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A comment on ‘Agpaicity revisited: pattern recognition in the chemistry of nepheline syenite rocks’ by Dagbert et al.
Geochimica et Cosmochmka
Acta,1977, Vol.41,
pp. 439 lo 440.
Pergamon Press. Printed inGreatBritain
CRITICAL COMMENT A comment on ‘Agpaicity revisited: pattern recognition in the chemistry of nepheline syenite rocks’ by Dagbert et al. (Received
3 June 1976; accepted
in revisedform
17 August 1976)
Abstract-The conclusions of DAGBERT et al. (1975) on agpaicity are based on chemical and mineralogical data which may be characteristic of peralkalinity but not necessarily of agpaicity. Lack of significant variations in K in correspondence analysis can be explained on petrogenetic grounds and do not imply insignificance in agpaitic processes. The agpaitic coefficient merely indicates peralkalinity.
IN A recent paper ‘Agpaicity revisited: pattern recog- sively used in naming different varieties of nepheline nition in the chemistry of nepheline syenite rocks’, syenites, their abundances are only of limited use in distinguishing peralkaline (Na + K/Al > 1) from DAGBERT et al. (1975) used correspondence analysis non-peralkaline (Na + K/Al < 1) varieties and, as inin an attempt to describe variations in the chemistry of nepheline syenites as defined by mineralogical cri- dicated previously, cannot be used as a criterion of agpaitic rocks. The presence of modal aegirine may teria. Unfortunately, the tlements used in the correindicate a peralkaline rock, although many instances spondence analysis are all major elements, characterare known of non-peralkaline rocks containing modall istic of peralkalinity but not necessarily of agpaicity; the mineralogical criteria used by Dagbert et al. are aegirine, e.g. the gneissic nephaeline syenites of Methnot necessarily diagnostic of either peralkaline or uen Township, Ontario (HELIX, 1960, p. 130). Dagbert et al. consider that all nepheline syenites which agpaitic rocks; and finally USSING’s (1912) definition plot in Streckeisen’s PAF diagram constitute a ‘petroof agpaicity used in their paper to draw an isoagpailogical entity’. While this may be the case for rocks city diagram is no longer applicable. Their conclusion (p. 1503) that K plays no significant role in without exotic mineralogies, whether peralkaline or not, this ‘entity’ should not include the agpaitic types the definition of agpaicity can be readily explained on petrogenetic grounds which account for the lack which, as attested by their distinctive chemistry and of significance in variation of K. In view of wide- mineralogy, are a result of a rare and extreme petrospread misconceptions regarding the terms ‘agpaitic’ genetic process. Indeed, there seems little justification in Dagbert et a/.‘~ using STRECKEISEN’S (1967) PAF and ‘peralkaline’ and the processes each represents classification for rocks such as naujaites which are (cf. EDGAR, 1974) the points raised above require true agpaites and cannot be adequately represented clarification. Agpaitic rocks are defined on the basis of their on this triangle. I (unpublished manuscript) have major and minor element chemistry as well us on compiled a file of 2387 analyses of feldspathoidal normative igneous rocks of which only 13% are pertheir exotic mineralogy. GERASSIMOVSKI (1956, p. 497) alkaline. This figure contradicts the 30% peralkaline lists 9 distinctive chemical characteristics of agpaitic nepheline syenites which distinguish these rocks from rocks claimed by Dagbert et al. (Fig. 2) based on (1938) estimated distribution. Of the non-agpaitic varieties. In addition to the excess of JOHANNSEN’S 13% peralkaline rocks probably only one-half can be alkalies to alumina and the predominance of Na,O over K,O, he stresses the high