Trachyte

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trachyte

[′tra‚kīt]
(petrology)
The light-colored, aphanitic rock (the volcanic equivalent of syenite), composed largely of alkali feldspar with minor amounts of mafic minerals.

Trachyte

 

a cenotypal extrusive, usually porphyritic rock. Porphyritic phenocrysts and microlites embedded in volcanic glass are represented by sanidine; neutral and acid plagioclase, biotite, pyroxene, or amphibole are encountered in lesser amounts. Trachyte is the extrusive equivalent of syenite. It consists of up to 60 percent silica and up to 10 percent alkalies. The rock is rough to the touch. There are glassy trachytes, such as obsidians and pumices, and trachytic tuffs. Trachytes grade into liparites, andesites, and basalts. They are found in the Caucasus and, outside the USSR, in Italy and France. Trachyte is a relatively rare rock. (See also.)

References in periodicals archive ?
Three reference suites from Iceland, Moorea and Tristan da Cunha illustrate (respectively) the tholeiitic series containing basalt to rhyolite, the alkaline series with alkali basalts to trachytes and a strongly alkaline series, beginning with basanite and ultimately yielding phonolite; few oceanic islands contain tholeiiteseries rocks.
2] conditions, slightly Ne-normative basalts can yield highly evolved peralkaline, Hy- and Q-normative trachytes through a two-stage fractionation process occurring in deep and then shallow-level magma chambers.
86]Sr ratios in Kerguelen trachytes supports assimilation of hydrothermally altered oceanic crust in subsurface magma chambers (Gagnevin et al.
Separation of clinopyroxene and plagioclase phenocrysts resulted in phenocryst-poor magma enriched in Na, K, and Si and depleted in Mg and Fe relative to the parental magma, such as the phenocryst-poor latites and trachytes.
Phenocryst-poor latite and trachyte are interpreted as the result of mineral separation, and phenocryst-rich tephrite porphyry and olivine diabase porphyry are the result of phenocryst accumulations in the magma chamber.
From oldest to youngest (Elston and Snider, 1964), they are diabase porphyry, olivine diabase porphyry, fine-grained diabase, latite or trachyte, tephrite porphyry, and rhyolite.
Rocks are K-feldspar trachytes determined by modal classification.
Rocks were classified as K-feldspar trachytes on modal observations (Table 1), but sample PPG-084i chemical composition suggests high-K calc-alkaline rhyolites (Table 2, Figure 7).
The intrusions form laccoliths (or stocks - a debatable Black Hills subject), sills, and locally dikes composed of rhyolite, porhyritic monzonite, quartz monzonite, phonolite, and trachytes of quartz, biotite, and hornblende.
1990) stated that the late-stage trachyte porphyries are the most consistently mineralized igneous rocks in the Golden Reward mine area.
Those rocks are modal alkaline-feldspar trachytes with anorthoclase and sanidine crystals in the matrix (Pardo, 2004).
Those are hypocrystalline alkaline trachytes and rhyolites (Pardo, 2004), that suggest that volatile release was continuous through the permeable conduit walls while magma rising occurred.