The petrogenesis of the Kangaˆmiut dyke swarm,
W. Greenland
A.C. Cadman
a,b, J. Tarney
b,*, D. Bridgwater
c, F. Mengel
d,1,
M.J. Whitehouse
e, B.F. Windley
baKingston Uni6ersity,Penrhyn Road,Kingston upon Thames,KT1 2EE,UK bDepartment of Geology,Uni6ersity of Leicester,Leicester LE1 7RH,UK
cGeological Museum,Øster Voldgade5-7,Copenhagen,Denmark dDanish Lithosphere Centre,Øster Voldgade10,1350Copenhagen,Denmark eSwedish Museum of Natural History,Box50007,S-104 05Stockholm,Sweden
Received 22 February 1999; accepted 9 December 1999
Abstract
Previous studies have shown that the 2.04 Ga Kangaˆmiut dyke swarm of SW Greenland was injected into an active tectonic environment associated with the formation of the Nagssugtoqidian orogenic belt. Major and trace element modelling of the swarm shows that its chemical evolution was controlled by simple clinopyroxene – plagioclase fractionation. However, such trends — although typical of continental flood basalts and mafic dyke swarms — are at variance with their mineralogy and petrography, which show that locally hornblende is the dominant primary ferromagnesian mineral. Modelling of intradyke fractionation alone shows that hornblende could locally have been an important crystallising phase within several dykes. Normal basaltic fractionation must have occurred before dyke injection at the exposed crustal levels, where the influx of water into the dykes is believed to be responsible for the transition from clinopyroxene – plagioclase (tholeiitic) to hornblende – plagioclase9oxides (calc – alkaline) crystallisa-tion. Overall geochemical trends are dominated by tholeiitic fractionation because (1) hornblende fractionation tended to buffer chemical composition; (2) the presence of water in the surrounding country rocks may have resulted in the advection of heat away from the dyke and consequently resulted in rapid crystallisation, particularly in thin dykes. There is no evidence from trace element data, and particularly Pb isotopic ratios, of any significant assimilation of country rocks occurring during clinopyroxene – plagioclase fractionation, although this does not preclude contamina-tion of the mantle source prior to magma generacontamina-tion. It is likely that the incompatible element enrichment within the dykes resulted from subduction-related mantle metasomatism. The Kangaˆmiut dyke swarm was both a syn-tectonic and thermal event, which triggered it may be linked to passage of a slab window underneath the metasomatised region, or a mantle plume ascending under a subduction zone. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Kangaˆmiut dykes; Greenland; Geochemistry; Precambrian; Petrology; Petrogenesis
www.elsevier.com/locate/precamres
* Corresponding author. Tel.: +44-533-523937; fax: +44-533-523918. E-mail address:[email protected] (J. Tarney).
1Present address: Conoco Inc., 600, North Dairy Ashford, Houston, TX 77079, USA.
1. Introduction
In recent years there have been great advances in our understanding of the mainly Proterozoic hypabyssal dyke swarms intruded into Precam-brian cratons. Much information has come from studies of (mainly Phanerozoic) continental flood basalt (CFB) provinces which have similar chem-istry. Although field evidence of such a relation-ship is often lacking (see arguments expressed in Ross, 1983; Tarney, 1992; Cadman et al., 1995), it can sometimes be deduced that flood basalts were fed from the extensive dyke systems which are now exposed in Precambrian cratons (e.g. Baragar et al., 1996).
Despite very detailed research on both phenom-ena, the degree to which various petrogenetic processes such as fractional crystallisation, crustal contamination and mantle metasomatism control their chemistry is still hotly debated. An obvious difficulty in any petrogenetic analysis of dykes is that the same process may have operated on the magma at different stages in its genesis; for exam-ple, crystal fractionation within a basaltic magma may take place both prior to dyke injection (e.g. within a magma chamber) and subsequently within the dykes themselves, the relative influence of fractionation within each environment may be very difficult to ascertain. Hence although many studies of intradyke petrogenetic processes have been undertaken (e.g. Gibb, 1968; Komar, 1972, 1976; Ross, 1983, 1986; Platten and Watterson, 1987; Blichert-Toft et al., 1992; Ernst and Bell, 1992) the degree to which basalt petrogenesis may be controlled by hypabyssal processes within mafic dykes is still uncertain.
Study of the Kangaˆmiut dyke swarm offers an excellent opportunity to help resolve some of these questions. Earlier workers have noted that unlike the vast majority of continental mafic swarms, the dykes were injected into an overall contractional environment (e.g. Escher et al., 1976) and throughout much of their extent horn-blende is the dominant primary ferromagnesian mineral (Korstga˚rd, 1979; Bridgwater et al., 1995). However, the major element chemistry of the swarm suggests a normal tholeiitic Fe-enrich-ment trend (Escher et al., 1975; Bridgwater et al.,
1995). As hornblende is associated with calc – al-kaline fractionation, it would appear that the petrogenetic processes governing the chemistry of the dykes may be unrelated to the crystallisation processes within the dykes themselves. The field setting and unusual petrographical characteristics of the Kangaˆmiut dyke swarm also require that models developed to explain the petrogenesis of other dyke swarms are applied in order to test their validity. In this paper we seek to undertake comprehensive major and trace element modelling of the chemistry of the Kangaˆmiut dykes with a view to understanding the processes governing their formation.
2. Field relationships and geological setting
The Kangaˆmiut dyke swarm was emplaced into the high-grade gneisses of the Archaean craton of SW Greenland (Fig. 1), occurring in an area spanning 200 km south of and 100 km to the north of the Nagssugtoqidian orogenic boundary (Bridgwater et al., 1995). Although originally re-garded as an ensialic orogen, many more recent studies of this orogeny suggest that it took the form of a continent – continent collision between 2.1 and 1.7 Ga (Kalsbeek et al., 1987; Marker et al., 1995; Kalsbeek and Nutman, 1996a,b; Kriegs-man et al., 1996). Contacts between the dykes and country rocks are usually sharp with little evi-dence of crustal remelting or absorption at the margins of the dykes. The host rocks for the dyke swarm are mainly tonalitic and granodioritic gneisses. Additionally, there is a tendency for paragneiss to be associated with zones of high strain (see Fig. 1). These paragneisses are gener-ally intermediate to highly siliceous in composi-tion and sulphide-rich. Smaller quantities of marbles are also present. The protoliths of the paragneisses are uncertain, but based on our field observation and compositional character, proba-bly consisted of volcano-sedimentary sequences interbedded with small amounts of limestone.
