metaconglomerate

Metaconglomerate is a metamorphic rock formed by the metamorphism of a sedimentary conglomerate. In sedimentary form, a conglomerate consists of rounded clasts embedded in a finer-grained matrix; after metamorphism, the rock is subjected to intense pressure and heat that recrystallize minerals and deform the texture. The result is a stone in which original pebbles may survive as porphyroclasts or become elongated, with a recrystallized, quartz- and mica-rich matrix.

Texture and mineralogy: The degree of foliation or lineation varies with metamorphic grade. At low to medium grades, metaconglomerates often display a schistose or sub-parallel fabric with flattened pebbles and aligned mica. In higher-grade variants, recrystallization produces a more massive appearance, and new minerals such as garnet or staurolite may appear in favorable chemistries.

Formation and environment: Metaconglomerates form under burial- or contact-related metamorphism, from greenschist to amphibolite facies or higher, depending on tectonics. Fluid activity during metamorphism promotes mineral recrystallization and cement dissolution, enhancing pebbly texture but obscuring the original sedimentary features such as bedding.

Distribution and significance: Metaconglomerates occur in many metamorphic belts around the world and are studied to reconstruct pressure-temperatures, deformation history, and tectonic processes. They are distinguished from unmodified conglomerates by their metamorphic textures and mineralogy.

See also: metamorphic rock, conglomerate, quartzite, schist.