Specific Metamorphic Rocks

The commonly used names that specify a particular rock type are listed below. As a rule, these names take precedence over the textural names –

• Marble :- A metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone.
• Quartzite :- A metamorphic rock composed predominantly of quartz. The protolith is typically sandstone. Some confusion may result from the use of this term in sedimentary petrology for a pure quartz sandstone.
• Greenstone/ Greenschist :- A low-grade metamorphic rock that typically contains chlorite, actinolite, epidote and albite. The first three minerals are green, which imparts the color to the rock. Such a rock is called greenschist if foliated and greenstone if not. The protolith is either a mafic igneous rock or graywacke.
• Amphibolite :- A metamorphic rock dominated by Hornblende + Plagioclase. Amphibolite may be foliated (gneissose) or non-foliated. The photolith is either a mafic igneous rock or graywacke.
• Serpentinite :- An ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.
• Blueschist :- A blue-amphibole-bearing metamorphosed mafic igneous rock or mafic graywacke. This term is so commonly applied to such rocks that it is even applied to non-schistose rocks. Glaucophane is the most common blue amphibole, and glaucophane schist has been commonly applied to rocks known to contain it.
• Eclogite :- A green and red metamorphic rock that contains clinopyroxene and garnet (omphacite + pyrope). The protolith is typically basaltic. Eclogites contain no plagioclase.
• Calc-silicate rocks (granofels or schist) :- A rock composed of various Ca-Mg-Fe-Al silicate minerals, such as grossular, epidote, tremolite, vesuvianite etc. The protolith is typically a limestone or dolostone with silica either originally present a clastic grains or introduced metasomatically.
• Skarn :- A calc-silicate rock formed by contact metamorphism and silica metasomatism from a pluton into an adjacent carbonate rock. Tectite is a synonym.
• Granulite :- A high-grade rock of pelitic, mafic or quartzo-feldspathic parentage that is predominantly composed of OH-free minerals. Muscovite is absent, and plagioclase and orthopyroxene are common.
• Migmatite :- A composite silicate rock that is heterogeneous on the 1 to 10 cm scale, commonly haing a dark gneissic matrix (melasome) and lighter felsic portions (leucosome). Migmatites may appear layered or the leucosome may occur as pods or form a network of cross-cutting veins.