Sources of Proterozoic mafic dyke swarms: constraints from ThTa and LaYb ratios

Pre(mnbriun Resenrth Precambrian Research 81 (1997) 3-14

ELSEVIER

Sources of Proterozoic mafic dyke swarms: constraints from Th/Ta and La/Yb ratios Kent C. Condie * Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801 USA

Received 9 October 1995; accepted 10 May 1996

Abstract Th/Ta and La/Yb ratios in mafic dykes provide an important constraint on their magma sources and magma evolution. These element ratios clearly indicate at least three sources must be available for some swarms and that few swarms can be accounted for by only one source. Some dyke magmas appear to reflect mixing of mantle plume sources with Ta-depleted Archean subcontinental lithosphere. Systematic secular compositional changes in Th/Ta or La/Yb ratios are not observed in dyke swarms ranging in age from 2.45 to 1.14 Ga from the southern Superior Province, and in the giant Mackenzie swarm (1.27 Ga) there is no change in either element ratio as a function of distance from an inferred plume source. Paleoproterozoic dyke swarms range from those dykes with low Th/Ta and La/Yb ratios, indicating a strong contribution of depleted or primitive mantle, probably in a plume source, to picrite/norite dykes with very high ratios, perhaps indicative of a source in the Archean subcontinental lithosphere. There is an overall shift in composition of dykes from high Th/Ta ratios in the Paleoproterozoic to low ratios in the Neoproterozoic, reflecting a decrease in importance of Archean subcontinental lithospheric sources and an increase in importance of plumes containing enriched mantle components (recycled sediments and oceanic lithosphere). The delayed appearance of enriched mantle components in dyke sources until the Neoproterozoic may reflect the time it takes to recycle oceanic lithosphere through the lower mantle, beginning in the Late Archean. Keywords: dykes; dyke swarms; magma sources; trace element ratios; mantle domains

1. Introduction Precambrian mafic dyke swarms are widespread on the continents and provide an important means of tracking major periods of continental rifting and supercontinent fragmentation (Ernst et al., 1995). Some, such as the Mackenzie swarm in the Canadian Shield, appear to emanate from a point suggesting a plume source (LeCheminant and Heaman, 1989). Other swarms have been emplaced parallel to major * E-mail address: [email protected]

rifts. In some instances, mafic dyke swarms are related in time and space to continental flood basalts and may represent part of the magmatic plumbing system of the flood basalts (Fahrig, 1987). As with flood basalts, the magma sources of mafic dyke swarms are difficult to identify and controversy has arisen between those who favor a subcontinental lithospheric source and those who favor mantle plumes (Tarney, 1992; Menzies, 1992; Brewer et al., 1992; Arndt et al., 1993). The single most important problem in distinguishing between these two sources is the fact that basaltic magmas from

0301-9268/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 0 1 - 9 2 6 8 ( 9 6 ) 0 0 0 2 0 - 4

4

K.C. Condie/Precambrian Research 81 (1997) 3-14

plumes can be contaminated by continental crust, resulting in trace element and isotopic characteristics similar to basalts produced in the subcontinental lithosphere. Uncertainties in the distributions of incompatible elements in the subcontinental lithosphere compound the problem. Most geochemical studies of dyke swarms, however, conclude that continental contamination has been minimal (Weaver and Tarney, 1981; Condie et al., 1987; Tarney, 1992; Seymour and Kumarapeli, 1995) As the data base for trace elements and radiogenic and stable isotopes increases for continental flood basalts, mafic dyke swarms, and mantle xenoliths, it is possible to evaluate more fully models for the sources of continental mafic magmas. In this contribution, use is made of two incompatible element ratios to characterize and constrain better the sources of Proterozoic mafic dyke swarms. Results clearly suggest that mantle plumes and subcontinental lithosphere have both served as sources for Proterozoic mafic dyke swarms. 2. Th/Ta and La/Yb indices for mafic dyke sources Incompatible trace element distributions in basalts are commonly used to characterize magma sources since these elements are transferred in large part into magmas during melting. The distributions of incompatible elements in basalts on primitive mantlenormalized graphs (spidergrams) are particularly characteristic of different magma sources (McCulloch and Gamble, 1991; Carlson, 1991). For instance, NMORB (normal ocean ridge basalts) shows significant depletion in the most incompatible elements (e.g.., Rb, Ba, Th, K) characteristic of a depleted mantle source, whereas subduction-related basalts show enriched incompatible elements with negative Nb and Ta anomalies characteristic of metasomatized mantle wedges (Fig. 1). Oceanic island basalts show variable enrichments in incompatible elements, often with positive Nb and Ta anomalies, probably reflecting mantle plume sources, some of which contain a large component of recycled oceanic crust (Hart and Zindler, 1989; Hart et al., 1992). Submarine basalts from the Ontong-Java plateau have element distributions similar to NMORB, but show less depletion in the most incompatible elements (Fig. 1).

, ,,

=~

,,,

~NMORB Are B a s a l t OIB " ~ - ~ --x ~x~- - ' - " O n t o n g - J a v a

~,

-

100 x- -x -x-

=

~, ~ , , ,

x-

.

Basalt

~

10

0,- 4

1 L L J ~ f L t l I L t l t ~ I I [ I f ~ [ I [ , I , [ , I , I

Rb Ba Th K Ta Nb L a C e

Sr P Hf Zr Eu Ti Yb Y

Fig. 1. Primitive-mantle normalized incompatible element distributions in basalts from four tectonic settings. NMORB, normal ocean ridge basalt (Sun and McDonough, 1989); OIB, oceanic island basalt (Sun and McDonough, 1989); Arc basalt (average island arc basalt from McCulloch and Gamble, 1991); OntongJava basalt (Mahoney et al., 1993). Primitive mantle values from Sun and McDonough (1989).

