex, both deformed during the Laxfordian tectonic event and cut by later (unfoliated) granite veins - road cutting on the A838 just north of ...
Metamorphic minerals alter with the changing P-T condition with time without reaching complete phase equilibrium, making P-T-t path tracking possible. From 1910 Ma (i.e. 1910 million years ago) to 1840 Ma, the rock experienced an increase in P-T conditions and formed mineral garnet , which is attributed to burial and heating.
In this belt magmatism is known to have occurred between 1.42 and 1.04 Ga depending on location. As with the Gneiss Belt, metamorphism is believed to have occurred at approximately 1.16 Ga. [4] [7] The Granulite Terrane is composed of meta-igneous gneisses including anorthosite massifs. Anorthosites form in plutons and are composed mostly of ...