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Dacite (/ˈdeɪsaɪt/) is avolcanic rock formed by rapid solidification oflava that is high insilica and low inalkali metal oxides. It has a fine-grained (aphanitic) toporphyritic texture and is intermediate in composition betweenandesite andrhyolite. It is composed predominantly ofplagioclasefeldspar andquartz.
Dacite is relatively common, occurring in many tectonic settings. It is associated with andesite and rhyolite as part of thesubalkalinetholeiitic andcalc-alkaline magma series.
The worddacite comes fromDacia, a province of theRoman Empire which lay between theDanube River andCarpathian Mountains (now modernRomania andMoldova) where the rock was first described.[1]
The termdacite was used for the first time in the scientific literature in the bookGeologie Siebenbürgens (The Geology of Transylvania) by Austrian geologistsFranz Ritter von Hauer and Guido Stache.[1][2] Dacite was originally defined as a new rock type to separate calc-alkaline rocks with oligoclase phenocrysts (dacites) from rocks with orthoclase phenocrysts (rhyolites).[1]
Dacite consists mostly ofplagioclase feldspar andquartz withbiotite,hornblende, andpyroxene (augite orenstatite). The quartz appears as rounded, corrodedphenocrysts, or as an element of the ground-mass.[3] The plagioclase in dacite ranges fromoligoclase toandesine andlabradorite.Sanidine occurs, although in small proportions, in some dacites, and when abundant gives rise to rocks that formtransitions to the rhyolites.[4]
The relative proportions offeldspars andquartz in dacite, and in many other volcanic rocks, are illustrated in theQAPF diagram. This defines dacite as having a content of 20% to 60% quartz, with plagioclase making up 65% or more of its feldspar content.[5][6][7][8] However, while theIUGS 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 andalkali metal oxides (K2O plusNa2O). TheTAS classification puts dacite in the O3 sector.
In hand specimen, many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white feldspars, and black crystals of biotite and hornblende. Other dacites, especially pyroxene-bearing dacites, are darker colored.[4]
In thin section, dacites may have anaphanitic toporphyritic texture. Porphyritic dacites contain blocky highly zonedplagioclase phenocrysts and/or rounded corrodedquartz phenocrysts. Subhedralhornblende and elongatedbiotite grains are present.Sanidine phenocrysts andaugite (orenstatite) are found in some samples. The groundmass of these rocks is oftenaphaniticmicrocrystalline, 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.
Dacite usually forms as an intrusive rock such as adike orsill. Examples of this type of dacite outcrop are found in northwesternMontana and northeasternBulgaria. Nevertheless, because of the moderately high silica content, daciticmagma is quite viscous[9] and therefore prone to explosive eruption. A notorious example of this isMount St. Helens in which dacite domes formed from previous eruptions.Pyroclastic flows may also be of dacitic composition as is the case with theFish Canyon Tuff ofLa Garita Caldera.[10]
Dacitic magma is formed by the subduction of young oceanic crust under a thickfelsic continental plate. Oceanic crust is hydrothermally altered causing addition of quartz and sodium.[11] As the young, hot oceanic plate is subducted under continental crust, the subducted slabpartially melts and interacts with theupper mantle through convection and dehydration reactions.[12] The process of subduction creates metamorphism in the subducting slab. When this slab reaches the mantle and initiates the dehydration reactions, minerals such astalc,serpentine,mica andamphiboles break down generating a more sodic melt.[13] 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.[14] 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 process by which dacite forms has been used to explain the generation of continental crust during theArchean eon. At that time, the production of dacitic magma was more ubiquitous, due to the availability of young, hot oceanic crust. Today, the colder oceanic crust that subducts under most plates is not able to melt prior to the dehydration reactions, thus inhibiting the process.[15]
Dacitic magma was encountered in a drillhole during geothermal exploration onKīlauea in 2005. At a depth of 2488 m, the magma flowed up the wellbore. This produced several kilograms of clear, colorless vitric (glassy, non-crystalline) cuttings at the surface. The dacite magma is a residual melt of the typicalbasalt magma of Kīlauea.[16]
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Dacite is relatively common and occurs in various tectonic and magmatic contexts:
Thetype locality of dacite is Gizella quarry nearPoieni, Cluj inRomania.[1] Other occurrences of dacite in Europe are Germany (Weiselberg), Greece (Nisyros andThera), Italy (inBozen quartz porphyry, andSardinia),Austria (Styrian Volcano Arc), Scotland (Argyll),[20]Slovakia, Spain (El Hoyazo nearAlmería),[21] France (Massif de l'Esterel)[22] andHungary (Csódi Hill).[23]
Sites outside Europe includeIran,Morocco,New Zealand (volcanic region ofTaupo),Turkey,USA andZambia.[citation needed]
Dacite is found extraterrestrially atNili Pateracaldera ofSyrtis Major Planum onMars.[24]
v. Richthofen's Namen gleichfalls ganz fallen zu lassen, dafür liegt wol nicht derselbe Grund vor. Dass die Oligoklasgruppe der "Quarztrachyte", dies muss der Name für die ganze Reihe bleiben, von der Orthoklasgruppe oder den "Rhyoliten" getrennt werden müsse, dafür plaidirte Roth gleichfalls schon in seiner Arbeit. Unser Nachweis der Altersverschiedenheit spricht nur um so dringender dafür. Für den Geologen genügen vielleicht die Namen "jüngerer" und "älterer" Quarztrachyt. Soll jedoch entsprechend der Sonderbezeichnung für die jüngere Gruppe, auch für die ältere Gruppe der Quarztrachyte ein besonderer Name eingeführt werden, so möchte der Name "Dacit" vielleicht entsprechend sein, da die Gruppe im alten Dacien eine besonders hervorragende Rolle zu spielen scheint).
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Flett, John Smith (1911). "Dacite". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 7 (11th ed.). Cambridge University Press. p. 728.
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