Crust-mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies

LITHOS CD

Lithos 40 (1997) 189-202

Crust-mantle interaction in the evolution of the Ilimaussaq Complex, South Greenland: Nd isotopic studies

Ross Stevenson a, * , B.G.J. Upton b, A. Steenfelt ’

a Geological Survey of Cam&, 601 Booth St. Ottawa, Ont., Canada KIA OE8 b Department of Geology and Geophysics, University of Edinburgh, Edinburgh, EH9 3.M UK

’ Geological Survey of Demnark and Greenland, 0ster Voldgode 10, DK-1350 Copenhagen K, Denmark

Received 12 October 1995; accepted 21 April 1997

Abstract

Sm-Nd isotopic compositions were determined for the peralkaline Ilimaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial E~,,( = 0), whereas a few samples trend towards cNd values as low as - 6 at the time of intrusion (1143 Ma). This latter value, horn a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilfmaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust. 0 1997 Elsevier Science B.V.

Keywords: Nepheline syenite; Crustal assimilation; Nd isotopes; Greenland

1. Introduction

The evolution of silica-undersaturated, alkaline igneous complexes remains poorly understood de-

spite the number of studies devoted to the subject.

l Corresponding author. Present address: GEOTOP/Jkparte- ment des Sciences de la Terre, Universid du Qu&ec a Mont&l. P.O. Box 8888, St. A., Mont&al, Que., Canada H3C 3P8.

The Gardar Igneous Province of South Greenland contains a number of undeformed/unmeta-

morphosed examples of such complexes and pro- vides an ideal area for their study (Emeleus and

Upton, 1976; Upton and Emeleus, 1987). The Ilimaussaq Peralkaline Complex (Fig. la) is unique among these alkaline complexes because of its re- markable igneous layering and exotic peralkaline mineralogy, which have inspired numerous petrolog-

0024-4937/97/$17.00 8 1997 Elsevier Science B.V. All rights resel PII SOO24-4937(97)00025-X

!-fed.

190 R. Stevenson et al. / Lithos 40 f 1997) 189-202

ical and mineralogical studies (Sorensen and Larsen, Ga orogenesis and involved little or no older

1987, Larsen and Sorensen, 1987, see also Report of (Archean) recycled crust (van Breemen et al., 1974;

the Geological Survey of Greenland, ~103, 1981). It Patchett and Bridgwater, 1984; Kalsbeek and Taylor,

was from this locality that the term agpaitic was 1985). Pb isotope studies of the Tugtut@ Younger

originally defined by Ussing (1912) as a measure of Giant Dike Complex which lies within the lineament

the degree of alkalinity (Na + K/Al at% > 1.2) of also indicate that the underlying crust is Proterozoic

the nepheline syenites of the Ilimaussaq Complex. (Taylor and Upton, 1993).

This paper presents Sm-Nd data from the Ilimaussaq Complex in order to investigate its magmatic evolution and the role of crust-mantle interac-

tion in its formation. Foland et al. (1993), Land011 et

al. (1994) and Land011 and Foland (1996) have demonstrated that the intrusion of nepheline syenites into the crust can result in the formation of granites through direct crustal melting or by combined crustal assimilation-fractional crystallization of the undersat-

urated magma. The undeformed, unmetamorphosed and highly differentiated nature of the Ilimaussaq Complex make it an ideal subject on which to study the effects of combined crustal assimilation and fractional crystallization. Crustal assimilation in the

Ilimaussaq Complex was suggested by Blaxland et al. (1976) on the basis of a Sr isotope study and a

number of the samples used in this study are splits of the same samples used by Blaxland thus allowing direct comparison of the data from the two different studies.

2. Geology

Late Gardar magmatism along the lineament com- menced with the intrusion of a massive dyke com-

plex (the Older Giant Dyke Complex) composed of a cogenetic suite ranging from alkali gabbro through

augite syenite, pulaskite, foyaite to peralkaline sodalite foyaite (Upton et al., 1985). This was followed by the still larger, Younger Giant Dyke Complex,

composed of mildly alkaline troctolitic gabbros which produced, in different localities, both silica-oversaturated products (quartz syenites and alkali granites) and undersaturated products (foyaites; Upton

and Thomas, 1980). Continued lithospheric exten- sion along the axis of the Julianehib Batholith gave rise to a swarm of smaller dykes ranging from basaltic, through intermediate compositions to

comenditic and phonolitic. Major intrusions of syen-

ite, quartz syenite and alkali granite in the Narssaq area (immediately west of Ilimaussaq) may have been partly synchronous with the dyke swarm emplacement. Fault-bounded basins containing se-

quences of sandstones and subaerial basalts (the Eriksfjord Formation) occur in association with the TugtutBq-Ilimaussaq-Nunataq lineament.

