Migmatite
A rock was originally referred to as a migmatite , which consists of two or more petrographically distinguishable parts. One part shows features of a metamorphic rock , another, mostly lighter part, features of igneous rock . Today it is understood as a partially melted rock (or anatexite ), the light igneous part represents the former, re-solidified rock melt.
Structure according to structure
According to Mehnert (1968), migmatites are divided into four structural elements:
- The paleosome is that part of a migmatite that was not subject to partial melting and therefore contains the genetically older solid phases of the rock. Nevertheless, the paleosome can react chemically with the rock melt during the melting process. Unlike the mobile melt, the paleosome is fixed in place. Its composition and structure testify to the highly metamorphic parent rock that existed before the Anatexis.
- The formerly liquid, melted phase of the rock, i.e. the younger formation, is called the neosome .
- Is the neosome rich in quartz and feldspars , e.g. B. plagioclase and alkali feldspar and thus of a light (mostly red) color, it is called a leukosome . The leukosome shows an igneous structure .
- If it is dark in color and is therefore rich in cordierite , biotite and hornblende , it is called a melanosome . These parts usually form a dark border around the leukosome. Since the melanosome is the difficult-to-melt remnant of the parent rock from which the leukosome emerged, it is also known as
Distinctions according to the degree of metamorphosis
Only partially melted migmatites are also known as metatexites . These are rocks whose neosome parts are aplitic , pegmatitic or leucogranitic type.
In the further increase, almost completely melted migmatites, which can be considered almost magmatic, but still show a slight adjustment, are called diatexites .
The anatexis of various parent rocks is called Syntexis .
Occurrence and occurrence
The occurrence of migmatites of granitic composition indicates minimum temperatures of the metamorphosis of 650 ° C. Basic rocks generally only melt at higher temperatures.
Its occurrence in highly metamorphic areas of basement areas with older, at least Proterozoic rocks is typical . For Europe, the Baltic Shield and the Moldanubia are outstanding references .
In Central Europe, migmatites occur in the Black Forest (especially in the Kandel and Schauinsland ), in the Upper Palatinate Forest and in the Bavarian Forest . In the Alps there are tertiary migmatites, for example in Ticino (in the so-called southern steep zone, immediately north of the Insubric seam ) and in the Ötztal-Stubai-Kristallin .
Natural stone types
- Paradiso (India, Tamil)
- Brasil Tropical (Brazil, Minas Gerais)
- Multicolor Red (India)
- Aurora Borealis ( Norway , Finland )
See also
literature
- Karl Richard Mehnert Migmatites and the origin of granitic rocks , Developments in Petrology, Volume 1, Amsterdam, Elsevier 1968, 2nd edition 1971.
- Edward W. Sawyer: Atlas of migmatites . In: The Canadian Mineralogist, Special Publication, 9 . NRC Research Press, 2008, ISBN 978-0-660-19787-6 .
- Roland Vinx: Rock determination in the field . Elsevier Spektrum Akademischer Verlag, Munich 2005, ISBN 3-8274-1513-6 .
- Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Ferdinand Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