Two features of normalized multi-element diagrams important in distinguishing basalt sources are Nb and Ta anomalies and the slope of the incompatible element distributions. These can be quantified by using element ratios, which also have the advantage of minimizing the effects of fractional crystallization, partial melting, and other magmatic processes. A ratio of a LIL element (Rb, Ba, Th, K) to Nb or Ta is a measure of the Nb-Ta anomaly and in this study, the Th/Ta ratio is selected since: (1) Th is the least mobile of the LIL elements and thus most likely to record source characteristics, and (2) most published analyses of Ta by INAA are more accurate than many published Nb analyses by XRF. Because the REE are also typically resistant to remobilization (Taylor and McLennan, 1985), the La/Yb ratio is selected as a measure of the slope of primitive-mantle normalized incompatible element distributions. In some instances when Ta and Yb data were not available, they were estimated as follows from published Nb and Y data (in ppm): Ta = Nb/17 and Yb = Y/10. Condie (1994) showed that a graph of Th/Ta versus La/Yb is useful in characterizing basalts from various tectonic settings as summarized in Fig. 2. NMORB typically exhibit Th/Ta ratios of 0.5 to 1.3 and La/Yb ratios of 0.8 to 2 with submarine plateau basalts (as represented by Ontong-Java) having somewhat higher Th/Ta ratios but similar La/Yb ratios. In contrast, most oceanic island basalts, as

K. C. '

Condie / Precambrian Research 81 (1997) 3-14

5

t

tic lO 5 F,I.-

PM ~ Ontong-Java Plateau

E]LC

HIMU[]

EM 2 E~M

I

PSCL

1

~ ~ ~ M O R B -

1

-

,

=

"~ R

,

R

5 La/Yb

Hawaii ,

,

• Atlantic Islands

I

10

30

Fig. 2. Th/Ta versus La/Yb graph showing distribution of basalts from various tectonic settings. DM, depleted mantle; PM, primitive mantle; PSCL, post-Archean subcontinental lithosphere; LC, lower continental crust; UC, upper continental crust; HIMU, high U/Pb mantle source; EM 1 and EM 2, enriched mantle sources; FOZO, lower mantle plume component (area on the graph beneath and around the word FOZO). Data from McDonough (1990), Condie (1993, 1994), Weaver (1991), Sun and McDonough (1989); Rudnick (1992), Hart et al. (1992), Hauri et al. (1994), Hart et al. (1995) and references cited in these publications. Other data sources given in Fig. 1.

represented by Hawaiian and South Atlantic island basalts, have La/Yb ratios of 5-20, but have Th/Ta ratios similar to NMORB. Subduction-related basalts show a considerable range in both ratios (Fig. 2), with the highest ratios generally found in continental-margin arcs. Using published data from young basalts, it is possible to characterize mantle domains as defined by Hart and Zindler (1989) and Hart et al. (1992) based on Sr-Nd-Pb isotope ratios. Both Th/Ta and La/Yb ratios are near one in depleted mantle (DM), whereas the HIMU mantle component (mantle with high U/Pb ratios) has a high La/Yb ratio (ca. 18) and a low Th/Ta ratio (ca. 1.5) (Fig. 2). HIMU is important in some volcanics of the South Atlantic. Two enriched mantle components have similar La/Yb ratios of about 25 and Th/Ta ratios near 2 (EM 1 = 2; EM 2 = 1.8). Most investigators now agree that EM 2 is probably young abyssal sediments that have been recycled into the mantle (Hart and Zindler, 1989; Hauri et al., 1994; Hart et al., 1995). The source of EM 1 is less certain: it could represent recycled ancient sediments or/and strongly depleted lithosphere. Sr-Nd-Pb isotopic data suggest the presence of a worldwide plume component common in oceanic is-

land basalts, referred to as FOZO (Hart et al., 1992; Hauri et al., 1994), and located in the lower mantle. FOZO stands for 'focal zone' and refers to NdPb--Sr isotopic trajectories that tend to converge at a point. An estimate of the composition of FOZO in terms of TNTa and La/Yb ratios, from published trace element distributions in oceanic basalts with isotopically characterized mantle sources, is shown in Fig. 2. Although FOZO falls on or near a mixing line between DM and HIMU in both isotopic space and on the Th/Ta-La/Yb diagram, He isotope data preclude it being a simple mixture of these two end members (Hauri et al., 1994). Also shown in Fig. 2 are average post-Archean lower continental crust and post-Archean subcontinental lithosphere. Lower crust is estimated from the average composition of mafic granulite xenoliths from the lower crust (Rudnick, 1992; K. Condie, unpubl. data), and subcontinental lithosphere is the average of spinel lherzolite xenoliths thought to be representative of post-Archean subcontinental lithosphere (McDonough, 1990). Because there is considerable variation in the compositions of both mafic granulite and spinel lherzolite xenoliths, there remains uncertainty in the average values of lower crust and post-Archean subcontinental lithosphere shown in Fig. 2. In terms of current ideas of mantle dynamics, it would appear that FOZO and the HIMU component occur in mantle plumes. HIMU includes a significant fraction of oceanic crust that has sunk to the bottom of the mantle (Hart, 1988; Hart et al., 1992). As a mantle plume rises and its head mixes with surrounding depleted mantle (Hill et al., 1992), its composition should define mixing lines between HIMU-EM, FOZO, and DM on the Th/Ta-La/Yb diagram. The fact that the South Atlantic and some of the Hawaiian data fall below such a line probably reflects uncertainty in the Th/Ta ratio of the EMHIMU end members (Fig. 2). The DM component is particularly prominent in the Ontong-Java plateau basalts. The relatively unmixed tail of a plume may have a composition near the FOZO or HIMU-EM end members. Entrainment studies indicate that mixing of plume components will occur chiefly in the lower mantle at elevated temperatures, and thus that depleted sources must also occur in the lower mantle (Hauri et al., 1994; Kerr et al., 1995). Post-Archean

6

K.C. Condie/Precambrian Research 81 (1997) 3-14

subcontinental lithosphere (PSCL) falls on or near DM-HIMU or DM-FOZO mixing lines. If spinel lherzolite xenolith average is representative of the post-Archean subcontinental lithosphere, this could mean that after the end of the Archean new continental lithosphere formed by the underplating of plume material beneath the continents, a mechanism that was suggested long ago by Brooks et al. (1976). One important limitation in using the Th/TaLa/Yb diagram to constrain magma sources in the mantle is the fact that both of these ratios can be raised by assimilation-fractional crystallization and by decreasing degrees of melting in the mantle (Defant and Nielsen, 1990; McKenzie and O'Nions, 1991). Model calculations using the open-system TRACE computer program of Defant and Nielsen (1990) suggest that assimilation-fractional crystallization can raise Th/Ta and La/Yb ratios by up to a factor of 3 or 4 for assimilation of upper continental crust and up to a factor of 2 for lower continental crust. The La/Yb ratio of a basaltic magma is also elevated if garnet is left in the melting residue. The fact that some basalts with high La/Yb ratios have positive end ratios suggests that they come from relatively depleted mantle sources, and for these basalts, the La/Yb ratio is not indicative of the source La/Yb ratio, but rather of a small degree of melting leaving garnet in the restite.