The Ilimaussaq Complex is among the alkaline intrusions representing the culmination of magmatic

activity along the TugtutGq-Ilimaussaq-Nunataq lineament (Upton et al., 1990). This lineament consti- tutes a narrow, continental rift segment which has

been subsequently uplifted and eroded to a depth of several kilometers. The lineament lies axially along an earlier Proterozoic batholith of granitoids dating

from 1.8-1.7 Ma (van Breemen et al., 1974; Patchett and Bridgwater, 1984). This (Julianehib) batholith comprises a suite of I-type granitoids emplaced during subduction from the south towards the Archean craton (Chadwick et al., 1994). Isotopic studies (Nd, Sr) indicate that the Julianehib Batholith and the Proterozoic crust, through which the Ilimaussaq- Nunataq lineament passes, were generated and ac- creted on to the Greenland margin during the 1.8- 1.7

The Ilimaussaq Complex, measuring ca. 17 X 8 km, was among the last of the alkaline complexes to

be emplaced. The complex was intruded into the Julianehib Batholith granitoids and the uncon- formably overlying basalts and sandstones of the Eriksfjord Formation which formed the roof of the

complex (Fig. la>. The Ilimaussaq Complex has been dated at 1143 + 20 Ma by Rb-Sr whole-rock - mineral work by Blaxland et al. (1976) and by

Paslick et al. (1993) with a 1130 + 50 Ma age from a Sm-Nd whole-rock - mineral isochron. The coher- ent Rb-Sr isochron indicates that the pluton has not been affected by any widespread hydrothermal alteration although local remobilization of Rb and Sr cannot be ruled out.

The formation of the complex involved three discrete intrusive events. The fist of these, involving benmoreitic magma, involved crystallization to form

R. Stevenson et al. / Lithos 40 (1997) 189-202 191

a silica-undersaturated augite syenite (Larsen and Sorensen, 1987). The major minerals are alkali feldspar, augite, acmitic pyroxene, fayahtic olivine, kaersutite, magnetite and biotite. The subsequent units were emplaced within the augite syenite by stoping (Nielsen and Steenfelt, 1979) leaving only a thin, remnant sheath of augite syenite, up to 2 km broad, around the western and southern margins of the complex (Fig. lb).

The second event saw the emplacement of quartz syenite and alkali granite sheets in the uppermost parts of the augite syenite (Bailey et al., 1981a). The larger sheets are regarded as having formed from

separate magmatic intrusions while smaller occur- rences enveloping agpaitic rocks adjacent to the country rocks are thought to be products of local contact-assimilation (Steenfelt, 198 1).

The third and final event was the intrusion of a per-alkaline, iron-rich phonolitic magma from which the agpaitic suite crystallized (Wensen and Larsen, 1987). This magma is believed to have been generated as a highly-evolved, low temperature, residual fraction at the top of a large underlying layered gabbroic to syenitic complex. The agpaitic magma appears to have crystallized as a closed system that gave rise to a series of cumulate sequences. Beneath

b. I limaussaq complex

Augite Syenite

0 1000 2QOOm

Fig. 1. Geological map and cross-section of the Ilimaussaq intrusion showing location of samples used in this study (map and cross-section

simplified after Andersen et al., 1981).

192 R. Stevenson et al. / Lithos 40 (I 997) 189-202

the roof, an upper border group crystallized down-

wards (Ferguson, 1970). This cumulate sequence

shows progressive differentiation from the top down from pulaskite, foyaite, through sodalite foyaite to a

sodalite-rich syenite (naujaite). The major minerals of the agpaitic units include alkali feldspar, acrnitic

pyroxene, Na-amphibole, eudialyte, sodalite and

nepheline. The naujaite is considered to have devel- oped as a flotation cumulate from accumulation of buoyant sodalite cumulus crystals and typically comprises 30-40%, and locally up to 90%, sodalite crystals (Sorensen and Larsen, 1987).

A suite of floor cumulates began growing up-

wards from the bottom of the magma chamber at the same time as the upper group was developing, but it

continued forming after the growth of the upper group had ceased. The exposed floor cumulates are

represented by macro-rhythmically layered, eudialyte-bearing nepheline syenites (kakortokites), al-

though cumulates complementary to, and contempo- raneous with, the naujaites are believed to be buried below the present level of erosion. That the kakortokites accumulated subsequent to termination of the down-growth of naujaites is attested by the mineral

chemistry and the presence of autolithic masses of naujaite, up to 100 m across, that had become de- tached from the contemporary roof and sunk to become enveloped by up-growing kakortokites (Lar-

sen and Sorensen, 1987). The kakortokites are composed of K-feldspar, nepheline, arfvedsonite, eudia-

lyte, aegirine and, sometimes, aenigmatite cumulates. Twentynine macro-rhythmic units, each about 10 m

thick, are exposed (Bohse and Andersen, 1981). Complete units commence at the base with arfvedsonite-rich (black) kakortokite, grading up into feldspar-rich (white) kakortokite, with the occasional intervention of eudialyte-rich (red) kakortokite between them.

The kakortokite succession passes up into still more extremely fractionated eudialyte-poor syenites (lujavrites) that constitute a sandwich horizon between the kakortokites and the overlying naujaites. The lujavrites are composed of microcline, albite, nepheline, eudialyte, aegirine and arfvedsonite in a matrix of natrolite, analcime and sodalite (Larsen and Sorensen, 1987). Stoped masses of naujaite are also common within the lujavrites, due to magmatic erosion of the overlying roof cumulates. Whereas the

pulaskites represent the earliest facies of the agpaite

body, the lujavrites are the youngest and most evolved components.