contents of Zr, Nb, considered to have agpaitic chemical characteristics, REE, Ti, Sr, Th, U, Cl, F and HzO. SORENSEN although for many of these rocks such chemical criteria are often unavailable. (1960,1974) and GERA~~IMOVSKI (1974) have expanded on this theme. These geochemical characteristics For their correspondence analysis Dagbert et al. result in the crystallization of Na-Zr-Ti-RE silicates consider ten elements of which only the alkalies, aluwhich, in the absence of chemical analyses, provide mina and titania are characteristic of agpaitic rocks. diagnostic minerals of the agpaitic suite. SBRENSEN Considering that in the 187 analyses used, those from (1974) lists these minerals. Many of these, such as the classical agpaitic massif of Lovozero, Khibiiny in eudialyte, contain appreciable F + Cl contents. the Kola Peninsula, USSR, Ilimaussaq and the other Using the classifications of STRECKEISEN (1967) and alkaline complexes of the Gardar province of S. JOHANNSEN(1938), Dagbert et al. (p. 1499) state that Greenland were numerous, suggests that omission of rocks of the nephaeline syenite type (ss) may be differ- Zr. Cl. F and other characteristic elements of agpaitic entiated on the basis of the relative abundances of nephaeline syenites, which may account for 2-3x of microcline, perthitic orthoclase, and aegirine. While the total of each analysis, must affect any conclusions the abundances of these minerals have been extendrawn from the correspondence analysis. 439
440
Critical Comment
Using the same statistical technique Dagbert et al. index’ in the absence of diagnostic geochemical and mineralogical evidence of agpaicity (EDGAR,1974). As attempt to relate agpaicity with trends of chemical variation. As a criterion of agpaicity they have used Dagbert et al. indicate the use of a parameter involving elements other than alkalies and alumina is desirUSSING’S(1912) classical definition of an agpaitic rock able. From geochemical and experimental evidence as one with atomic (Na + K)/Al > 1.2. Unfortunately, this definition is now virtually never used, EDGARand PARKER(1974, Figs. 8 and 9) show that weight per cent TiO, + ZrOz + F + Cl and weight since in many agpaitic complexes the ratio is less than 1.2, although the other geochemical characteristics are per cent ZrO, + F + Cl increase with increasing peralkalinity for plutonic nepheline syenites from the present (cf. SPIRENSEN, 1960). Using Ussing’s definiIlimaussaq and Khibiiny complexes. tion, Dagbert et al. show (Fig.4) that of the 5 complexes they have chosen only the rocks of Lovozero A. D. EDGAR are agpaitic, whereas the compositions of those of the Department of Geology, University of Western Ontario, classical agpaitic complexes of Ilimaussaq, and KhiLondon, Ontario, N6A SB7, biiny fall below the isoagpaicity line. Canada As the behaviour of K is not recorded in any of their correspondence analysis trends, Dagbert er al. conclude (p. 1503) that the traditional form of the agpaitic coefficient is unsatisfactory. The excess of Na REFERENCES relative to K in most agpaitic rocks (GERASSIMOVSKI, BAILEYD. K. and SCHAIRER J. F. (1964) Feldspar-liquid 1956) is a consequence of several complex petrogeneequilibria in peralkaline liquids-the orthoclase effect. tic processes. Early in their formation continuous and Amer. J. Sci. 262. 