Continental subduction in Paleoproterozoic: Insights from ∼ 1.9 Ga nepheline syenite in the North China craton
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Nepheline syenite
Nepheline
Nepheline syenite, which has been reported by L. F. Ransome to occur in Brookville, N. J., was not found despite careful search, including several visits to the locality and examination of the originally described specimens. Ransome9s "nepheline syenite" is a granophyric differentiate of diabase. The only known nepheline syenites in the eastern states south of the Beemerville, N. J., complex are in Augusta County, Va.
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The igneous succession in Mount Royal is outlined emphasizing the fact that there were two distinct advances of a nepheline syenite magma.Those rocks which are definitely known to belong to the older nepheline syenite and those known to belong to the younger are described separately.A number of types are described which belong either to the first or to the second intrusion, but whose relations are not definitely known. These include most of the nepheline syenite pegmatites, nordmarkite pegmatites, soda-rich monzonites, syenites and an unusual porphyry intermediate between a shonkinite and an essexite.Seven chemical analyses are given.
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Concentrations of radioactive minerals are found in the eudialyte- bearing nepheline syenites of the Ilimaussaq complex in South West Greenland. The radioactive minerals may occur as accessories in the melanocratic nepheline syenite, called lujavrite. The lujavrite is considered to be the youngest rock of the complex, in which it occupies a considerable volume. The U content averages about 150 ppm. In restricted areas concentrations of 500 to 2000 ppm have been found. The Th content is 2 to r times that of U. Radioactive minerals may also occur as local concentrations in inclusions of older, recrystallized nepheline syenite in the lujavrite and as concentrations in recrystullized zones of deformation in some of the oldest nepheline syenites of the complex. Only the lujavrite may be considered as possible low-grade U and Th sources. The other occurrences are mainly of genetical interest. The most important radioactive mineral is steenstrupine, with about 0.3% U and 2.7% Th. Steenstrupine may alter to various poorly defined radioactive alteration products. Monazite, britholite, microlite, and thorianite have been found in small amounts. The eudialyte of the nepheline syenites is faintly radioacti ve. Steenstrupine is found in rocks, in which the eudialyte has been altered hydrothermally. (auth)
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A bstract The fundamental properties of nepheline syenite, such as thermal expansion and fusion characteristics, are given. Test specimens of twelve bodies, comprising various flux combinations of nepheline syenites and potash feldspars with talc, whiting, magnesium carbonate, and pyrophyllite, were fired to seven different cones. The firing linear shrinkage curves of combinations of nepheline syenite with the. various materials are somewhat similar to such combinations with potash feldspar except that the syenite combinations are active at lower temperatures. That the sintering ranges of nepheline syenites are longer than those of potash feldspar is explained thermochemically.
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Supercontinent
Dharwar Craton
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The Kaleybar nepheline syenite intrusion forms the largest silica undersaturated alkaline exposure in northwestern Iran. It consists of various rock types ranging from nepheline syenite to nepheline diorite that were emplaced during Eocene-Oligocene times, corresponding to the Alpine orogeny. The essential rock-forming minerals in nepheline syenite are plagioclase, K-feldspar, nepheline and amphibole. Clinopyroxene is the dominant phase in nepheline diorites. Titanian garnet occurs as an uncommon accessory phase forming reddish to deep brown individual grains. Chemically it is intermediate between Ti-andradite (67 to 78 mole %) and grossular (21 to 33 mole %) with TiO2 contents ranging from 1.5 to 5.0 wt %. Stoichiometry and R-mode factor analysis on garnet chemistry show that the dominant exchange vectors are Si-Ti and Al-Fe substitutions in the tetrahedral and octahedral crystal sites, respectively. A magmatic origin of the investigated Ti-garnet is suggested on the basis of mineralogical criteria and chemical properties.
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Nepheline syenite
Nepheline
Porphyritic
Trachyte
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