Green-land (e.g. Bridgwater et al., 1990). Within SW Greenland, the study of fault movement history shows that the dykes are demonstrably younger than the 2.2 Ga high magnesian and tholeiitic
‘MD’ [‘m6 etad6olerite’] dykes which intrude the
southern part of the craton (Berthelsen and Bridg-water, 1960; Hall and Hughes, 1990). U – Pb zir-con SHRIMP analysis dated two the Kangaˆmiut
dykes at 204698 Ma and 203595 Ma (Kalsbeek
and Nutman, 1996a,b; Nutman et al., 1999).
Re-cent 40Ar/39Ar data confirm this 2040 Ma age
(Willigers et al., 1999).
Structural studies by Escher et al. (1976) showed that dyke orientation veered from NNE – SSW to NE – SW approaching the Nagssugtoqid-ian orogenic boundary (Fig. 1), with a second subordinate set of ESE-trending intrusions also being present. Escher et al. (1976) suggested that the two sets were coeval, and that they were intruded along conjugate shear fractures. How-ever, later studies of field and cross-cutting rela-tionships suggest three distinct crosscutting sets,
the E – W/ESE – WNW set is oldest, being crosscut
by the NNE – SSW and subsequently the NE – SW
dykes (Mengel et al., 1996). The NE – SW set constitute the main set of intrusions and include the two dykes dated by Nutman and Kalsbeek (1996) at ca. 2.04 Ga. North of Itilleq Fjord, Kangaˆmiut dykes were emplaced into Archaean E – W trending amphibolite facies zones.
3. Mineralogy and petrology
The mineralogical and petrographic variations within the Kangaˆmiut dyke swarm have been well
documented from previous studies, Windley
(1970) showed that the dykes near Sukkertoppen (Fig. 1) were often composites of hornblende quartz dolerite, amphibolite and garnet
amphibo-lite, with microporphyritic hornblende-quartz
margins. Cross-cutting relationships between
Kangaˆmiut dykes show that the development of internal mineral foliations within the earlier intru-sions are syn-intrusive. Fahrig and Bridgwater
(1976) also noted the existence of zoned
Kangaˆmiut dykes. To the north, between
Kangaˆmiut village and Søndre Strømfjord (Fig. 1) an additional texture is observed in some of the most highly fractionated intrusions, which contain garnet – albite – quartz – diorite pods. It is impor-tant to realise that these pods are sometimes situated within unsheared portions of the dyke, suggesting their origin to be igneous rather than metamorphic (Fahrig and Bridgwater, 1976), and that at least some of the dykes crystallised from wet magmas (Bridgwater et al., 1995).
Recent work has shown petrographic differ-ences associated with relative age and dyke trend (Mengel et al., 1996); the older E – W dykes con-tain olivine, clinopyroxene orthopyroxene and plagioclase. The NNE-trending dykes contain ig-neous clinopyroxene, plagioclase, rare
orthopy-roxene and primary hornblende in chilled
margins. The NE-trending dykes contain igneous clinopyroxene, plagioclase, igneous hornblende phenocrysts in chilled margins and felsic patches of dioritic composition, within the centres of some wider intrusions.
Petrographically, the Kangaˆmiut dykes range from those with fresh igneous textures (including
those with primary hornblende phenocrysts) to those with recrystallised static or dynamic meta-morphic textures (essentially amphibolites) (Men-gel et al., 1996). The grade of metamorphic overprinting generally increases northwards. Ini-tial metamorphic reactions resulted in the
forma-tion of amphibole, biotite and garnet.
Metamorphic pyroxene is recorded in the north-ernmost areas of the swarm around and to the south of Itertoˆq Fjord, representing upper amphi-bolite – granulite metamorphism (Mengel et al., 1996).
Structurally the Kangaˆmiut dykes show strong evidence for syn-shear emplacement. Escher et al. (1976) recorded consistent stepping directions, oblique offsets of country rock bands across dykes and oblique internal foliations. They con-cluded that the E – W and NNE – SSW trending dykes were injected in conjugate shears, although more recent work suggests they were intruded under a sinistral transpressive regime (Mengel et al., 1996; Hanmer et al., 1997). Similarly Windley (1970) noted that some Kangaˆmiut dykes con-tained unfoliated chilled margins with strongly deformed dyke centres, suggesting that the dykes had acted as loci of shear whilst still hot and rheologically weak. Very similar field relationships were observed during this study.
Structural and mineralogical work therefore strongly suggests that Kangaˆmiut dyke emplace-ment, shear zone formation and aqueous fluid transport were coeval or near coeval processes (Korstga˚rd, 1979; Bridgwater et al., 1995), at least
within the later ca 2.04 Ga NE-trending
Kangaˆmiut intrusions. Mengel et al. (1996) fur-ther noted that that the two earlier dyke sets could not be positively be classed as being of Kangaˆmiut age at the present time due to a lack of precise age data.
4. Geochemistry and petrogenesis
4.1. Analytical techniques
X-ray fluorescence spectrometry (XRF) at the University of Leicester. Major elements were de-termined on glass discs (fusion beads), and trace elements on powder pellets, using analytical pro-cedures detailed in Marsh et al. (1983) and Weaver et al. (1983). Selected samples were also analysed for the rare earth elements (REE) La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb and Lu by ICP – MS on a Prototype VG Elemental
Plas-maQuad II+ at the Centre for Analytical
Re-search in the Environment, Imperial College (Silwood Park). Standard operation procedures were used. Samples and reference materials were first dissolved using nitric acid into aqueous solu-tions. A procedural blank was also prepared and analysed. Each sample and reference material was measured three times and the results averaged. Reference materials were run in between batches of five samples to check for machine drift.