* Traverse 4

<, × • o

2

Traverse 8 Traverse 11, Traverse 15 T r a v e r s e 16

12 EM2~ fl] EM I

t~ HIMU

tI-

•

1 0.9 0.8 0.7 0.6 0.5

•o

X~xX 8

110

2'0

30

La/Yb

Fig. 3. Th/Ta versus La/Yb graph showing distribution of samples from the Great Abitibi Dyke (1141 Ma), SE Superior Province, Canada. Direction of emplacement is from Traverse 4 to 16. Data from Ernst (1989).

3. Proterozoic mafic dyke swarms in terms of Th/Ta and La/Yb ratios

Representative samples from Ernst (1989) from six geochemical traverses across the dyke show a very small range in both Th/Ta and La/Yb ratios (Fig. 3), and there are no apparent differences in these ratios either along or across strike. A similar plot (not shown) shows no differences between the two compositional units. Thus, the fractionation that produced the two compositional units did not affect the Th/Ta and La/Yb ratios, suggesting that intra-dyke fractional crystallization does not appreciably change these ratios, and hence, they can be used as source indicators. The relatively high La/Yb ratios (11-13), yet low Th/Ta ratios (0.7-1) suggest a mantle plume source with a HIMU or/and EM component for the Great Abitibi Dyke magma.

3.1. Intradyke variations

3.2. Intraswarm variations

There is currently only one large dyke that is well characterized geochemically: the Great Abitibi Dyke in the southeastern Canadian Shield emplaced as part of the Abitibi swarm at 1141 Ma. This dyke, which is >600 km long with an average width of 0.25 km ranges in composition from olivine tholeiite to monzodiorite, and comprises two compositional units that probably represent two magmatic pulses (Ernst et al., 1987; Ernst, 1989). One unit is restricted to the center of the dyke, occurs along half the dyke length, and varies in composition along strike. Petrologic and anisotropy of magnetic susceptibility (AMS) studies indicate that the dyke was intruded subhorizontally towards the northeast (Ernst, 1989).

3.2.1. Matachewan swarm

The Matachewan dyke swarm occurs in the central and southern Superior Province in Canada, trends north to northwest, and was emplaced at two different times in the Paleoproterozoic: 2446 and 2473 Ma (Heaman, 1995). The swarm, which is the second largest in the Canadian Shield covering an area of 500 x 700 km, is largely composed of tholeiites. A southern convergence of the dykes is consistent with a focal point to the south, perhaps related to rifting of the southern Superior Province leading to deposition of the Huronian Supergroup (Fahrig, 1987). The dykes are Fe-rich quartz tholeiites with plagioclase megacrysts and they exhibit sig-

K. C. Condie / Precambrian Research 81 (1997) 3-14 T * 10

Wawa Subprovinee Quetico Subprovince Abitibi Subprovince

o

UC

o•

o ~o

5 o o

o* *-o

PM

~o

1

~

HIMU

FOZO ~CM

EM2

LC

EM I

PSCL

5 La/Yb

10

30

Fig. 4. Th/Ta versus La/Yb graph showing distributionof dykes from the Matachewan swarm (2473 and 2446 Ma) in the southern Superior Province, Canada. Data from Condie et al. (1987) and Nelson et al. (1990). Other informationgiven in Fig. 2.

nificant trace element variations (Condie et al., 1987; Nelson et al., 1990). Although the swarm crosses three Archean subprovinces, the dykes do not show changes in composition at subprovince boundaries (Nelson et al., 1990). On a Th/Ta versus La/Yb diagram, the Matachewan swarm defines an array of increasing ratios leading away from depleted (DM) or primitive mantle (PM) end members (Fig. 4). Although there is no apparent change in ratios at the subprovince boundaries, a larger range of ratios occurs in dykes crossing the Abitibi subprovince. The variation in Th/Ta and La/Yb ratios is suggestive of mixing between a depleted or primitive mantle end member and an end member with high Th/Ta and La/Yb ratios. On a linear graph, the Th/Ta and La/Yb ratios, as well as other incompatible element ratios in the swarm, define hyperbolic or linear distributions consistent with a mixing model (Langmuir et al., 1978). Two models merit consideration: (1) open system fractionation/assimilation of basalt in an upper crustal magma chamber, and (2) mixing of mantle plume and lithosphere sources. Nelson et al. (1990) have shown the feasibility of the open-system magma model based on trace element calculations. An equally plausible model, however, is one in which a depleted or primitive mantle source (in a the asthenosphere or in a plume) variably mixes with Archean subcontinental lithosphere with a Th/Ta ratio of about 15 and a La/Yb ratio of 8.

7

The fact that a continental fragment was probably rifted from the southern edge of the Superior Province in the Paleoproterozoic does not render support for a plume at 2470-2450 Ma in that the rifting appears to have occurred 350-370 My after intrusion of the Matachewan swarm (Roscoe and Card, 1993). But, of course, all mantle plumes do not necessarily cause continental fragmentation. The Preissac and Kenora-Kabetogama dyke swarms eraplaced at 2200-2100 Ma in the same area may have been derived from the plume that fragmented the Superior Province at this time. 3.2.2. M a c k e n z i e s w a r m

The giant Mackenzie tholeiitic dyke swarm in the Canadian Shield emplaced at 1267 4- 2 Ma (LeCheminant and Heaman, 1989) is the largest dyke swarm known. The swarm fans out radially from an apparent focal point near the southern coast of Coronation Gulf in the Arctic and extends for >2400 km into northern Ontario, reaching a maximum width of 1800 km. The dyke swarm is genetically related to the Coppermine River flood basalts northeast of Great Bear Lake and also to the Muskox layered intrusion, all of which are generally interpreted to come from a mantle plume that was located beneath what is now Victoria Island (LeCheminant and Heaman, 1989). Magnetic susceptibility studies indicate that magma was injected vertically within 500 km of the focal point, and then traveled horizontally to greater distances (Ernst and Baragar, 1992). Baragar et al. (1995) have shown that the Mackenzie dykes become more fractionated with distance from the focal point, with Mg numbers decreasing from 70--35 in dykes close to the focal point to values of 55-30 in dykes beyond 800 km. They show that the swarm is consistent with a mantle plume source with the dykes being intruded in a radial fracture system developed by collapse over the plume as magma is withdrawn. Dykes at progressively greater distances from the focal point were emplaced later than those near the focal point. Th/Ta and La/Yb ratios calculated from data obtained by Gibson et al. (1987) and Baragar et al. (1995) are shown in Fig. 5 as a function of distance from the plume focal point. Three factors are clear from the data: (i) all of the data plot in a limited area on the graph; (2) there is no change in

