Dacite


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dacite

[′dā‚sīt]
(geology)
Very fine crystalline or glassy rock of volcanic origin, composed chiefly of sodic plagioclase and free silica with subordinate dark-colored minerals.

Dacite

 

a magmatic rock that is the effusive equivalent of granodiorite and quartz-diorite. Dacite consists of a finegrained or vitreous basic mass and phenocrysts of plagioclase and quartz, more rarely, of hornblende, biotite, and pyroxene, and sometimes, sanidine. Where albitization processes are widely developed, dacite turns into quartz al-bitophyre. Dacites were ejected by volcanoes in the Paleozoic (in the Urals, for example), Mesozoic, and Cenozoic (in the Caucasus, for example). They are also found among modern lavas on Kamchatka.

References in periodicals archive ?
A: andesite, BA: basaltic andesite, BTA: basaltic trachyandesite, and D: dacite. Adjusted composition (% m/m) and CIPW norm calculated applying SINCLAS program [14,15].
This rock that spans the compositional fields of both andesite and dacite (see examples on page 92) has less silica than the dacite of block F, but it is also a volcanic flow rock.
The upper volcanic assemblage comprises basalt and andesite flows, felsic and heterolithic volcanic breccia, bedded tuff, argillite, chert, and bodies of porphyritic dacite. The flows are variably massive, vesicular, pillowed and autobrecciated, and in places contain interbeds of tuff and argillite.
The magma body was also the source of trachyte and dacite plugs and pipes intruded into the upper crust, leading to the formation of skarns with garnet, pyroxene, amphibole and magnetite in the Trepca orebodies.
In western North America, dacite stone tools and the 'reduction flakes' produced during their manufacture (shown here) represent the most ubiquitous artifacts associated with archaeological sites in this area.
A number of lithological units were logged over the 20 m that the Devico system drilled -- drilling commenced in dacite, then into basalt and a narrow sediment was also observed.
There was also considerable within-plot heterogeneity in slope and aspect due to the irregular lobate structure of the flow surface and the presence of [leq]1-m diameter dacite blocks that protrude above the flow surface (Eppler, 1984).
High-silicon lavas such as dacite and rhyolite tend to form short, gooey flows and steep volcanic peaks.
It is easy to see, then, how the discovery in 1951 of self-reversing minerals in the dacite (silca-rich volcanic rocks) lava fields of Mt.
These volcanic strata can be divided into the Majiashan, Lingxiang, and Dasi formations from bottom to top, with compositions of (trachy-) basalt, (trachy-) andesite, (trachy-) dacite, and rhyolite [22, 26, 36].
The oldest rock type is massive dacite (e.g., along the Zbirozsky potok Creek) and andesite (e.g., Tyrovicke skaly Cliffs), followed by dacite with phenocrysts and rhyolite (Kohoutov).
However, the grains of tachylithic scoria and vesiculated sideromelane that were considered as the juvenile fraction of the ash for this eruption (squares in figure 7), show a narrow distribution in composition ranging from andesite to dacite with silica contents between approximately 58 and 62 wt.%.