Pb isotope results from two dykes were gener-ated at the University of Oxford, see Table 2 for details.
4.2. Geochemical results
Representative chemical analyses of the
Kangaˆmiut dykes are given in Table 1. Multi-ele-ment mantle normalised diagrams of these analy-ses (Fig. 2) show signatures comparable with other continental dyke swarms and flood basalt provinces: the patterns show ‘lithospheric’ charac-teristics, being enriched in incompatible elements when compared to primordial mantle or chondrite abundances (Weaver and Tarney, 1981; Arndt et al., 1993). They show Nb depletion relative to the light rare-earth elements (LREE) and large-ion-lithophile (LIL) elements, but significantly not the extreme Nb depletions which are associated with island arc basalts (see Thompson et al., 1983 for comparison of continental flood basalt and island arc signatures).
The consistency between patterns for both LIL-and for high-field-strength (HFS) elements show that despite undergoing amphibolite-facies meta-morphism, the chemistry of the dykes has not been strongly affected by element remobilisation.
4.3. Inter-dyke crystallisation processes
Despite the extensive fieldwork and mineralogi-cal studies done on the Kangaˆmiut dyke swarm, geochemical analysis has hitherto been restricted to major elements, some trace elements and REE. These provided the first indication that the dyke swarm maintained a tholeiitic fractionation trend throughout most, if not all, of their magmatic evolution (Windley, 1970; Escher et al., 1975; Fahrig and Bridgwater, 1976; Bridgwater et al., 1995). Hall and Hughes (1990, 1993) noted that the Kangaˆmiut dyke swarm showed no real differ-ence in this respect from the extension-related dolerites and gabbros of the MD dyke swarm, despite their evidently different mineralogy and tectonic environment of formation. Figs. 3 – 7 show that for major, LIL and transition metal elements, the Kangaˆmiut dykes have very simple and consistent chemical trends. For example, the correlation of CaO and MgO (Fig. 3) is very similar to that described by Cox (1980) for conti-nental flood basalt geochemical evolution and attributed to the fractionation of clinopyroxene and plagioclase. Modelling of these trends using the most primitive Kangaˆmiut dyke sample
(416009, based on Mgc and Zr concentration;
Table 1) as the most primitive magma showed that the trends could be precisely mimicked by the fractional crystallisation of a 1:1 clinopyroxene – plagioclase crystal assemblage. In total, the range in highly incompatible trace elements such as Zr suggest that approximately 82% fractionation oc-curred, a value close to several other dyke swarms and flood basalt sequences (e.g. Ahmad and Tar-ney, 1991). The validity of this hypothesis was further checked using multi-element mantle nor-malised diagrams (Fig. 7) for incompatible ele-ments and REE. The diagrams show very good agreement between the modelled solutions and the evolved samples. It is evident from the REE data that garnet was not involved in the crystal frac-tionation assemblage, as even small amounts would strongly deplete the heavy rare earth con-centrations of the most evolved dykes.
Table 1
Representative analyses of Kangamiut Dykes, W. Greenland
416025 416009 416001 416004 416005 416022 416018 416007
Sample 416024 416019
57 32 20 24 25 53 49
56 25
Loc.No. 50
SiO2 53.2 51.8 49.7 50.9 51.2 50.8 51.4 50.7 57.4 49.1
2.70 0.84 1.37 1.63 2.29 1.65
1.95 2.65
TiO2 1.73 3.34
13.2
Al2O3 11.9 14.3 13.6 12.7 12.3 13.1 13.0 12.7 12.7
Fe2O3 14.9 17.4 10.9 14.1 15.7 17.6 15.6 15.5 13.8 17.7
0.26 0.18 0.20 0.23 0.25 0.23
0.19 0.21
MnO 0.20 0.23
4.32
MgO 3.98 8.91 6.43 5.53 4.51 5.23 4.15 2.80 3.78
8.05 12.81 9.68 9.48 8.52 9.48
CaO 8.23 7.38 5.81 8.04
2.8 2.1 3.1 3.0 2.9 2.7
0.09 0.19 B0.05 B0.14 0.36 B0.13
B0.25 B0.26
H2O 0.32 B0.38
Total 100.1 99.9 100.2 100.0 100.1 100.4 100.0 100.1 100.4 99.9
Trace elements in ppm(determined by XRF)
39 40 38 34 41 36
112 136 191 100 250 161
Cu 34 336 128 308
171 143 218 178 164 166
386 550
Rare earth elements in ppm(determined by ICP-MS)
41 29 9.68 12.08 18.94 11.87 16.08 39.51 34.77 37.63
La
80 13.70 30.33 29.60 30.68 32.08
86 84.44
Ce 73.85 90.75
– 2.58 4.07 4.16 4.249
– 4.48
Pr 10.49 9.60 11.62
34 10.50 17.53 18.21 19.58
Nd 39 19.36 42.15 40.57 50.30
Sm – – 2.69 4.11 4.74 5.13 5.27 8.32 9.86 11.66
– 0.90 1.42 1.60 1.87 1.64
Eu – 2.62 2.93 2.93
– 2.96 4.30 5.184 5.90
– 5.70
Gd 7.76 10.35 11.99
Table 1 (Continued)
416024
Sample 416025 416009 416001 416004 416005 416022 416018 416007 416019
57 32 20 24 25 53
Loc.No. 56 49 25 50
416003 416004 416008 416010 416011 416013
416002 416010
Sample 416020 416021 416023 Metased
Loc.No. 21 22 24 27 33 37 40 33 50 51 54 Av (n=17)
50.6
SiO2 50.9 51.2 48.4 50.4 50.4 50.4 49.1 50.7 51.2 51.1 49.0
1.63 2.09 2.43 2.02 2.20 1.94
1.57 2.37
0.21 0.25 0.26 0.24 0.22 0.23
0.22 0.24
MnO 0.22 0.22 0.26 0.19
MgO 6.34 5.70 5.21 5.29 5.54 5.63 5.87 5.87 6.96 5.60 4.08 13.37 7.68 8.75 8.74 9.40 9.57 9.78
9.67 9.51
CaO 10.6 9.76 8.13 9.40
3.0
Na2O 4.4 2.6 3.2 3.0 3.1 2.7 2.7 2.5 2.5 2.7 2.1
0.50