1198-1206. gradual crystallization, together with fractionation of BOWENH. L. (1945) Phase equilibria bearing on the origin anorthite, nepheline and the Tschermak component and differentiation of alkaline rocks. Amer. J. Sci. 243A, 75-90. of pyroxenes (KOGARKOand POLYAKOV,1967) results DAGBERTM., PERT~C~WSKY R., DAVID M. and PERRAKJLT in residual liquids depleted in Al and enriched in Na G. (1975) Agpaicity revisited: pattern recognition in the relative to K. An example of this process in the early chemistry of nepheline syenite rocks. Geochim. Cosmostages is the ‘plagioclase effect’ (BOWEN, 1945) chim. Acta 39, 1499-1504. whereas at later stages the ‘orthoclase effect’ (BAILEY EDGARA. D. (1974) On the use of the term “Agpaitic”. Mineral. Mag. 39, 729-730. and SCHAIRER,1964) producing liquids with Na > K, EDGARA. D. (unpublished manuscript) A data file on normay operate. The foyaites of the classic Ilimaussaq mative feldspathoidal rocks. complex (MACWNALD, 1974) are examples of such EDGARA. D. and PARKERL. M. (1974) Comparison of mechanisms and produce peralkaline but not agpaitic melting relationships of some volcanic and plutonic peralkaline undersaturated rocks. Lithos 7, 263-273. rocks. In situ gravity differentiation of predominantly FERCUSON J. (1964) Geology of the Ilimaussaq alkaline inNa-rich minerals of the foyaites produces the agpaitic trusion, South Greenland. Medd. Greenland 172(4), l-81. naujaites and kakortokites; the former by flotation FERGUSON J. (1970) The differentiation of agpaitic magmas: ’ of sodalite, the latter being cumulus aggregates of the Ilimaussaq intrusion, South Greenland. Can. perthite, aegirine, riebeckite-arfvedsonite, eudialyte Mineral. 10(3), 335-349. V. I. (1956) Geochemistry and mineralogy and nepheline (FERGUSON, 1964, 1970; SORENSEN, GERASSIMOVSKI of nepheline syenite intrusions. Geochemistry 5,494510. 1969). Agpaitic trends in phonolites (cf. VARET,1969) GERA~~IMOVSKI V. I. (1974) Trace elements in selected may result from gas differentiation and increasingf,*, groups of alkaline rocks. In The Alkaline Rocks, (editor both processes tending to increase Na relative to K. H. Sorensen), pp. 402411. Wiley. These processes produce peralkaline liquids or’ HEWITTD. F. (1960) Nepheline syknite deposits of Southem Ontario. Rep. Ontario Dept Mines 69(8), 1-194. crystalline assemblages with Na > K. Under quiesJOHANNSENA. (1938) A Descriptive Petrography of the cent conditions accumulations of the elements characIgneous Rocks, Vol. IV. University of Chicago Press. teristic of agpaicity may occur resulting in the crystalK~CARKOL. M. and POLYAKOV A. I. (1967) The problems lization of predominantly Na-rich complex silicates. of genesis of the agpaitic nepheline syenites (in Russian). Geokhimiya 2, 131-143. Thus the absence of K in agpaitic rocks, and their R. (1974) The role of fractional crystallization characteristic minerals is due primarily to lack of con- MACDONALD in the alkaline rocks. In The Alkaline Rocks, (editor H. centration of K relative to Na in the liquids producSorensen), pp. 442458. Wiley. ing these rocks. In glassy volcanic rocks of agpaitic SORENSENH. (1960) On the agpaitic rocks. Int. Congr. Geol. Copenhagen Part XIII, pp. 319-327. affinities the K is preferentially incorporated in the SC~RENSEN H. (1969) Rhythmic igneous layering in peralkaglass phase (EDGAR and PARKER, 1974, Table3), line intrusions. Lithos 2, 261-283. whereas in plutonic rocks K may be accommodated SORENSEN H. (1974) Alkali syenites, feldspathoidal syenites in K-feldspars or be dissipated in late-stage pneumaand related lavas. In The Alkaline Rocks, (editor H. tolytic alteration products. Sorensen), pp. 15-2 1. Wiley. STRECKEISEN A. L. (1967) Classification and nomenclature In view of the processes forming peralkaline and of igneous rocks: Ne;es Jahrb. Mineral. 107, 144-240. agpaitic rocks, it is hardly surprising that Dagbert N. V. (1912) Geology of the country around et al. (p. 1503) did not record the behaviour of K on USSING Julianehaab, Greenland. Medd. Groenland 38, l-376. any of their correspondence analysis trends. The so- VARETJ. (1969) Les pyroxenes des phonolites du Cantal. called agpaitic coefficient is really only ‘a peralkaline New-es Jahrb. Mineral. Monatsh. 174184.
