Dacite is commonly occur volcanic rock types of rock categorize and is intermediate in composition between andesite and rhyolite. In this slide will discuss about the short notes on the chemical composition of dacite, texture, geological context and formation and distribution of the dacite rock.

1.  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 tholeiitic and calc-alkaline magma series
Dacite
Figure 1

2. • Dacite consists mostly of plagioclase feldspar and quartz with biotite, hornblende,
and pyroxene (augite or enstatite).
• The quartz appears as rounded, corroded phenocrysts, or as an element of the
ground-mass. The plagioclase in dacite ranges from oligoclase to andesine and
labradorite. Sanidine occurs, although in small proportions, in some dacites, and
when abundant gives rise to rocks that form transitions to the rhyolites.
• The relative proportions of feldspars 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.
Composition

3. Figure 2
Chemical classification and nomenclature of fine-grained crystalline rocks using the total alkali
silica (TAS) diagram (after Le Bas et al., 1986
• 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 (K2O
plus Na2O). The TAS classification
puts dacite.

4. Texture
• 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.
• 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.

5. Geological context and formation
• Dacite usually forms as an intrusive rock such as a dike or sill.
• Examples of this type of dacite outcrop are found in northwestern Montana and
northeastern Bulgaria. Nevertheless, because of the moderately high silica
content, dacitic magma is quite viscous and therefore prone to explosive
eruption.
• A notorious example of this is Mount St. Helens in which dacite domes formed
from previous eruptions. Pyroclastic flows may also be of dacitic composition as is
the case with the Fish Canyon Tuff of La Garita Caldera.
• 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

6. • 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 partially melts 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, mica and amphiboles 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.

7. Distribution
• Dacite is relatively common and occurs in various tectonic and magmatic contexts:
• In oceanic volcanic series. Examples: Iceland (Heiðarsporður ridge), Juan de Fuca Ridge
• In calc-alkaline and tholeiitic volcanic series of the subduction zones of island arcs and active
continental margins. Examples of dacitic magmatism in island arcs are Japan, the Philippines,
the Aleutians, the Antilles, the Sunda Arc (Mount Batur), 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 volcanic series, often in association with tholeiitic basalts and intermediary
rocks.
• 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 Thera), Italy (in Bozen quartz
porphyry, and Sardinia), Austria (Styrian Volcano Arc), Scotland (Argyll),Slovakia, Spain (El
Hoyazo near Almeríaz, France (Massif de l'Esterel) and Hungary (Csódi Hill).
• Sites outside Europe include Iran, Morocco, New Zealand (volcanic region of Taupo), Turkey,
USA and Zambia.
• Dacite is found extraterrestrially at Nili Patera caldera of Syrtis Major Planum on Mars.

8. References
• "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological
Survey: Rock Classification Scheme. 1: 1–52. 1999.
• "Dacite" (PDF). Hungarian Natural History Museum. Retrieved 23
February 2018.
• https://en.wikipedia.org/wiki/Dacite
Dacite power point presentation
by Vasudevan M Master science in Geology