K2O 0.86 0.46 0.39 0.39 0.53 0.42 0.41 0.24 0.40 0.66 0.72
0.15 0.18 0.18 0.17 0.33 0.19
0.16 0.17
P2O5 0.09 0.14 0.33 0.10
1.32 0.02 B0.27 B0.16 B0.07 0.14 B0.15
H2O B0.25 B0.19 B0.17 0.18 0.44
101.35 100.1 100.0 100.1 99.9 100.2 100.2
99.9 100.0
Total 99.9 100.2 99.94
Trace elements in ppm(determined by XRF)
Sc 33 37 50 34 43 37 41 44 43 36 44 34
323 497 572 448 356 369
350 582
124 131 139 126 151 143
126 121
Zr 65 127 197 65
18 8 14 9 12 8
Nb 13 10 3 5 15 6.4
544 149 180 136 169 115
173 128
modelled for TiO2 and K2O with respect to Zr
content (Fig. 4). The higher distribution coeffi-cients of Ti, K and Zr for hornblende result in much more constrained fractionation trends and
weaker increases in TiO2 and K2O concentration
with increase in Zr. To increase contents of all three elements would have required much larger proportions of plagioclase fractionation, which in turn could be expected to result in much stronger depletions of Sr with increase in Zr content (Fig. 6). Similarly, a higher plagioclase to hornblende ratio in the fractionation assemblage would
re-quire hornblende to have considerably higher dis-tribution coefficients for Y. The disdis-tribution coefficients values for Cr, Sc and Ni would also have to be considerably higher and unrealistic when compared to the published ranges of values (see Rollinson, 1995).
4.4. Intra-dyke crystallisation processes
Fig. 2. Incompatible element (a) and rare earth (b) multi-ele-ment mantle normalised diagrams for the representative sam-ples from the Kangaˆmiut dyke swarm, as presented in Table 1. Primordial Mantle abundances from Sun and McDonough (1989). Chondrite values from Nakamura (1974). Ho value estimated from Y concentration.
nocrysts. This zone became more melanocratic and finer grained towards the centre of the dyke, though the centre of the dyke was still more leucocratic and coarser grained than the dyke margin.
This dyke was selected for special study as it contained many of the best examples found dur-ing this study of the textural features noted in previous studies of the Kangaˆmiut dykes (e.g. Windley, 1970). Analysis of the marginal and leucocratic zone of this intrusion showed substan-tial chemical variation. Zr content varies between 102 – 243 ppm, and the centre of the dyke is substantially more siliceous (57.3 wt.%) than the
Fig. 3. Biaxial plot of (a) CaO vs MgO and (b) Fe2O3vs MgO
showing fractionation trend of Kangaˆmiut dykes. Dashed line connects analyses from the margin [m: sample 416005] and centre [c: sample 416007] of a sheared Kangaˆmiut dyke. See text for full details.
modelled for TiO2 and K2O with respect to Zr
phe-Fig. 4. Biaxial plots showing fractionation trends for (a) TiO2% vs Zr ppm; and (b) K2O% vs Zr ppm. Line with white
filled circles correspond to fractionation compositions derived from sample 416009. The fractionation crystallisation assem-blage used consisted of a 1:1 ratio of clinopyroxene and plagioclase. Numbers adjacent to circles give amount of frac-tionation. Line with dark filled circles: same as above but for 1:1 crystal fractionation assemblage of plagioclase and horn-blende other details as for Fig. 3.
elements within this dyke are closely comparable to the general trends of the Kangaˆmiut dyke swarm and the modelled fractional crystallisation solution.
In order to test this hypothesis the crystallising assemblage used in modelling inter-dyke chemical variation was applied to this dyke, using the marginal sample 416005 as the parent magma composition. Zr was again taken to be perfectly incompatible in order to estimate the amount of
fractional crystallisation required (i.e. 60%).
Simple clinopyroxene+plagioclase fractional
crystallisation produced poor fits with the
analysed composition from the leucocratic zone
(sample 416007). Concentrations of TiO2 and
K2O were too high whereas LREE concentrations
are too low (Fig. 8a, b). If hornblende is added to
Fig. 5. Biaxial plots showing fractionation trends for: (a) Y ppm vs Zr ppm; and (b) Cr ppm vs Zr ppm. Other details as for Fig. 3 and Fig. 4, except line modelling hornblende-plagio-clase fractionation not shown.
margin (50.9 wt.%). Such a trend in silica enrich-ment is uncommon in continental flood basalts, where silica content is buffered during fractiona-tion (Cox, 1980). The centre of the dyke is de-pleted in Fe2O3 and TiO2 relative to its margin. Similar trends are not evident within other Kangaˆmiut dyke samples within this study (Table 1), and are usually associated with calc – alkaline fractionation. The presence of (albite-rich) plagio-clase phenocrysts within the dyke centre may
explain the higher Na2O and Sr contents but
Fig. 6. Biaxial plots showing fractionation trends for: (a) Sr ppm vs Zr ppm; (b) Sc ppm vs Zr ppm; and (c) Ni ppm vs Zr ppm. Other details as for Fig. 3 and Fig. 4.
ation. These results therefore show that horn-blende fractionation dominated intradyke pro-cesses, but considerably more fractionation than using a ‘tholeiitic’ fractionation assemblage of clinopyroxene and plagioclase is required to pro-duce a given range in concentrations.