8

K.C. Condie/Precambrian Research 81 (1997) 3-14

f • o x +

10

400 km from FocalPoint~ 500 km 600 km 800 km 1000-1500km 2000-2500km NE Trajectory

* 10

[]

•

Bunger 1 Bunger 2 Bunger 4

uc e8

UC

Z~ ZX

5

~r7

mPM

•

Z~

•

t~

[] PM

el-

1

~

m HIMU

I=~

EM2

o •

EM2

Lc

[]

PSCL

EMI

* 4

FO2~ ~DM

i

i

HIMU

~

E~ PSCL ,

1

1

'' 10

30

La/Yb ,

, 5

,

,

b

,

I 10

i 30

La/Yb Fig. 5. Th/Ta versus La/Yb graph showing distribution of dykes from the Mackenzie swarm (1267 Ma), Canada. Data are keyed to distance from inferred focal point NE of Great Bear Lake. Data from Gibson et al. (1987) and Baragar et al. (1995). Other informationgiven in Fig. 2.

either element ratio as a function of distance from the source; and (3) four samples have anomalously high Th/Ta ratios. Two of the three anomalous samples (at 500 km) have been shown by the studies of Baragar et al. (1995) to be crustally contaminated. The other two samples, which come from dykes in the extreme northeastern part of the swarm north of Hudson Bay jare probably also crustally contaminated. Again, it would appear, that with minor exceptions, fractionation processes in crustal (or subcrustal) magma chambers associated with plumederived magmas did not appreciably affect the Th/Ta and La/Yb ratios. The fact that the Mackenzie dykes have Th/Ta and La/Yb ratios very similar to the FOZO plume component supports an origin for the Mackenzie plume in the deep mantle, with little if any entrainment of other mantle components. 3.2.3. B u n g e r Hills s w a r m

Just how complex a dyke swarm can get geochemically is illustrated by the Bunger Hills swarm in East Antarctica. This swarm, which was intruded at 1140 Ma, includes five distinct suites of mafic dykes ranging in composition from tholeiites to picritic ankaramites. In spite of the small geographic

Fig. 6. Th/Ta versus La/Yb graph showing distribution of dykes from the Bunger Hills swarm (1140 Ma), East Antarctica.Of the several populationsdefinedby Sheratonet al. (1990), only three had sufficientdata to plot. Other informationgivenin Fig. 2.

area and limited time interval in which the dykes were intruded, their complex geochemistry requires multiple sources together with varying amounts of open system fractionation/assimilation (Sheraton et al., 1990). The dykes have a remarkable range of initial Sr (0.703-0.717) and Nd (SNd = +6.8 tO --18,6) isotopic ratios, as well as incompatible element distributions. Sheraton et al. (1990) suggest that at least three mantle source components are necessary in generating the Bunger Hills dykes: depleted mantle, a source with a negative Nb anomaly (lithosphere?), and a Nb-rich oceanic island-type source. Three of the Bunger Hills populations for which sufficient data are available are shown in Th/TaLa/Yb space in Fig. 6. Bunger 1 has a low Th/Ta ratio, even lower than HIMU-EM sources, Bunger 2 a high Th/Ta ratio requiring a Ta-depleted component in the source, and Bunger 4 is similar to a FOZO source. Bunger 1 and 4 dykes may have been derived from a plume with distinct FOZO and HIMU-EM components. Bunger 2 dykes, on the other hand, could have come from the Archean subcontinental lithosphere, with melting caused by heat coming from the plume. 3.2.4. Scourie s w a r m

Perhaps no other swarm of Precambrian dykes have been studied mor~ extensively than the Paleoproterozoic Scourie swarm in the Lewisian Complex of NW Scotland (Weaver and Tarney, 1981; Tar-

K. C. Condie / Precambrian Research 81 (1997) 3-14

hey and Weaver, 1987; Tarney, 1992). This swarm, which was intruded into Archean crustal rocks ranging from granulite to amphibolite grade contains four distinct lithologies: bronzite picrites, olivine gabbros, norites, and quartz tholeiites. Over 90% of the dykes are quartz tholeiites. U-Pb baddeleyite ages indicate intrusion of the picrites and norites at 2418 Ma and the tholeiites and perhaps the gabbros mostly at 1992 Ma (Heaman and Tarney, 1989). All compositions of dykes are enriched in incompatible elements and show notable negative Nb-Ta anomalies on primitive-mantle normalized diagrams (Tarney, 1992). The picrites and norites also show strong negative P anomalies. Differences in incompatible element distributions preclude any one group being derived from another by fractional crystallization (Tarney and Weaver, 1987). Geochemical studies by Tarney and Weaver provide the following constraints on dyke origin: (1) a minimum of two sources is required, one for the tholeiites and gabbros and another for the picrites and norites; (2) the tholeiites and gabbros may be derived from the same source melting at different depths; (3) norites could be derived from the same source as the picrites by a smaller degree of melting; and (4) crustal contamination plays little or no role in the origin of any of the Scourie dykes. These authors favor an incompatible-element enriched subcontinental lithospheric source for the Scourie dykes. The picrites and norites come from a more refractory part of a source that has been enriched in LILE, whereas the tholeiites and gabbros come from an undepleted source. From published data, examples of the four Scourie dyke compositions are shown a Th/Ta-La/Yb diagram in Fig. 7. The picrites and norites define a distinct high Th/Ta population, reflecting a source with a strong Ta-depleted geochemical component, in agreement with the lithospheric source proposed by Tarney and Weaver (1987). The gabbros and tholeiites each define a subgroup with lower Th/Ta and La/Yb ratios. Although the sublinear distributions of data in all four groups are consistent with fractionation/assimilation, it seems unlikely that these rocks are crustally contaminated because the dyke host rocks are too low in Th and Rb to act contaminants, as previously pointed out by Tarney and Weaver. Although the tholeiites and gabbros may come from a less enriched part of litho-

'I

10

I-

[

* o x •

9

Picrite Norite Olivine Gabbro Tholeiite

~UC cq'c,

5

¢

I-

a~PM

oo •

DM i i

xx

x x

x

LC

FOZO

EM2 HIMU

•

EMI

PSCL i

r

1

i

i

5

i

~ r I

10

30

La/Yb

Fig. 7. Th/Ta versus La/Yb graph showing distribution of dykes from the PaleoproterozoicScourie swarm, NW Scotland. Data from Weaver and Tarney (1981). Other information given in Fig. 2. sphere (Tarney and Weaver, 1987), these dykes could equally come from a FOZO plume source, somewhat contaminated by a Ta-depleted lithosphere. This is especially true for the tholeiites, which define a linear array that could be a mixing line. 3.3. I n t e r s w a r m variations 3.3.1. Paleoproterozoic s w a r m s

