The crystallisation history of the Igdlerfigssalik nepheline syenite intrusion, Greenland

The crystallisation history of the Igdlerfigssalik nepheline syenite intrusion, Greenland MAILIORIE POWELL

L1THOS

Powell, M. 1978: The crystallisation history of the Igdlerfigssalik nepheline syenite intrusion, Greenland. Lithos 11, 99-120. Oslo. ISSN 0024-4937. During slow cooling of plutonic igneous rocks the initial high temperature minerals cry,~:ta!lisedfrom the magma continue to re-equilibrate with each ,3ther to varying degrees with falling temperature. Thermodynamic studies of mineral equilibria are used to calculate crystallisation temperatures for the cumulus assemblage ol-cpx-mt-ne-fsp in th: Igdlerligssalik syenites and to calculate composition parameters for the original magmas. Cumulus crystailisation occurred in the range 900-980"C. Nepheline and alkali feldspar continued to equilibrate in some rocks down to 650°C, while macroscopic exsolution in alkali feldspar and titanomagnetite continued to temperatures below 600°C. Oxygen activities during the crystallisation of the cumulus minerals were be:ow

magnetite-wus~ite. M. PoweU, Department of Earth Sciences, The UnDersity, Leeds 2, England.

Studies of plutonic igneous rocks frequently tend to under-emphasise the importance of slow cooling in the production of the complex mineralogical textures so commonly observed. For example a highlevel stock-like magma body some 10-15 km across may take of the order of one million years to cool to the ambient regional geotherm from initial magmatic temperatures o? 1000°C or greater. During this interval the initial high temperature minerals crystallised from the magma continue to re-equilibrate with each other to varying degrees with faUing temperature. At subsolidus temperatures interaction with late stage hydrothermal iluids may result in recrystallisation and sometimes the growth of new minerals such as biotite. These processes may account for the observed 'metamorphic' textures in many plutonic rocks presently exposed at the Earth's surface. Thermodynamic studies of mineral equilibria in plutonic rocks provide a basis for interpreting their complex cooling histories. Element distributions between mineral pairs are frequently sensitive functions of temperature and therefore such distributions can be used as geothermometers, if the thermodylaamics of the appropriate balanced chemical reactiions can be correctly f3rmulated. The blocking temperature recorded by a particular mineral pair is a function of the relative mobilities of the elements involved. Therefore if a plutonic rock has a sufficiently large number of phases it should be possible to formulate several geothermometers which will record different stages in the

cooling history. Mineral equilibria studies can also be used to determine composition parameters such as asio2 and ao2 for the magmas which c~stallised to form the plutonic rocks. This study was initiated to investigate the cooling history of a high-level 'nepheline syenite intrusion, the Igdlerfigssalik centre of the Igaliko Complex, Gardar Province, Greenland.

Igdlerfigssalik The Igdlerfigssalik centre is one of four composit ~, intrusions forming the Igaliko complex in the east of the Precambrian Gardar Province of South Greenland (Emeleus & Upton 19"~'6). A general description of the rocks and struetme of the intrusion is given in Emeleus & Harry (1970). The Igdlerfigssalik syenites are divided into two groul~s separated in time by the intrusion of a regional dyke swarm. These two groups are further subdivided on the basis of field relations (Emeleus & Harry 1970) into intrusive units (Fig. l). Group I

Group 2

[SI 1

augite syenite

! S12[ |SI3J [S14

nepheline syenite

IsIsI ]Sl6}

augite syenite nepheline syenite

IS17/ The earliest units within each o1" the two groups,

LITHOS I1 (1978)

100 MarjoriePowell

2

~km

SO2 ' ++ 4. + 4" 4- 4-~" 4"~ "1" "4 4 4, "r 4. 4. ÷ 4. . ' +4`4.÷4.

,.

4.,1

4.44-+4.+4-04.04.

I

4"

4+

• .,

4-

÷ ÷ 4. 4 " 4 4 . 4. ~ 4"

4-

4-

4.

÷

4-4-4-4.4

÷

+

N

.4-4.4,4-

÷ 44.4+

+'(',0.*',1. ÷

/i,

4~82

~35~0

Igdlerfigssalik syenites SII,2,3,4,5,7

S17

SI6

•

Gardar supracr ustl!ds

O Banment granite Older Igaliko syenit~;s [ ~ Superficial deposits Glacier --- Fault ~ Dip of igneous layel,ing

m above

+. +

FJORD

,

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+

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+ ~

++++÷

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+ + +++++

+ + ,~ ÷ 4-+4 ÷÷+4+ ÷

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+ + ÷ ÷ 4++++++4.+÷( + + 4. ++4+÷+4.++ +

÷ + 44.

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,, .21-2.1111 Fig. 1. Geological map of the lgd~errig,salik intrusion, aher Emeleus & Ha~rry (1970). Analysed specimens referred to in Tables 2-9 are located with black dots and their GGU numbers indicated.

SII and S14, are aagite syenites with only small amount:.; of nephe',ine, whereas the later unit=, contain much more abundant nepheline. A striking feature of the centre is the annular (ovoid) outcrop pattern displayed by the later unit:; (Fig. 1).. The mechanism of emplacemer~t favoured by Emeleus & Harry (19'70) is one ofcombined rin~ dyke intrusion and block subsidence. For each unit

a la~rge cylindrical or steep-sided conical block has been downfaulted and the space formed filled by syerfite magma that moved up the ovoid fracture and over the top of the subsided mass of earlier rocks. The ~brm of the final intrusion, whether a rir~g dyke (e.g. SI6) or a stock (e.g. SI7), would depend on the amocnt of central subsidence ;and the extent of subsequent erosion. Repealed subsid-

LITHOS 11 (1978) ence about approximately the same centre Frodueed successively younger intrusions appearing towards the middle, culminating in the well-preserved steepsided stock of SI';'. The earlier syenites SI 1-3 which outcrop on the northern margins ef the centre, probably also had annular (ovoid) outcrop patterns, but these have been largely obscured by the intrusion of the Group 2 syenites. Each of the units of lgdlerfigssalik, apart from the ring dyke S16, crystallised from a high-level stoeklike magrm, body, which by virtue of its size and the relatively low viscosity ofdry phonolitic-trachytic magmas (about l0 s poise at 1000°C, calculated from the data of Bottinga & WeiH 1972) must initially have been in a state of vigorous, perhaps turbulent, natural convection. Crystal settling, aided by convection currents, resulted in the formation of a series of rhythmically and cryptically layered cumulates. Mineral layering and igneous lamination are steeply dipping in the outer part of each unit but became shallower inwards (Fig. 1). Layering in S17 forms a series of saucershaped structures. The total pressure of crystallisation was probably in the range 1-2 kb on the basis of the thickness of adjacent contemporaneous supracrustal rocks.

Mineralogy The major phases in the syenitic rocks are Ca-Na clinopyroxene, olivine, titaniferous magnetite, apati~te, nepheline, alkali feld:3par, amphibole, biotite and aenigmatite. Calcite, ca,crinite, sodalite, analcite, astrophyllite, sphene, iron sulphides and eudialyte occur as accessory minerals. Eudi~lyte is restr;,cted in occurrence to pegmatites, while aenigmatke occurs only in the ,outer 1.5 km of St5 and the r~ng dyke SI6. The crystallisation interval of each syenite specimen can be divided into three stages - cumulus, intercumulus and subsolidus. Curnuh~s phases crystallised from the main body of magma (Wager & Brown 1967). During their nucleation and growth, magmatic liquid was volumetrically more abundant than crystals, resulting in unzoned crystals. Cumulus crystals are recognised by their approximately euhedral outlines (Fig. 2), although these can be obscured by later overgrowths of the same phase or different phases, crystallised from the intercumulus liquid. The common cumulus assemblage of the syenites is: Ca-rich clinopyroxene, +olivine, apatite, rna~etite, alkali feldspar, +nepheline. These cumulus phases crystallised 7 - Lithos 2/78

Igdlerfigssalik nepheline syenite

101

Fig. 2. Rtctangular sectioned, fi~ely perthitic, cumulus alkali feldspar (grey) with fine a!bite rims (white). The isotropic area in the centre of the fiod of view is intercumulus sodalite. Crossed polars. Photomic~rograph dimensions: !.!6 cm x0.77 cm.

approximately contemporaneously from the main body of magma. Their apparently different orders of crystallisation in the layered syenites merely reflect different rates of crystal settling plus the vagaries of convection. There is a conspicuous lack of cumulus hydrous phases. Intercumulus phases crystallised from magrnatic liquid trapped between the cumulus crystals (Wager & Brown 1967). During their growth liquid was subordinate in proportion to the surrounding crystals, resulting in rapid changes in liquid composition with progressive crystallisation. Two distinct textural features resulting from intercumulus crystallisation are the development of compositionally zoned rims around existing cumulus cr/stals (Fig. 3), and the nucleation and growth of phases, some of which are not present as cumulus phases, with a poikilitic or interstitial habit (Figs. 2 and 4). All the cumulus crystals show var ous scales of zoning which is most apparent, how~/er, in the pyroxenes where outer zones to cumulus crystals are much deeper green in colour and frequently show considerable Na enrichment. I ntercumulus minerals include amphibole, biotite, ~enigmatite, sodalite, nepheline, alkali feldspar, pyroxene and magnetite. Intercumulus olivine is very r~ze. Subsolidus phases crystallised in the absence of magmatic liquid but in the presence of an aqueous fluid phase. Two distinct textural features resr~l~ing from subsolidus crystaUisation are the growth of new minerals and the alteration of existing minerals. There is convincing textural evidence in some rocks for subselidus growth of biotite, which occurs as radial fringes on titanomagnetite grains (Fig. 5). These biotite fringes only occur where the fitano-

102 MarjoriePowell

Fig. 3. Semi-euhedral pale green cumulus Ca-elinopyroxene (grey) mantled by deep green intercumulus Cz~-Na-clinopyroxene (dark grey-black). The light partially altered areas are semi-euhedral cumulus nepheline. Ordinary light. Photomicrograph dimensions: i.16 cm x0.77 cm.

