Pyrometamorphism

Pyrometamorphism, from the Greek pyr/pyro = fire, meta = change; morph = shape or form, is a term first used by Brauns (1912a, b) to describe high temperature changes which take place at the immediate contacts of magma and country rock with or without interchanges of material. The term was applied to schist xenoliths in trachyte and phonolite magma of the Eifel area, Germany, that had undergone melting and elemental exchange (e.g. Na₂O) with the magma to form rocks consisting mainly of alkali feldspar (sanidine, anorthoclase), cordierite, spinel, corundum, biotite, sillimanite and (relic) almandine garnet ± andalusite. Brauns (1912a, b) considered the essential indicators of pyrometamorphism to be the presence of glass with the implication that temperatures were high enough to induce melting, the formation of pyrogenic minerals (i.e. crystallized from an anhydrous melt), replacement of hydrous minerals by anhydrous ones, the preservation of the crystal habits of reacted minerals and of rock textures.

Pyrometamorphism related to intrusion of mafic magma and the burning of organic material is expected to result in anisotropic thermal expansion and associated positive volume changes in contact rocks due to reaction and melting of the mineral phases. Columnar jointing, cracking and dilation are typical of the structures developed. Temperature variation can be extreme with thermal gradients of several hundred degrees developed over a few meters or even tens of centimeters, particularly in the case of combustion, and lightning strike metamorphism where extreme temperature gradients occur over a few millimeters.

Pyrometamorphosed quartzofeldspathic rocks (sandstone, shale, claystone) and sediments (sand-silt, clay, glacial till, diatomaceous earth), and their metamorphosed equivalents (phyllite, schist, gneiss), are characterised by the presence of tridymite, mullite/sillimanite, cordierite, orthopyroxene, clinopyroxene, sanidine-anorthoclase, plagioclase (oligoclase–anorthite), corundum, hercynite-rich spinel, magnetite, ilmenite, hematite, pseudobrookite, sulphides and in carbonaceous protoliths, native metals. Ti-rich biotite and osumilite are less common; sapphirine is rare. These minerals are usually associated with acidic (rhyolitic) to intermediate (dacitic) glass that is frequently abundant enough for the rocks to be termed buchites and paralavas. Partly melted granite-granodiorite may contain tridymite, Ca-plagioclase, orthopyroxene and magnetite.

Siliceous carbonate rocks can be divided into protoliths that contain variable proportions of mainly dolomite, calcite and quartz that produce high temperature metamorphic assemblages of Ca-silicates, Ca- + CaMg-silicates and CaMg- + Mg-silicates with increasing amounts of dolomite.

Examples of anthropogenic pyrometamorphism are numerous and often closely analogous to natural pyrometamorphic processes and products. In this chapter, products and conditions of anthropogenic and biomass pyrometamorphism are described and include: bricks and ceramics derived from a variety of compositions; fused rocks associated with burning spoil heaps, in situ gasification; slags produced from non-metallic blast furnaces, iron ore smelting, surface burning, drilling and artificial fulgurites.

Metastable melting and high temperature disequilibrium reaction mechanisms are important processes in pyrometamorphism. Because of kinetic factors such as low diffusion rates, low fluid pressure and short-term heating, reaction textures in pyrometamorphic rocks do not generally achieve thermodynamic equilibrium and disequilibrium mineral assemblages arrested in various stages of up-temperature reaction are typically preserved. It is only with a coarsening of grain size during annealing at high temperatures, that thermodynamic equilibrium is approached during pyrometamorphism. Using light optics, the initial stages of mineral reactions can rarely be resolved because they occur over very small distances and the reaction products are typically extremely fine grained.