4.5. Crustal contamination
The influence of crustal contamination as a cause of trace element enrichment in continental flood basalts and mafic dyke swarms remains a matter of controversy after many years of debate. Although it is adhered to by many petrologists (e.g. Thompson et al., 1982, 1983; Seifert et al., 1992; Arndt et al., 1993), others have preferred
Fig. 7. (a) Multielement diagram and (b) rare earth element diagrams for samples 416009 and 416019 including modelled solution for 82% clinopyroxene-plagioclase fractionation, us-ing data from Table 1. Primordial mantle abundances from Sun and McDonough (1989). Chondrite values from Naka-mura (1974). Ho value estimated from Y concentration. the fractionating assemblage, much better fits
fraction-Fig. 8. Incompatible element and rare earth multi-element mantle normalised diagrams for samples 416005 and 416007 and modelled solutions for: (1) 60% clinopyroxene – plagioclase fractionation (triangles) and (2) 71% clinopyroxene – plagio-clase – hornblende – magnetite fractionation (diamonds). Pri-mordial Mantle abundances from Sun and McDonough (1989). Chondrite values from Nakamura (1974). See Table 3 for distribution coefficients used.
Bridgwater et al. (1995) preferred crustal con-tamination as the enriching mechanism on iso-topic grounds.
2. Contamination may be more subtle. For ex-ample, the concept of contamination by
selec-tive element diffusion (Watson, 1982;
Blichert-Toft et al., 1992), rather than by bulk mixing, has made it a lot more difficult to discount crustal contamination on mass bal-ance arguments.
Although some recent highly integrated studies using many different analytical techniques have successfully resolved the influence of both pro-cesses (e.g. Shirey et al., 1994; Hawkesworth et al., 1995; Gibson et al., 1996), when using only major and trace elements such problems must be taken into consideration. However, the very good fit of fractional crystallisation modelling obtained for a variety of elements does suggest that large amounts of contamination did not occur during or after fractionation of the magma. The fact that the dykes were intruded over a wide area into different rock types, but have similar spidergram patterns also suggests that significant amounts of contamination did not occur.
It is much more difficult to rule out massive selective contamination of primary magmas prior to fractionation of the Kangaˆmiut dyke magma. It is possible for contamination to have occurred in sub-crustal magma chambers (e.g. Cox, 1980). However, the general absence of depleted MORB-type magmas in the continental crust does suggest that the mantle source regions of flood basalts are enriched prior to their ascent into the crust, as contamination within the crust is unlikely to be a highly efficient process unless magmas are able to flow turbulently and hence mix thoroughly — an unlikely occurrence in tholeiitic basaltic melts (Kerr et al., 1995a).
4.5.1. Pb isotopic ratios of Kangaˆmiut dykes Pb-isotopic ratios in mafic rocks are a particu-larly sensitive indicator of possible crustal
assimi-lation and contamination during magma
emplacement, given the order of magnitude lower concentration of Pb in mantle derived rocks and the strongly contrasting and distinctive isotopic signatures developed in old continental crustal Pb pre-contamination of the mantle source (mantle
metasomatism) as a more viable enrichment mechanism (e.g. Sheraton and Black, 1981; Ellam and Cox, 1991; Hergt et al., 1991; Tarney, 1992) or both (Gibson et al., 1996). The difficulty in resolving which process is dominant has two prin-ciple causes.
reservoirs (e.g. Moorbath and Welke, 1969; Dickin, 1981). In a related study, whole-rock Pb-isotopic ratios for 20 samples from two composite dykes south of Sondre Strømfjord are presented
in Table 2 and Fig. 10. In the 207Pb/204Pb vs
206Pb/204Pb diagram, these data define a regression
line corresponding to an age of 2.0890.10 Ga
(2s; MSWD=22), which is indistinguishable
from the more precise U – Pb zircon ages of ca. 2.04 Ga (Kalsbeek and Nutman, 1996a,b). The
single stage m1 value (
238 U/204
Pb) of 7.9 for this regression is typical for mantle derived magmas in this region. In the 208
Pb/204
Pb vs 206
Pb/204 Pb dia-gram, a roughly linear trend corresponding to a
Th/U ratio of about 4 (typical of mantle-derived
mafic rocks) is observed. Plotted for comparison in both diagrams are whole-rock Pb-isotopic ra-tios from Archaean gneisses in the
Nagssugtoqid-ian orogen (Whitehouse et al., 1998). These data show a considerable range of compositions, reflecting their complex crustal history. At the time of emplacement of the Kangaˆmiut dyke swarm at ca. 2.04 Ga, the composition of these gneisses would lie to the left of their present position in the diagram, along isochrons parallel to that defined by the dykes themselves, retaining
a broad range in 207Pb/204Pb ratios which has
clearly not influenced the isotopic compositions of the dykes to any significant extent.
4.6. Mantle melting and metasomatism
Metasomatic modification of mantle sources has been preferred by many petrologists to crustal contamination due to the much lower degree of
Table 2
Pb isotope data for Kangaˆmiut dykesa
Samplec 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb m1(2.04 Ga) m2 k2
65°56.5%N 53°29.0%W
158074 18.088 15.438 38.127 7.82 9.65 3.91
7.87 4.47
158076 16.161 15.219 35.871 3.69
24.585 45.189 7.75 27.10 3.85
158077 16.216
7.97 19.68
158078 21.824 15.984 42.373 3.95
158078 21.828 15.992 42.409 7.99 19.69 3.96
5.86
158081 21.097 15.834 39.915 7.86 17.73 3.08
19.066 15.561 39.218
158082 7.82 12.27 3.91
158083 19.104 15.565 39.189 7.82 12.37 3.86
15.540
18.823 3.89
158084 38.914 7.84 11.62
18.899 15.530 39.084 7.80 11.83
158086 3.95
66°01.4%N 53°28.5%W
15.203 35.918 7.79 5.02 3.37
16.367 158119
18.258 15.518 38.346
158120 7.94 10.10 3.94
158121 18.994 15.593 39.196 7.90 12.08 3.96
158123 19.041 15.601 38.859 7.91 12.21 3.66
20.667 15.777 40.491
158125 7.85 16.57 3.62
14.87
158127 15.790 41.167 7.91 16.15 4.11
4.05
aSamples were analysed using a VG-54E mass spectrometer at the University of Oxford using previously described analytical
procedures (Whitehouse, 1990). Pb-isotope ratios have been corrected for mass fractionation of ca.−0.15% per atomic mass unit, based upon replicate analyses of NBS 981 Pb, and have an overall analytical error of ca.90.1% (2s). Model parameters:m1and m2 are the modelled238U/204Pb ratios respectively before (assuming a single stage of evolution) and since 2.04 Ga;k2 represents
Fig. 9. Log Nb/Y vs log Nb plot showing Kangaˆmiut dykes and modelled partial melts: all partial melts from Fertile MORB Mantle (FMM) composition of Pearce and Parkinson (1993). Numbers next to symbols and lines denote percent mantle melting. Mantle mineralogical composition: grey cir-cles-14% Cpx, 43% Opx, 43% Ol. Grey squares — 14% Amph, 43% Opx, 43% Ol. Black circles — 5% Cpx, 47.5% Opx, 47.5% Ol. Black squares — 5% Amph, 47.5% Opx, 47.5% Ol. Solid enclosure-data field for Labrador Kikkertavak dykes. Dashed enclosure — data field for West Greenland sediments collected during this study.