Fig. 4. Strongly cleaved poikilitic biotite enclosing cumulus magnetite (black), olivine (pale grey), Ca-clinopyroxene (deeper grey) and apatite. The light areas are intercumulus alkali feldspar. Ordinary light. PhotomicrograpF dimensions: 1.16 cm x0.77 cm.

LITHOS 11 (1978)

Fig. 5. Subsolidus fringes of biotite (dark grey) on cumulus magnetite (black) in contact with alkali feldspar (white). Magnetite grains in the right of the field of view enclosed in intercumulus amphibole (dark grey) are prevented from reaction with the adjacent alkali feldspar. Ordinary light. Photomicrograph dimensions !.16 cm × 0.77 cm.

on the mineralogical and mineral chemical variations in Group 2 syenites, excluding the ring dyke SK6. S16 has been excluded because of the difficulty of placing the available specimens in stratigraphical sequence. Electron microprobe analyses have been made of all the phases, except apatite and rarer accessory minerals, in over 50 syenite specimen,;, encompassing all seven units of the lgdlerfigssalik centre. To represent the range of variation two specimens have been chosen from each of S14, SI5 and SI7. Mineral analyses for these specimens are presented in the appropriate tables. The assemblages in these 'average' syenite specimens are given in "i'able 1.

Pyroxene magnetite was originally in contact with aa alkali feld:;par grain suggesting that magnetite and feldspar, plus added water, are being converted to blot ite. Blue-green amphibole fringes around olivine grains, which are particularly common in S':~5,may also be the product ofso!id state reaction. Interstitial cam:rinite raay have developed by reaction between neplaeline and a COn rich fluid phase. The mineral most commonly altered is nepheline, to a micaceous mineral, gieseckite. Alk:~li feldspar is occasiiona|iy altered to :~ericite.

Mineral ,chemistry Due to the poor erposure of Group I syenites, the discussion~ in the following sections will be based

Cumulus pyroxenes in the lgdlerfigssalik syenites are essentially CaMgSi206-CaFeSi206 solid solutions with varying amounts ofTi, Zr, Mn and AI as minor constituents. Acmite (NaFe3+Si206) contents are uniformly low (Table 2). The augite syenilte cumulus pyroxenes are pale brown, while those from the nepheline syenites are normally pale green. Pyroxenes crystallised from the intercumulus liquid show varying degrees of sodium enrichment and are much ,deeper green in colour. Pyroxenes from pegmatites in the syeni~:es all approach acmite in composition. The range of Na enrichment in S14, S15 an(~l Slit cumulus ar~d intercumulus pyroxenes is shown ~n Fig. 6. An important feature of the chemistry of the sye:Jite pyroxenes is that Na enrichment only occurs in the:intercumulus environ-

lgdlerfigssalik nepheline syenite

LITHO5 I 1 (~978)

103

Table 1. Mineral assemblages in selected syenite specimens from the lgdlerfigssalik intrusion. Unit

G G U No.*

Cumulus

Intercumulus

Subsolidus

S14

54331 87 ! I 0

rot, ap, oi, px, fsp, ne rot, ap, ol, px, fsp, ne

amph, px, fsp, ne amph, px, fsp, ne

bi

S!5

41981 43827

rot, ap, ol, px, fsp, ne rot, ap, oi, fsp, ne

amph, aen, sod, px, fsp, ne amph, px, ne, fsp

amph, anal amph, bi

S17

43910 43850

rot, ap, ol, px, fsp mt, ap, ol, px, fsp, ne

amph, bi, px, fsp amph, bi, px, fsp ne

bi

43884

ab, or, '~x, px, amph, anal, astro, ap, eud

Peg.

Abbreviations: mr-magnetite, a p - a p ~ i t e , ol-olivine, px-pyroxene, fsp-feldspar, ne-:ephe!ine, amph-amphibole, bi-biotite, sod-sodalite, aen-aenigmatite, anal-analcite, ab-albite, or-orthoclase, Peg-pe~matite. * Gmnlands Geologiske Undersogelse

Table 2. Pyroxene analyses. S14

S15

S17

Peg. 43884

54331 c

54331 i

87110 c

87110 i

41981 c

41981 i

43827 i

43910 c

43910 i

4385~ c

43850 i

SiOz AizO3 TiO2 ZrO2 FeO* Fe203* MgO MnO CaO NazO K,O

50.80 1.69 0.99 0.00 10.27 1.97 10.21 0.47 22.48 0.84 0.00

50.99 2.50 0.86 0.00 11.47 0.21 9.68 0.39 22.50 0.82 0.00

49.61 !.31 0.70 0.21 t9.13 2.50 4.22 0.65 21.88 !.02 0.00

49.39 093 0.34 i.03 19.06 6.60 i.76 0.87 17.51 3.11 0.01

49.88 1.49 0.64 0.08 15.69 2.20 6.38 0.66 21.75 !.00 0.01

49.87 49.54 11.64 1.43 ~.22 0.68 0.91 0.19 1".:.77 12.08 g.97 5.26 ~.66 7.12 (z78 0.56 15.97 21.50 3.95 1.52 0.02 0.01

49.77 2.22 0.91 0.00 1 i.92 3.40 8.45 0.55 22.30 0.91 0.00

49.99 2..31 0.79 0.00 15.63 1.63 8.&) 0.5(~ 20.1~. 0.88 0.00

49.23 1.24 0.52 0.10 18.31 3. i 2 4.33 0.65 21.84 1.02 0.00

49.05 1.12 0.00 0.15 i'L01 6.97 1.42 i.38 18.86 2.51 0.00

51.40 !.02 !.09 0.59 !.14 28.78 0.23 0.35 2.13 12.54 0.00

Total

99.72

99.42

101.23

100.61

99.78

100.76

100.35 100.47

99.27

PFeO +

12.04

11.66

21.38

25.00

17.67

25.84

! .947 0.06 ! 0.021 0.004 0.628 0.074 0.247 0.022 0.920 0.078 0.000

! .969 0.044 0.010 0.020 0.636 0.198 0.g05 0.029 0.748 0.:~40 0.001

1.951 0.069 0.019 0.002 0.513 0.065 0.372 0.022 0.912 0.076 0.000

1.979 0.030 0 007 0.018 0.590 0.268 0.098 0.026 0.679 0.304 0.001

99.89

100.43

99.99

16.81

14.98

17.09

21.11

25.28

27.04

1.924 0.065 0.020 0.004 0.392 0.154 0.412 0.018 0.895 0.114 0.000

i .908 0. 100 0.026 0.000 0.382 0.098 0.483 0.018 0.916 0.068 0.000

! .9M0.105 0.023 0.000 0.505 0.047 0.466 0.016 0.836 0.066 0.000

1.946 0.058 0015 0.002 0.605 0.093 0.255 0.022 0.925 0.078 0.000

1.963 0 053 0.000 0C03 0,636 021C 0,085 0.047 0.809 0.195 0000

1.985 0.046 0.032 0.01 i 0.037 0.837 0.013 0.011 0.088 0.939 0.000

Recalculated on 6 oxygens Si AI Ti Zr Fe 2+ Fe 3÷ Mg Mn Ca Na K

i .936 0.076 0.028 0.000 0.327 0.057 0.580 0.015 0.918 0.062 0.000

1.946 0. ! 13 0.025 0.000 0.366 0.006 0.551 0.013 0.920 0.06B 0.000

* Fe a + contents calculated assuming a stoichiometry of 4 cations; + PFeO-- All Fe as FeO; c = Cumulus; i = Intercumulus.

104 Marjorie Powell

LITHOS !1 (1978)

Mn

Fig. 6. The range (4 compositional variation in SI4, S15 and S17 cumulus and intercumulus pyroxenes in terms of Mg-FEZ*+ Mn--Na (atom %). Dashed lines indicate the range of variation in individual syenite specimens. Analyses given in Table 2 are indicated by their GGU numbers. Solid circles are cumulus pyroxenes, open circles intercumulus pyroxenes. SI4

SI5

SI7

OLIVINE

'5 - ® 54331

Mg .2 _

B

Mg

.5

-

U

-

Ig

4382~

3850

CUMULUS PYROXENE

54331 •

'6

B

==41981

@

Q

~1981

"4 "3 0 '2

87110

--

'1

0 L_,____

~

lO00m t

Fig. 7. V~riation in Mg content (cations per 4 and 6 oxygens respectively) of cumulus olivine and Ca-clinopyroxene wiLth distance from the outer contact in S14, Si5 and S17. Analyses given in Tables 2 and 3 are indicated b9 the ix GGU number~i.

ment and may be reiated to progressive oxidation of the trapped liquid. The variation in Mg content of cumulus pyroxenes with progressive fractionation in SI4, SI5 and S17 is shown schematically in Fig. 7. SI4 and SI7 cumulus pyroxenes show progressive Mg depletion and concomitant Fe 2+ enrichment with distance from the outer contacts of the units, while S15 cumulus pyroxenes are uniformly Mg poor. Cumulus pyroxene core compositions reflect changing

compositions in the main body of magma and therefore the discontinuities in Fig. 7 confirm that the units as mapped in the field represent separate intrusions of magma. However the different pyroxene trends do not necessarily reflect different initial magma compositions. They may simply reflect erosion to different levels in the various units. Thus S15 may be presently exposed at a high level and SI4 alta low leve!.

Igdlerfigssalik nephelinesyenite 105

LITHOS 1i (I 978) Table 3. Olivine analyses.

