Chapter 4 Geochemistry and Significance of Mafic Dyke Swarms in the Proterozoic

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Chapter 4

GEOCHEMISTRY AND SIGNIFICANCE OF MAFIC DYKE SWARMS IN THE PROTEROZOIC J. TARNEY

INTRODUCTION

Mafic dyke swarms are an important feature of the Proterozoic and in parts of some stabilised Archaean cratons may be the only significant geological event in perhaps 2 Ga. Elsewhere, in less stable regions, the dyke swarms are affected by Proterozoic orogenic activity and can potentially be important time markers. Proterozoic swarms tend to be voluminous in terms of the number, width and length of dykes. The genesis of each swarm clearly constitutes a major thermal event affecting the Earth’s mantle. Moreover, because dyke swarms are often parallel to major transpressional shear zones, and the dykes may be subsequently affected by these shear zones, there is clearly an important tectonic control on their genesis. What is more surprising is that recent careful and precise U-Pb dating (e.g. LeCheminant and Heaman, 1989; Heaman and Tarney, 1989) is beginning to indicate that emplacement of any particular swarm took place over a very restricted time period. So that not only has a large amount of thermal energy to be concentrated in order to melt the mantle, but the energy has to be delivered quickly and then apparently shut-off. To accomplish this rapid burst of activity is in itself an important constraint on mantle processes. The aim of this chapter is to summarise some of the more significant features of Proterozoic mafic dyke swarms, and try to account for these features in the context of mantle evolution. There are some clear similarities - but also some differences - with Phanerozoic continental flood basalt provinces, and comparisons will be made with both continental and oceanic flood basalt provinces, where relevant. The injection of mafic dyke swarms at intervals throughout the Proterozoic provides a useful window to monitor mantle evolution, particularly the subcontinental lithosphere, which appears to be the dominant source component of most dyke swarm magmas. There is always the question, whether or not the lithosphere is the dominant source (Weaver and ’hrney, 1981), of the extent to which the magma compositions have been modified by other processes such as fractional crystallisation en route to the surface (”hrney and Weaver, 1987), fractional crystallisation in RTF (periodically replenished, tapped and fractionated) magma chambers (Cox, 1988), assimilation with fractional crystallisation (‘‘AFC”: DePaolo, 1981), thermal erosion of deep crust by mafic magmas (Huppert and Sparks, 1985) or more substantial

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ponding of magmas near the Moho with melting, assimilation, storage and homogenisation (“MASH’: Hildreth and Moorbath, 1988) before emplacement. Even without crustal involvement it is potentially possible to account for the range of compositional variation in terms of some combination of the following: partial melting, dynamic melting (Langmuir et al., 1977; Wood, 1979), disequilibrium melting (Bkdard, 1989), veined mantle sources (Brney et al., 1950), mantle lithosphere enrichment processes (Hawkesworth et al., 1990), or contributions from heterogeneous mantle reservoirs (MORB, HIMU, EM1, EM2, PREMA: Hart and Zindler, 1989). Moreover these mantle sources could be further modified by subduction zone processes (Saunders et al., 1980, 1991; Sun et al., 1989) or subducted sediment contamination (Weaver et al., 1986; Hergt et al., 1989). All the above are possible factors to consider, but not all are necessarily likely in the context of Proterozoic crust-mantle evolution. FORM AND FEATURES

From a physical viewpoint there is nothing unusual about dykes (Emerman and Marrett, 1990): low-viscosity magmas will naturally form sheets and it takes less energy to propagate a fracture than to deform host country rocks to accommodate rounded diapiric forms. Fractures can propagate rapidly and opening fractures can be rapidly filled with fluid magma. A recurrent problem is the extent to which intruding dykes have propagated laterally or vertically. We know that dykes can penetrate laterally for many tens of kilometers from recent magmatic centres in Iceland (Sigurdsson, 1987) and as much as 200 km from the Tertiary centres in NW Scotland, and that their compositions can remain essentially constant throughout this length. Similarly the 2150-Ma “Long Dyke” in West Greenland is compositionally uniform for ca. 400 km (Kalsbeek and ’Bylor, 1956). But the spectacular 1270 Ma Mackenzie Swarm in Canada (Fahrig, 1987; LeCheminant and Heaman, 1989) radiates outwards for almost 2500 km from the “centre” marked by the Muskox Intrusion, and it is conceptually difficult to imagine why magma should penetrate laterally for such huge distances. Conversely, if the dyke magmas are penetrating vertically it is necessary to specify that the source compositions and magma-generating processes must remain constant over similar distances, which is equally difficult to conceive. The volume of magma involved in the Mackenzie Swarm is estimated at 90000 km3 (Fahrig, 1987), comparable with some Phanerozoic continental flood basalt provinces. If this volume of magma were held in some central magma chamber it would be expected to assimilate crust or differentiate to produce more silicic magmas, which are absent. Cadman et al. (1990) noted that there is evidence of lateral propagation around the Tkrtiary plutonic centres of NW Scotland, which have fractionated/assimilated to produce silicic compositions (and dykes), but Proterozoic dykes in the same area (and elsewhere) show none of these features. Hence, if there are central igneous complexes feeding Proterozoic dyke swarms,