were calculated firstly for 1 – 9% partial melting for peridotitic sources containing 14% and 5% clinopyroxene. In each case the rest of the source was assumed to be comprised of equal amounts of olivine and orthopyroxene. Following the ap-proach of Pearce and Parkinson (1993), clinopy-roxene was taken to melt at twice the rate of olivine and orthopyroxene.
Fig. 9 shows these batch melt compositions plotted with Kangaˆmiut dyke samples and the previously discussed modelled cpx-plag fractional crystallisation solution. A line for 50% olivine fractionation is also shown. It is clear from the figure that the Nb and Y concentrations of pri-mary Kangaˆmiut dyke magmas could be pro-duced from moderate degrees of melting of a peridotite composition similar or perhaps slightly less depleted than the Fertile Morb Mantle com-position used. This approximation is not unrea-sonable when considering the evidence that by Paleoproterozoic times a degree of upper mantle depletion had occurred (e.g. DePaolo, 1981; Ben Othman et al., 1984).
The evidence that the Kangaˆmiut dyke magmas contained primary hornblende suggests that the magmas may have become ‘wet’ from mantle modified by subduction-related metasomatic pro-cesses. Thus identical melting calculations were performed using the same compositions and melt-ing rates but replacmelt-ing clinopyroxene with amphi-bole. The compositions derived had very low
Nb/Y ratios and were unsuitable as primary
mag-mas for the Kangaˆmiut dykes (Fig. 9). The lack of very strong island-arc type Nb anomalies or inter-dyke calc – alkaline fractionation trends, and their
similarities in Nb/Y and Nb values to other dyke
swarms which do not show evidence of intradyke
calc – alkaline fractionation (e.g. the Early
Proterozoic Kikkertavak dyke swarm from
Labrador; Fig. 9) also suggests that the magmas and their source were originally no more strongly hydrated than other CFB-type magmas. However, this does not rule out the presence of small quan-tities of hydrous minerals such as amphibole or phlogopite being present in a metasomatised mantle source.
input of crustal material required. For example, Hergt et al. (1991) showed that up to 3% input of sediment subducted into the mantle could account for the trace element signature of Mesozoic Gondwanaland basalts. In other cases, mantle metasomatism is preferred because the local crustal types may not make suitable contaminants for the production of the dyke geochemical signa-ture (e.g. Weaver and Tarney, 1981).
When considering the case for metasomatism, it is first necessary to approximate the chemical composition of the mantle source prior to metaso-matism. Studies of mafic magmatism in arc ter-ranes using island arc basalts and boninites (Pearce and Parkinson, 1993) has shown that this could be successfully modelled using elements least likely to be enriched by sediment, fluid or crustal input. Therefore, an initial approach was to calculate batch melting curves for the element
Nb and the ratio Nb/Y for a variety of possible
When considering the nature of a chemical component potentially added via subduction to the mantle source prior to magmatism, the best
analogues are likely to be found amongst the substantial amounts of paragneisses preserved within the Nagssugtoqidian orogenic belt (Fig. 1).
Fig. 10.208Pb/204Pb and207Pb/204Pb vs 206Pb/204Pb diagrams illustrating whole-rock Pb-isotopic compositions of the Kangaˆmiut
dykes in this study (open circles). Data from Archaean gneisses in the Nagssugtoqidian orogen are shown for comparison (crosses and squares). Plotted symbols are generally larger than the analytical error. Growth curves assumem1=7.9 andk1=4. Data from
Table 3
Distribution coefficients used in modelling for the major, trace and rare earth elementsa
K2O TiO2 Nb Zr Y Sr Sc Ni Cr La
Plagioclase 0.17 0.04 0.01 0.05 0.03 1.8 – – – 0.14
0.40 0.01 0.10 0.90 0.06
0.04 2.5
Clinopyroxene 4 5 0.8
0.96
0.08 0.08 0.32 0.10 0.03
0.14 0.09
Plagioclase 0.08 0.07 0.08
Clinopyroxene 0.15 0.31 0.50 0.51 0.61 - 0.68 0.65 0.62 0.56
0.34 0.91 1.01 1.10 1.4
0.34 0.64
Hornblende 0.48 0.97 0.89
–
Magnetite – – – – – – – – –
aValues from Henderson (1982) and Rollinson (1995)
Thus 14 samples of metasediments were collected from the Kangaksiak and Nordre Strømfjord re-gions (Fig. 1) and analysed in order to ascertain their average composition. Following Hergt et al. (1991), this average composition was added to the FMM mantle composition in the ratio 3:97. Cal-culations show that magmas compositionally sim-ilar to the Kangaˆmiut dykes could be produced through second stage fractional melting of the source, using 5% melting increments. However, uncertainty over the ages of the paragneisses and the strong likelihood that the metasomatic com-ponent will be formed by the complex addition of fluids or melts to the mantle wedge rather than by simple bulk mixing means that we can only ad-vance metasomatism as the most likely cause of
the incompatible-element enrichment in the
Kangaˆmiut dyke magmas.