There is considerable variation in Th/Ta and La/Yb ratios between Paleoproterozoic dyke swarms (Fig. 8). Swarms range from those with low element ratios, such as the Kenora-Kabetogama swarm, indicative of a strong depleted or primitive mantle input, to those with very high element ratios, similar to ratios characteristic of the upper continental crust and perhaps of the Archean subcontinental lithosphere. Swarm distributions on the Th/Ta versus La/Yb graph range from sublinear, as shown by the Matachewan swarm, to irregular and equidimensional. The sublinear groups appear to reflect either assimilation-fractional crystallization processes that were operative before intrusion, or mixing of mantle sources, such as a Ta-depleted component (high Th/Ta ratio) with either a plume or a plume component in the lithosphere (Fig. 8). As previously discussed, the plume-lithosphere mixing is possible for the Matachewan swarm, and such mixing also has been proposed for the Grenville swarm (590 Ma) in eastern Canada based on incompatible element ratios (Seymour and Kumarapeli, 1995). None of the Paleoproterozoic dyke swarms lies on or near DM-

10

K.C. Condie/Precambrian Research 81 (1997) 3-14

* Preissac 2167 & 2214 Ma

z, • v + x

10

]

Kikkertavak 2235 Ma I Metachewan 2446 & 2473 Ma Commonwealth Bay 1600 Ma p Kenora-Kabetogama 2075 Ma / Scourie PicritelNorile 2418 Ma / Scourie OI Gab/Tholeiite 1992 Ma/ 4-++

5

Norite Swarms 10 UC

Tholeiit

m

e

v ~ . oPM v m[:M

•

~

#_

oxx x _

xX

v

x

l

~ eC)/~ClE/ [3 e~,~'~ X "

xz,

z&

L

5

ee*x$ m %++ tz • •

UC

LC

F-O2D

PO2~

EM2 m DM

HIMU

PSCL

HIMUa

[]

PSCL

'=1

'

'

5

....

1'0

EM2 l 30

La/Yb I

1

,

,

,

5 La/Yb

.

.

.

.

ll0

30

Fig. 8. Th/Ta versus La/Yb graph showing distribution of Paleoproterozoic dyke swarms. Data from Weaver and Tarney (1981), Condie et al. (1987), Nelson et al. (1990), Buchan et al. (1993), Schmitz et al. (1995), Sheraton et al. (1989) and references cited in these publications. Other information given in Fig. 2.

HIMU-FOZO mixing lines suggesting that among the dyke data available, there are no Paleoproterozoic representatives of post-Archean type mantle plume sources. Populations with limited ranges in Th/Ta and La/Yb ratios are difficult to explain by mixing or contamination in that they require a very uniform amount of mixing or the same amount of fractionation/assimilation. Those dykes with Th/Ta ratios >2 are particularly puzzling in that if they represent a plume source, one is faced with explaining how Ta depletion developed in the plume. Perhaps the most promising models for dyke swarms with uniform compositions are those that involve mixing of mantle plume and lithosphere, with intrusion occurring at only one stage in the mixing process, such that dyke magmas within a given swarm are rather uniform in composition. Two major compositions of dykes are recognized in the Archean and Paleoproterozoic, sometimes occurring in the same swarm. These are tholeiites and norites. Incompatible element distributions in these two dyke groups require two different sources, both of which must be available at the same time in some swarms (Hall et al., 1987; Hall and Hughes, 1987, 1990; Tarney, 1992). The norite source must be refractory in nature in terms of major elements, yet enriched in incompatible elements, especially LIL

Fig. 9. Th/Ta versus La/Yb graph showing the distribution of Paleoproterozoic norite and tholeiite dyke swarms, Data from Weaver and Tarney (1981), Sheraton and Black (1981), and Hall and Hughes (1990) and references cited in these publications. Other information given in Fig. 2.

elements, and Archean subcontinental lithosphere, with a Ta-depleted geochemical component, is an obvious candidate (Hall and Hughes, 1990; Tarney, 1992). On the Th/Ta versus La/Yb diagram, tholeiite and norite dyke swarms define two populations: the norites typically have Th/Ta and La/Yb ratios each >3, whereas the tholeiites generally exhibit Th/Ta ratios <5 with variable La/Yb ratios (Fig. 9). The high element ratios in the norites are consistent with a source that has a Ta-depleted component, probably the subcontinental lithosphere as suggested by Hall and Hughes (1990). However, if post-Archean spinel lherzolite xenoliths (PSCL in Fig. 9) are representative of the post-Archean subcontinental lithosphere as suggested by McDonough (1990), then clearly they cannot come from this source which has very low Th/Ta ratios. It may be important, in this regard, that all of the Paleoproterozoic norite dyke swarms studied so far are intruded into Archean crust, which is known to be, or is probably underlain by a thick, depleted Archean lithosphere (Boyd and Mertzman, 1987; Durrheim and Mooney, 1994). Perhaps it is this Archean subcontinental lithosphere that carries a Ta-depleted geochemical component from which the norite dykes are derived, whereas tholeiites typically come from mantle plume sources. The occurrence of norite and tholeiite compositions in the same swarm is also accounted for if plumes supply the heat to

K. C. Condie / Precambrian Research 81 (1997) 3-14

partially melt the lithosphere to produce the norite magmas. The 'normal' post-Archean subcontinental lithosphere, on the other hand, may be produced by underplating of plumes (Brooks et al., 1976), thus accounting for the low Th/Ta ratios in post-Archean spinel lherzolite xenoliths. An important question that comes out of this study is why post-Archean plume sources (HIMUEM) are not represented among the Paleoproterozoic dykes? It could be due to inadequate sampling and the appropriate dyke swarms have not yet been analyzed. If sampling is representative, however, mantle plume sources may be recorded in some Paleoproterozoic dykes, but they have gone unrecognized because Archean and at least earliest Proterozoic plumes differed in composition from most postArchean plumes. For instance, if slab recirculation was largely limited to the upper mantle during the Archean as suggested by McCulloch (1993), plumes derived from the lower mantle during the Archean, and perhaps also during the earliest Proterozoic, would carry primitive mantle (PM) or depleted mantle (DM or FOZO) geochemical signatures (or mixtures thereof), and would not lie along mixing lines with HIMU and EM sources on the Th/Ta - La/Yb plot. The Kenora-Kabetogama dyke swarm (Fig. 8) has Th/Ta and La/Yb values very close to primitive mantle (Th/Ta = 2; La/Yb = 1.4) and could be derived from a plume with primitive mantle composition. 3.4. Neoproterozoic swarms