The magma body from which the agpaitic cumulates formed was tabular, with lateral dimensions of

roughly 8 X 17 km but a thickness probably little

greater than the observed thickness of the exposed agpaites (ca. 1.5 km). With a total volume of ca. 200 km3, inferred to represent about a 2% residue of a parental transitional basalt magma, it is necessary to

propose some 10,000 km3 of such a parent (Larsen and Sorensen, 1987). Bailey et al. (1981b) concluded that a compositional continuum may have existed

from augite syenite to the agpaites and that extensive fractionation could have taken place beneath the present erosion level. Crystallization in excess of

99% of the augite syenite magma would have been required to produce the final lujavrites.

Samples from each of the three intrusive units

were analyzed in order to investigate the role of crustal interaction in the modification of the agpaitic rocks and the formation of the saturated rocks. In all,

eighteen samples were analyzed, including two samples of augite syenite from the first intrusion, three samples of alkali granite from the second set of intrusions and two pulaskite, three foyaite, two nau-

jaite, three lujavrite and three kakortokite samples from the final agpaitic intrusion.

3. Results

The Sm and Nd analyses were performed at the Geological Survey of Canada following the proce- dures outlined by ThCriault (1990). The Sm-Nd data are reported in Table 1 and plotted in Fig. 2 as &nd against the age of intrusion. Fig. 2 also includes a

summary of Nd data available from the Gardar and Ketilidian terranes. Overall, the Ilimaussaq samples show a range from &nd =Oto -6atthetimeof

intrusion with the lowest and the highest cNd values being found in the augite syenite samples. The augite syenite sample (150781) with &nd = -6 was taken from near the contact with the enclosing Ketilidian granites, while sample 150782 with the cNd = 0 came from within the pluton. The alkali granites have &nd values from - 1.2 to -2.4, which are

lower than those of most of the agpaitic samples.


The agpaitic rocks have &Nd values varying between 0 and - 2, with the exception of a lujavrite (150885,

&Nd = -3.9) which borders the Gardar supracrustal rocks on the northern side of the complex. The lujavrites, which form the sandwich horizon of the complex, have lower cNd values than the bottom or roof cumulates. The Ilimaussaq &Nd values are well below the +2 values determined for the Eriksfjord basalts of the Gardar supracrustal sequence (Paslick et al., 19931, but within the lower half of the range for mafic intrusions preceding Ilimaussaq (A.N. Hal- liday and co-workers, personal communication).

The range in Sm and Nd concentrations is extremely large (4.7 to 233 ppm and 27 to 1200 ppm respectively), and yet, the 147Sm/144Nd ratios remain relatively constant for any one rock type. This constancy may be a reflection of the fractionating mineral assemblage which served to buffer the i47Sm/144Nd ratio, but increased the Rb/Sr ratios in later units of the complex (Blaxland et al., 1976). A fractionation assemblage dominated by feldspars and feldspathoids could satisfy the above criteria. Feldspars and feldspathoids strongly fractionate Rb from Sr, but do not strongly discriminate between

Table 1

Sm-Nd data for the Ilimaussaq complex

Sample/type Sm @pm) Nd (ppm) 143Nd,144Nd b cNd ’ s7Sr/s?r d Rb/Sr d

Augite syenites

GGU-150781

GGU-150782

GGU- 150782 repeat

Pulaskites

GGU-1.50704

GGU- 150685

Foyaites

GGU-186406

GGU-186407

GGU-150769

Naujaites

GGU-150743

GGU- 150698

Kakortokites

GGU-150791

GGU-15079

GGU-150731

Lujavrites

GGU-150885

GGU-150795

GGU- 150794

Alkali Granite

GGU-86339

GGU-186401

GGU-150831

4.70 27.1 0.1048 0.511645 + 8 -5.9 0.70638

12.3 66.4 0.1117 0.512002 + 9 0 0.70380

12.3 66.3 0.1121 0.511999 + 5 0

0.15

0.27

24.5 134 0.1106 0.511960 f 7 -0.6

21.6 123 0.1060 0.511918 + 5 -0.8 0.70670 10.3

26.5 153 0.1050 0.511932 f 7

47.6 267 0.1076 0.511950 + 7

43.5 241 0.1089 0.511960*5

-0.4

-0.4

-0.4

5.61 34.2 0.0990 0.511880+4

5.84 36.5 0.0967 0.511866+5

-0.5

-0.4

59.3 334 0.1072 0.511936 f 5 -0.6 0.70183 7.48

223 31197 0.1180 0.512045 f 5 -0.1 0.70991 0.46

74.8 372 0.1215 0.512060 + 9 -0.3 0.70776 1.64

36.8 220 0.1010 0.511723 + 8 -3.9

158 897 0.1066 0.5 11907 f 7 - 1.1 0.72024

155 843 0.1109 0.511924 + 7 -1.4 0.72050

9.76

11.3

26.0 139 0.1129 0.511919 f 9 - 1.8 0.68216

61.6 369 0.1009 0.511797 f 5 - 2.4

99.2 582 0.1030 0.511875 f 6 -1.2

18.5

a ‘47Sm/‘44Nd 20 less than 0.5%.

’ Present day Nd ratios with 2c+ in-run errors. A value of 143Nd/144Nd = 0.511878 + 15 (2~) for 29 analyses was obtained account for

the high value of the standard obtained during the period of this study.

’ ENd at the time of crystallization. Errors associated with replicates of the Nd standard suggest a reproducibility within 0.5 eNd units. The

replicate of sample GGU-150782 falls well within the stated reproducibility.

d Initial s7Sr/86Sr ratios and Rb/Sr ratios from the data of Blaxland et al. (1976).