6O

S14

SI5

S!7

543.31 87110

41981 43827

43910

43850

SiO2 FeO MgO MnO CaO

31.'79 30.00 54.14 64.34 10,73 1.15 2.32 3.94 0.30 0.63

29.21 64.34 0.77 4.38 0.69

31.12 58.23 7.91 2.85 0.37

29.27 59.74 0.27 8.89 0.53

Total

99.28 100.06

99.39 100.07 100.48 98.70

29.63 64.31 !.08 4.44 0.61

Ne

Recalculated on 4 oxygens Si Fe z+ Mg Mn Ca

1,000 !.425 0,503 0,062 0.010

1.004 1.801 0.057 0.112 0.023

0.992 1.827 0.039 0.126 0.025

0.995 1,807 0,054 0.126 0,022

~.992 !.552 Q.376 0.077 0,013

i.001 1.708 0,014 0.257 0.019

Table 4. Average magnetite analyses.

GGU No.

Mole Vo Uivospinel

54331 87110 41981 43827 43910 43850

70 44

'~d :,o

V

k/ ,to

V

V eo

SO

Ks

Fig. 8. Compositions of coexisting cumtflus nephelines and alkali feldspar'~ from Tables 5 and 6 projected into Petrogeny's Residua System, nepheline-kalsilite-silica.

mole percent ulvospinel (2(FeO+RO).TiO2) in Table 4 represent averages of many microprobe analyses made with a focussed beam on a closely spaced grid ~ystem across the exsolved grains. The calculation of ulvospinel contents from aver'age analyses of exsolved magnetites is considerefl in Powell & Powell (1977a). Average ulvospin~! contents fall in the range ~5(+ 5) to 35(+5) mole percent.

62

Nepheline

83

Olivine is a common cumulus pha,,e it, the syenites but its dis~tribution is sporadic due to the mineral layering characteristic of the syenites. However it ar~pears to crystallise throughout each unit. Major element variations ,'an be expressed in terms of Fe, Mg, Mn and Ca (Table 3). Compositions range from Fa71.aFo25.2Tp31-,ao.s in 54331 (S14) to FassFol TptaLa ~ in 43850 (S17) with considerable Mn enrichment in the most iron rich olivines. Compositiona~ variations with progressive fractionation in S14, S15 and SI7 parallei those of cumulus pyroxene (Fig. 7).

Nepheline is frequently a euhedral cumulus phase in S15 and S17 but also occurs interstitially. Subordinate amounts of nepheline, of presumed cumulus origin, occur as blebs in cumulus alkali feldspar in S14. In some specimens of S14, nepheline and feldspar occur as intimate intergrowths suggestive of euteetic crystallisation. All Igdlerfigssalik nepheline analyses (Table 5) have Si significantly greater than the stoichiometric formula. These excess silica nephelines can be adequately represented in Petrogeny's Residua System, nephcline-kalsilite-silica, for example Hamilton (1961) and Powell & Powell (1977b). The compositional range of cumulus nepheline coexisting with curr,ulus alkali feldspar is shown in Fig. 8. There is remarkably little variation in nepheline compositions with progressive fractionation in S14, S15 and SI7.

Magnetite

Alkali feldspar

Titaniferous magnetite is a ubiquitous cumulus phase in tihe syenites. Subsolidus oxidation and exsolution, resulting in blebs and lamellae of ilmenite, makes estimation of original bulk compositions difficult. Average analyses expressed as

Perthitic alkali feldspar is a ubiquitous ctJmu',u~ and interstitial phase in the Igdlerfigssalik syenites. The exsolution lamellae make estimation of origit~al bulk compositions difficult. The analyses given in Table 6 are average mi~croprol~ analyses ofexsolved

Olivine

106

MarjoriePowell

L I T H O S 11 (1978)

Table 5. Nepheline analyses. S!4

S15

SI7

Peg. 43884

54331

87110

41981

43827

43850

C

C

C

C

¢

SiO2 A!203 Fe2Off CaO BaO Na20 K20

43.91 33.02 0. ! 3

45.47 3 ! .98 0.68 0.01 0.00 16.40 5.47

45.35 31.43 0.90 0.08 0.00 16.99 4.49

44.24 32.25 0.30

45.50 32.27 0.67

0.00

0.00

0.00 15.61 5.09

46.05 3 ! .55 0.52 0.06 0 00 !6 65 522

0.00 16.81 , 5.80

0.00 16.39 5.43

Total

99.1 !

100.05

100.01

99.24

99.40

100.26

!.086 0.901 0.012 0.000 0.000 0.760 0.167

!.090 0.891 0.016 0.002 0.000 0.792 0.138

!.068 0.918 0.005 0.000 0.000 0.787 0.179

1.084 0.906 0.012 0.000 0.000 0.757 0.165

1.35

Recalculated on 4 oxygens Si AI Fe Ca Ba Na K

!.057 0.937 0.002 0.035 0.000 0.729 0.156

1.,~98 0.887 0.009 0.002 0.000 0.769 0.159

as,A~s,o,: Calculated using the data of P o w e l i & P o w e l l ( 1 9 7 7 b ) . 1073 K 0.55 0.56 0.62 0.57 1273 K 0.60 0.61 0.65 0.62 * All Fe as Fe203; c = c u m u l u s .

Table 6. Feldspar analyses. S[4

Si5

:!I!7

Peg.

54331 c

87110 c

41981 c

43827 c

43827 i

.:13910 i

43910 c

43850 c

43884

43884

SiO2 AI2Oa Fe203* CaO BaO Na20 K20

62.66 21.13 0.20 2.50 8.46 2.26

67.14 18.52 0.13 0.08 0. ! 2 6.29 8. i I

65.36 18.29 0.00 0.00 0.00 4.61 11.31

67.38 18.84 0,07 0.10 0.06 6,13 7.87

66.63 19.01 0.11 0.14 0.12 6.75 6.87

65.45 19.73 0.51 0.99 0.76 7.40 5.37

65.25 20.09 0.36 1.07 6.70 6.22

66.29 18.80 0.00 0. ! 1 -5.85 8.7C

65.22 17.97 0.30 0.00 0.00 1.13 14.20

69.12 18.79 0.00 0.00 0.00 11.50 0.11

Total

97.21 +

100.39

100.07

100.45

99.63

100.21

99.69

99.75

98.82

99.52

3.008 0.991 0.002 0.005 0 O01 0.531 0.,~48

2.993 1.007 0.004 0.007 0.002 0.588 0.39.4

2.939 1.045 0.017 0.048 0.0 ! 3 0.644 0.308

2.934 ! .065 0.012 0.052 0.584 0.357

2.993 1.001 0.000 0.005 0.512 0.501

3.023 0.982 0.010 0.000 0.000 0.102 0.840

3.027 0.!)70 0.4)00 0.000 0.000 0.976 0.006

using the data of Thomps(~n & Waldbaum (1969). 0.587 0.528 0,.5'~1 0.561 0.479 0.547 -

0.604 0.588

0.567 0.537

Recah:ula~ted on 8 oxygens Si Ai Fe Ca Ba Na K

2.862 l. ! 38 0.007 0, 122 0.749 0.132

aNaAISi3Oa: Calculated 1073 K 1273 K

-

3.008 0.978 0.004 0.004 0.002 0.546 0.464

2.9':'8 0.982 0.00t~~ 0.000 0.00f 0.407 0.657

* All Fe as Fe203; c = cumulus; i = intercumul~us.

* Total low due to brc,ad exsolution lam¢llae in perthite.

Igdlerfigssalik nephelinesyenite I07

LITHOS !1 (1978) Table 7. Amphibole analyses. S14

S15

SI7

Peg.

54331 Br

87110 Br

41981 Br core

41981 B-G rim

43827 Br

43827 G-Br

43827 B ss.

~39!0 Br

43850 Br

43854 B-G

SiO2 Al2Oa TiO2 ZrO2 FeO* MgO MnO CaO BaO Na20 K20 F'

39.99 10.54 4.47 0.33 ! 8.67 7.66 0.41 ! 1.43 2,67 1,56 0.16

41.30 6.87 ! .59 0.87 28.79 2.86 0.78 8.65 0.00 3.96 !.48 1.15

44.09 4.32 2.50 0.34 27.75 3.59 0.69 7.80 0.00 4.37 !.66 1.26

46.65 2.35 0.39 0.82 32.27 0.99 1.34 4.78 0.00 5.88 1.82 i.04

44.33 5.58 ! .82 0.56 26.22 5.12 0.00 8.52 0.00 4.18 !.55 I.! I

44.3i 4.84 2.26 0.49 29.28 2.99 0.89 6.87 0,00 4,97 !.57 -

46.51 3.93 0.37 0.31 27.73 4.93 0.67 6.08 0.00 5.62 1.54 1.17

38.14 10.37 3.84 0.30 21.53 6.34 ~J.44 11.06 0.00 2.55 1.50 0.30

38.95 8.92 3.56 0.21 29.02 1.82 1.03 9.69 3.57 1.53 0.24

46.93 3.06 1.20 0.22 29.76 1.44 2.02 1 15 8.41 1.74 !.04

Total

97.89

98.30

98.37

98.33

98.99

98.47

98.86

96.37

98.54

96.97

7.106 0.821 0.303 0.027 3.740 0.862 0.094 1.347

7.641 0.454 0.048 0.065 4.420 0.242 0.186 0.839

7.006 1.040 0.216 0.043 3.465 1.206 0.000 !.443

7.080 0.912 0.272 0.038 3.913 0.712 0.120 !.!76

7.o81 ~).590 ).!48 3.0 ! 8 4.073 0.351 0.280 0.202

0.000

0.000

0.000

! .366 0.341

1,867 0.380

1,281 0.313

1,540 0.320

6.101 1.955 0.462 0.023 2.880 1.511 0.060 1.896 0.000 0.791 0,306

6.313 1.704 0.434 0.017 3.934 0.440 0.141 !.o~3

0.000

7.384 0.736 0.044 0.024 3.682 1.166 0.090 1.034 0.000 1.730 0.312

Recalculated on 23 oxygens Si AI Ti Zr Fe z + * Mg Mn Ca Ba Na

K

6.178 1.920 0.519 0.025 2.412 !.764 0.054 !.892 0.800 0.307

6.735 1.321 0,195 0.069 3.926 0.695 0.108 !.51 ! 0.000 1.252 0.308

-

!. 122 0.316

-

2,669 0.363

* All Fe as FeO: Br = Brown; B - G = Blue-green; G - B r = Green-brown; E = Blue; ss. = subsolidus.

grains. Anorthite contents are low but increase with increasing albite content. The range of compositions of cumulus alkali feldspar, projected onto the binary Ab-Or, coexisting 'with cumulus nepheline, is shown in Fig. 8. There is no clear trend of feldspar compositional variation in each unit.