Although Neoproterozoic dyke swarms have Th/Ta and LaJYb distributions with as much variation as Paleoproterozoic swarms, there are some interesting differences. There is an intriguing overall shift in composition from higher Th/Ta ratios in the Paleoproterozoic to lower ratios in the Neoproterozoic (Figs. 8 and 10). Also, there are at least three dyke swarms that fall on or near DM-HIMUFOZO mixing lines recording plume sources similar to modem plume sources. The Abitibi and Banger Hills 1 swarms suggest that a HIMU component was important in mantle plumes by 1 Ga, and numerous swarms show a FOZO component by this time. As mentioned above, if slab recirculation in the Archean was largely limited to the upper man-

11

Amata 800 Ma Abitibi 1141 Ma Bunger Hills 1140 Ma Mackenzie 1267 Ma SW US 1080 Ma Kulgera 1093 Ma Stuart 1076 Ma Sudbury 1139 Ma

10

x

ts

+ ~o

• • + o o ~~O

x • •

•

=

=

A E] ,~ t, PSCL

@

EM 2 m

~m LC

o',, %Io= o o F<>ZO

CM

IF

ia

x × e,

I.-

UC

v v v

HMUI -s

EMI

A& • Vk• •

30 LalYb

Fig. 10. Th/Ta versus La/Yb graph showing distribution of Neoproterozoic dyke swarms. Data from Zhao et al. (1994), Ernst (1989), Condie et al. (1987), Baragar et al. (1995); Sheraton et al. (1990), Cadman et al. (1993), Hammond (1990) and unpublished data, and references cited in these publications. Other information given in Fig. 2.

tle (McCulloch, 1993), slabs would not begin to sink below the 660-km discontinuity until the Late Archean/Paleoproterozoic. Thus, the appearance of a HIMU source in the Neoproterozoic may reflect the time it takes to recycle oceanic lithosphere to the bottom of the mantle and then back to the base of the lithosphere as part of a plume. The absence of Neoproterozoic dykes with high Th/Ta ratios would seem to mean that there are no examples derived from lithospheric sources with extreme Ta depletion. In this light, it is interesting that most of the Neoproterozoic dykes shown in Fig. 10 are intruded into juvenile post-Archean crust, and thus lack the thick Archean lithospheric root that may be depleted in Ta. Those swarms that intrude Archean crust, such as the Abitibi, Sudbury, Amata, and Stuart swarms, are emplaced around the margins of Archean cratons, which may also lack a thickened Archean lithospheric root.

4. Secular compositional changes in dyke swarms in one region Dyke swarms of several ages are found in some geographic regions. The southern Superior Province in Ontario is one such region that records five periods of dyke swarm intrusions (Condie et al., 1987;

12

K.C. Condie/Precambrian Research 81 (1997) 3-14

o • ÷

10

Sudbury 1139 Ma Preissac 2167 & 2214 Ma r Abitibi 1141 Ma I Metaehewan 2446 & 2473 Ma J Kenora-Kabetogama 2075 Ma I

ii

1

1

5. Conclusions

t

z~

++_H_+ +

DM

tic

[B

~/A zx t~ ~1'41'5

PM

I--

lithosphere.The source of the Abitibi swarm appears to be a HIMU-dominated mantle plume, whereas the Sudbury swarm may come from the Archean subcontinental lithosphere.

EM2

I

m

LC

FOZO

HIMU~

PSCL i

•

I~

E

o i

i

5 La/Yb

i

,

i

i

I

10

30

Fig. 1 l. Th/Ta versus La/Yb graph showing distribution of Proterozoic dyke swarms in the southern Superior Province, Canada. Data sources given in Figs. 8 and 10 and other information in Fig. 2.

Schmitz et al., 1995). From oldest to youngest these are the Matachewan(2473 and 2446 Ma), KenoraKabetogama (2075 Ma), Preissac (2214 and 2167 Ma), Abitibi (1141 Ma), and Sudbury (1238 Ma) swarms. On the Th/Ta versus La/Yb diagram, there are major compositional differences observed between dyke swarms in this region (Fig. 11). However, there are no systematic secular changes in composition apparent. The Kenora-Kabetogama and Abitibi dykes have low Th/Ta ratios, yet low and high La/Yb ratios, respectively. Other swarms have elevated Th/Ta ratios and variable La/Yb ratios. It is noteworthy that at 2200-2100 Ma, dyke swarms from two different sources were intruded into the southern Superior Province: the Kenora-Kabetogama dykes in the west and the Preissac dykes in the center. Both of these swarms converge to the south. Perhaps the Kenora-Kabetogama dykes come from a plume composed of a primitive mantle component (Fig. 11), and the Preissac dykes from the Archean subcontinental lithosphere, partially melted by heat coming from this plume. This plume also may have been responsible for fragmenting the Superior Province at 2200-2100 Ma (Roscoe and Card, 1993). Although both the Matachewan and KenoraKabetogama swarms may come from plumes composed of primitive mantle, the Matachewan magmas must have also undergone either crustal contamination or mixing with the Archean subcontinental

So what can we learn from Th/Ta and La/Yb distributions in Proterozoic mafic dyke swarms to help better understand the nature of their sources and their fractionation histories? These ratios are not a Pandora's Box and are far from being free from interpretational ambiguities. They do, however, provide a relatively simple and informative way to constrain dyke sources. Also, all four elements can be accurately analyzed in many samples at minimal costs by instrumental neutron activation (INAA), This is in contrast to isotopic analyses where few samples are quite costly and time-consuming. Since intradyke fractionation does not appreciably affect either the Th/Ta or La/Yb ratio, as indicated by results from the Great Abitibi Dyke, the ratios are sensitive to magma source. Although it is unlikely that crustal contamination accounts for much of the variation in Th/Ta and La/Yb ratios in dyke magmas, more complicated processes like assimilation-fractional crystallization and varying degrees of partial melting in the source can account for some variation in these ratios. Although the La/Yb ratio should be depthrelated because of the residual garnet effect, the fact that it correlates with Th/Ta ratio suggests that it definitely records information about mantle sources irrespective of depth. Perhaps the reason for this is that the degree of melting is almost always great enough to remove garnet from the residue. One of the most astonishing features of dyke swarms is the intraswarm variation in incompatible element distributions and ratios. Th/Ta and La/Yb relationships clearly indicate that at least three sources must be available for the production of some swarms, and that few swarms can be accounted for by only one source. A common scenario seems to involve mixing of a multi-component mantle plume, together in some instances with Ta-depleted Archean subcontinental lithosphere. The Th/Ta ratio is particularly sensitive to the recognition of a Ta-depleted cojmponent in dyke sources. The high Th/Ta ratio i~ Paleoproterozoic