194 R. Stevenson et al. / Lithos 40 (1997) 189-202

4 Eriksfjord

Basalts 2 I

0 & Nd

-2

Ketilidian granitoids

n Augite Syenite 0 Pulaskite

0 Foyaite 7 X Naujaite 0 Kakortokite + Lujavrite a Alkali Granite

1.8 2.0

Fig. 2. cNd versus time diagram showing the range of .sNd values from 0 to about -6, suggesting that samples from the margin of the complex have interacted with crust of Ketilidian age. Also shown are the ranges of isotopic compositions for Etiksfjord basalts (Paslick et al., 19931, mafic intrusions of the TugtutCq-Ilimaussaq Lineament (A.N. Halliday and co-workers, personal communication) and for Ketilidian granitoids and sediments (Patchett and Bridgwater, 1984). The lack of mantle isotopic values (.sNd = + 2 to + 6) in the complex implies significant crustal assimilation.

Sm and Nd. This also implies that minor components up to several percent REE oxides, but its REE profile of the fractionating assemblage, such as aegirine, shows only a slight light REE enrichment (Harris arfvedsonite and eudialyte, either behaved similarly and Rickard, 1987). to the feldspar& with respect to Sm and Nd, or had Fig. 3 relates the isotopic composition of the insignificant effect. Eudialyte, for example, can have samples to their respective Nd concentrations. It is

Fractionation > 0

l Lujavrite

A Alkali Granite

-6

1 /Nd

Fig. 3. .sNd versus l/Nd concentration (ppm). The majority of the samples plot between 0 and -2 and are probably related through fractional crystallization and assimilation of Ketilidian crust. Alkali granites and samples from the margin of the complex extend towards the field of Ketilidian crust suggesting that these reflect greater assimilation.

n Augite Syenitc

0 Pulaskite

0 Foyaite

X Naujaite

0 Kakortokite


apparent from the figure that most of the samples show little correlation between .eNd and Nd concentration, although there is a tendency for samples with lower Nd concentrations (and which are from close to the margins of the complex) to have more negative .sNd values. For example, augite syenite sample 150781, from the outer margin of the complex, has a low Nd concentration with an en,, value of -6, whereas a lujavrite sample (1508851, which is also from the outer margin (northern portion) but contains more Nd, has an cNd value of only - 3.9. In the northern portion of the complex, the lujavrites intruded and brecciated the roof zone and are in contact with the Eriksfjord Formation (Steenfelt, 1981). This, and the lower Nd concentration of the sample probably led to its having a lower gNd value than the other two lujavrite samples. The cNd values of these samples are inferred to reflect country-rock assimilation along the margins. The alkali granite values also fall, more or less, along this trend and it is probable that these oversaturated rocks were produced through a larger-scale contamination of basalt or augite syenite magmas by melts derived from the country rocks.

Although most of the agpaitic samples have simi-

lar ‘Nd values within determinative error ( f 0.5 gNd units), there are some slight and constant differences between individual units. Among the roof cumulates, the early formed pulaskites have slightly lower eNd values than the foyaites and naujaites ( - 0.6 to - 0.8 for the pulaskites compared to - 0.4 to - 0.5 for the foyaites and naujaites). This could reflect slightly greater contamination of the pulaskites at the top of the magma chamber, possibly through digestion of augite syenite blocks that had themselves been crustally contaminated. The pulaskites were followed by the foyaites and the naujaites which have slightly higher &Nd values. These latter units may be less contaminated, perhaps because of the overlying layers of pulaskite and augite syenites which insulate them from the country rock.

The kakortokites may be petrographically subdi- vided into black arfvedsonite-rich, red eudialyte-rich and white feldspar-rich layers (Sorensen and Larsen, 1987) represented by samples 150731, 150792 and 150791, respectively. Samples 150792 and 150791 are from the same macro-unit, whereas 150731 was taken from another unit. The three samples show a slight negative correlation between cNd and Nd con-

centrations from .sNd = -0.1 and 1200 ppm Nd in a eudialyte-rich sample to cNd = -0.6 and 334 ppm Nd in feldspar-rich sample. The residual nature of the lujavrite magma and reaction with its wall-rocks can explain the more negative cNd (- 1 to -4) values of this suite.

4. Discussion: Crust and mantle influences

Silica-undersaturated magmas are invariably mantle-derived and agpaitic complexes often have isotopic compositions reflecting a depleted mantle source (e.g., Kramm and Kogarko, 1994). However, the low &Nd values of Ilimaussaq (maximum of 0) relative to the expected +4 to + 6 values of depleted mantle at that time (DePaolo, 1981a) is prob- lematic. Are these near-chondritic values representa- tive of an enriched mantle source for the complex or were the initial cNd values originally higher and subsequently lowered by crustal assimilation?

Upton and Emeleus (1987) and Macdonald and Upton ( 1993) suggested that the Gardar mantle source was metasomatically enriched in K, Ba, Sr, BEE, C and halogens prior to melting. Upton (1987) also suggested that mantle metasomatism was responsible for the enrichment of a suite of Gardar mantle xenolitbs in K, Ba and BEE. Such an enrichment would be expected to be accompanied by a lowering of the sNd values in the mantle. Fig. 2 shows that the isotopic compositions of the Gardar basaltic intrusions vary from -4 to + 4 (A.N. Halliday and co-workers, personal communication) indicating that depleted mantle compositions were present at the time of Gardar activity. Basalts from the Eriksfjord Formation have cNd values of +2 to +3 which imply a depleted mantle reservoir (Paslick et al., 1993). More importantly, a eudialyte separate from Ilimaussaq has an initial &nf = + 5 compared to mantle and crustal values of + 10 and - 2, respectively for the era (Patchett et al., 1981). This indicates that the Ilimaussaq parental magma was not derived from a chondritic mantle (&nr = 0).