Amphibole Amphibole in the lgdlerfigss~lik syenites is mainly intercumulus but occasionally subsolidus. Colours range from deep red-brown through green-brown to deep blue-green and there is ,t corresponding wide range of compositional variation (Table 7). In some rocks traces of blue-green amphibole occur as outer zones to green-brown amphibole, indicating that the blue-green variety crystallised later. Brown or green-brown amphibole commonly mantles cumulus clinopyroxene and in some cases may have been formed by reaction between the pyroxene and the

intercumulus liquid. Blue-green a;nphibole has two distinct textural parageneses; in some syenites it is poikilitic while in others it occurs as fringes around olivine grains. It is possible that the latter texture developed by subsolidus growth, Blue-green intercumulus amphibole is common only in S15, where it is associated with interstitial aenigmatite. All the Igdlerfigssalik amphiboles have a completely filled A site. For. simplicity they can be • -cons:idered in terms of the fallowing.end members as in Deer et al. (1966): A X Y Z Na Ca2 Mgs SiTAI Edenite CA Pargasite Na Ca2 Mg4AI Si6AI2 Na NaCa Mg5 Sis Richtedte Na NaCa Mg4M Si~AI Katophorite ,AA Na NaCa Mg3AI2 SieAI2 Mboziite Na Na2 Mg4AI Sis Eckermannite where CA=calcic a~nphibole, A A=alkali amphi-

Marjorie lowell

108

LITHOS 11 (1978) Table 8.

S~ 8

•8

..~,

.4

-2

7

"B

"6

-4

6

"2

-'-T-'-r--r-"l~

S17

Pg Hff 5a33Yo

AA .6

4382~

.q

o~ ,o," d'

a

G

oo

SiO2 A1203 TiO2 ZrO2 FeO* MgO MnO CaO BaO

j

Na

+ 2 Ri K .2

,.lip 043827

iVIb

)Kt

js IJ'41981

.4

_

"6

F

s*

-8

w ~L

3

r.K

43910

43850

35.03 13.19 6.75 0.00 25.33 7.70 0.26 0.00 0.39

33.38 !1.18 4.08 0.00 34. ! 5 1.82 0.87 0.00

0(3850 --

'L87110

a

Biotite analyses.

43884 ---

Fig. 9. Si versus Na+K (cations per 23 oxygens) for lgdlerfigssalik amphiboles. Analyses given in Table 7 are indicated by their GGU number(,.. Dashed lines join amphibole analyses from the same specimen. A - S14, brown, C) - S15, green-brown, • - S15, blue-green, [] - S17, brown, W - Pegmadtes, blue-green, Ed - Edenite, P g - Pargasite, Ri - Richterite, K t - Katophorite, Mb)Mboziite, Ek - Eckermannite.

bole, N a = N a + K , M g = M g + M n + F e 2+, and A! = AI + Fe 3 ÷. Further constituents in many of the amphiboles are Ti and Zr in the octahedral sites and 0 2 - and F - substituting for OH - in the vapour site. The range of compositional variation of Igdlerfigssalik amphiboles is shown in Fig. 9, a plot of Si versus N a + K . Blue-green aml:)hiboles are much richer in Si and alkalis than the brown and greenbrown varieties. An important correlation is that those syenites with alkali rich ~ntercumulus amphfibole, notably S15, also show extreme Na enrichment in the pyroxene crystallised from the intercumulus liquid.

Biotite Biotite has two distinct textural parageneses; as a poikilitic intercumnlus phase in SI7 and as subsolidus fringes to titanomagnetite grains in the augite syenites. Analyses of intercumulus biotite from SI7 are given in Table 8. All of the syenite biotites irrespective of paragenesis are iron rich phlogopite-annite-oxyannite solid solutions (Wones & Eugster 1965) and may have substantial l=e3 + contents charge balanced by O 2- substituting tbr O H - in the hydroxyl site.

Na20

0.35

0.14

K20 F

8.84 0.21

9.02 0.00

Total

98.05

94.64

Recalculated on 22 oxygens Si Ai Ti Zr Fe 2+ Mg Mn Ca Ba Na K

5.406 2.400 0.783 0.O00 3.269 1.771 (?.034 0.000 0.024 0.105 i .741

5.624 2.221 0.517 0.000 4.812 0.457 0.124 0.000 0.046 ! .939

(' All Fe as FeO.

Aenigr~atite Aenigmatite occurs as a strongly pleochroic (redbrown 1o black) poikilitic or interstitial phase in the outer part of S15 and in the ring dyke S16. Analyses from 41981 (SI5) are given in Table 9. Fe 3+ contents have been estimated assuming a stoichiometry of 14 cations. An important substitution operating on the basic aenigmatite end member Na2 FesTiSi6020 appears to be Fe 2 +Ti = Fe a + Fe a +. The aenigmatite in 41981 shows small compositional variations; AI, Ti, Zr, Fe 2 +, Ca and Ba decreasing and Si, Fe 3 ÷ and Na increasing from core to rim.

Crystallisation conditions During cumulus crystalli,~a~don crystals were volumetrically subordinate to magma and therefore the composition of the magma constrained the compositions of the phases crystallising from it. Progressive variations in the compositions of cu-

lgdlerfigssalik nepheline syenite 109

LITHOS !1 (1978)

Table 9. Aenigmatite analyses. 41981 core

rim

5iO, A!203 TiO 2 ZrO z FeO* Fe2Oa* MgO MnO CaO BaO Na20 K20

39.80 1.20 7.85 0.12 35.94 5.04 0.23 1.24 0.68 0. ! 4 7.04 0.00

40.19 1.04 7.51 0.07 35.43 6.17 0.20 1.26 0.44 0.00 7.29 0.00

Total

99.28

99.60

PFeO +

,40.47

40.98

Recalculated on 20 oxygens Si Ai Ti Zr Fe z + Fe a + Mg Mn Ca Ba Na K

5.752 0.204 0.853 0.008 4.344 0.548 0.049 0.152 0.105 0.008 1.973 0.000

5.779 0.176 0.8 ! 2 0.005 4.261 0.668 0.043 0.154 0.067 0.000 2.032 0.000

* Fe ~+ contents calculated assuming a stoichiornetry of 14 cations. + All Fe as FeO.

mulus phases within individual units of the Igdlero figssalik cemre thus reflect the changing conditions and degree of fractionation of the main body of magma. In the intercumulus environment the trapped liquid plus surrounding crystals behaved as an essentially closed system on a scale of several centimetres. Thus compositional variations in the intercumulus minerals only reflect the extent of evolution ofthe intercumulus liquid in these isolated microenvironments. The initial compofition of the trapped liquid and thus the initial compositions of the intercumulus phases will however reflect the stratigraphical position of the particular syenite specimen within the unit. An attempt is made in this section to account for the changes in the chemistry of the cumulus phases and thus the chemistry of the main bodiy of magma

:for each unit of the Igdlerfigssalik centre and to account for the mineralogies! and mineral chemical variations within individual rocks, which reflect the ,:hanging con¢itions between and within the cumulus and intercumulus environments, in terms of temperature, as~o2, ao z and Mg/Fe ratio.

Geothermometry A common cumulus assemblage in the Igdlerfigssalik syenites is. olivine, Ca-clinopyroxene, magnetite, nepheline and alkali feldspar. Four geothermometers can be applied to this assemblage. Detailg of these geothermometers are published elsewhere and therefore the methods will not be considered in detail.

Olivine-clinopyroxene geothermometry: An olivine-Ca-clinopyroxene geothermometer was developed by PoweU & Powell (1974) based on the thermodynamics of an Fe-Mg exchange reaction: 2CaMgSi206 + Fe2SiO4 = 2CaFeSi206 + Mg2SiO4 Wood (1976) considers that there are considerable uncertainties involved in the calibration ,ff tMs geothermometer. However the olivine-clincpyroxene pairs from the Igdlerfigssalik syenites are within the composition range of the data used to calibrate the geothermometer (apart from the Mn contents of some of the olivines, which should not affect the results greatly) and therefore it is considered that the calculated temperatures should reflect the c~stallisation conditions of the olivine-clinopyroxene pairs. Application of the Powell & Powell geothermometer to all available olivine-clinopyroxene pairs from the Igdlerfigssalik syenite- gives temperatures in the range 900-980°C (Table 10). These temperatures should be close to the original magmatic temperatures and are consistent with water-saturated liquidus temperatures, at 1 kb, of about 900°C experimentally determined by Edgar & Parker (1974) for a range of peralkaline undersaturated rocks. The syenite liquidus temperatures must have been somewhat higher than 900"C as it is unlikely that the Igdlerfigssalik magmas were initially water saturated.

Nepheline-alkali feldspar geothermometry: A nepheline-alkali feldspar geotherraometer has been developed by Powell & Powell (1977b) based on the thermodynamics of an N a - K exchange reaction between nepheline and alkali feldspar:

Marjorie lowell

110

LITHOS ! I (1978)

1.0

.8

&

31132

~X.S~lto

'6

501

XAb .4

!

A

•

• it

•

•04

.08

1

.12

0

xSi02 I~ x Kals: .14

"04

"08

.12

0

"04

.15

"08

"12

.16

0

.04

"08

-12

0

04

.17

08

.12

.18

Fig. 10. Nepheline-Aikali Feldspar geothermometer diagram. XAb, feldspar versus Xs~o2, nepheline for a range of XK,j,la,e values in the nepheline. Temperature contours in degrees Centigrade. The dashed line indicates phase separation in the alkali feldspar. The compositions of,;everal coe×isting cumulus nepheline-alkali feldspar pairs from Table I I are indicated by solid symbols and their G G U numbers.

