Blueschist

Blue slate or glaucoma slate are often bluish rocks that have metamorphosis at relatively low temperatures and high pressures (less than 400 ° C GPa ⁻¹ ). Such rocks are mainly found in active or former subduction zones of maximal neoproterozoic age (0.8 billion years BP) or younger due to the increasingly cooler mantle and the overlying earth's crust . The bluish color is caused by the mineral glaucophane , an amphibole , which is not always present, so that blue slate is not necessarily bluish in color. The blue slate are characteristic of rocks that have undergone a metamorphosis, so that the so-called blue slate facies (see also metamorphic facies ) are named after them .

Blue slate facies

The starting rocks for blueschist are basalts and rocks with a composition similar to basalt. These rocks are subject to metamorphosis at around 200 - 400 ° C and 6 - 12 kilobars . The special metamorphic conditions lead to the formation of a socialization of minerals, the occurrence of which can serve to determine the metamorphic facies.

The eponymous blue amphiboles are stable over a wide pressure-temperature range, which can extend into the field of the green schist facies . The occurrence of these minerals is therefore not a sure sign of the blue schist facies. However, if they occur with the also bluish lawsonite , with aragonite and / or jadeitic pyroxene , this is a sure sign of the blue schist facies. Other peculiarities are the absence of biotite , andalusite and sillimanite and of feldspars other than albite .

As with all metamorphic facies, the formation of the characteristic minerals depends on the composition of the parent rock. In Meta basiten the minerals formed next Glaukophan lawsonite and chlorite , sphene , epidote , phengite , paragonite and omphacite . Meta gray wacken prevail quartz and jadeite before next Lawsonite, Phengite, glaucophane and chlorite. Typical of carbonate rocks ( marble ) is the appearance of aragonite, while in metapelites phengite, paragonite, carpholite , chlorite and quartz are characteristic.

literature

Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
Myron G. Best: Igneous and Metamorphic Petrology . 1st edition. WH Freemann & Company, San Francisco 1982, ISBN 0-7167-1335-7 , pp. 399 f .

Web links

Subdivision according to the concept of metamorphic facies. Learning portal for the mineral and rock determination internship at the Free University of Berlin

Individual evidence

^ Richard M. Palin, Richard W. White: Emergence of blueschists on Earth linked to secular changes in oceanic crust composition . In: Nature Geoscience . tape 9 , no. 1 , January 2016, ISSN 1752-0894 , p. 60–64 , doi : 10.1038 / ngeo2605 ( nature.com [accessed August 9, 2017]).
↑ The prefix meta- denotes a transformed rock in petrology: a metapelite is a metamorphic pelite (claystone)

[1] Richard M. Palin, Richard W. White: Emergence of blueschists on Earth linked to secular changes in oceanic crust composition . In: Nature Geoscience . tape 9 , no. 1 , January 2016, ISSN 1752-0894 , p. 60–64 , doi : 10.1038 / ngeo2605 ( nature.com [accessed August 9, 2017]).

[2] The prefix meta- denotes a transformed rock in petrology: a metapelite is a metamorphic pelite (claystone)