Neither the Nd nor the Hf data on their own allow us to differentiate between an enriched mantle source or crustal assimilation as the cause of the observed isotopic compositions. However, the initial 87Sr/s6Sr ratio of 0.7096 determined by Blaxland et al. (1976)

196 R. Stel~enson et al. / Lithos 40 f lYY7I 189-202

lies within the range of values from the surrounding

Ketilidian crust. Together, the depleted mantle val-

ues for other, more mafic, Gardar intrusions, the relatively high 87Sr/8hSr values of the Ilimaussaq complex, and its lower cNd and cut values, strongly

implicate crustal assimilation rather than mantle enrichment as the cause of the observed isotopic signa-

tures. This, however, does not mean that mantle

enrichment did not take place. If metasomatic enrichment occurred just prior to melt formation. there would have been insufficient time for radioactive

decay to produce a change in the isotopic ratios and

subsequent isotopic variations would have been solely

due to crustal assimilation. The following discussion

and subsequent modeling assumes that the Ilimaus- saq parental magma was derived from a depleted mantle but that the derivative magmas were variably

contaminated by the earlier Proterozoic crust. The most contaminated Ilimaussaq sample (an

augite syenite), with the cNd value of -6, lies along the Nd isotope evolution trend for Ketilidian crust,

defined by the granite and metasedimentary isotopic data of Patchett and Bridgwater (1984). This is

compatible with the complex having assimilated

Ketilidian crust which isotopically resembles the Ju- lianehib Batholith (Fig. 2). The predominance of

lower cNd values among samples close to the margins of the complex indicates that the assimilation was greatest along the margins and decreased towards the interior.

The overall similarity of the Nd isotopic composition of the least contaminated augite syenite and

those of the agpaites supports the suggestion of

Larsen and Sorensen (1987) that the augite syenites and agpaites are related by fractional crystallization from the same parental magma. However, although

similar in respect to their Ed,, values, the augite syenites have lower 87Sr/86Sr ratios than the agpaites (0.703 vs. 0.709, respectively, Blaxland et al.,

1976). Blaxland et al. (1976) interpreted the higher initial Sr ratios of the agpaites as the products of preferential leaching of “Sr from feldspars in the surrounding granitoids by the reactive fluids emanat- ing from the Ilimaussaq magmas. Direct assimilation was dismissed because the large amount of crust necessary would have yielded a silica over-saturated bulk composition.

There is no direct evidence available to determine

whether contamination occurred by bulk crustal assimilation or by leaching as suggested by Blaxland et

al. (1976). However, several observations point to

bulk crustal assimilation. Feldspars contain very little Nd and leaching of feldspars alone would have been

insufficient to alter the isotopic composition of the Nd-rich agpaitic magma. Consequently, leaching

and/or assimilation of minerals other than feldspars

is required (e.g. micas, amphiboles, apatite; cf. Tay-

lor and McLennan, 1985). The intrusion of mantle- derived melts into the crust is often accompanied by

crustal melting and the formation of granites, but the ~~~ values of the alkali granites are too high for

these to have been derived solely from the melting of

Ketilidian crust. Furthermore, the granites are peral-

kaline with high contents of F, Zr, Nb and Be (Hamilton, 1964) and, thus, resemble highly evolved residues of crystal fractionation rather than rocks

generated from melting of Ketilidian crust. The cNd values of the alkali granites lie between that of the

crust and those of the uncontaminated augite syenite

and agpaites and thus the most likely origin of these granites is by combined fractional crystallization and

crustal assimilation. These observations favor bulk

contamination rather than a selective process. Foland et al. (19931, Land011 et al. (1994) and Land011 and

Foland (1996) have also demonstrated that granites associated with nepheline syenites can be derived either by crustal melting or combined crustal assimilation and fractional crystallization (AFC) of an un-

dersaturated magma. The differences in the Sr and Nd isotopic compo-

sitions of the augite syenites and the agpaites are

also compatible with a bulk crustal AFC process. The relatively large change in Sr isotopic composition compared with the small change in sNd values

between the augite syenites and the agpaites probably reflects the greater concentration of Nd (> 100

ppm) with respect to Sr (< 150 ppm) in the latter compared to the augite syenites (< 66 ppm Nd, > 300 ppm Sr). Interaction of the two magmas with Ketilidian crust containing an average of 35 ppm Nd (Taylor and McLennan, 1985; Patchett and Bridgwa- ter, 1984) and 600 ppm Sr (van Breemen et al., 19741 would have resulted in greater contamination effects on the cNd values in the augite syenites and on the initial Sr ratios in the agpaites (see Fig. 4~). However, the amount of assimilation experienced by


the parental magma of the augite syenites and agpaites was insufficient to drive it to an oversaturated composition.