Acta,1977, Vol.41,
pp. 439 lo 440.
Pergamon Press. Printed inGreatBritain
CRITICAL COMMENT A comment on ‘Agpaicity revisited: pattern recognition in the chemistry of nepheline syenite rocks’ by Dagbert et al. (Received
3 June 1976; accepted
in revisedform
17 August 1976)
Abstract-The conclusions of DAGBERT et al. (1975) on agpaicity are based on chemical and mineralogical data which may be characteristic of peralkalinity but not necessarily of agpaicity. Lack of significant variations in K in correspondence analysis can be explained on petrogenetic grounds and do not imply insignificance in agpaitic processes. The agpaitic coefficient merely indicates peralkalinity.
IN A recent paper ‘Agpaicity revisited: pattern recog- sively used in naming different varieties of nepheline nition in the chemistry of nepheline syenite rocks’, syenites, their abundances are only of limited use in distinguishing peralkaline (Na + K/Al > 1) from DAGBERT et al. (1975) used correspondence analysis non-peralkaline (Na + K/Al < 1) varieties and, as inin an attempt to describe variations in the chemistry of nepheline syenites as defined by mineralogical cri- dicated previously, cannot be used as a criterion of agpaitic rocks. The presence of modal aegirine may teria. Unfortunately, the tlements used in the correindicate a peralkaline rock, although many instances spondence analysis are all major elements, characterare known of non-peralkaline rocks containing modall istic of peralkalinity but not necessarily of agpaicity; the mineralogical criteria used by Dagbert et al. are aegirine, e.g. the gneissic nephaeline syenites of Methnot necessarily diagnostic of either peralkaline or uen Township, Ontario (HELIX, 1960, p. 130). Dagbert et al. consider that all nepheline syenites which agpaitic rocks; and finally USSING’s (1912) definition plot in Streckeisen’s PAF diagram constitute a ‘petroof agpaicity used in their paper to draw an isoagpailogical entity’. While this may be the case for rocks city diagram is no longer applicable. Their conclusion (p. 1503) that K plays no significant role in without exotic mineralogies, whether peralkaline or not, this ‘entity’ should not include the agpaitic types the definition of agpaicity can be readily explained on petrogenetic grounds which account for the lack which, as attested by their distinctive chemistry and of significance in variation of K. In view of wide- mineralogy, are a result of a rare and extreme petrospread misconceptions regarding the terms ‘agpaitic’ genetic process. Indeed, there seems little justification in Dagbert et a/.‘~ using STRECKEISEN’S (1967) PAF and ‘peralkaline’ and the processes each represents classification for rocks such as naujaites which are (cf. EDGAR, 1974) the points raised above require true agpaites and cannot be adequately represented clarification. Agpaitic rocks are defined on the basis of their on this triangle. I (unpublished manuscript) have major and minor element chemistry as well us on compiled a file of 2387 analyses of feldspathoidal normative igneous rocks of which only 13% are pertheir exotic mineralogy. GERASSIMOVSKI (1956, p. 497) alkaline. This figure contradicts the 30% peralkaline lists 9 distinctive chemical characteristics of agpaitic nepheline syenites which distinguish these rocks from rocks claimed by Dagbert et al. (Fig. 2) based on (1938) estimated distribution. Of the non-agpaitic varieties. In addition to the excess of JOHANNSEN’S 13% peralkaline rocks probably only one-half can be alkalies to alumina and the predominance of Na,O over K,O, he stresses the high contents of Zr, Nb, considered to have agpaitic chemical characteristics, REE, Ti, Sr, Th, U, Cl, F and HzO. SORENSEN although for many