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they must be located at depth - but they have never been uncovered by erosion. The related question of whether the extensive Proterozoic dyke swarms were matched by equally extensive eruptive volcanic sequences is usually circumvented by assuming that these ancient Proterozoic terrains are deeply eroded and the evidence removed. However, major volcanic sequences (not connected with dyke swarms) are commonly preserved in both the Archaean and the Proterozoic. Cases like the Coppermine River Lavas, which are closely associated with the Muskox Intrusion at the centre of the Mackenzie Swarm, are rare. B r n e y and Weaver (1987) noted that dykes were more abundant and thicker in granulite terrains than in juxtaposed amphibolite facies terrains. This suggests an alternative explanation: simply that the dykes did not usually reach the surface. This carries a number of further implications: (a) that the source of the dyke magmas is shallow, thus lacking the hydraulic “head” to reach the surface, or (b) that, being Fe-rich, the magmas were too dense to reach the surface, or (c) that the magmas were underplated into deep sub-crustal Moho magma chambers, from which they had only limited opportunity to ascend (as in (a)). These all have a bearing on models for Proterozoic dykes (see below), but it should be noted that this is a major difference with continental flood basalts which are dominantly extrusive and appear to have a limited number of associated dykes. In a recent study of Proterozoic dyke magnetic anisotropy flow fabrics in Labrador, Cadman et al. (1992) found that initial flow fabrics were vertical, and then later replaced by horizontal fabrics. This would seem to imply that each dyke fracture initially fills vertically, then propagates laterally: if there was an escape route to the surface, vcrtical flow would dominate. Dyke swarms reflect significant extension of the continental crust, and dyke densities indicating extension of the order of 5-10% are not unusual (Cadman et al., 1990). Fahrig (1987) suggested that the Proterozoic dyke swarms around the Canadian Shield could represent “failed arm” extensional rifts in modern plate tectonic parlance. However, the width of many dyke swarms exceeds several hundred kilometers, even over 1000 km in the case of the Mackenzie Swarm (Fahrig, 1987), which is wider than most modern failed arms. Of course in the Basin and Range Province and in the Aegean, extension and crustal thinning occurs over lateral distances of several hundred kilometers, but these are not failed arms. So while the cause of the extension may be related to mantle processes, the tectonic environment has yet to be established. LeCheminant and Heaman (1989) have proposed that the Mackenzie Swarm is centred over a 1000 km diameter mantle plume head, following the model of White and McKenzie (1989) for continental flood basalts. For Early Proterozoic dyke swarms in Greenland and Scotland (Nielsen, 1987; Hall et al., 1990), it is not so easy to link them to possible failed arm rifts or plume heads. They are more closely associated with shear zones which became the focus of later Mid-Proterozoic orogenic activity. It is to be noted that shear zones may have a major transtensional as well as a transpressional component, and that in some cases (e.g. the Gulf of California) they can be associated with large degrees of extension.