5. AFC modeling

The possible role of the AFC process in the formation of the Ilimaussaq complex may be considered with respect to the isotopic and elemental variations shown in Fig. 4. Fig. 4a illustrates that the decrease in cNd values from the augite syenite,

a.

Basalt-augite syenite-agpaite model

&Nd

-6 7

0.01

Basalt-alkali magma granite model

Ketilidian ’ r-0.42-0.38

, Granites

0.1 Rb;Sr 10 100

4

2

0

&Nd -2

-4

-6

Eriksfkxd basalt b.

Basalt-augite syenite- agpaite model average agpaitic

granite model r-0.42-0.38

Midian lraniteb

0.1 0.01 0.001 0.0001

l/Nd ppm

4 I I

Eriksfjord basalt

ENd :-- •~;zte~“. c’

Y -2 -- Augite syenite average agpaitic

magma

-4 -- Ketilidian Granites

-6 7

0.7 0.702 0.704 0.706 0.708 0.71

Initial Sr87/86

through the kakortokites to the lujavrites is accompanied by an increase in the Rb/Sr ratios (increasing fractionation). This trend excludes the highly contaminated samples from the margin of the complex (one augite syenite and one lujavrite). The Rb/Sr ratios are derived from splits of the same samples used by Blaxland et al. (19761, except those of the foyaites which are from Ferguson (1970). The correlation between eNd values and Rb/Sr ratios probably reflects assimilation-fractional crystallization processes in which assimilation occurred concomi- tantly with feldspar-dominated fractionation.

Among the agpaites, crystallization from both the top and the bottom in the complex concentrated the incompatible elements towards a diminishing lujavrite residual fraction trapped close to the top of the complex. This has been documented by Andersen et al. (1981) for Zr/Y and Zr/U ratios which decrease up through the kakortokites to the lujavrites (U and Y increase and Zr decreases in the residual melt as a result of eudialyte precipitation). Similarly, the Rb/Sr ratios increase from pulaskite (Rb/Sr = 5) through naujaite (Rb/Sr > 20) and from the bottom upwards from kakortokite (Rb/Sr < 12) to lujavrite (Rb/Sr > 10, Blaxland et al., 1978) as a result of greater Sr partitioning into the feldspar structure. Fig. 4a shows that the .Q, values of the roof and bottom cumulates generally decrease as the Rb/Sr ratio increases with progressive crystallization. How- ever, lower values of &Nd characterize the pulaskite

Fig. 4. AFC modeled curves are fitted to the isotopic and elemen-

tal data from this study and Blaxland et al. (1976). (a) Rb/Sr vs.

.cNd, (b) cNd vs. l/Nd and (c) cNd vs. initial *7Sr/*6Sr ratios.

The curves are fitted to demonstrate the possibility of producing

the augite syenite and agpaitic parental magmas through crustal

assimilation and fractional crystallization of a transitional basalt

parent with a composition similar to that of the Eriksfjord Forma-

tion lavas. The data require that the initial ratio of assimilation to

fractional crystallization was ca. 0.3 from basalt to augite syenite,

and subsequently decreased towards 0.2 for the main agpaite

body. However, the lujavrites and samples close the margin of the

complex experienced greater degrees of assimilation. The discrep-

ancy between the model and the agpaitic data points is ascribed to the fact that the agpaites are cumulates and do not reflect melt

compositions. In particular, Nd, Rb and Sr concentrations may

have been modified by crystal accumulation. AFC trends for the

alkali granite genesis from a basaltic parent are also modeled and

involve greater degrees of assimilation.

I98 R. Stewnson et cd. / Lithos 40 CIY97) 189-202

samples relative to those of the more fractionated

foyaites and naujaites. Again, this may reflect greater contamination of the pulaskite due to the digestion of

roof-xenoliths. An attempt to model the Nd and Sr isotopic

compositions, Rb/Sr ratios and Nd concentrations

using the AFC equations of DePaolo (1981a) is also

illustrated in Fig. 4. This modeling is presented with

several caveats. First of all the Ilimaussaq complex is largely composed of cumulates, whereas the AFC

equations of DePaolo (1981b) model a liquid line of

descent. Thus elemental concentrations determined by the fractionation equations could under-estimate

or exceed those observed in the samples due to the presence of cumulus phases such as sodalite, eudia-

lyte, etc. This leads to a second problem in that the distribution coefficients for Rb, Sr and the REE are not well known or are completely unknown for many

of the cumulus phases in the complex (eudialyte,

sodalite, arfvedsonite, and even nepheline). The AFC

modeling used distribution coefficient (01 values consistent with the feldspar-dominated assemblage indicated by the Rb-Sr data. For Nd, Sr and Rb, the

D values ranged from 0.01, 2 and 0.1 for the

basalt-augite syenite-agpaite model to 0.01. 4 and 0.01 for the basalt-alkali granite model. Values for the ratio of the mass of material assimilated to the

mass of material crystallized (I) were substituted in

Table 2

Assimilation-fractional crystallization parameters

order to obtain a best-fit model. This model was then

evaluated with respect to other AFC studies of un-

dersaturated magmas (e.g. Foland et al., 1993). Better constraints can be placed on the composi-

tions of the parental magma and contaminant. The augite syenite and agpaite units of the Ilimaussaq

complex are believed to be residues derived by

fractionation of a transitional basaltic magma (Lar-

sen and Sorensen, 1987; Macdonald and Upton,

1993). The transitional basalts from the Eriksfjord

Formation were chosen as a starting point from which to model the isotopic evolution of the augite

syenite and the initial agpaitic magma, because chemical and isotopic data exist for this suite (see