of these rocks such chemical criteria are often unavailable. (1960,1974) and GERA~~IMOVSKI (1974) have expanded on this theme. These geochemical characteristics For their correspondence analysis Dagbert et al. result in the crystallization of Na-Zr-Ti-RE silicates consider ten elements of which only the alkalies, aluwhich, in the absence of chemical analyses, provide mina and titania are characteristic of agpaitic rocks. diagnostic minerals of the agpaitic suite. SBRENSEN Considering that in the 187 analyses used, those from (1974) lists these minerals. Many of these, such as the classical agpaitic massif of Lovozero, Khibiiny in eudialyte, contain appreciable F + Cl contents. the Kola Peninsula, USSR, Ilimaussaq and the other Using the classifications of STRECKEISEN (1967) and alkaline complexes of the Gardar province of S. JOHANNSEN(1938), Dagbert et al. (p. 1499) state that Greenland were numerous, suggests that omission of rocks of the nephaeline syenite type (ss) may be differ- Zr. Cl. F and other characteristic elements of agpaitic entiated on the basis of the relative abundances of nephaeline syenites, which may account for 2-3x of microcline, perthitic orthoclase, and aegirine. While the total of each analysis, must affect any conclusions the abundances of these minerals have been extendrawn from the correspondence analysis. 439
440
Critical Comment
Using the same statistical technique Dagbert et al. index’ in the absence of diagnostic geochemical and mineralogical evidence of agpaicity (EDGAR,1974). As attempt to relate agpaicity with trends of chemical variation. As a criterion of agpaicity they have used Dagbert et al. indicate the use of a parameter involving elements other than alkalies and alumina is desirUSSING’S(1912) classical definition of an agpaitic rock able. From geochemical and experimental evidence as one with atomic (Na + K)/Al > 1.2. Unfortunately, this definition is now virtually never used, EDGARand PARKER(1974, Figs. 8 and 9) show that weight per cent TiO, + ZrOz + F + Cl and weight since in many agpaitic complexes the ratio is less than 1.2, although the other geochemical characteristics are per cent ZrO, + F + Cl increase with increasing peralkalinity for plutonic nepheline syenites from the present (cf. SPIRENSEN, 1960). Using Ussing’s definiIlimaussaq and Khibiiny complexes. tion, Dagbert et al. show (Fig.4) that of the 5 complexes they have chosen only the rocks of Lovozero A. D. EDGAR are agpaitic, whereas the compositions of those of the Department of Geology, University of Western Ontario, classical agpaitic complexes of Ilimaussaq, and KhiLondon, Ontario, N6A SB7, biiny fall below the isoagpaicity line. Canada As the behaviour of K is not recorded in any of their correspondence analysis trends, Dagbert er al. conclude (p. 1503) that the traditional form of the agpaitic coefficient is unsatisfactory. The excess of Na REFERENCES relative to K in most agpaitic rocks (GERASSIMOVSKI, BAILEYD. K. and SCHAIRER J. F. (1964) Feldspar-liquid 1956) is a consequence of several complex petrogeneequilibria in peralkaline liquids-the orthoclase effect. tic processes. Early in their formation continuous and Amer. J. Sci. 262. 