Table !0. Olivine - Ca-clinopyroxene geothermometry. Calculated temperatures for Group 2 syenites, PTot,t = 1000 bars. G G U No.

Unit

nMg

nFe

mFe

mMg

XAt

- I n KD

T°C

41948 54331 63824 63783 58214 63829 87112 87110 41915

4 4 4 4 4 4 4 4 4

0.335 0.503 0.332 0.242 0.341 0,291 0.307 0.057 0.147

!.562 1.425 1.580 1.701 1.570 1.639 ! .623 ! .80 ! 1.7 ! 2

0.380 0.327 0.355 0.354 0.350 0.421 0.417 0.628 0.553

0.533 0.580 0.536 0.546 0.572 0.484 0.508 0.247 0.370

0.099 0.097 0.117 0. ! 14 0.097 0.1 !1 0.092 0. i 07 0.093

1.878 !.614 1.972 2.383 2.018 1.863 i .868 2.520 2.053

955 960 960 950 950 960 950 940 940

41923 41981 41972 43840

5 5 5 5

0.083 0.039 0.078 0.057

1.774 1.827 !.8 ~7 !.793

0.621 0.5 ! 3 0.573 0.535

0.242 0.372 0.326 0.344

0.122 O. 106 0.098 O. ! 12

2. ! 20 3.525 2.584 3.007

960 905 930 930

41944 46232

6 6

0.397 0.137

!.534 1.738

0.326 0.568

0.521 0.324

0.167 0.110

1.821 !.979

980 960

43910 43861 43864 43850

7 7 7 7

0.376 0.378 0.106 0.014

1.552 1.555 1.738 1.708

0.382 0.372 0.495 0.605

0.483 0.513 0.372 0.255

0.132 0.135 0.111 O.114

1.652 1.736 2.529 3.940

975 975 940 900

n - Cations per 4 oxygens (olivine). m - Cations per6 oxygens (clinopyroxene). XAI ~--"XAI + XTI "~" X c r -].-XFe3 + o n M i, cpx. KD = n M__s. mF¢ rife ol m M8 cpx

Igdlerfigssalik nepheline syenite

LITHOS I I (1978)

111

Table 11. Compositions of coexisting cumulus nepheline and alkali feldspar and calculated equilibration temperatures from Power & Powell (1977b). Unik

G G U No.

Nepheline Xr.~

4

41915

4

87110

5 5

41981 43807

5

43827

5

43840

6 7

419~14 43832

7 7

43850 43853

7 7

54101 54108

Feldspar Xsio,*

XN.

0.148 0.150 0.162 0.159 0.167 0.145

0.08 0.08 0.07 0.09 0.08 0.10

0.614 rim 0.516 core 0.546_

0.153 0.138 0.169 0.162 0.130 rim 0.158 core 0.167 0.179 0.179

0.07 0.08 0.07 0.08 0.1 ! 0.10 0.08 0.06 0.06

core 0.150 rim 0.165 core 0.154

0.08 0.09 0.10

rim core rim core

rim core rim core

0.407 0.537 0.508 0.588 0.531 0.517

rim core rim core

0.685 0.598 rim 0.600 core 0.512 0.561 0.462 0.431

T°C

650 ± 80 650 ± 75 750±90 900_+ 100 725 ± 75 725 ± 85 750 ± 80 7~, ± 8¢) < 675 80O_+9O

8O0_+90 1300+_!30 { 100± 130 1050+ 120 825 + 90 925 + I00 650 + 80 775 _.+75 "r75 .+ 75

* The uncertainty in Xs~o2 is of the order of ±0.01.

KAISi308 +~1 KNa3Al,Si40t 6 = = NaAISiaOs + 1 KKaAi,,Si,O t ~ A simple numerical expression for the geothermometer cannot be derived because the calculations involve simultaneous solution of two equations which are transcendental in several unknowns. However, the geothermometer is presented graphically (Fig. 10) in terms of XAb, feldspar versus Xs~o2, nepheline for a range of XK.,.mt. values in the nepheline. Application of this geothermometer to all available cumulus nepheline-alkali feldspar pairs f~om the lgdlerligssalik syenites gives temperatures in the range 650 to 1050°C (Table 11). If it is assumed that nephcline and alkali feldspar crystallised approximately contemporaneously with olivine and clinopyroxene, on textural evidence, at temperatures in the range 900-980°C, then these calculated temperatures must reflect considerable re-equilibration of nepheline and feldspar with falling temperature in some specimens.

Temperature of cessation cf exsolution in perthites: The temperature of cessation of macroscopic exsolution in perthitic alkali feldspar [;rains can be

determined from the thermodynamics of the reaction: NaAISi3Os - NaAISiaOs alkali fsp plagioclase fsp Storraer (1975) developed a geothermometer ba~-.d on this reaction which did not ta~e into accowlt the solution of small amounts of Ca in the alkali feldspar. This geothermometer has been reforn, u~at~ (Powell & Powell 1977c) and the preferred ~eothermometer equation is: - xzK.~(6423 + 2718 XN~.~) T(K)Rln Kv + xzK.~(-- 4.63 + 1.54 XN,.~ where XNa,AF

KD - - ' ~ XNa,PL

for a pressure of 1000 bars (the estimated pressme of crystallisation of the syenites). A graphical representation of the geothermometer is shown in Fig. I I. Application of this geothermometer to the compositions of hosts and lameilae from coarseJy exso red perthites in SI4 (Table 12) gives temperature~ in the range 575-65C°C for the cessation of macroscopic exsolution in the syenite alkali feldspar:;.

112 MarjoriePowell !.0

-

LITHOS 11 (1978)

~f""------"~"-

!'0

t'O

I'0

•9

"9

-.

XNa, PL

.7 -6

.6

(~a :004 .s

AF

g C a e~ •4 .2

I .3

I .,t

t. .5

.5 i .6

.4

. .2

I .3

I

t

•~i

.5

~

'6

.,4

"2

7 xF=o.06 t .:)

I .4

I .S

~~x ~a: 0'08 I .6

.4

-.2

i .3

I .,t

I .s

I .6

X Na. AF

Fig. 11. Coexisting Alkali Feldspar-Plagioclase ge ~thermometer diagrams. Temperature contours are in degrees Centigrade. Analyses of hosts and lamellae from coarsely ,~:xsolved perthites in S14, given in Table i 2. are indicated by solid symbols and their GGU numbers. Table 12. Temperatures of ,cessation of exsolution in S14 perthites. G G U No.

Cations per 8 oxyger~s Ca + Ba

Na

K

T°C

63822

0. l114 0.04 !

0.814 0.504

0.094 PL 0.469 AF

650

63783

0.117 0.068

0.842 0.499

0.042 PL 0.406 AF

575

58214

0.141 0.085

0.798 0.583

0.070 P'L 0.340 AF

600

63829

0.127 0.086

0 808 0.596

0.041 PL 0.304 AF

575

87112

0.144 0.056

0.830 0.468

0.038 PL 0.493 AF

580

Temperature of cessation of exsolution in titanomagnetite grains: All cumulus titanomagnetite grains ';o the Igdlerfigssalik syenites conllain blebs and lamellae of ilmenite due to subsolidus oxidation and exsolution. The temperature of cessation of macroscopic exsolution has been calculated using a thermodynamic formulation of the Buddington & Lindsley (1964) Fe-Ti oxide geothermometer (Powell & Powell 1977a). The geothennometer equation used is: - 8155 T

+ 4.59 = in

X u ~ • Xa~m

macroscopic exsolmion in magnetite grains are, given in Table 13 and lie in the range 500-670°C, comparable to the temperatures calculated for the cessatioa of macroscopic exsolution in alkali feldspars.

Summary: From the above geothermometry it can be concluded that the temperature interval over which c:rystallisation and re-equilibration processes left their imprint on the syenites was large. Temperatures of 900-980°C seem reasonable for cumulus crystallisation of olivine, magnetite, apatite, pyroxene, nepheline and alkali feldspar. However temperatures calculated for some nepheline-alkali feldspar pairs are much lower, down to 650°C, suggesting re-equilibration of the two phases with falling l:emperature, lntercumulus liquid may still have been present down to at least these temperatures and this would have facilitated N a - K exchange between nepheline and alkali feldspar. In the temperature range 500-650°C, macroscopic exsotution in both alkali feldspar and tkanomagnetite ceased. A hydrothermal fluid, rather than intercumulus liquid, at lower temperatures (less than 600°C), probably promoted growth of subsolidus biotite, blue amphibole and cancrinite. Calculated temperatures of 420-460°C for crystallisation of coexisting feldspars in pegraatites, using the alkali feldspar soivus of Thompson & Waldbaum (|969L support such low temperatures for late stage fluids.

XM, " X . m

where for example Xu~ is the mole fi'action of ulvospinel in ~Ihe magnetite host. A graphical representation of the geothermometer is shown in Fig. 12. Calculated temperatures for cessation of

Calculation of Fe/Mg for the Igdlerfigssalik magmas The cumulus mineralogy magnetite, Ca-clinopyroxene, + olivine, alkali feldspar, + nepheline is more

lgdlerfigssalik nepheli~esyenite 113

LITHOS 1! (! 978) I<)

Roeder .& Emslie (1970) an~ Roeder (1974) coneluded from experimental studies of olivine--liquid equilibria in basaltic systems at 1 bar that KD was essentially independent of temperature at low pressures. Also Roeder's (1974) experimental data show no clear compositional dependence of KD, at ~,~st in basaltic systems. Therefore for the Ig~ierfigssalik syenite magmas it seems reasonable to assume that:

.w.O..~

"8

_.6

g

"K

o_>

Yw~

.2-

0

i

i

•9

"8

i

tO

XIImenite

Fig. 12. Coexisting Iron-Titanium Oxides geothermometer diagram. Compositions of hosts and lamellae from exsolved titanomagnetite grains, gi'ven in Table 13, are indicated by solid symbols. Table 13. Calcalated temperatures for cessation of exsolu-

tion in titanomagnetite grains. Unit

GGU No.