Table 2 for AFC parameters). The average agpaitic magma composition used for the modeling was de-

rived from a weighted average of the agpaites (Ferguson, 1970) while the isotopic value was aver-

aged from the agpaitic samples. The isotopic and

compositional data for the Ketilidian crust are de-

rived from Patchett and Bridgwater (1984) and van Breemen et al. (1974) except for the Nd content

which is from Taylor and McLennan (1985). Fig. 4a illustrates two AFC models; one for the

derivation of the augite syenite and agpaitic units and another for the alkali granites. The basalt-augite

syenite-agpaite magma model suggests that the ENd-Rb/Sr values of the augite syenite magma

Sample/type Eriksfjord alkali basalt Augite syenite parental magma Agpaitic parental magma Alkali granite Ketilidian crust

Input data

F

r

D Nd

Ds, D t4Yh

Nd/‘“‘Nd

&rjd Sr

CNd

CSr

CRb

Rb/Sr

0.3 0.01

;.I

0.5 I 1265

2.2 0.702

18

500

35

0.07

0.4

0.3

0.0 I

2

0. I 0.51 I164 0

0.7036

66

350

95

0.27

0.15

0.3 to 0.21

(I.01

0. I

0.5 I I 160 0.6

0.7096

334

I18

I87

1.58

0.5-0.1

0.42 to 0.38

0.01

4

0.01

.511073 0.5 10859

- 1.78 - 687/86 0.71 I

139 35

21 600

390 100

18.5 0.17

Concentration and isotopic data from Blaxland et al. (19761, Ferguson ti970), Paslick et al. (1993). Sorensen and Larsen (1987) and this study, except for average concentration of Nd in contaminant which is from Taylor and McLennan (1985). AFC calculations from DePaolo

(198lb).


might be produced by an assimilation/crystallization ratio (r-1 of 0.3 after 40% of the magma has crystallized (12% assimilation with respect to the initial amount of magma). Subsequent production of the average agpaitic magma requires a steadily decreasing rate of assimilation from 0.3 to 0.21 after at least 85% crystallization (now a total of 17% assimilation). The model illustrates that successive formation of the augite syenite and agpaitic magmas was accompanied by increasing cumulative assimilation with a progressively decreasing rate of assimilation ( r). The decrease in the r values reflects the diminishing degree of assimilation in the later, lower temperature, stages of crystallization. This could be due to: (al the decreasing energy available to assimilate crust as the magma cools, (b) the increasing insula- tion provided by the augite syenite shell around the complex and (c) later contamination possibly having occurred through assimilation of the augite syenite as the agpaitic magma stoped through it during emplacement.

This model illustrates how AFC of a basaltic magma might produce the isotopic composition of the augite syenite and average agpaitic magma only. Modeling of the individual agpaitic units is ham- pered by the fact that they represent cumulates and not the liquid compositions. In addition, the lujavrite magma may have been accompanied by a late-stage alkaline fluid resulting in the remobilization of Rb and possibly Sr because calculated initial 87Sr/86Sr ratios for the lujavrites are higher than those for the surrounding granites (0.72 vs. 0.7 11; van Breemen et al., 1974). Such a remobilization may have also contributed to the high Rb/Sr ratios of the lujavrites. Nevertheless, it is evident that additional assimilation is required to explain the lower cNd values of the lujavrites. This is consistent with field evidence showing that the lujavrite magma was highly reactive (possibly a consequence of higher concentrations of fluorine) as attested by the presence of large numbers of xenoliths in varying stages of digestion (Steenfelt, 1981). F-rich fluids could also result in the remobilization of Nd and other REE. However, the constant and comparable 147Sm/‘44Nd ratios of the lujavrites with the other units indicates a melt- controlled process rather than a fluid-controlled process. The presence of fluorine, however, would cer- tainly have aided the assimilation of crustal material

by lowering melting temperatures and viscosities (Dingwell et al., 1993).

The lower cNd values (but not as low as crustal values) of the alkali granites suggests the possibility that assimilation of silica-rich rocks by a basaltic, or perhaps augite syenitic magma led to silica over- saturation and generation of granitic magma. For the modeling in Fig. 4a and b we have chosen to model the alkali granite from the same basalt composition used above for both the convenience and because a hot, large-volume basaltic magma would have been far more efficient in assimilating crustal material, than the augite syenite melt. The modeling indicates that this requires an assimilation/crystallization ratio of 0.42 that decreases to 0.38 towards the end of crystallization. This represents 35 to 40% assimilation with respect to the initial mass of magma. The modeling reflects the fact that in order to form the alkali granites from the same basaltic composition as that of the agpaites, greater assimilation was required to make the magma silica-saturated. It was also necessary to change the distribution coefficients of Sr (2 to 4) and Rb (0.1 to 0.01) in order for the model to fit the alkali granite data. The increase in the D value for Sr (and the decrease in the D for Rb) is compatible with a greater amount of plagio- clase in the fractionation assemblage of the granite.