1198-1206. gradual crystallization, together with fractionation of BOWENH. L. (1945) Phase equilibria bearing on the origin anorthite, nepheline and the Tschermak component and differentiation of alkaline rocks. Amer. J. Sci. 243A, 75-90. of pyroxenes (KOGARKOand POLYAKOV,1967) results DAGBERTM., PERT~C~WSKY R., DAVID M. and PERRAKJLT in residual liquids depleted in Al and enriched in Na G. (1975) Agpaicity revisited: pattern recognition in the relative to K. An example of this process in the early chemistry of nepheline syenite rocks. Geochim. Cosmostages is the ‘plagioclase effect’ (BOWEN, 1945) chim. Acta 39, 1499-1504. whereas at later stages the ‘orthoclase effect’ (BAILEY EDGARA. D. (1974) On the use of the term “Agpaitic”. Mineral. Mag. 39, 729-730. and SCHAIRER,1964) producing liquids with Na > K, EDGARA. D. (unpublished manuscript) A data file on normay operate. The foyaites of the classic Ilimaussaq mative feldspathoidal rocks. complex (MACWNALD, 1974) are examples of such EDGARA. D. and PARKERL. M. (1974) Comparison of mechanisms and produce peralkaline but not agpaitic melting relationships of some volcanic and plutonic peralkaline undersaturated rocks. Lithos 7, 263-273. rocks. In situ gravity differentiation of predominantly FERCUSON J. (1964) Geology of the Ilimaussaq alkaline inNa-rich minerals of the foyaites produces the agpaitic trusion, South Greenland. Medd. Greenland 172(4), l-81. naujaites and kakortokites; the former by flotation FERGUSON J. (1970) The differentiation of agpaitic magmas: ’ of sodalite, the latter being cumulus aggregates of the Ilimaussaq intrusion, South Greenland. Can. perthite, aegirine, riebeckite-arfvedsonite, eudialyte Mineral. 10(3), 335-349. V. I. (1956) Geochemistry and mineralogy and nepheline (FERGUSON, 1964, 1970; SORENSEN, GERASSIMOVSKI of nepheline syenite intrusions. Geochemistry 5,494510. 1969). Agpaitic trends in phonolites (cf. VARET,1969) GERA~~IMOVSKI V. I. (1974) Trace elements in selected may result from gas differentiation and increasingf,*, groups of alkaline rocks. In The Alkaline Rocks, (editor both processes tending to increase Na relative to K. H. Sorensen), pp. 402411. Wiley. These processes produce peralkaline liquids or’ HEWITTD. F. (1960) Nepheline syknite deposits of Southem Ontario. Rep. Ontario Dept Mines 69(8), 1-194. crystalline assemblages with Na > K. Under quiesJOHANNSENA. (1938) A Descriptive Petrography of the cent conditions accumulations of the elements characIgneous Rocks, Vol. IV. University of Chicago Press. teristic of agpaicity may occur resulting in the crystalK~CARKOL. M. and POLYAKOV A. I. (1967) The problems lization of predominantly Na-rich complex silicates. of genesis of the agpaitic nepheline syenites (in Russian). Geokhimiya 2, 131-143. Thus the absence of K in agpaitic rocks, and their R. (1974) The role of fractional crystallization characteristic minerals is due primarily to lack of con- MACDONALD in the alkaline rocks. In The Alkaline Rocks, (editor H. centration of K relative to Na in the liquids producSorensen), pp. 442458. Wiley. ing these rocks. In glassy volcanic rocks of agpaitic SORENSENH. (1960) On the agpaitic rocks. Int. Congr. Geol. Copenhagen Part XIII, pp. 319-327. affinities the K is preferentially incorporated in the SC~RENSEN H. (1969) Rhythmic igneous layering in peralkaglass phase (EDGAR and PARKER, 1974, Table3), line intrusions. Lithos 2, 261-283. whereas in plutonic rocks K may be accommodated SORENSEN H. (1974) Alkali syenites, feldspathoidal syenites in K-feldspars or be dissipated in late-stage pneumaand related lavas. In The Alkaline Rocks, (editor H. tolytic alteration products. Sorensen), pp. 15-2 1. Wiley. STRECKEISEN A. L. (1967) Classification and nomenclature In view of the processes forming peralkaline and of igneous rocks: Ne;es Jahrb. Mineral. 107, 144-240. agpaitic rocks, it is hardly surprising that Dagbert N. V. (1912) Geology of the country around et al. (p. 1503) did not record the behaviour of K on USSING Julianehaab, Greenland. Medd. Groenland 38, l-376. any of their correspondence analysis trends. The so- VARETJ. (1969) Les pyroxenes des phonolites du Cantal. called agpaitic coefficient is really only ‘a peralkaline New-es Jahrb. Mineral. Monatsh. 174184.