X.u~

X..

T°C

4 4 6 6 7 7

63822 58283 41944 41978 43832 43861

0.33 0.20 0.11 0.08 0.29 0.29

0.99 0.94 0.98 0.96 0.96 0.96

550 660 500 520 670 670

or less ubiquitous, but the compositions of the cumulus marie phases change markedly within units, notably in their Fe/Mg ratios. Olivine is an early crystallising cumulus phase in many of the Igdlerfigssalik syenites, occurring in all of the units. The Fe/Mg ratios of these cumulus olivines should be related to the Fe/Mg ratios of the magmas which crystallised them via the equilibrium relation for the reaction: MgzSiO4 + 2FeO = 2 ~ g O + FezSi04 ol

liq

i~

ol

=constant

As a remit the variation of Fe/Mg in the Igdlerfigssalik syenite magmt:s should parallel the variation in Fe/Mg ratio of the cumulus olivines. Therefore the variation in Mg content of cumulus olivines within $14, SI5 and S!7 shown schematically in Fig. 7 directly reflects the changing Mg content of the syenite magmas. The Mg content of the magma decreased markedly in SI4 and S17 with progressive crystallisation but remained consistendy low throughout S15.

Activity of silica and activity of oxygen during cumulus crystailisation The reaction: _23FesO4 + 3iOz = F%SiO4 + ~ Oz

(1~

is the well kr~own Quartz-Fayalite-Ma[~netitc (QFM) oxygen buffer (Wones 8: Gilbert 1969). Free quartz is not present in the Igdlerfigssalik syenites, although fayalitic olivine and titanomagnetite are common cumulus phases. However the SiOz component in (1) can be considered as a component of the magma from which the cumulus phases cryslallised, and therefore the equilibrium relation for (1) can be used to determine as~ 2 in the magma as a function of a% and temperature; th~s is the SFM oxygen buffer. (For a standard state of pure Oz at I bar and the temperature of interest this corresponds to the convention that ai=f~xiT~, wheref~ is the activity of pure 02 at the temperatur:: and pressure of interest. This ai is equivalent to the f~ of, for example, Buddington & Lindsley (1964).J The equilibrium relation for (1) is: o 2 1 --A_G _inaF, z s i ° 4 - 3 in a~so4+ ~ln aoz_ln as~oz KI

(2~ RT

\X MILol " XF¢,liq/

+2In (TMs'~ • fYF~-~ -=21RKD+21R K~ TF¢/liqk'l f M~]o|

and AG° = 1904____22_ 6.47 RT T (Cannichael et al., in press).

114

Marjorie Powell

LITHOS il (,|97~.) Nepheline and alkali feMspar are common cumulus phases in the syenites and therefore the reaction:

-10

NaAISiO4 + 2SIO2 ---NaAISi3Os

o-3s

-;s

.

,,,0

.,.I

-45

:20 0

-2

-4

-6

Ln a s i 0 2 Fig. 13. Ln aoz versus !n as~o~ for cumulus crystallisation in the lgdlerfigssalik syenites, contoured for temperature in

degrees Kelvin The shaded vertical bands represent the range of lnas~2 values at each temperature for liquids in equilibrium with the assemblage nepheLine-alkali feldspar. The left side of each band is the value of In asm2 for the end member reaction.

(4)

can be used to calculate the activity of silica in the magma. In the preceding section it was shown that nepheline and feldspar have re-equilibrated with falling temperature in some rocks and therefore it could be argued that the present compositions of these phases cannot be used to determine a sio2 in the main body of magma. Fortunately the equilibrium relation for (4) is comparatively insensitive to the compositions of the nepheline and feldspar and therefore asio2 can be determined adequately without knowiag the original compositions of the phases. The Gibbs energy of this reaction was calculated from the thermodynamic data of Robie & Waldbaum (1968) and Kelley (1960). AG°= - 6 5 0 0 - - 1.06T

T in K

The equilibrium relation for (4) is: - AG ° R~

= In

aNaAiSi308

2In as~2

aNaAI sio 4

Olivine is assumed to be ideal, ori a basis of mixing on sites, arid therefore: • , 2 i:lFe 2 SIO4 = '~ Favalite "-- X2Fa

aFe304 -" X Magnelile

1

-57126 I I X 2mt . .., T t- 1!).41 + I -_7g--+..,masio, X Fa

"

(3)

From (3) a graph of lnaoz versus lnasio2 wilt be a series of parallel lines with slope + 3 and intercept of A(T,X).

(5)

As Xc, in the Igdlerfigssalik alkali feldspars is generally small (Table 6): RT In aNa^,si3os ~ RT In XNa+ X~. [7782 - 3.86T + 2XN,( -- 1359 -- 0.77T)]

"V'3/a" _L RTIn aNaAl SiO4 -- RT In vl/4 "'Na, K-site "~'Na,Na-site T + RTln,,l/, INs,K-site ,,s/4 INa, Na-site

or

In ao2 = A(T,X) + 3In a sio2

aNaAISl308

0.26+-1n 2 aNaAiSi04

Using the data of Thompson & Waldbaum (1969) for a pressure of 1000 bars. For mixing on sites in nephelines:

Rearranging (2) gives:

at lnasioz=O

1635

inas~2= ~

T

Magnetite is assumed to be ideal, on a basis of molecular mixing (Katsura et al. 1975), and therefore:

lna°2-

Rearranging gives:

Values of In x2ma ..

For the lgdlerfigssalik nephelines this reduces to: RTI n aNaAiSiO4 ~

D T l n V l / 4Ha, K-site V 3Na, 1 4 Ha-site

2 5 0 0 XK K-site (1

4

'

- Xs,.Na-site)

X6Fa

for all coexisting cumulus olivines and titano~agnefites from the Igdlerfigssalik syenites rail in the range 0 to - 3. Fig. 13 is a plot of In ao~ versus lnas~oz contoured for temperature. The band for each temperature represents the range of A(T,X) values at that temperature. If lnao2 were known In asioz would be uniquely defined at each temperature and vice versa.

using the data of Powell & Powell (1977b) and assuming that ~'S,,.Na.site=1. Calculated values of as,A, sio4 and aN,AiSi~O8 for cumulus nepheline and alkali feldspar are gwen in Tables 5 and 6 respectively. Although nepheline and alkali feldspar are markedly non-ideal the activity product term in (5) will always be small (½ In a/a in the range 0 to -0.1

L I T H O S {1

Igdlerfigssalik neptieline syenite 115

(i978)

-2O

:a't -'°

: .-f .-" , .-~.

/

-30 t~l

!""

O

•

c .J -40

.

• "

•

/ -50

-"

""

,/-)/ .,

'""""

IL~ ~ " /"

/ ~ . . f / i ,,~"

ico-, I

,~ , '

700

Y"

.7"

""

I 900

Q

,

m

I

', :I

v, ,

O ",

-20

{I ]100

X°C Fig. 14. Lnaoa versus T°C for the cumulus assemblage ncpheline-aikali feldspar-olivine-magnetite, calculated from the intersections in Fig. ! 3. Shown for comparison are the positions of the synthetic oxygen buffers NNO, Q F M andl M W (Ganguly 1972). The stippled area rewesents the cumulus crystallisation conditions for. the lgd]erfigssalik syenites.

for the composition range of nephelines and feldspars in the Igdlerfigssalik syenites (Tables 5 and 6)) and therefore to a first approximation In asm 2 will be that defined by the end member reaction. Vertical bands indicating the activity of silica in a liquid in equilibrium with alkali feldspar and nepheline are shown intersecting the SFM buffer bands in Fig. 13. Values of In ao2 at each temperature can be obtained from the intersections and are plotted against temperature in Fig. 14. Shown for comparison in Fig. 14 are the positions of the synthetic oxygen buffers MW, NNO and QFM. The graph shows that for those syenites with the cumulus assemblage nepheline-alkali feldspar-olivine-magnetite the activity of oxygen was below that defined by the MW oxygen buffer at likely magmatic temperatures. Thus conditions in the magma during cumulus crystallisation were ve~ reducing.

A petrogenetic grid for the syenites Olivine-magnetite-clinopyroxene--nepheline-alkali feldspar is a common cumulus assemblage in all the units of the Igdlerfigssalik centre. Temperatu':es of cumulus crystallisation were in the range 900-980°C for all units and for the above cumulus assemblage ao2 was below that defined by the synthetic oxygen

buffer magnetite-wustJte. Coexisting cumulus nepheline and alkali feldspar buffered the activity of silica in the main body of magma. Nepheline and alkali feldspar commonly occur as both cumulus and intercumulas phases in the same syenite specimen and therefore t.~e activity of silica must have been buffered throughout the crystallisation interval. The restriction of extreme sodium enrichment to pyroxenes crystallised from the intercumulus liquid suggests that changes in ao2 may be important in detemdning the nature of the crystallisation path Although intercumulus crystallisation occurred in an essent~vily closed system, on a scale of sever.-.! centLmetres, the initial composit;on of the trapped liquid should reflect the degree of fractionation of the main body of magma. Thus there should be some correlation between the products of intercumulus crystallisation and the extent of fractionation in the main body of magma. For example S15 pyroxenes show the most extensive degrees of sodium enrichment of all tt,:e Igdlerfigssalik syenite pyroxenes and the S15 magma was also the most fractionated, at the present level of exposure. The foilowir~g concerns ~he development of a lnao2-T petrogenetic grid for the igdlerfigssalik syenites in terms of a model system in NazO-FeO-AI2Os-SiO2-O-H, involving the components: NaAISi3Os-Alkali feldspar (fsp,t; NaAISiO,-Nepheline (ne); Fe~SiO~-Olivin~." (ol); FesO4-

