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Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.
Dacite is relatively common, occurring in many tectonic settings. It is associated with andesite and rhyolite as part of the subalkaline tholeiite and calc-alkaline magma series.
The term dacite was used for the first time in the scientific literature in the book Geologie Siebenbürgens ( The Geology of Transylvania) by Austrian geologists Franz Ritter von Hauer and Guido Stache. Dacite was originally defined as a new rock type to separate calc-alkaline rocks with oligoclase phenocrysts (dacites) from rocks with orthoclase phenocrysts (rhyolites).
The relative proportions of and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram. This defines dacite as having a content of 20% to 60% quartz, with plagioclase making up 65% or more of its feldspar content. (2025). 9780521880060, Cambridge University Press. ISBN 9780521880060 However, while the IUGS recommends classifying volcanic rocks on the basis of their mineral composition whenever possible, dacites are often so fine-grained that mineral identification is impractical. The rock must then be classified chemically based on its content of silica and alkali metal oxides (Potassium oxide plus Sodium oxide). The TAS classification puts dacite in the O3 sector.
In thin section, dacites may have an aphanitic to porphyritic texture. Porphyritic dacites contain blocky highly zoned plagioclase phenocrysts and/or rounded corroded quartz phenocrysts. Subhedral hornblende and elongated biotite grains are present. Sanidine phenocrysts and augite (or enstatite) are found in some samples. The groundmass of these rocks is often aphanitic microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz or tridymite; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline.
Dacitic magma is formed by the subduction of young oceanic crust under a thick felsic continental plate. Oceanic crust is hydrothermally altered causing addition of quartz and sodium. As the young, hot oceanic plate is subducted under continental crust, the subducted slab partial melting and interacts with the upper mantle through convection and dehydration reactions. The process of subduction creates metamorphism in the subducting slab. When this slab reaches the mantle and initiates the dehydration reactions, minerals such as talc, Serpentine group, mica and break down generating a more sodic melt. The magma then continues to migrate upwards causing differentiation and becomes even more sodic and silicic as it rises. Once at the cold surface, the sodium rich magma crystallizes plagioclase, quartz and hornblende. Accessory minerals like pyroxenes provide insight to the history of the magma.
The formation of dacite provides a great deal of information about the connection between oceanic crust and continental crust. It provides a model for the generation of felsic, buoyant, perennial rock from a mafic, dense, short-lived one.
The type locality of dacite is Gizella quarry near Poieni, Cluj in Romania. Other occurrences of dacite in Europe are Germany (Weiselberg), Greece (Nisyros and Santorini), Italy (in Bozen quartz porphyry, and Sardinia), Austria (Styrian Volcano Arc), Scotland (Argyll), Slovakia, Spain (El Hoyazo near Almería), France (Massif de l'Esterel) and Hungary (Csódi Hill).
Sites outside Europe include Iran, Morocco, New Zealand (volcanic region of Taupo), Turkey, United States and Zambia.
Dacite is found extraterrestrially at Nili Patera caldera of Syrtis Major Planum on Mars.
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