5. Discussion
Cox (1980) advocated a model for continental flood basalt sequences which suggested that such fractionating chambers may lie at the crust-mantle boundary. The major difficulties with such a model are that the Kangaˆmiut dykes are spread over a wide area and therefore the dimensions of an underlying magma chamber would have to be huge. However, the alternative explanation — that major dyke swarms are injected laterally outward from a smaller focal area (e.g. Ernst and
Baragar, 1992) appears only to be suitable for the Kangaˆmiut dykes if one rejects the arguments of Escher et al. (1976) that the dykes were intruded into conjugate shears.
The existence of supposedly pre-emplacement fractionation trends within the Kangaˆmiut dyke swarm is at first sight puzzling: dykes (and sills) should be ideally suited to processes such as in situ crystal fractionation, which can cause strong decoupling of compatible and incompatible ele-ments (Langmuir, 1989; Nielsen and DeLong, 1992) due to their large aspect ratios. In situ crystallisation relies on fluids being able to escape from crystallising boundary layer zones at the margins of an intrusion, and migrating back into
the main unfractionated magma (Langmuir,
1989). The presence of dioritic pods within several Kangaˆmiut dykes is entirely compatible with this mechanism. Their presence strongly suggest the mobility of residual late-stage, highly-fractionated fluids migrating upwards into a semi-solidified crystallising mush during the final stages of dyke solidification.
ferromag-nesian minerals (Henderson, 1982; Rollinson, 1995). As Fig. 4 shows, even appreciable amounts
of hornblende+plagioclase fractionation may not
result in large amounts of chemical variation. Thus, within the Kangaˆmiut dykes, in situ
horn-blende+plagioclase fractionation may not
de-stroy or blur the trends produced by any earlier
episodes of clinopyroxene+plagioclase fractional
crystallisation (Table 3).
In contrast to overall trace element geochemical trends, the highly fractionated, SiO2-rich, Fe2O3-,
TiO2-poor compositions in the centres of some
Kangaˆmiut dykes noted in other studies (see Bridgwater et al., 1995) and modelled here, cer-tainly suggest that fractionation processes (i.e.
hornblende+plagioclase9oxide phases)
simulat-ing calc – alkaline trends were locally volumetri-cally significant (e.g. 50 – 70%) within many wider dykes. Other mechanisms may also have helped to preserve the dominance of clinopyroxene – plagio-clase fractionation trends in the Kangaˆmiut dykes. Repeated injections of melts into the cen-tres of dykes from a clinopyroxene – plagioclase fractionating magma chamber may dominate lo-cal intradyke hornblende – plagioclase fractiona-tion in some dykes. Indeed, the presence of margin parallel, alternating zones of dioritic and doleritic material in the Kangaˆmiut dykes in the Sukkertoppen area (Windley, 1970) would appear to suggest the dominant process within some dykes alternated between melt injection and intra-dyke fractionation. The observed accumulation of plagioclase phenocrysts within the centres of large Kangaˆmiut dykes would also tend to counteract the effects of plagioclase fractionation during in-tra-dyke crystallisation. This accumulation most probably resulted from flow differentiation pro-cesses (Komar, 1972, 1976).
Finally, fractionation within the Kangaˆmiut dykes may have been severely impeded by rapid cooling of the intrusion after it was injected into the crust. The dehydration of hydrous assem-blages within the surrounding country rocks may cause strong advective circulation processes. This would lead to the rapid dissipation of heat from the dyke and inward cooling. In this case, rapid
cooling may result in the boundary layer migrat-ing quickly towards the centre of the intrusion, inhibiting the escape of residual fluids. In thin intrusions, crystallisation may occur throughout the whole dyke simultaneously, and intradyke fractionation would therefore be strongly impeded or non-existent. The single difficulty with such arguments is that strong advection may be ex-pected to result in LIL-element mobility and their transferral between dyke and country rock. How-ever, the general coherence of incompatible ele-ments and ratios within the dyke swarm (Fig. 2 and Fig. 4) and within individual dykes (Fig. 8), and particularly the Pb isotope results (Table 2) strongly suggest that no such LIL-element mobil-ity or transfer between country gneisses and dykes occurred. As most Kangaˆmiut dykes possess un-deformed chilled margins, it is probable that the quick freezing of magma upon intrusion would act as a barrier to element transfer. Alternatively, it is possible that the transferral of water into the dyke magmas at greater depth was accompanied by element mobility. Bridgwater et al. (1985) pointed out that this may be a viable contamina-tion mechanism for the high-Mg dykes injected in the SW region of the Archaean craton. Hence the LIL-enriched nature of the Kangaˆmiut dykes may in part be due to such a contamination mecha-nism. However, it should be noted that LIL-ele-ment enrichLIL-ele-ment relative to primordial mantle is a feature of all continental mafic dyke swarms, the vast majority of which did not crystallise from wet magmas.
a figure is only reasonable if all the modelled clinopyroxene and plagioclase fractionation oc-curred (e.g. within a central magma chamber) prior to dyke emplacement. Plagioclase fractiona-tion within the dyke must have been counteracted by flow differentiation processes and the accumu-lation of phenocrysts within the dyke centre. This is not unreasonable for porphyritic dykes, where flow acummulation processes can result in a net addition of fractionating phases to a dyke centre which is otherwise more evolved due to in situ fractionation processes (e.g. Ernst and Bell, 1992). If flow differentiation did not occur, then the amount of plagioclase fractionated would be ex-pected to be at least comparable to the amount of hornblende fractionation. Hence this would sug-gest a minimum of 65% intradyke fractionation in the modelled example. This figure is not much less than the amount of fractionation experienced by the swarm as a whole (82%). Thus it would ap-pear that dykes in general are as capable as a single large fractionating magma chambers in pro-ducing compositional dispersion. So it may not be necessary always to invoke large sub-crustal magma chambers (Cox, 1980).
