The agpaitic rocks; an overview

University of Copenhagen, Institute of Geology, Copenhagen, Denmark

It is now generally agreed that the term "agpaitic" should be restricted to peralkaline nepheline syenites (and phonolites) containing minerals such as eudialyte and rinkite, that is complex silicates of Zr, Ti, the rare earth elements (REE), and F and other volatiles. There are, however, cases of transition into more common types of nepheline syenites containing zircon, titanite, ilmenite, etc. The agpaitic rocks are characterized by extremely high contents of rare elements such as Li, Be, Nb, Ta, REE, Zr, Th, etc. and of volatiles, first of all F and Cl. This gives rise to a wealth of mineral species, more than 500 in the Lovozero and Khibina complexes of the Kola peninsula, about 250 in Mont Saint-Hilaire, Quebec, Canada, and about 200 in the type locality, the Ilimaussaq complex, South Greenland. These rocks have very long melting intervals and solidus temperatures as low as 500 to 400 degrees C. They are accompanied by a gas phase rich in methane and other hydrocarbons and most probably also by sodium-rich fluids as indicated by the presence of minerals such as ussingite (NaAlSi ₃ O ₈ .NaOH) and villiaumite (NaF) and of pegmatites and hydrothermal veins rich in sodium and rare and volatile elements. Agpaitic nepheline syenites are considered to have been formed by consolidation of melts oversaturated in alkalis, especially sodium, under conditions preventing the volatiles from escaping. These melts have been derived by extreme fractionation processes in alkali basaltic or nephelinitic magmas. The main stage of crystallization of the melts is characterized by minerals such as nepheline (sometimes also sodalite), alkali feldspars, arfvedsonite, aegirine and eudialyte, but the most highly developed, hyperagpaitic lujavrites of the Ilimaussaq complex have been formed from melts with extreme concentrations of sodium and volatiles resulting in the formation of naujakasite instead of nepheline, ussingite instead of sodalite and alkali feldspars, and steenstrupine instead of eudialyte. During the late stages of crystallization, sodium-rich fluids are the cause of late- and postmagmatic alteration and of the formation of hydrothermal mineralizations. The late stages are characterized by water-soluble sodium-rich minerals of which more than 80 have been found in the Khibina and Lovozero complexes.

This record provided courtesy of AGI/GeoRef.

This article has been cited by other articles:

				M. A. W. Marks, J. Schilling, I. M. Coulson, T. Wenzel, and G. Markl The Alkaline-Peralkaline Tamazeght Complex, High Atlas Mountains, Morocco: Mineral Chemistry and Petrological Constraints for Derivation from a Compositionally Heterogeneous Mantle Source J. Petrology, June 1, 2008; 49(6): 1097 - 1131. [Abstract] [Full Text] [PDF]

				G. Graser and G. Markl Ca-rich Ilvaite-Epidote-Hydrogarnet Endoskarns: a Record of Late-Magmatic Fluid Influx into the Persodic Ilimaussaq Complex, South Greenland J. Petrology, February 1, 2008; 49(2): 239 - 265. [Abstract] [Full Text] [PDF]

				T. Andersen and H. Sorensen Stability of naujakasite in hyperagpaitic melts, and the petrology of naujakasite lujavrite in the Ilimaussaq alkaline complex, South Greenland Mineralogical Magazine, April 1, 2005; 69(2): 125 - 136. [Abstract] [Full Text] [PDF]

				I. V. Pekov and N. V. Chukanov Microporous Framework Silicate Minerals with Rare and Transition Elements: Minerogenetic Aspects Reviews in Mineralogy and Geochemistry, January 1, 2005; 57(1): 145 - 171. [Full Text] [PDF]

				M. Marks, M. Marks, and G. Markl Ilimaussaq 'en miniature': closed-system fractionation in an agpaitic dyke rock from the Gardar Province, South Greenland (contribution to the mineralogy of Ilimaussaq no. 117) Mineralogical Magazine, October 1, 2003; 67(5): 893 - 919. [Abstract] [Full Text] [PDF]

				F. Ridolfi, F. Ridolfi, A. Renzulli, P. Santi, and B. G. J. Upton Evolutionary stages of crystallization of weakly peralkaline syenites: evidence from ejecta in the plinian deposits of Agua de Pau volcano (Sao Miguel, Azores Islands) Mineralogical Magazine, August 1, 2003; 67(4): 749 - 767. [Abstract] [Full Text] [PDF]

				V. Stahle, M. Koch, C. A. McCammon, U. Mann, and G. Markl OCCURRENCE OF LOW-Ti AND HIGH-Ti FREUDENBERGITE IN ALKALI SYENITE DIKES FROM THE KATZENBUCKEL VOLCANO, SOUTHWESTERN GERMANY Can Mineral, December 1, 2002; 40(6): 1609 - 1627. [Abstract] [Full Text] [PDF]

				A. A. Finch, K. M. Goodenough, H. M. Salmon, and T. Andersen The petrology and petrogenesis of the North Motzfeldt Centre, Gardar Province, South Greenland Mineralogical Magazine, December 1, 2001; 65(6): 759 - 774. [Abstract] [Full Text] [PDF]

				G. MARKL, M. MARKS, G. SCHWINN, and H. SOMMER Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland J. Petrology, December 1, 2001; 42(12): 2231 - 2257. [Abstract] [Full Text] [PDF]

				S. Salvi, S. Salvi, F. Fontan, P. Monchoux, A. E. Williams-Jones, and B. Moine Hydrothermal Mobilization of High Field Strength Elements in Alkaline Igneous Systems: Evidence from the Tamazeght Complex (Morocco) Economic Geology, May 1, 2000; 95(3): 559 - 576. [Abstract] [Full Text] [PDF]