Plutonite

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Typical appearance of a plutonic rock (here a granodiorite )

Plutonites , plutonic rocks or deep rocks are igneous rocks that are formed at great depths through extremely slow cooling of magmas . The resulting rock bodies, which can reach the earth's surface through uplift processes and erosion, are called plutons or, if they reach very large dimensions, batholiths . Since the temperatures in the formation depths of several kilometers , which are typical for plutonites, are relatively high , the crystallization proceeds more slowly than with magma that has penetrated to the surface of the earth ( lava ) and from which the volcanic rocks emerge. This is why plutonites have a typically large-grain texture.

Together with the sub-volcanic rocks , the plutonites are intrusive rocks .

composition

Through partial rock melt and igneous differentiation - as a result of processes such as fractional crystallization - different types of plutonites arise from an initially uniformly composed magma. As the minerals gradually crystallize out, the composition of the residual magma that has not yet crystallized changes in the direction of a higher silicate content , which in turn continuously shifts the composition of the plutonites still formed from this residual melt to higher silicate contents and results in the following, rather rough, crystallization sequence:

Peridotite → gabbro → diorite → syenite → granodiorite → granite → alkali feldspar granite
Example of the classification of mafic plutonites

By removing darker parts of the mixture, the plutonites become lighter and lighter in the above order. For example, the dark, black-green peridotite at the other end of the row is juxtaposed with the much lighter, albeit differently colored, granite, and this increasing brightness is one of the most important first distinguishing features of the plutonites. In addition, their specific gravity decreases in the order mentioned, while the silica content increases. Granite and diorite are considered to be "acidic" rocks because of their high silica content, while gabbros and peridotites, with their lower silica content, are considered "basic". The terms “acidic” and “basic” are, however, borrowed from old miners' practice, so they have nothing directly to do with the pH value .

Felsic plutonic rocks are proportionate to their composition from quartz , alkali feldspar , plagioclase feldspar and Foiden in a qapf diagram shown mafic the other hand, the other diagrams in about the content of olivine , pyroxene and hornblende use for classification.

structure

The structure of plutonites is characteristic. They consist almost exclusively of crystallized minerals, i.e. they have a fully crystalline structure , and most of these crystals can be seen with the naked eye. Plutonitische crystals are due to the slow cooling usually much time to train and therefore are already mostly with the naked eye visible. The rock matrix consequently shows a medium to coarse-grained, at most partially porphyry structure , whereby minerals that crystallize out early can develop almost idiomorphically .

Often there is no directional texture , but in some cases flow processes create flow textures in which crystals are regulated and trace the last movements of the magma. Through crystallization processes and the sinking of crystals, a more or less distinct banding occurs in some plutonites. There are no cavities between the crystals , so they are bubble-free.

meaning

Plutonites are often responsible for the formation of mineral and ore deposits , which, however, can also be formed by other processes. Surface outcrops of plutonites can only be seen after the cover layer has been removed, usually combined with uplifts and folds of the rock layers. The weathering forms of plutonites are predominantly rounded and soft. Plutonites, especially the granite, are used for crushed stone and cobblestones in which the individual crystals can be clearly seen.

Examples of plutonites

literature

  • Myron G. Best: Igneous and Metamorphic Petrology . WH Freemann & Company, San Francisco 1982, ISBN 0-7167-1335-7 , pp. 33 ff .
  • Werner Zeil: Brinkmann's outline of geology, first volume: General geology . 12th edition. Ferdinand Enke Verlag, Stuttgart 1980, ISBN 3-432-80592-6 , p. 195 ff .

Web links

  • Gert Klöß: Introduction to Petrography / Petrology: Chapter C, Plutonites. (pdf, 968 kB) Institute for Mineralogy, Crystallography and Materials Science at the University of Leipzig, December 20, 2007, archived from the original on December 7, 2008 .;
  • Rock description and rock identification. Lecture notes. (Microsoft Word, 550 kB) (No longer available online.) ETH Zurich ;formerly in the original;
  • Ralph Delzepich: Plutonite. Geology at RWTH Aachen University, archived from the original on February 26, 2011 (winter semester 1999/2000).;
  • Jan W. Krieger: Deep rocks. In: Jan's Mineral and Fossil Pages. July 7, 2002 .;