Ultrametamorphism

regional metamorphism of rocks in the deep zones of the earth’s crust, accompanied by the development of migmatites. As a result of ultrametamorphism, meta-morphic rocks, such as gneisses, pyroxene-plagioclase schists, and amphibolites, are subjected to secondary, often retrogressive, metamorphism, leading to their granitization at temperatures of 650°-800°C and lithostatic pressures of 4-10 kbar (0.4-1 giganewton/m²). In this process, the pyroxenes are replaced by hornblende, the hornblende by biotite, and the plagioclase by potassium feldspar and quartz. As a result, the general chemical composition of the rocks is significantly altered: potassium and silicon are introduced along with rubidium, zirconium, lanthanum, and cerium, while sodium, lithium, chromium, nickel, cobalt, zinc, titanium, vanadium, molybdenum, yttrium, and gold are removed.

The granitization of rocks during ultrametamorphism, leading to the formation of migmatites, involves anatexis and extensive replacement of the migmatites by acidic magma saturated with volatile components along bedding and schistosity planes and in cracked and brecciated zones. The zones of ultrametamorphism are areas of deep generation of granitic magma that is enriched by volatile components and becomes able to penetrate strata of metamorphic rock. Ultrametamorphism is characteristic of the orogenic stage of development of géosynclinal mobile zones.

The term was proposed in 1909 by the Swedish geologist P. Holmquist.