Pyrometamorphism

Pyrometamorphism occurs at very high temperatures (800 - > 1000 °C) and low pressures (< 2 kb) and typically results in the formation of "burnt" and fused rocks termed buchites, paralavas, clinkers and fulgarites. It is typically associated with shallow basaltic intrusions (contact aureoles, xenoliths,) combustion of carbonaceous matter, lightning strikes, and is also found in meteorites. During pyrometamorphism, the sequence of heating and cooling is greatly condensed favouring the preservation of a variety of stranded reaction microstructures that reflect disequilibrium reaction kinetics with metastable and mineral crystallisation.

Pyrometamorphism is a type of contact metamorphism (sanidinite facies) involving very high temperatures that may cause fusion in suitable lithologies at very low pressures. The high temperatures are attained by flow of mafic magma through conduits, by way of spontaneous combustion of coal, carbonaceous sediments, oil and gas, and through the action of lightning strikes. Temperature gradients are typically extreme, varying by several hundred degrees over a few metres or even centimetres. Relatively short periods of heating and cooling create an environment dominated by metastable melting and rapid mineral reaction rates driven by significant temperature overstepping of equilibrium conditions. This results in the formation of a large variety of minerals, many of which are metastable and are only found in pyrometamorphic rocks.