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Finally, an observation worth making is that in some cases Proterozoic dykes are so abundant that extension exceeds 30%. This usually occurs near major shear zones. Despite the fact that such dykes may be emplaced into hot high-grade granulite or amphibolite facies gneisses, the country rock never shows signs of melting, even when the dykes are high-temperature picrites. This cautions against assumptions that mafic magma injection into the deep crust can cause crustal melting and severe contamination of the mafic magmas. It would appear that Proterozoic dykes rarely have sufficient superheat to promote such crustal melting. CHRONOLOGY

Although dyke swarms have been exceptionally useful tools in separating major phases of Proterozoic orogenic activity, achieving precise dates has proved to be much more difficult. With K-Ar dating there are many problems of argon loss and excess argon (e.g. Evans and Tarney, 1964); with Rb-Sr and Sm-Nd whole-rock dates there are uncertainties over inherited source characteristics, crustal contamination and metamorphic re-equilibration; and the latter can also affect U-Pb zircon or rutile dates. Even with very careful and detailed work (e.g. Chapman, 1979; Sheraton e t al., 1990), age uncertainties of several tens or even hundreds of m.y. can result. Fortunately, U-Pb baddeleyite dates (e.g. Heaman and Thrney, 1989) and SmNd mineral isochrons (Waters et al., 1990) now seem able to provide dyke ages with much higher precision. So it has now been established that the Scourie dyke swarm in NW Scotland comprises two distinct phases of emplacement, close to 2.42 and to 2.00 Ga, but the proportion of dykes in each phase is not known. In Canada the Matachewan Swarm has now been similarly constrained to 2.45 Ga and the Mackenzie Swarm to 1.27 Ga (LeCheminant and Heaman, 1989). Halls (1987) has made a compilation of dyke ages worldwide and has identified concentrations of dyke ages at certain periods, as well as periods of apparent inactivity. It is difficult to know whether the spectrum of dyke ages (?errorchrons) that has been reported from many cratons - covering almost the whole of Proterozoic time - will eventually be narrowed down to discrete pulses with global significance. For instance the 2.4-Ga-suite is also represented in Antarctica (Sheraton and Black, 1981), by the Great Dyke in Zimbabwe and the Jimberlana Dyke in Western Australia (Hatton and Von Gruenewaldt, 1990). It is interesting to note that the high-Mg noritic compositions are dominant at this age. PETROLOGICAL CHARACTERISTICS

Amongst most Proterozoic dyke swarms it is possible to recognise several petrological types. In NW Scotland, for instance, four main types are readily distinguished (Xirney, 1973): quartz dolerites, olivine gabbros, norites and bronzite

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picrites, with some minor more alkalic types, but with the dolerites (tholeiites) forming over 80% of all dykes. The noritic and picritic rock types represent an important magma type in the Late Archaean-Early Proterozoic, and Hall and Hughes (1987) have stressed their possible petrogenetic affinities with modern island arc boninites (Crawford et al., 1989). Xrmed “siliceous high-magnesian basalts” (SHMB) by Sun et al. (1989) to distinguish them from komatiites, this magma type seems to be more common in the Early Proterozoic, whereas more alkalic dykes appear in the later Proterozoic. The majority of Proterozoic dykes worldwide are aphyric iron-rich tholeiites comprising augite, plagioclase and titanomagnetite and minor hypersthene. Most are oversaturated quartz tholeiites, but they range to olivine tholeiites that are still quite Fe-rich. Some swarms, like those in southern Greenland (Nielsen, 1987) and Labrador (Cadman et al., 1990) are plagioclase phyric, some spectacularly so. Tholeiitic dykes that were emplaced at considerable depth, such as those in NW Scotland and (some in) West Greenland, may have large, though variable, amounts of primary hornblende (5 biotite), and kaersutite in the case of the olivine gabbros, indicating high p~~~ conditions during crystaIlisation. As these dykes were emplaced into “dq” granulite-facies host rocks, they cannot have acquired their fluids locally so their high water contents must have been inherited at the source. Interestingly, despite such high water contents, Proterozoic dykes never follow calc-alkaline fractionation trends (i.e. co-magmatic dykes of andesitic or dacitic character are absent), with the implication that high P H ~ O is not the only factor determining fractionation trends. Indeed the mantle sources beneath the old Archaean cratons seem to be quite reduced (Daniels and Gurney, 1991). H i g h p ~ does , ~ suppress plagioclase crystallisation, so it may be that the rarity of plagioclase phenocrysts in most dyke swarms is in part attributable to h i g h p ~ , ~ . Conversely shallow level dykes that have lost water could precipitate plagioclase in profusion (cf. Phinney et al., 1988); in any case high water pressures could generate basic melts with high normative plagioclase contents (Yoder and Tilley, 1962). The noritic and picritic dykes of the Scottish Lewisian are notable for their coarse grainsize and strong across-dyke symmetrical and asymmetrical petrological variations (Tmrney and Weaver, 1987), which are attributable to flowage in turbulent low-viscosity magmas and crystal settling in inclined dyke-sheets that were cooling slowly in hot country rocks. The mineralogy of these high-magnesian, low-alumina dykes is dominated by olivine and/or orthopyroxene, with smaller amounts of clinopyroxene, plagioclase but always with significant amounts of phlogopitelbiotite. Hornblende is quite rare. It is clear that these two types of dyke magma represent separate lineages, although both range from olivine-rich through to olivine-poor and silica-saturated. One is more Fe-rich and “fertile” in terms of major element components; the other is Fe-poor and refractory. Hornblende occurs in one, phlogopitebiotite in the other. Interestingly this difference is commonly apparent throughout both space and time in continental igneous sequences (e.g. Ellam and Cox, 1989), the

