Nephelines and analcimes in some alkaline igneous rocks |
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| Authors: | J F G Wilkinson H D Hensel |
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| Institution: | (1) Department of Geology and Geophysics, University of New England, 2351 Armidale, Australia;(2) Department of Geology, Australian National University, 2601 Canberra, Australia |
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| Abstract: | The paper presents electron microprobe analyses of nephelines and analcimes in alkaline igneous rocks ranging from theralite and basanite to mugearite and tinguaite. With few exceptions, the nephelines are Si-rich types whose Qz (quartz) components exceed those defining the limits of excess SiO2 in solid solution in the Ne-Ks-Qz-H2O system at 700°C and 1 kbarP
H
2
O. Unlike the nephelines in the basanites which show only limited grain-to-grain compositional variation, those in theralites and tinguaites from the differentiated Square Top intrusion, New South Wales, and in a New Zealand tinguaite vary significantly in Ne (nepheline), Ks (kalsilite) and Qz, even within individual samples, and they also may be strongly zoned. The rims of zoned nephelines are enriched in Si and Fe3+, relative to core compositions. These zoning trends contrast with the composional trend of successive  bulk nepheline fractions in the Square Top sequence theralite  tinguaite whereby Qz decreases. The nephelines coexist with high-temperature alkali feldspars. In the Ne-Ks-Qz system they plot on the Ne side of the Barth compositional join defined by the omission solid solution series with end-members K2Na6Al8Si8O32 (the Buerger ideal composition; Ne75Ks25 mol%) and  2Na6Al2Si10O32 (=cavity cation vacancy). The compositions of most natural nephelines are restricted to the field defined by Ne and the Barth join, compositions more K-rich than the ideal composition being relatively rare. The compositions of nephelines on the Ne side of the join are controlled by a number of factors which include the physical conditions attending nepheline crystallization and the compositions of the alkaline hosts. Interstitial analcimes from the Square Top intrusion display extensive NaAl Si substitution and their compositions extend from analcime of natrolite composition to compositions slightly more Si-rich than ideal NaAlSi2O6. Groundmass analcimes in the basanites, mugearite and New Zealand tinguaite have relatively constant compositions which approach stoichiometric NaAlSi2O6. An unusually Si-rich deuteric analcime (60.2% SiO2) is also present in vugs in the New Zealand tinguaite. Experimental and other evidence, includingP/T data defining the coexistence of analcime and silica-undersaturated silicate melt in the NaAlSiO4-KAlSiO4-SiO2-H2O system, and inferred solidus temperatures of the various hosts (they would have exceeded the stability range of analcime) preclude a primary magmatic origin for the interstitial and groundmass analcimes. These are interpreted as subsolidus phases produced by nepheline interaction with deuteric and/or hydrothermal fluids. Analyses of nephelines and their derivative analcimes indicate that the latter may form from both Si-rich and more Si-poor nephelines. |
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