Study of mantle source processes and contami-nation mechanisms is constrained by the limited amount of chemical and isotopic data involved in this study. However, mantle metasomatism rather than crustal contamination appears the more vi-able mechanism for the production of the incom-patible element signature of the Kangaˆmiut dykes. It is also worth considering the role of the metasomatic component in facilitating mantle melting. One of the central problems in the gene-sis of Proterozoic dyke swarms (and continental flood basalts) is the production of large volumes of melt within the relatively short timescale of 5 – 10 Ma (e.g. LeCheminant and Heaman, 1989). McKenzie and Bickle (1988) showed that large scale mantle melting could be produced by adia-batic decompression of anomalously hot, dry as-thenosphere during stretching of the overlying lithosphere. Similarly, Thompson and Gibson (1991) suggested melting would occur when a plume ascended to the base of previously thinned
lithosphere. In the context of Proterozoic dyke swarms, many mid-Proterozoic to Phanerozoic swarms seem ideal candidates for plume-induced magmatism (e.g. LeCheminant and Heaman, 1989; Baragar et al., 1996). Some spread out from a point in arc-like fashion, and have tholeiitic to transitional alkaline geochemistry, which may suggest a contribution from a source enriched relative to primordial mantle or small degree melts generated at greater depths perhaps plume-induced.
However, there are a number of unresolved problems in applying plume magmatism to Early Proterozoic dyke swarms: in regions such as Scot-land, Labrador and Greenland mafic magmas ap-pear to have been injected repeatedly into Archaean terranes between 2.4 and 2.0 Ga (see review of age data in Cadman et al., 1993); this frequency of dyke intrusion is not observed be-tween 1.3 and 2.0 Ga (Cadman et al., 1993). Given the smaller size of Archaean cratonic nu-clei, it appears improbable that plumes would ascend repeatedly underneath the same section of continental lithosphere on numerous occasions within 0.4 Ga, and yet fail to do so during the next 7 Ga.
Additionally, the chemistry of Early Protero-zoic swarms is dominantly tholeiitic or noritic (e.g. Weaver and Tarney, 1981). Although there is considerable debate on whether mantle plumes have depleted or enriched signatures relative to primordial mantle (e.g. Anderson, 1994; Kerr et al., 1995b), it is generally agreed that the ‘litho-spheric’ (i.e. low Nb/La, high LIL-/HFS-element)
signatures of continental basalts cannot be
derived from the plume itself. Hypotheses involv-ing mixinvolv-ing between plume-derived melts with melts from the continental lithosphere fail to ex-plain the lack of an obvious plume endmember in the trace element composition of Early Protero-zoic dyke swarms.
However, any alternative hypothesis must first address the thermal requirements for large-scale mantle melting needed to generate the volumes of magmas required. One alternative is that the bulk
swarm magmatism are derived from wet sub-lithospheric mantle. Using the hydrous peridotite solidus of Olafsson and Eggler (1983), Gallagher and Hawkesworth (1992) showed that large de-gree melting of wet lithosphere is possible under ‘normal’ mantle geothermal conditions (i.e. with a
potential asthenospheric temperature [Tp]=
1280°C).
A likely source of the metasomatic fluids or melts needed to hydrate the sub-continental litho-sphere may be produced by subduction processes. Subduction can also result in back-arc rifting (Tarney et al., 1981), a process several authors have suggested is an important control on mantle melting and the location of some Phanerozoic flood basalt provinces (e.g. Cox, 1978; Smith, 1992). Given the smaller size of Archaean cratonic nuclei, back-arc environments are likely to be
considerably more important in the Archaean/
Early Proterozoic than in Phanerozoic times. Continuous subduction at one or more edges of the Archaean cratons would cause continuous addition of metasomatic agents to a proportion-ately large area of the uppermost mantle (litho-sphere and astheno(litho-sphere) beneath the cratonic nucleus. Why, then, was basic magmatism not also persistent if this continually replenished, hy-drated lithosphere was not stable under normal geothermal conditions?
The answer must be that anomalously cold subducting plate temporally stabilised the base of the hydrated lithosphere by reducing its tempera-ture to below the hydrated peridotite solidus. Large-scale melting would only occur once sub-duction stopped or ridge subsub-duction resulted in a ‘slab window’ passed beneath the metasomatised region, perhaps accompanied by lithospheric stretching. The upwelling of the underlying as-thenosphere would result in a rise in temperature towards ‘normal’ geothermal conditions and large degrees of melting of the hydrated lithosphere/ up-per asthenosphere (Fig. 11).
An alternative explanation would be to have a mantle plume interact with a subduction zone as has been suggested by Hollings et al. (1999) for the Archaean Superior Province, and indeed for the Recent Colombia River CFB province (Taka-hashi et al., 1998) in the NW United States, once regarded as a manifestation of back-arc activity (cf. Carlson and Hart, 1987).
Bickle (1978) suggested that the higher heat
flow in Archaean/Proterozoic times may be
com-pensated for by either faster spreading rates or a greater number of tectonic plates. The existence of a multitude of micro-plates may mean that ridge subduction was a more common process in Ar-chaean or Early Proterozoic times than at later stages in Earth’s history. This may in turn explain
the frequency of mafic magmatic events in the North Atlantic Craton areas of Greenland, Labrador, and Scotland during Early Proterozoic times.
Acknowledgements
The Danish Lithosphere Centre (DLC) is thanked for fully supporting fieldwork in Green-land during the summer of 1994 and partially supporting the analytical costs involved in this study. ACC would like to thank the Geological Society of London for the Fermor Fellowship (1992 – 1995) and subsequently Kingston Univer-sity for post-doctoral research funding (1995/96). REE determinations by ICP were undertaken
with the support of NERC Grant ICP/87/1295.
This work is part of a study of the Nagssugtoqid-ian Oregon in West Greenland conducted by the Danish Lithosphere Centre. The centre is funded from the Danish National Research Foundation.
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