Abstract
Mineral compositional relationships have been studied in a variety of alkaline basic rocks from small, highlevel intrusions. These small intrusions must have cooled quickly as the phases are all zoned, especially those forming the matrix of some of the rocks which seem to have formed under conditions of almost perfect fractional crystallization.
The compositions of nephelines from the various rocks define an overall evolutionary trend in which Na2O, K2O and CaO decrease and SiO2 increases. The most SiO2-rich nephelines plot in the nepheline plus feldspar stability field and must have crystallized metastably from the melt. Clear, interstitial analcime is definitely not a primary phase. It appears to be restricted to rocks which crystallized primary SiO2-rich nepheline and formed from this phase by subsolidus recrystallization processes.
Most of the rocks studied appear to have crystallized plagioclase as the first felsic mineral. In some rocks this phase is zoned from ∼An70-60, through anorthoclase compositions to rims of composition ∼Or50 and this is the only feldspar present in the rocks. In other rocks the “plagioclase” shows zoning to similarly potassic compositions (Or40–50) but this is accompanied by separate grains of orthoclase cryptoperthite. The zoning trends of the coexisting “plagioclase” and alkali feldspar allow the feldspar critical end point compositions to be estimated for these rocks. Rocks containing strongly zoned plagioclase as the only feldspar species are believed to have evolved towards relatively K2O-rich low-temperature melting compositions due to the presence of mafic components.
The crystallization histories of the rocks are considered in terms of three planes in the An-Ne-Ks-Qz system i.e. Ne-Ks-Qz; (Ne60Ks40)-An-Qz; and An-Ab-Or. Mineral and bulk rock compositional data together with textural criteria are used to modify phase equilibrium relations in these simple experimental systems to take account of the additional components (e.g. mafics) present in natural magmas.
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References
Bowen NL, Schairer JF (1938) Crystallization equilibrium in nepheline-albite-silica mixtures with fayalite. J Geol 46:397–411
Brown FH (1970) Zoning in some volcanic nephelines. Am Mineral 55:1670–1680
Carmichael ISE (1964) Natural liquids and the phonolitic minimum. J Geol 4:55–60
Carmichael ISE, Turner FJ, Verhoogen J (1974) Igneous petrology. McGraw-Hill, New York
Dollase WA, Thomas WM (1978) The crystal chemistry of silicarich, alkali-deficient nepheline. Contrib Mineral Petrol 66:311–318
Greig JW, Barth TFW (1938) The system Na2O·Al2O3·2SiO2 (nephelite, carnegieite)-Na2O·Al2O3·6SiO2 (albite). Am J Sci 35A:93–112
Gibb FGF, Henderson CMB (1978) The petrology of the Dippin sill, Isle of Arran. Scot J Geol 14:1–27
Hamilton DL (1961) Nephelines as crystallization temperature indicators. J Geol 69:321–329
Hamilton DL, MacKenzie WS (1965) Phase-equilibrium studies in the system NaAlSiO4 (nepheline)-KAlSiO4 (kalsilite)-SiO2-H2O. Mineral Mag 34:214–231
Henderson CMB, Gibb FGF (1972) Plagioclase-Ca-rich-nepheline intergrowths in a syenite from the Marangudzi complex, Rhodesia. Mineral Mag 38:670–677
Henderson CMB, Gibb FGF (1977) Formation of analcime in the Dippin sill, Isle of Arran Mineral Mag 41:534–437
Kim K-T, Burley BJ (1971) Phase equilibria in the system NaAl-Si3O8-NaAlSiO4-H2O with special emphasis on the stability of analcite. Can J Earth Sci 8:311–337
Lindsley DH, Smith D (1971) Chemical variations in the feldspars. Carnegie Inst Washington Yearb 69:274–278
MacKenzie WS (1972) The origin of trachytes and syenites. Progr Expt Petr (NERC) 2:46–50
Mohammed ARO (1982) Mineralogy and petrology of Eilean Mhuire, Shiant Isles. Unpubl Ph D thesis, Univ of Cambridge
Morse SA (1980) Basalts and phase diagrams. Springer-Verlag, Berlin, Heidelberg, New York
Nash WP, Carmichael ISE, Johnson RW (1969) The mineralogy and petrology of Mount Suswa, Kenya. J Petrol 10:409–439
Norris G (1973) Phase relations in petrogeny's residua system with the addition of anorthite. Unpubl Ph D thesis, Univ of Manchester
Parsons I, Brown WL (1983) A TEM and microprobe study of a two-perthite alkali gabbro: implications for the ternary feldspar system. Contrib Mineral Petrol 82:1–12
Roux J, Hamilton DL (1976) Primary igneous analcite — an experimental study. J Petrol 17:244–257
Saha P (1959) Geochemical and X-ray investigation of natural and synthetic analcites. Am Mineral 44:300–313
Schairer JF (1957) Melting relations of the common rock-forming oxides. J Am Ceram Soc 40:215–235
Schairer JF, Yoder HS Jr (1960) The nature of residual liquids from crystallization, with data on the system nepheline-diopside-silica. Am J Sci 258A:273–283
Schairer JF, Yoder HS Jr (1961) Crystallization in the system nepheline-forsterite-silica at 1 atmosphere pressure. Carnegie Inst Washington Yearb 60:141–144
Stewart DB, Roseboom EH Jr (1962) Lower temperature terminations of the three-phase region plagioclase-alkali feldspar-liquid. J Petrol 3:280–315
Thompson RN, MacKenzie WS (1967) Feldspar-liquid equilibria in peralkaline acid liquids: an experimental study. Am J Sci 265:714–734
Tilley CE (1959) A note on the nosean phonolite of the Wolf Rock, Cornwall. Geol Mag 96:503–504
Tuttle OF, Bowen NL (1958) Origin of granite in the light of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O. Mem Geol Soc Am 74
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Henderson, C.M.B., Gibb, F.G.F. Felsic mineral crystallization trends in differentiating alkaline basic magmas. Contr. Mineral. and Petrol. 84, 355–364 (1983). https://doi.org/10.1007/BF01160287
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DOI: https://doi.org/10.1007/BF01160287