The AFC parameters determined above are tested by applying the same contamination rates (r values) to plots of Nd isotopic compositions with Nd concentrations (Fig. 4b) and Sr isotopic compositions (Fig. 4~). In Fig. 4b and c, the basalt-augite syenite-agpaite model uses the same amount of assimilation and the same D values as Fig. 4a and results in generally good agreement with the data. Here again, individual units such as the augite syenites, naujaites or lujavrites may not lie exactly along the modeled AFC trend because they represent cumulates rather than melt compositions. Cumulates of eudialyte, sodalite or apatite may drive the samples to higher Nd concentrations. The basalt-alkali granite model for Nd isotopes and concentrations in Fig. 4b accords well with the data for two out of the three granite samples. Fig. 4c shows only the basalt-augite syenite-agpaite model because Sr isotopic data are not available for the granites. The poor correlation in this diagram may result from local alteration of the Sr isotopic compositions due to remobilization of Rb

200 R. Steuenson et al. / Lithos 40 (1997) 189-202

and/or Sr (e.g., the lujavrite samples). Nevertheless, it is apparent that the same AFC parameters can be

used to model more than one variable (Nd and Sr isotopes and concentrations) in the Ilimaussaq Com-

plex indicating that the models are at least internally consistent.

Foland et al. (1993) calculated the effects of

crustal contamination in the system quartz-nephe-

line-kalsilite and demonstrated that, for highly undersaturated melts, r values close to 0.5 are required

to achieve silica-saturation whereas melts with

smaller values of r remain under-saturated. For Ilimaussaq, the calculated r values of 0.42 for the alkali granites and 0.3 for the augite syenite and

agpaites are in good agreement with these findings and also with those of Land011 et al. (1994) and

Land011 and Foland (1996) for other nepheline syenite-granite associations. The assimilation rate of 0.42

was apparently sufficient to drive the magma composition across the alkali feldspar divide into the

silica-oversaturated field, whereas the lesser contamination rate of 0.3 kept the product silica-under-

saturated. This might have been accomplished if the

initial magma was phonolitic, (i.e., with a composition close to the nepheline-feldspar minimum) and whose composition, following wall-rock assimilation, moved closer to the alkali feldspar join in the

residua system. However, concurrent alkali feldspar fractionation would have tended to drive the bulk

composition of the magma back towards the phonolite minimum and maintain silica-undersaturation in opposition to the effect of assimilation. Evidence for

this can be found in the kakortokite samples which show a smooth decrease in &nd with increasing

Rb/Sr (Fig. 4a). The data indicate that the feldspar- rich layers are the most contaminated which may imply that assimilation enhanced crystallization and fractionation of feldspar. Because the average compositions of the agpaites (in particular the kakor-

tokites) plot close to the phonolite minimum, incre- mental assimilation would tend to move the compositions towards the quartz-saturated field, while con- comitant fractional crystallization (dominated by feldspar) would act in the opposing sense, tending to drive the compositions back towards the minimum, providing that the Ab-Or saturation plane was never crossed.

This model is in general harmony with that pro-

posed by (Larsen and Sflrensen, 1987). The intrusion

of a large body of transitional basalt magma into the crust is inferred to have formed a large deep-seated chamber in which extensive differentiation and as-

similation of crustal material occurred. Larsen and Sorensen (1987) suggested that such a magma cham-

ber may have been stratified with the more differen-

tiated (and contaminated) melts, from which the

agpaitic rocks formed, concentrated at the top. They also suggested that the augite syenite (benmoreite)

magma formed at a lower level, prior to agpaitic magma formation, and subsequently ascended to its present position. The alkali granites may have formed

in a similar fashion where greater degrees of crustal assimilation drove the composition of the syenitic magma across the silica-saturated boundary. The re-

sultant quartz trachytic and rhyolitic (comenditic)

magma ascended to form the Ilimaussaq quartz syenites and alkali granites of the second stage, and were

subsequently followed by the comparatively less-

contaminated agpaitic (phonolitic) magma.

6. Summary

The Nd isotopic data presented in this paper suggest that the Ilimaussaq Complex evolved from a

mantle source with an cNd value of at least f2. Once emplaced in the Ketilidian crust, the transitional basalt parental magma produced the augite

syenite and agpaitic magmas through a combination of fractional crystallization and crustal assimilation.

AFC modeling suggests that the assimilation/crystallization ratio was relatively low (r = 0.3) during

production of the augite syenite and agpaitic magmas. However, the ratio is inferred to have increased in the final lujavrite residues in which volatile components were concentrated (Sorensen and Larsen,

19871, producing a highly reactive magma capable of vigorous wall-rock assimilation. A still higher degree of assimilation (r = 0.42) by some portions of the differentiating magma at depth led to silica-over- saturation and the formation of the alkali granites. Samples close to the margin of the complex, however, predominantly show the effects of direct mix- ing with the Ketilidian wall-rocks rather than AFC effects.

R. Stevenson et al. / Lithos 40 (I 997) 189-202 201

Acknowledgements

The authors wish to thank P.J. Patchett for mak- ing samples available for analysis and A.N. Halliday for providing unpublished data for other Gardar igneous rocks. This paper benefitted from reviews by J.H.J. BCdard and an anonymous reviewer. R.S. thanks the Natural Science and Engineering Re- search Council of Canada for the support of a visit- ing scientist fellowship during tenure at the Geologi- cal Survey of Canada. We thank the Director of the Geological Survey of Denmark and Greenland for the use of GGU material and permission to publish this paper.

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Documents

Crust-mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies