metasedimentary

Metasedimentary rocks are metamorphic rocks that originated from pre-existing sedimentary rocks through metamorphism. This process involves heat, pressure, and chemically active fluids, which alter mineralogy and texture. Metamorphism typically occurs during regional tectonic events, such as continental collision, or during localized contact with intruding magmas. As a result, the original sedimentary textures are often replaced by foliated or layered structures, though some low-grade rocks may retain hints of their sedimentary past, such as bedded planes, mud cracks, or ripple marks.

Common metasedimentary rocks include slate and phyllite, derived from shale or mudstone, as well as schist and gneiss, which can originate from shale, mudstone, sandstone, or limestone. Carbonate protoliths produce marble from limestone or dolostone, while quartzite forms from sandstone, and metaconglomerate derives from conglomerate. The degree of metamorphism governs the texture: slate exhibits slaty cleavage; phyllite has a sheen; schist shows platy minerals aligned into schistosity; and gneiss displays banded or layered compositional segregation. Mineral makeup includes micas (muscovite, biotite, chlorite), quartz, feldspar, and often graphite or calcite; higher grades may host garnet, staurolite, or kyanite.

Metasedimentary rocks are distinguished from meta-igneous rocks by their sedimentary origins, although the boundary can blur with strong metamorphism. They are useful in reconstructing past environments and tectonics, and many are economically important as sources of marble, quartzite, slate, and other dimension stones, as well as indirect indicators of metamorphic conditions.