Dacite
Dacite ( dacite also ) is an igneous rock, specifically a felsic (acidic ) volcanic. Dacitic magmas are subalkalisch and belong to kalkalkalischen volcanic series ( with moderate potassium content). They are also metaluminos (A ' / F < 0.33), supersaturated in SiO2 and quartz - normative.
Paläovulkanische dacites are called Quarzporphyrit.
Etymology and history
The name " dacite " is derived from the former Roman province of Dacia. It was first used in 1863 by Franz von Hauer and Guido Stache in their scientific work geology of Transylvania, to enable them rich in quartz and oligoclase leading Trachytvarietät (previously known yet Quarztrachyt ) to distinguish from ordinary trachytes. They related this to a notice published in the same year description of the rock with the results of microscopic examinations by Ferdinand Circle in the meeting reports of the Imperial Academy of Sciences in Vienna.
The type locality is situated in the valley of the Sebes Körös near the village Poieni (formerly Kissebes ) in the northern Apuseni Mountains in Romania. There are numerous quarries are in dacite, and in upcoming trachytes, the aufschlossen comprehensively at the time of Hauer and Stache the rock.
The occurrence between Poieni and the adjacent Bologa (formerly Sebesvár ) won a short time after the scientific description of rocks on the basis of its physical and technical quality of great economic importance. From the main quarry operation in this region to be delivered from 1872 operators huge amounts of paving stones for roads and footpaths. Significant places of delivery in the former Hungary were Budapest, Békéscsaba, Brassó, Debrecen, Gyula, Kolozsvár, Orosháza, Szarvas Szeged (focus of deliveries ), Szolnok and other cities. Furthermore, the railroad used the rock in the form of blocks for bridges Alfölder the train and as railway ballast. In 1881 they built a railway siding for the newly developed extraction points on the left flank of the Körös Valley.
Classification
In the Streckeisen diagram dacite takes QAPF fields 4 and 5, ie Plagioklasanteil on the feldspar content is greater than 65 % and its modal quartz content varies between 20 and 60 %. Are the mineral constituents can not be seen, as it is defined on the basis of chemical analyzes in the TAS field O3, ie its proportion by weight of SiO2 is typically 63-68 % (but can be higher ) and its weight percentage of Na2O and K2O is a total of less than 7-8 %.
Composition
Dacite consists of about 66 percent by weight of silicon dioxide. He is a Ergussäquivalent of granodiorite and tonalite of. In a dense matrix are phenocrysts of plagioclase and quartz. As part dark mixture occurs mainly hornblende.
Mineral constituents
- As phenocrysts act: Quartz - rounded and corrosion. His frequent occurrence is an important distinguishing feature of the andesites
- Plagioclase - oligoclase to andesine / labradorite often zoned, twinned and may also show signs of corrosion. Does the zoning sometimes concentric glass inclusions
- Sanidine - with frequent occurrence arise transitions to rhyodacite and rhyolite
- Biotite - brown
- Hornblende - brown, greenish brown, or edenitische hastingsitische composition. Furthermore, you may also have:
- Augite - green
- Orthopyroxene ( hypersthene / enstatite )
- Garnet ( almandine )
- Cordierite
- Olivine
- Magnetite, titanomagnetite, hematite and ilmenite, as well as further
- Zircon, titanite, apatite and fluorite
Overall, the structure of dacites of completely crystalline forms varies based on mixed forms ( devitrified types) to pure glassy forms such as dacite obsidian, pitch stones and Bunsen.
Chemical composition
The following table gives the average values of chemical analyzes of 651 dacites with the translation in the CIPW norm again, as well as two examples of the type locality in the Apuseni and the Pinatubo (Philippines) for the composition of the trace elements:
External appearance and varieties
Dacite is fine-grained and usually has a pale gray to reddish- brown color, bearing in mind that its appearance is highly dependent on the potential of glass. Its color index is usually between 12 and 14 ie 12-14% are dark mafic minerals. Dacites with a color number <5 are Leukodazite, with a color index > 25 Meladazite.
Hornblende - biotite dacite and dacite are usually colored gray, brownish or yellowish. Augite dacite and dacite enstatite are dark in color.
Responsible for the final coloring, in addition to phenocrysts with their characteristic intrinsic color finely divided pigment minerals such as hematite, goethite and chlorite.
Formation and association
Dacite formed by rapid cooling of viscous magma that emerges as lava at temperatures of about 800 to 1000 ° C. There has been explosive ejected among others in 1980 by the Mount St. Helens and 1991 by the volcano Pinatubo as dacite ash. Dacite but also can make transitions and massive intrusions in volcanic centers.
Although dazitisches magma towards Rhyolite is poorer in silica, it can still achieve a higher viscosity - in the danger of the outbreak of activity it is rhyolitic magmas thus in every way. The high explosive potential dazitischer magmas can be explained due to the high crystal content in the already quite siliceous melt. So it may come to Plinian eruptions of enormous proportions, go by which vast amounts of tephra and hot volcanic gases high into the stratosphere.
For dacites there is as yet no unified development model, and it is quite likely that several mechanisms contribute to the formation of Dazitmagmen. The following mechanisms are commonly used for Inselbogendazite:
- Fractional crystallization of basic precursors (eg basalts, basaltic andesites ).
- Mixing of magma rhyolitischem with basic magma; conversely, it is also possible that basic magmas have been heavily contaminated by silica- rich rocks.
- Education dazitischer melting of basaltic rocks by melting; latter originate either from amphibolite facies crustal areas, or from the subducting plate eklogitfaziellen ( Slab ).
Dacites are usually associated with andesites and trachytes.
Occurrence and localities
Dacites are relatively common and occur in different tectonic and magmatic contexts:
- In oceanic Vulkanitserien. Examples: Iceland, Juan de Fuca Ridge
- In kalkalkalischen and tholeiitic Vulkanitserien the subduction zones of island arcs and active continental margins. Examples of dacitic magmatism in island arcs are Japan, the Philippines, the Aleutian Islands, the Antilles, the Sunda Arc, Tonga and the South Sandwich Islands. Examples of dacitic magmatism in active continental margins are the Cascade Range, Guatemala and the Andes (Ecuador and Chile).
- In continental Vulkanitserien, often in association with Tholeiitbasalten and intermediate rocks.
Locations of dacite in Europe are Germany ( Weisel mountain at top churches in the Saarland ), Greece ( Nisyros and Thera ), Italy ( turned on in Bozen quartz porphyry, and Sardinia), Austria ( Styrian Volcanic Arc ), Romania ( Transylvania), Scotland ( Argyll ), Slovakia, Spain ( El Hoyazo at Almeria) and Hungary.
Non-European localities are still in Iran, Morocco, New Zealand ( Taupo Volcanic Zone ), Turkey, United States and Zambia.
Dacite is also to be extraterrestrial exist on Mars.
Use
Dacite is used for flooring and paving and is under different names in the trade, such as under " Kosice " (Slovakia ), " Szob " ' (Hungary ) or " Yazd Red" (Iran). There are colored variants that have been used for centuries as gemstones.