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phlogopite reflecting the fact that SHMB have enhanced levels of potassium and other lithophile elements (e.g. Sun et al., 1989). However, a potentially important observation (Hall and Hughes, 1990a) is that the high-Mg picritic and noritic suites are dominant in the Early Proterozoic, and are petrologically similar to many of the large layered intrusions such as Stillwater, Bushveld, Great Dyke, Jimberlana, etc. (Hatton and Von Gruenewaldt, 1990) that are found in the Late Archaean-Early Proterozoic. The tholeiitic magma type seems to have been available contemporaneously with the noritic type, and indeed may have been locally present as one of the inhomogeneous magma pulses that characterises these large intrusions. Conversely alkaline magma types are quite rare in Early Proterozoic dyke suites, but on almost every craton (e.g. North America, Condie et al., 1987; Greenland, Nielsen, 1987; Antarctica, Sheraton et al., 1990; see other compilations in Halls and Fahrig, 1987) they become much more important in the later Proterozoic and particularly about 1.1 Ga. This results in a much more diverse assemblage of magma types in the later Proterozoic (see Sheraton et al., 1990). Tarney and Weaver (1987) suggested that this resulted from continual additions of plume material to the base of the lithosphere, providing a greater diversity of mantle compositions for later thermal events. These systematic changes in dyke types throughout the Proterozoic must reflect in some way the processes of mantle evolution. It is important to note, of course, that large noritic intrusions of late Mesozoic age occur in the Himalayas (Chilas Complex: Khan et al., 1989), so it could be that noritic magmatism is more strictly linked to regions of recent active crustal growth rather than to an absolute time scale. GEOCHEMISTRY

A large body of geochemical and isotopic data has accumulated for Proterozoic dyke suites worldwide. It is probably easiest to summarise these data by comparison with the Early Proterozoic Scourie dyke suite which has a large petrological diversity (Tamey, 1973), but is quite well characterised chronologically (Heaman and Tarney, 1989) and its trace element (Weaver and Tarney, 1981, 1983; Wood et al., 19Sl), Sr-, Nd- and Pb-isotopic (Waters et al., 1990) and O-isotopic (Cartwright and Valley, 1991) composition is very well known. The trace element geochemistry of these dykes is summarized by the mantle-normalised spiderdiagrams (Fig. 1) and chondrite-normalised R E E plots (Fig. 2). The Scourie quartz dolerites and olivine gabbros show a clear Fe-enrichment trend, and this is matched by a wide range of REE and trace element abundances, though Weaver and Tarney (1981) argued that this range could not have resulted from simple magma chamber crystal fractionation. Indeed they suggested that the greater H RE E depletion and the more picritic nature of the olivine gabbros was a result of melting at greater depth, of an essentially similar source, but with small amounts of garnet in the residue. The shapes of the REE and spider patterns

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SCOURIE QUARTZ DOLERITE and OLIVINE GABBRO DYKES

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Fig. 1. a. Mantle-normalised trace element patterns for Scourie Iow-Ti quartz dolerite dykes (open symbols) and olivine-gabbro dykes (closed symbols), after Weaver and Tarney (1981). The patterns of the two dyke types are very similar, but the olivine gabbros lack Sr anomalies but show strong Y depletion, possibly attributable to garnet, instead of plagioclase, being residual in the source. b. Patterns for norites and bronzite picrites are similar to each other, but appear more fractionated than those in (a) because of incompatible element enrichment of a more refractory (hanburgitic) host (Weaver and Tarney, 1981). Small negative Sr and P anomalies probably reflect source as plagioclase and apatite are only late-crystallising phases in these dyke types.

