Tholeiite


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Related to Tholeiite: trachyte

tholeiite

[thō′lē·ə‚īt]
(petrology)
A group of basalts composed principally of plagioclase, pyroxene, and iron oxide minerals as phenocrysts in a glassy groundmass.
Any rock in the group.

Tholeiite

 

(named for the village of Tholey, in the Saar, Federal Republic of Germany), a volcanic rock devoid of porphyritic inclusions. Tholeiite is composed of basic plagioclase (labradorite), pyroxenes (pigeonite, augite, hypersthene), basaltic hornblende, quartz, and sometimes olivine. The interstices between the components contain particles of unaltered volcanic glass, with microlites and dendrites of plagioclase and ore mineral. The entire mix is known as a tholeiitic structure. Upon decomposition of the glass, tholeiite is converted into quartz-potassium-feldspar granophyres characteristic of traprock. Tholeiite is the most widely distributed of the petrochemical basalts, which are slightly supersaturated with silica; it is contrasted with alkaline (olivine) basalt, which is incompletely saturated with SiO2.

The study of tholeiite is of significant theoretical value in solving problems related to magma formation, since tholeiite reflects the composition of the upper mantle in regions where magmatism has occurred.

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References in periodicals archive ?
(1986) but overlap into true basalt, as is common for quartz tholeiite derived from sub-continental lithospheric mantle (Dorais et al.
The first chemical analyses of magmatites of the Nicoya Complex were made by Dengo (1962a) and Weyl (1969), concluding oceanic tholeiites.
8d) of Pearce and Cann (1973), classify the gabbros as both island arc tholeiite (IAT) and depleted mantle oceanic floor basalt (N-MORB).
Like the plutons, the Bryant Point Formation is mainly calk-alkaline but straddles the boundary with the tholeiite field (Fig.
Compared to arc tholeiites, calc-alkalic magmas have higher abundances of incompatible large ion lithophile (LIL) elements reflecting enrichment in the mantle wedge source.
Keywords: Bagh complex; Melange; volcanic rocks; Tholeiite; Alkaline; Petrogenesis
Jal-Niat amphibolites are generally fine grained, banded and strongly foliated and fall in tholeiite group and show enrichment in HFSE (Zr, Y, P, Nb) and depletion in LILE (Rb, Pb, Th, K).
Gabbro intrusions of the Cobequid Shear Zone are of olivine tholeiite composition and appear geochemically related to the basalt.
Olivine precipitation appears to begin and end early in tholeiite rock suites creating a restricted range of olivine Fo contents (e.g., [Fo.sub.90] - [Fo.sub.70] for Kilauean tholeiites).
%) within the range of continental tholeiite (Pearce and Cann 1973).
Among these, the best candidates for LIPs are thick tholeiite sequences that contain komatiites.
In these areas, calc-alkaline arc volcanic rocks like those of the Goulette Brook Formation are transitional between older primitive arc tholeiite and younger back-arc basalt, and were probably generated during arc rifling.