Gang rock

Complex sub-volcanic dike on the Isle of Arran , Scotland. The gangue rock (a dolerite ) is more resistant to erosion than its surrounding rock and therefore smells like ribs from the subsoil.

Dike rocks (often also called sub- volcanic rocks or microplutonites ) are igneous rocks that have solidified in crevices at a relatively shallow crust. In terms of their structural features, they are therefore between the deep rocks ( plutonites ) and the effusion rocks ( volcanites ).

The dike rocks are grouped together with the plutonites under the name intrusive rocks .

Formation and characteristics of the dike rocks

The dike rocks represent a magma that, starting from a deep magma chamber, has penetrated into crevices relatively close to the surface of the earth and solidified there. Although the term “corridor” implies a structure that is narrowly limited in two spatial directions, corridors are generally only narrowly limited in one spatial direction. It is actually a rather platy rock body, bounded by approximately parallel surfaces and a few centimeters to a few hundred meters wide or thick, but can withstand laterally over many kilometers. Depending on their spatial position, such corridors are called dykes , if they are approximately vertical (saiger) or as sills (storage corridors), if they are more or less horizontal (solitary). Due to the shallow depth of the crust and the correspondingly relatively rapid cooling and solidification of the magma, no larger crystals (mineral grains) can form. Only when the melt had cooled down relatively slowly to a certain extent in the magma chamber could larger crystals grow there, which are then present in the otherwise finely crystalline gangue as so-called intruders . This is called the porphyry structure and is also found in volcanic rocks. However, the basic mass (“ matrix ”) of igneous dike rocks tends to be more coarse-grained than that of volcanic rocks. The same applies to the basic mass in so-called Intersertal structures (special case: ophitic structure ), which only occur in mafic rocks (including dolerite ) and are characterized by large, strip-shaped plagioclase crystals.

In some dike rocks, the chemical and mineralogical composition still largely corresponds to that of the initial magma of the magma chamber, so that the dike rock only differs in terms of its structure from the rock of the corresponding magma chamber (the corresponding pluton ). The dike rock microgranite (also called granite porphyry if the structure is appropriate ) is chemically and mineralogically hardly different from its plutonic equivalent, granite (or from its volcanic equivalent, rhyolite ). Such gangue rocks are referred to with the adjective aschist (from ancient Greek ασχιστός aschistos ' unsplit '). The collective terms subvulcanite and microplutonite are mainly used for ashy dike rocks.

In addition, there are also diaschiste (from διασχίζω diaschizo , ' dichotomous ') dike rocks that have emerged from melts that differ chemically clearly from their original magma. The reason for this deviation is a progressive differentiation of the magma while it is in the magma chamber. To apply Pegmatitgänge as the result of the injection of aggressive, highly mobile, with silica and incompatible elements (u. A. Rare earth enriched) residual melting pluton in columns of the surrounding rocks. In contrast to ashy dike rocks, pegmatites typically show a large to giant crystalline structure. Aplites , fine-grained, light-colored dike rocks that consist exclusively of quartz and feldspar also emerge from late magmatic differentials .

Lamprophyres are mesocrate to (rarely) ultramafic dykes with a porphyry structure, whereby the inserts consist exclusively of mafic minerals (predominantly biotite and / or amphiboles) and - if available - feldspars (often alkali feldspars) or foids only occur in the matrix. With this relatively exotic chemical and mineralogical composition, they are also considered to be diaschist gangue rocks. It is still unclear whether and to what extent lamprophyric magmas are differentiates.

Quarry in a wide dolerite dike in southern Sweden
Sample of microsyenogranite from the Leipzig area (" Beucha granite porphyry ")
A dolerite dike cuts an older granite pegmatite dike (southern Sweden)
Part of a warehouse corridor made of monchiquit (amphibole-rich, feldspar-free lamprophyre) in Quebec .

Demarcation

Igneous veins are not to be confused with mineral and ore veins. These arise from the precipitation of minerals from hot aqueous ( hydrothermal ) solutions. Pegmatite dikes can, however, already be seen as a borderline case between “real” igneous dykes and “real” hydrothermal dykes.

literature

Hans Cloos: Introduction to Geology , Textbook, 503p. (115–119), Brothers Borntraeger, Berlin 1963
A. Section iron: minerals and rocks , Hallwag-Taschenbuch, Bern-Stuttgart 1977

Web links

Geochemistry and petrography of late Variscan dike rocks of the Upper Bavarian Forest. (Thin section photos), www.isar-kiesel.de

Individual evidence

↑ RW Le Maitre (Eds.), A. Fahrtisen, B. Zanettin, MJ Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efermova, J. Keller, J. Lameyre, PA Sabine, R. Schmid, H. Sørensen, AR Woolley: Igneous Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks. Cambridge University Press, 2002, ISBN 978-0-521-66215-4 , p. 19.

[1] RW Le Maitre (Eds.), A. Fahrtisen, B. Zanettin, MJ Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efermova, J. Keller, J. Lameyre, PA Sabine, R. Schmid, H. Sørensen, AR Woolley: Igneous Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks. Cambridge University Press, 2002, ISBN 978-0-521-66215-4 , p. 19.