K.C Condie/Precambrian Research 81 (1997) 3-14

picrite and norite dyke swarms may reflect a source in the refractory Archean subcontinental lithosphere, if this lithosphere is depleted in Ta. If Archean subduction and slab recycling were limited to the upper mantle (i.e., above the 660-km discontinuity), some Paleoproterozoic dyke sources may be plumes with primitive or depleted compositions derived from the lower mantle. Evidence for HIMU and enriched (EM) plume sources does not appear until the Neoproterozoic, and may reflect the time it takes for slabs to sink from the 660-km discontinuity to the bottom of the mantle (beginning in the end of Archean), and then to return as part of a mantle plume.

Acknowledgements The author acknowledges Robert Baragar and Richard Ernst for making their unpublished data on the Mackenzie dyke swarm available, and Janet H a m m o n d Gordon for making her unpublished data available on mafic dykes from the SW United States and Australia. Mark Schmitz also sent a preprint of his paper on the Kenora-Kabetogama dykes. The manuscript was greatly improved from critical reviews by John Tamey and Peter Hall, although it should be emphasized that some of the interpretations proposed in the paper are not necessarily accepted by these reviewers.

References Arndt, N.T., Czamanske, G.K., Wooden, J.L., and Fedorenko, V.A., 1993. Mantle and crustal contributions to continental flood volcanism.Tectonophysics,223: 39-52. Baragar, W.R.A., Ernst, R.E., Hulbert, L. and Peterson, T., 1995. Integrated evolution of plume-related Mackenzie dykes and Coppermine River Flood basalts. In: G. Baer and A. Heimann (Editors), Physics and Chemistry of Dykes. Balkema, Rotterdam. Boyd, F.R. and Mertzman, S.A., 1987. Compositionand structure of the Kaapvaal lithosphere, southern Africa. Geochem. Soc., Spec. Publ., 1: 13-24. Brewer, T.S., Hergt, J.M., Hawkesworth, C.J., Rex, D. and Storey, B.C., 1992. Coats Lane dolerites and the generationof Antarctic continental flood basalts, Geol. Soc. London Spec. Publ., 68: 185-208. Brooks, C., James, D.E. and Hart, S.R., 1976. Ancient lithosphere: its role in young continental volcanism. Science, 193: 1086-1094. Buchan, K.L., Mortensen, J.K. and Card, K.D., 1993. NE-

13

trending Early Proterozoic dykes of southern Superior Province: multiple episodes of emplacement recognized from integrated paleomagnetism and U-Pb geochronology. Can. J. Earth Sci., 30: 1286-1296. Cadman, A.C., Heaman, L., Tarney, J., Wardle, R. and Krogh, T.E., 1993. U-Pb geochronology and geochemical variation within two Proterozoic marie dyke swarms, Labrador. Can. J. Earth Sci., 30: 1490-1504. Carlson, R.W., 1991. Physical and chemical evidence on the cause and source characteristics of flood basalt volcanism. Aust. J. Earth. Sci., 38: 525-544. Condie, K.C., 1993. Chemical composition and evolution of the upper continental crust: contrasting results from surface samples and shales. Chem. Geol., 104: 1-37. Condie, K.C., 1994. Greenstones through time. In: K.C. Condie (Editor), Archean Crustal Evolution.Elsevier, Amsterdam, pp. 85-120. Condie, K.C., Bobrow, D.J. and Card, K.D., 1987. Geochemistry of Precambrian marie dykes from the southern Superior Province of the Canadian Shield. Geol. Assoc. Can., Spec. Pap., 34: 95-108. Defant, M.J. and Nielsen, R.L., 1990. Interpretation of open system petrogenetic processes: phase equilibria constraints on magma evolution.Geochim. Cosmochim. Acta, 54: 887-102. Durrheim, R.J. and Mooney, W.D., 1994. Evolution of the Precarnbrian lithosphere: seismologicaland geochemical constraints. J. Geophys. Res., 99: 15,359-374. Ernst, R.E., 1989. The Great Abitibi Dyke, SE Superior Province, Canada. Ph. D. dissert., Carleton Univ., Ottawa, Ont., 578 pp. Ernst, R.E. and Baragar, W.R.A., 1992. Evidence from magnetic fabric for the flow pattern of magma in the Mackenzie giant radiating dyke swarm. Nature, 356:511-513. Ernst, R.E., Bell, K., Ranalli, G. and Halls, H.C., 1987. The Great Abitibi Dyke, SE Superior Province, Canada. Geol. Assoc. Can., Spec. Pap., 34: 123-135. Ernst, R.E., Buchan, K.L. and Palmer, H.C., 1995. Giant dyke swarms: characteristics, distribution and geotectonic applications. In: G. Baer and A. Heimann (Editors), Physics and Chemistry of Dykes. Balkema, Rotterdam, pp. 3-21. Fahrig, W.E, 1987. The tectonic settings of continental marie dyke swarms: failed arm and early passive margin. Geol. Assoc. Can., Spec. Pap., 34:331-348. Gibson, I.L, Sinha, M.N. and Fahrig, W.F., 1987. The geochemistry of the Mackenzie dyke swarm, Canada. Geol. Assoc. Can., Spec. Pap., 34: 109-121. Hall, R.R and Hughes, D.J., 1987. Noritic dykes of southern West Greenland:Early Proterozoic boninitic magmatism.Contrib. Mineral. Petrol., 97: 169-182. Hall, R.P. and Hughes, D.J., 1990. Noritic magmatism. In: R.P. Hall and D.J. Hughes (Editors), Early Precambrian Basic Magmatism. Blackie and Son, Glasgow, pp. 83-110. Hall, R.P., Hughes, D.J., Friend, C.R.L and Snyder, G.L., 1987. Proterozoic mantle heterogeneity: geochemical evidence from contrasting basic dykes. Geol. Soc. London Spec. Publ., 33: 9-21. Hammond, J.G., 1990. Middle Proterozoic diabase intrusions

