Abstract
Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di71 Hd18Ac11 to Di63Hd22Ac15, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or88 to Or95. Perthites are absent.
Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite.
A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites (sensu lato) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites.
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References
Abbott, M.J.: Petrology of the Nandewar volcano, N.S.W. Australia. Contrib. Mineral. Petrol. 20, 115–135 (1969)
Allen, C.C.: The malignites of Poohbah Lake, Ontario. J. Geol. 50, 134–152 (1942)
Barth, T.F.W., Ramberg, I.B.: The Fen circular complex. In: Carbonatites (P.F. Tuttle and J. Gittins, eds.), pp. 225–257. New York: Wiley Interscience 1966
Brøgger, W.C.: Die Eruptivgesteine des Kristiania-Gebietes. IV. Das Fengebeit in Telemark: Videnskap. Selsk. Skr. I. Mat. Naturv. Kl. 9, 1–408 (1921)
Campbell, C.D.: The Kruger alkaline syenites of southern British Columbia. Am. J. Sci. 237, 527–549 (1939)
Currie, K.L., Curtis, L.W., Gittins, J.: Petrology of the Red Wine alkaline complexes, central Labrador and a comparison with the Ilimaussaq complex, southwest Greenland. Geol. Surv. Can. Paper 75-1A, 271–280 (1975)
Daly, R.A.: Geology of the North American Cordillera at the forty-ninth parallel. Geol. Surv. Can. Mem. 38, Part 1, 448–455 (1912)
Davidson, A.: Nepheline-K-feldspar intergrowth from Kaminak Lake, Northwest Territories. Can. Mineralogist 10, 191–206 (1970)
Gomes, C. de B., Moro, S.L., Dutra, C.V.: Pyroxenes from the alkaline rocks of Itapirapua, Sao Paulo, Brazil. Am. Mineralogist 55, 224–230 (1970)
Hamilton, D.L.: Nephelines as crystallization temperature indicators. J. Geol. 69, 321–329 (1961)
Larsen, L.M.: Clinopyroxenes and coexisting mafic minerals from the alkaline Ilimaussaq intrusion, South Greenland. J. Petrol. 17, 258–290 (1976)
Lawson, A.C.: Malignite, a family of basic plutonic orthoclase rocks rich in alkalies and lime intrusive in the Coutchiching schists of Poohbah Lake. Univ. California Pub. Bull. Dept. Geol. 1, 337–362 (1896)
Le Bas, M.J.: Carbonatite-nephelinite volcanism. 347pp. New York: Wiley Interscience 1977
Miller, T.P.: Potassium-rich alkaline intrusive rocks of Western Alaska. Geol. Soc. Am. Bull. 83, 2111–2128 (1972)
Mitchell, R.H.: Composition of nepheline, pyroxene and biotite in ijolite from the Seabrook Lake complex, Ontario, Canada. N. Jb. Mineral. Monatsh. 9, 415–422 (1972)
Mitchell, R.H.: Potassium-argon geochronology of the Poohbah Lake alkaline complex, northwestern Ontario. Can. J. Earth Sci. 13, 1456–1459 (1976)
Mitchell, R.H., Platt, R.G.: Mafic mineralogy of ferroaugite syenite from the Coldwell complex, Ontario, Canada. J. Petrol. 19, 627–651 (1978)
Nash, W.P.: Mineralogy and petrology of the Iron Hill carbonatite complex, Colorado. Geol. Soc. Am. Bull. 83, 1361–1382 (1972)
Nash, W.P., Wilkinson, J.F.G.: Shonkin Sag laccolith, Montana. I. Mafic minerals and estimates of temperature, pressure, oxygen fugacity and silica activity. Contrib. Mineral. Petrol. 25, 241–269 (1970)
Nicholls, J., Carmichael, I.S.E.: Peralkaline acid liquids: A petrological study. Contrib. Mineral. Petrol. 20, 268–294 (1969)
Parsons, G.E.: Niobium complexes east of Lake Superior, Ontario. Ontario Dept. Mines Rept. 3, 1–73 (1961)
Pearce, T.H.: The analcite-bearing volcanic rocks of the Crowsnest Formation, Alberta. Can. J. Earth Sci. 7, 46–66 (1970)
Saether, E.: The alkaline rock province of the Fen area in southern Norway. Del. Kgl. Notsk. Vidensk. Selsk. Skr. 4, 150 pp.(1957)
Shand, S.J.: The problems of the alkaline rocks. Proc. Geol. Soc. S. Afr. 25, xix-xxxii (1922)
Shand, S.J.: The geology of the Pilansberg (Pilaan's Berg) in the Western Transvaal: A study of alkaline rocks and ring intrusions. Trans. Geol. Soc. S. Afr. 31, 97–156 (1928)
Shand, S.J.: Loch Borolan laccolith, Northwest Scotland. J. Geol. 47, 408–420 (1939)
Sørensen, H.: Alkali syenites feldspathoidal syenites and related lavas. In: The Alkaline Rocks (H. Sørensen, ed.), pp. 22–52. New York: John Wiley and Sons 1974
Stephenson, D.: Alkali clinopyroxenes from nepheline syenites of the South Qoroq Centre, South Greenland. Lithos 5, 187–201 (1972)
Streckeisen, A.: To each plutonic rock its proper name. Earth Sci. Revs. 12, 1–33 (1976)
Tilley, C.E.: Nepheline-alkali feldspar parageneses. Am. J. Sci. 252, 65–75 (1954)
Tilley, C.E.: Problems of alkali rock genesis. Quart. J. Geol. Soc. London 113, 323–360 (1957)
Tyler, R.C., King, B.C.: The pyroxenes of the alkaline igneous complexes of Eastern Uganda, Mineral. Mag. 36, 5–21 (1967)
Woolley, A.R.: The Loch Borralan alkaline igneous complex. Ph. D. Thesis. Univ. London (1965)
Woolley, A.R.: The pseudoleucite borolanites and associated rocks of the South-Eastern Tract of the Borralan complex, Scotland. Bull. British Museum (Nat. Hist.) Mineralogy 2, 288–333 (1973)
Yagi, K.: Petrochemical studies of the alkalic rocks of the Morutu district, Sakhalin. Geol. Soc. Am. Bull. 64, 769–810 (1953)
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Mitchell, R.H., Platt, R.G. Nepheline-bearing rocks from the Poohbah Lake complex, Ontario: Malignites and Malignites. Contr. Mineral. and Petrol. 69, 255–264 (1979). https://doi.org/10.1007/BF00372328
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DOI: https://doi.org/10.1007/BF00372328