-Magnetite (rot); NaFeSi2Oo-Pyroxene (px), NaFe3AISi3Oto(OH)2-Bio~tite (bi); NaNa2FesSie O22(OH)2-Amphibole (amph). For the purlxpses of the following calculations, which involve the construction of a petrogenet~c grid for the crystalhsation :interval of the syenites, it is convenient to adopt an 'average' IgdlerfigssaF,k syenite. Composition parameters for the sol~d phases of this hygothetical sy¢n~teare: XN, = 0.545, X K = 0.450,,Xc~ = 0.005 Alkali RTin~tNaAlsis% ----! 276 -- 2.16"1" feldspar: Nepheline: Xx.t,i,,® :0.16, Xsi), =0.09 1073 K XN.,¢.,.~" = 0,177; Xss.Na.sile = 0.942 1273 K = 0.22 !; = 0.928 Site distributions ~|culated using the method of Poweil & Powell { ~977b). aN,AJS~ =0.55 at 1073 K =0.60 at 1273 K Magnetite: aF,3o4 = X~,~, ~,i,, = 0.3 Olivine:

aFeZSiO* =

Pyroxene:

aN~=esi~,6 = X~,,,,,,; lot c u m u l u s pyroxene X , ~ i t , =:0.07 aNaF®yUSi~OIo(OH) 2 = X,~, " X~-,2 + =0.01

Biotite:

Xfaya~it e = 0 8 U

In the model sysltem as~: in the liquid is buffered by the assembktge nepheline-alkali feldspar:

116

LITHOS 11 (!~78)

M a r j o r i e Po~, Increasing ..$ -4

1-0

Xacmite -3

.2

.05

.I

-20

- -!0

E' -30

_

"Jt ~ -®I x

Px

=°'t - -15

u

0 t=" ._I

--?'-?-(

I

I

!

I

-40

Amph

-50

I I I I I 1

// J II I

/

6O0

-60

I

i

1.1

'00

1,

12

I I I

I I I I I 800

I.

eu

i f I I I I I I I I I I

900

-

-20

r'c t~ - -25

1000

I,i

10

O

01

, I I I I I I I ~ ! I I I I J

tO

8

Fig. 15. L n a o a - l f r petrogenetic grid for the Igdlerfigssalik syenites. Heavy solid and dashed lines delimit the boundaries of one phase fields of olivine, magnetite, pyroxene, amphibole and biotite. The light solid lines are contours for varying acmite contents of the pyroxene. The light dashed lines are hypothetical contours for varying 'eckermannite' (NaNa2FesSisO~2(OH)2) ,contents of the amphibole, comparable to the pyroxerie contours. Point A indicates the calculated cumulus crystallisation conditions. Points A','Y and Z are hypothetical, but have been positioned on the basis of the results of the geothermometry calculations and on the observed mineralogical and mineral chemical variations in syenites from the various units of Igdlerligssalik.

1__ x 104 K -1 T 0.26 Inas~2= - 1635 T

(end member reaction)

It is also necessary to model the variation of Pn2o with falling temperature in the syenite magma, as it is unlikely that Pa2o equalled PTot,~throughout the crystallisation interval. Assuming ideal mixing of components in the hypothetical fluid phase in equilibrium with the magma: a.2o=f.2o

PH2° r " ~-T.---=~.2o • X.2 o • total

A model was adopted in which Xu2o=0.1 at 1000°C and Xu2o = 1.0 at 600°C in this hypothetical ?uid for which: 7.56 In Xm,,o- 6593 T

T(K) at Ptotal= 1000 bars

assuming a reciprocal relationship between lr~ Xa2o and temperature. For pure H20 at 1000 bars: lnj~2o_: _ ! 27___33+ 7.92 T

calculated from the data of Burnham et al. (1969). Thus: In aa 20 = lnfazo + In X H z o = -5320 T - - + 0.36 The following reactions can be written beeween the chosen components in the model system: (a) (b) (c) (d) (e) (f) (g) (h)

3oi + 02 = 2mt + 3SiO 2 6px = 2rot + 3Na20 + ! 2SIO2 + ½02 2fsp+2mt + 2 H 2 0 = 2 b i + O 2 2fsp + 3ol + 2H20 = 2bi + 3SIO2 ! 2px + 4fsp + 4HzO = 4b:, + 1302 + 6Na20 + 24SIO2 amph + NazO + 2SIO2 -i- 02 = 5px + H20 6amph + 3½02 --- 10m~+ 9Na20 + 48SIO2 + 6H20 1 3amph + 5ne + 2H20 =: 5bi + 4-iNazO + 14SiO, +102

Each reaction is univariant in In a % - I/T space at constant pressure and mineral compositions. Fig. 15 is a ln ao2-1/T diagram in which univariant lines separate one pha~e fields of magnetite, olivine, biotite, pyroxene and amphibole. The calculation of these univariant lines is discussed below.

Igdlerfigssalik nepheline syenite I 17

L I T H O S ! 1 (1978)

(a)

Olivine-ma+3~etite

AG~= - 113521 + 38.57T T (K) (Carmichaei et al., in press). The equilibrimn re:lation for this reaction is: - AG~--

RT In

2 3 aF®304 a sio2 a~-. z t o o ,

ao 2

Substituting for as~oz and the activities of the solid phases: lnao, = -62033 4-18.63 T

T (K)

with magnetite of composition X,t=0.3 and a silicate liquid with a fixed activity of Na20 (Fig. 15). Because of the above assumptions there are considerable uncertainties attached to the positions of these contours. Fortunately the slope of the reaction is comparatively insensitive to the large uncertainties in the thermodynamic data used. If instead of remaining consent a~2 o increased in the liquid with falling temperature, a not unreasonable suggestion, then the contours in Fig. ! 5 would become more ~losely spaced, moving towards the Xl,~ = =0.07 contour.

(c) (b)

Pyroxene-magnetite

Using data for acrnite from Marsh (1975) and for the other components from Robie & Waldbaum (1968) and Kelley (1960) the equilibrium relation for (b) is: 0 =-~127750 24.82 + 21na~+3o++ 3 lnam,2o+ T + 12In as~ z + ~ln ao2 - 6In as~v®si2o6 The standard states used for NazO and SiOz are solid and glass respectively. The largest uncertainty in the above expression is in the thermodynamic data for acmite. Marsh (1975) gives an estimated uncertainty of + 2 kcals, on his Gibbs energy of formation of acrnite. Approximating av+3o+ by X,m on a mixing on sites basis and as~v+si:~6 by X,cmitc z and substituting for as~2 gives: O = 108130 30.35+ 3in am,2o+ ½In no2 T - 1 2 i n Xicmit, The position of the pyroxene-magnetite equilibrium in In no, - I f r sp~ce is thus dependent on aN,2O in the liquid and the acmite contm~t of the pyroxene. An estimate of am,=,Oduring the crystailisation of the cumulus assemblage ne-fsp-px-ol-mt can be made by substituting the composition of the cumulus pyroxene and the calculated cumulus conditions, i.e. X~m= 0.07, T-950°C, In no2 = - 34 from Fig. 14, giving lnas,:~j=-24.5. The simplest assumption is that am,2ochanges little during the fractionation of the syenite magma. Substituting for aN,zo gives: 4-208 + 24 In X,om lnaoz = - 216260 T This equation defines In aoz- I f r contours for pyroxenes of varying acmite compositions in equilibrium 8 - Lithos 2/78

Biotite-magnetite

AG~=32680+84.4T T (K), calculated from the QFM, NNO and G - C H buffered experimental data of Rutherford (1969, Fig. 4) in the system KAISizOB-NaAISi3OB-Fe-O-H, over the temperature range 600-690°C. The amount of Na in solid solution in the biotite in Rutherford's experiments was estimated from his Fig. 2. Fortunately large errors of _+0.05 in the Na content of the biotite only produce errors of _+500 cals. in the calculated Gibbs energy of reaction at each temperature. The activity of the Na-annite component in the biotite was approximated by a model of ideal mixing on sites and the activity of the feldspar component by the subregular solution model parameters of Thompson & Waldbaum (1969). Substituting the activities of the solid phases and au2o as a function of temperature into the equilibrium relation for (c) gives: lnaoz = -4521 T (d)

37.12

Biotite-olivine

Reaction (d) is a linear combination of reactions (a) and (c) and therefore combining the equilibrium relations in the appropriate way gives: 0 = - 57512 4-55.75 or T = ! o~32K T The reaction is independent of %2" The ",mcertainty in the calculated temperature may be of the order of + 50 K.

(e) Pyroxene-biotite Reaction (e) is a linear combination of reactions (b) and (c) and therefore combining the equilibrium relations in the appropriate way gives:

118

MarjoriePowell

~-44.49 + 8in X,¢,,. lnao2 = -75101 T As for the pyroxene-magnetite equilibrium the slope is relatively insensitive to uncertainties in the thermodynamic data, but the absolute positions of the contours are not known with any certainty. However, they are constrained to be consistent with the appropriate X,~mcontours in the magnetite field.