14

K.C. Condie /Precambrian Research 81 (1997) 3-14

in the SW United States as indicators of limited extensional tectonism. Geol. Assoc. Can., Spec. Pap., 38: 517-531. Hart, S.R., 1988. Heterogeneous mantle domains: signature, genesis and mixing chronologies. Earth Planet. Sci. Lett., 90: 273-296. Hart, S.R. and Zindler, A., 1989. Constraints on the nature and development of chemical heterogeneities in the mantle. In: W.R. Peltier (Editor), Mantle Convection. Gordon and Breach, New York, pp. 261-387. Hart, S.R., Hauri, E.H., Oschmann, L.A. and Whitehead, J.A., 1992. Mantle plumes and entrainment: isotopic evidence. Science, 256: 517-519. Hart, S.R., Blusztain, J. and Craddock, C., 1995. Cenozoic volcanism in Antarctica: Jones Mountains and Peter I Island. Geochim. Cosmochim. Acta, 59: 3379-3388. Hauri, E.H., Whitehead, J.A. and Hart, S.R., 1994. Fluid dynamic and geochemical aspects of entrainment in mantle plumes. J. Geophys. Res., 99: 24,275-24,300. Heaman, L.M., 1995. U-Pb dating of mafic rocks: past, present and future. Geol. Assoc. Can., Prog. Abstr., 20: A43. Heaman, L.M. and Tarney, J., 1989. U-Pb baddeleyite ages for the Scourie dyke swarm, Scotland: evidence for two distinct intrusion events. Nature, 340: 705-708. Hill, R.I., Campbell, I.H., Davies, G.F. and Griffiths, R.W., 1992. Mantle plumes and continental tectonics. Science, 256: 186193. Kerr, A.C., Saunders, A.D., Tarney, J., Berry, N.H. and Hards, V.L., 1995. Depleted mantle-plume geochemical signatures: no paradox for plume theories. Geology, 23: 843-846. Langmuir, C.H., Vocke, R.D., Jr., Hanson, G.N. and Hart, S.R., 1978. A general mixing equation with applications to Icelandic basalts. Earth Planet. Sci. Lett., 37: 380-392. LeCheminant, A.N. and Heaman, L.M., 1989. Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening. Earth Planet. Sci. Lett., 96: 3848. Mahoney, J.J., Storey, M. Duncan, R.A., Spencer, K.J. and Pringle, M., 1993. Geochemistry and geochronology of Leg 130 basement lavas: nature and origin of the Ontong Java Plateau. Proc. Ocean Drilling Prog., Sci. Results, 130: 3-22. McCulloch, M.T., 1993. The role of subducted slabs in an evolving earth. Earth Planet. Sci. Lett., 115: 89-100. McCulloch, M.T. and Gamble, J.A., 1991. Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet. Sci. Lett., 102: 358-374. McDonough, W.F., 1990. Constraints on the composition of the continental lithospheric mantle. Earth Planet. Sci. Lett., 101: 1-18. McKenzie, D. and O'Nions, R.K., 1991. Partial melt distributions from inversion of REE concentrations. J. Petrol., 32: 1021109t. Menzies, M.A., 1992. The lower lithosphere as a major source for continental flood basalts: a reappraisal. Geol. Soc. London Spec. Publ., 68: 31-39.

Nelson, D.O., Morrison, D.A. and Phinney, W.C., 1990. Opensystem evolution versus source control in basaltic magmas: Matachewan-Hearst dike swarm, Superior Province, Canada. Can. J. Earth Sci., 27: 767-783. Roscoe, S.M. and Card, K.D., 1993. The reappearance of the Huronian in Wyoming: rifting and drifting of ancient continents. Can. J. Earth Sci., 30: 2475-2480. Rudnick, R.L., 1992. Xenoliths-samples of the lower continental crust. In: D.M. Fountain, R. Arculus and R.W. Kay (Editors), Continental Lower Crust. Elsevier, Amsterdam, pp. 269-316. Schmitz, M.D., Wirth, K.R. and Craddock, J.E, 1995. Geochemistry of the Early Proterozoic mafic dykes of northern Minnesota and southwestern Ontario. In: G. Baer and A. Heimann (Editors), Physics and Chemistry of Dykes. Balkema, Rotterdam, pp. 219-233. Seymour, K.S. and Kumarapeli, P.S., 1995. Geochemistry of the Grenville dyke swarm: role of plume-source mantle in magma genesis. Contrib. Mineral. Petrol., 120: 29-41. Sheraton, J.W. and Black, L.P., 1981. Geochemistry and geochronology of Proterozoic tholeiite dykes of East Antarctica: evidence for mantle metasomatism. Contrib. Mineral. Petrol., 78: 305-317. Sheraton, J.W., Oliver, R.L. and Stuwe, K., 1989. Geochemistry of Proterozoic amphibolite dykes of Commonwealth Bay, Antarctica, and possible correlations with mafic dyke swarms elsewhere in Gondwanaland. Precambrian Res., 44: 353-361. Sheraton, J.W., Black, L.P., McCulloch, M.T. and Oliver, R.L., 1990. Age and origin of a compositionally varied mafic dyke swarm in the Bunger Hills, East Antarctic. Chem. Geol., 85: 215-246. Sun, S. and McDonough, W.E, 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geol. Soc. London Spec. Publ., 42: 313-345. Taylor, S.R. and McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell Scientific, Oxford, 312 pp. Tarney, J., 1992. Geochemistry and significance of mafic dyke swarms in the Proterozoic. In: K.C. Condie (Editor), Proterozoic Crustal Evolution. Elsevier, Amsterdam, pp. 151-179. Tarney, J. and Weaver, B.L., 1987. Geochemistry and petrogenesis of Early Proterozoic dyke swarms. Geol. Assoc. Can., Spec. Pap., 34: 81-94. Weaver, B.L., 1991. The origin of ocean island basalt endmember compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett., 104: 381-397. Weaver, B.L. and Tarney, J., 1981. The Scourie dyke suite: petrogenesis and geochemical nature of the Proterozoic subcontinental mantle. Contrib. Mineral. Petrol., 78: 175-188. Zhao, J., McCulloch, M.T. and Korsch, R.J., 1994. Characterization of a plume-related 800 Ma magmatic event and its implications for basin formation in central-southern Australia. Earth Planet. Sci. Lett., 121: 349-367.