Dacite

Dacite, volcanic rock that may be considered a quartz-bearing variety of andesite. Dacite is primarily associated with andesite and trachyte and forms lava flows, dikes, and sometimes massive intrusions in the centres of old volcanoes. Like andesite, dacite consists mostly of plagioclase feldspar with biotite, hornblende, augite, or enstatite and generally has a porphyriticstructure; additionally, however, it contains quartz as rounded, corroded crystals or grains, or as a constituent of the groundmass. The feldspar content of dacite ranges from oligoclase to andesine and labradorite; sanidine occurs also in some dacites and when abundant gives rise to rocks transitional to the rhyolites.

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.

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.

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QAPF Diagram Dacite field in blue





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Dacite sample, Plagioclase (white) and Biotite (dark). From "The University of Auckland"





Bibliografia



Le informazioni contenute in questa pagina sono tratte da:
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London


Foto
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Plagiolcase, Biotite and clinopyroxene in a dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
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Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
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Plagiolcase and Biotite in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase and Biotite (with plagioclase inclusions) in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
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Plagiolcase and Biotite (with plagioclase inclusions) in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase and clinopyroxene in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
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Plagiolcase and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase, Biotite and clinopyroxene in a Dacite from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase and Hornblende in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
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Plagiolcase quartz (rounded) and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase and Hornblende in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
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Plagiolcase and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase and Hornblende in a Dacite from Bulgaria. PPL image, 2x (Field of view = 7mm)
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Plagiolcase quartz (rounded) and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Plagiolcase and Hornblende in a Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 2x (Field of view = 7mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 2x (Field of view = 7mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)
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Feldspars altered by sericite in a hydrothermal altered Dacite. XPL image, 10x (Field of view = 2mm)