Position of the amphibole stability field An important assumption is that there is an amphibole stability field at low temperatures and low activities of oxygen. The experimental work of Bailey (1969) indicates that at the low oxygen activities of the wustite-magnetite and iron-wustite synthetic oxygen buffers, acmite is unstable with respect to amphibole in the presence of excess H20, between I and 4 kilobars. Amphibole only occurs as an intercumulus phase in the Igdlerfigssalik syenit,~s, supporting the idea of a lower temperature paragenesis. Thermodynamic data for suitable amphibole components are not available and therefore the relative position of the amphibole field can only be determined qualitatively. Fortunately several geometrical constraints enable estimation of the relative slopes of the field boundaries: (a) All invariant points must obey Schreinemaker's Rules (Korzhinskii 1959). (b) Oxygen must always be evolved on the low o~ygen activity side of a univariant line. dlnao 2 On the basis of (b), ~ must be negative for d I/T the amphibole-biotite univariant line and positive for" the amphibole-magnetite line. The slope of the pyroxene--amphibole boundary is unconstrained. On the basis of (a) the slope of the amphibole-biotite boundary must be steeper than the pyroxene-biotite contours. Detailed petrographic studies ot the syenites show that intercumulus amphibole probably began to crystallise as pyroxene crystallisation from the intercumulus liquid ceased. Also ~there is a strong positive correlation between the eckermannite contents of intercumulus amphiboles and the acmitc contents of associated intercumulus pyroxenes. The amphibole stability field shown in Fig. 15 has been constructed on the basis of these criteria. A set of hypothetical contours representing variations in the "eckermannite' content of the amphibole have been drawn in Fig. 15 parallel to the amphibole-magnetite and amphibole-biotite

LITHOS

!1 (1978)

field boundaries, comparable to the pyroxene contours considered earlier. The positions of the amphibole and pyroxene stability fields, for particular compositions of amphibole and pyroxene, can be visualised using the appropriate pyroxene arid amphibole contours. Fig. 15 cannot be used to predict the In ao2- I/T conditions of, for example, the appearance of amphibole in the assemblage. However, if the compositions of amphibole and pyroxene coexisting with a third phase, for example magnetite, are known then the l n a o 2 - I / T conditions would be defined for the assemblage. Fig. 15 has been constructed for fixed compositions of biotite, magnetite and olivine. If, for example, magnetite compositional variations were considered then a further set of contours would have to be added to the diagram. However, as the range of chemical variation of these solid phases is relatively small compared with those of amphibole and pyroxene, this assumption will not affect the interpretation of the diagram seriously. The In ao2-1/T cr./staUisation paths of those syenites crystallised from liquids in which asia2 was buffered by the assemblage nepheline-alkali feldspar can be interpreted using Fig. 15.

Interpretation There is a spectrum of crystallisation paths in In ao2-1/T space resulting in the appearance.of different intercumulus phases with widely varying compositions. The limits of this spectrum are: (l) Paths such as A-Z in Fig. 15 in which the final pyroxene crystallised from the intercumulus liquid is acmite rich, followed by an 'eckermannite' rich amphibole. (2) Paths such as A-Y in Fig. 15 in which the pyroxene is only weakly zoned with associated 'eckermannite' poor (i.e. 'pargasite' rich) intercumulus amphibole and biotite. Fig. 16 shows the positions of the stability fields from Fig. 15 relative to the synthetic oxygen buffers NNO, QFM and MW. Paths such as A-Z cut across the buffer curves and therefore the intercumulus environment becomes progressively more oxidising with falling temperature. For syenites crystallising along such paths the crystallisation interval must have been large, perhaps as much as 350°C, with final solidification at low temperatures, say 600°C. For paths such as A-Y oxygen activity decreases rapidly with falling temperature and parallels the synthetic oxygen buffer MW. The crystallisation interval, of syenites fi~llowing such paths is smaller, perhaps 200°C, and cessation of

Igdlerfigssalik nepheline syenite ! 19

LITHOS 11 (1978)

-20

I I

--10

I I I

Mt

I I I

Px

I

-30

I I I I

--15

I I

0 ¢g c ._1

I

?-..4

-40

I

/'

r O tt~

OI

I

Amph

- -20

/,

-50

!

Bi

I I

Fig. 16. Positions of the

one phase fields and point~ A, A', Y and Z from Fig. 15 relative to the synthetic oxy3en buffers NNO, QFM and MW.

I I I

_60J.

I °l ° 12

71° I 8i° 10 lX

9GO

lO00

I ,I

T'C

- -25

.

8

10 4 K -1

T Table 14. Crystallisation paths for the different units of Igdlerfigssalik.

Unit

Temperature of cessation of intercumulus crystallisation

Path in Fig. ! 5

S14 S15 S17

high low high

A-A A-Z A-Y

intercumulus crystaUisation occurs at quite high temperatures, say 750°C. All possible intermediate paths between A - Z and A - Y can be envisaged giving rise to different degrees of pyroxene zoning and the different intercumulus phases. The paths for thedifferent units of Igdlerfigssalik are summarised in Table 14. The reasons why SI7 syenites tended to crystallise along reducing paths while SIS syenites followed oxidising paths may reflect some intrinsic differences in their chemistry. Certainly the S15 mai~nas were more fractionated at the present level of exposure, as evidenced by the iron-rich nature of the cumulus mafic phases.

The above interpretation is based on a series of assumptions, particularly regarding the variation of anzo as a function of temperature and the constancy ofam,2o in the liquid with progressive fractionation. Doubtless the Igdlerfigssalik syenites crystallised under a spectrum of conditions involving, for example, different variations of am,-o and an2o with progressive crystallisation than t~ose chosen to produce the above model, resulting in displacement of the net in Fig. 15 for different syenites. However, Fig. 15 is important in that it can be used to account for both the variations in mineralogy and variations in the compositions of intercumulus pyroxene and amphibole, in terms of different crystallisation paths in In ao2 - l/T space, even if the precise values of In ao2 and T are not determined. Acknowledgements. - The author would like to thank Dr. C. H. Emeleus for supplying the specimens for this study; Professor G. M. Brown for the use of the Durham Geoscan Mk II electron probe microanalyser and the Director of the Geological Survey of Greenland for permission to work on the lgdlerfigssalik syenites and publish the results. Dr. R. Powell provided useful criticism of the manuscript. This work was undertaken while the author held an N.E.R.C. resear~:h studentship at Leeds University.

120

Marjorie Powell

Refi;rences Bailey, D. K. 1969: The stability of Acmite in the presence of H20. Am. J. Sci. 267-A, !-!6. Bottinga, Y. & Weill, D. F. ! 972: The viscosity of magmatic silicate liquids: A model for calculation. Am. J. Sci. 272, 438-475. Buddington, A. F. & Lindsley, D. H. 1964: Iron-titanium oxide minerals and synthetic equivalents. J. Petrol. 5, 310-357. Burnham, C. W., Holloway, J. R. & Devis, N. F. 1969: Thermodynamic properties of water to '.000°C and 10,000 bars. Geol. Soc. Am. Spec. Pap. 132. Carmi~chael, I. S. E., Nicholis, J. W., Spera, F. J., Wood, B. J. & Nelson, S. A. in press: High-temperature properties of silicate liquids: Application to the equilibration and ascent of basic magma. Roy. Soc. Lond. Phil. Trans. Ser. A. Deer, W. A., Howie, R. A. & Zussman, J. 1966: An Introduction to the Rock Forming Minerals, Longmans, London. Edgar, A. D. & Parker, L. M. 1974: Comparison of melting relationships of some plutonic and volcanic peralkaline undersaturated rocks. Lithos 7, 263-273. Emeleus, C. H. & Harry, W. T. 1970: The Igaliko nepheline syenite complex. General description. Meddr. Gronland Bd. 186, Nr. 3. Emeleus, C. H. & Upton, B. G. J. 1976: The Gardar period in southern Greenland, in Escher, A. & Watt, W. S. (Eds.): Geology of Greenland. Gronlands Geologiske Undersogelse, Denmark. Genguly, J. 1972: Staurolite stability and related parageneses: Theory, e,~periments and applications. J. Petrol. 13, 335-365. Hamilton, D. L. 1961: Nephelines as crystallisation indicators. J. Geol. 69, 321-329. Katsura, T., Wakihara, M., Hara, S. & Sugihara, T. 1975: Some thermodynamic properties in spinel solid solutions with the Fe30~ component. J. :;olid. State Chem. 13, 107-113. Kelley, K. K. 1960: Comr/butions to the data on theoretical metallurgy. Xill. High-temperati~re heat-content, heatcapacity and entropy data for the ,elements and inorganic compounds. Bull. I/.S. t~ur. Mince, 584. 232 pp.

Korzhinskii, D. $. 1959: Physicochemical Basis of the Analysis of the Paragenesis of Minerals, Consultants Bureau, Inc., New York. Marsh, J. S. 1975: Aenigmatite stability in silica-undersaturated rocks. Contrib. Mineral. Petrol. 50. 135-144. Powell, M. & Powell, R. 1974: An olivine-clinopyroxene geothermometer. Contr. Mineral. Petrol. 48, 249-263. Powell, R. & Powell, M. 1977a: Geothermometry and oxygen barometry using coexisting iron-titanium oxides. Mineral. Mag. 41, 257-263. Powell, M. & Powell, R. ! 977b: A nepheline-alkali feldspar geothermometer. Contr. Mineral. Petrol. 62, 193-204. Powell, M. & Powell, R. 1977c: Plagioclase-alkali feldspar geothermometry revisited. Mineral Mag. 41, 253-256. Robie, R. A. & Waldbaum, D. R. 1968: Thermodynamic properties of minerals and related substances at 298.15 K (25.0°C) and one atmosphere (1.013 bars) pressure and at higher temperatures. Bull. U.S. Geol. Surv. 1259. Roeder, P. L. 1974: Activity of iron ~ Id olivine solubility in basaltic liquids. Earth and Planet. Sci. Lett. 23, 397-410. Roeder, P. L. & Emslie, R. F. 1970: Olivine-liquid equilibrium. Contr. Mineral. Petrol 29, 275-289. Rutherford, M. J. 1969: An experimental determination of iron-biotite-alkali feldspar equilibria. J. Petrol. 10, 381408 Stormer, J. C. Jr. 1975: A practical two feldspar geothermometer. Am. Mineral. 60, 667-674. Thompson, J. B. Jr. & Waldbaum, D. R. 1969: Mixing properties of Sanidine crystalline solutions, lII. Calculations based on two phase data. Am. Mineral. 54, 811-838. Wager, L. R. & Brown, G. M. 1967: Layeredlgneous Rocks, Oliver & Boyd, Edinburgh. Wones, D. R. & Eugster, H. P. 1965: Stability of biotite: • Experiment, theory and application. Am. Mineral. 50, 1228-1272. Wones, D. R. & Gilbert, M. C. 1969: The fayalite-magnetite-quartz assemblage between 600 and 800°C. Am. J. Sci. 276-A, 480-488. Wood, B. J. 1976: An olivine-clinopyroxene geothermometer. Contrib. Mineral. Petrol. 56, 297-303. Accepted for publication June 1977 Printed April 1978