Metamorphism of Pelitic (Al-Rich) Rocks

Abstract Pelites are clay-rich sedimentary rocks that form in low-energy depositional environments, including lakes, the distal portions of continental shelves, and deep-sea abyssal plains. When metamorphosed, pelites show a great diversity in mineral assemblages and textures that are highly sensitive to changing pressure and temperature conditions within the Earth. As such, they have considerable value for petrologists who employ thermobarometry to interpret the tectonic evolution of geological terranes. Various forms of thermodynamically constrained phase diagrams may be used to investigate the stability of different parageneses as a function of pressure, temperature, and bulk composition. Contact metamorphism of pelites adjacent to igneous intrusions produces low-pressure mineral assemblages commonly with the index minerals biotite, andalusite, cordierite, K-feldspar, and sillimanite stabilizing up-grade, although this order of appearance varies with both pressure and bulk-rock composition. Staurolite, chloritoid, and/or garnet may occur in Al-rich pelites in this environment. Regional metamorphism of pelitic rocks during orogenesis produces the well-known Barrovian prograde succession, with index minerals chlorite, biotite, garnet, staurolite, kyanite, and sillimanite. Partial melting is expected to occur at upper amphibolite facies conditions in the presence of water or due to breakdown of muscovite and biotite mica, producing granite. Metamorphism of pelitic rocks during subduction produces chlorite (sudoite), carpholite and chloritoid at relatively low pressure, and Mg-rich garnet, talc, kyanite, and/or Si-rich white mica at high pressure. Whiteschists represent a rare subset of metapelites formed by subduction of extremely Mg- and clay-rich protoliths.