Metaconglomerates

Metaconglomerates are metamorphosed equivalents of sedimentary conglomerates. They form when a clastic, rounded-clast conglomerate is subjected to metamorphism during burial and tectonic deformation, or to contact metamorphism near heat sources. The term is used in petrology and regional geology to classify rocks bearing a history of high-temperature or high-pressure alteration while retaining coarse clasts.

In hand sample, metaconglomerates preserve clasts but the matrix is recrystallized; the texture ranges from non-foliated to foliated; clasts may be elongated or flattened; new minerals like muscovite or garnet can appear; the matrix often becomes a finer-grained mosaic of quartz and feldspar.

Optically and petrographically, they are distinguished by evidence of metamorphic minerals and textures, with clasts embedded in a metamorphic matrix; if strong foliation forms due to tectonism, the rock shows cleavage or lineation oriented parallel to the fabric.

Occurrence and significance: metaconglomerates occur in many mountain belts and terranes; they provide clues to metamorphic grade distribution and deformational history; they may overlie or intergrade with meta-sedimentary units; dating can constrain tectonic events.

Etymology and classification: 'meta-' indicates metamorphosed; 'conglomerate' refers to original coarse clastic sediment; classification recognizes protolith and metamorphic grade; the term can be used variably by authors, and regional naming conventions may differ.