are thus representative of the source. The REE patterns tend to be sigmoidal and convex-upward, and in fact not too dissimilar to those of Icelandic basalts (Thrney e t al., 1980). The patterns show moderate enrichment in the LREE and lithophile elements, distinct negative Nb anomalies and smaller negative Sr and Eu anomalies, and while the dolerites have small negative Ti anomalies, the olivine gabbros do not. By contrast the bronzite-picrite and norite REE patterns are concave-upwards (indeed more like patterns of ocean basalts from the FAMOUS area: Tarney

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et al., 1980). While alumina and the more compatible trace elements such as Zr, Y, Tb and Ti are much lower than in the dolerite dykes, as would be expected if the picro-norites were derived from a more refractory source or through much higher degrees of mantle melting (or both), there is much greater relative enrichment in the LREE and the lithophile elements K, Rb, Th, Ba, etc. The negative Nb anomaly is much more pronounced than in the dolerites. Weaver and Tarney (1981) argued that the picrite and norite dykes could not be related to each other by fractional crystallisation, but that they were both derived by partial melting of a similar refractory mantle source, the picrites resulting from melting at greater depth. The consistently greater K20/A1203 ratios in the picro-norites relative to the dolerites accounts for the almost ubiquitous presence of phlogopitebiotite rather than hornblende in these norites. Interestingly this

Geochemistryand SigniJcanceof ma$c dyke swarms in the Proterozoic

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PROTEROZOIC NORlTlC DYKES OOA Scotland V A E.Antarctica

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Fig. 3. Multielernent patterns for Early Proterozoic noritic dykes from Scotland (Weaver and Tarney, 1981), Greenland (Hall and Hughes, 1987, 1990a, b) and East Antarctica (Sheraton and Black, 1981), showing that chemical characteristics are veIy similar. Negative Nb, Sr, P and Ti anomalies most likely reflect the source, as mineral phases containing these elements are never on the liquidus of this dyke type.

is a feature of all noritic magmas (even modern ones such as Chilas), and the accompanying high Rb/Sr ratio, which is probably inherited from the source, means that there is rapid growth in s7Sr, and hence it is not surprising that most of the large noritic or SHMB intrusions such as Bushveld, Stillwater, Great Dyke, etc., have high (though variable) initial 87Sr/86Srratios, a feature which Hatton and Von Gruenewaldt (1990) ascribed to contamination of the mantle source with subducted sediment. Several pertinent petrogenetic observations can be made at this stage: (1) Hall and Hughes (1987, 1990a, b) have shown that abundant norite (BN) dykes accompany or predate the 2.1 Ga “MDl”, “MDY, “MD3” and slightly younger Kangamiut dolerite swarms in both West and East Greenland, and that the norites essentially have the same distinctive major and trace element compositions (cf. Fig. 3) as the Scourie norites. Moreover, most other Early Proterozoic norite dykes, from Antarctica (Sheraton and Black, 198l), South America (Wirth et al., 1990) and North America (Hall et al., 1987) are very similar. l3king account of the large noritic layered intrusions, this is an abundant magma type in the Early Proterozoic. But it was always available at the same time as tholeiite magma. How can two very different magma types be available essentially contemporaneously? How is it that they do not mix? It is very difficult to reconcile this observation with models of massive basaltic magma chambers underlying the lower crust, which should homogenize such diverse magma types. (2) Tholeiite dykes are rather uniform in composition throughout the Proterozoic. Surprisingly they are also similar to many tholeiitic flood basalts, from as far apart in space and time as Bsmanian and Karoo dolerites. This is il-

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lustrated in Fig. 4a (after Ahmad and Drney, 1991), and contrasts with the spectrum of other magma types shown in Fig. 4b. This magma type is not found beneath the oceans, but clearly is continually available beneath the continents from the Early Proterozoic (Fig. 4c) onwards. Hergt et al. (1989, 1991) persuasively argued, on trace element grounds, that the Tasmanian and Ferrar dolerites were derived from a mantle source pervasively contaminated by ca. 3% subducted sediment. But similar arguments would apply to the mantle source of the norites, which otherwise have quite different major element compositions. In theory it is possible to reconcile these two models by arguing that the dolerites are derived from sediment-contaminated asthenosphere and the norites from sediment-contaminated refractory lithosphere, but this fails to explain the bimodal distribution and the lack of intermediate members. ( 3 ) Students of continental flood basalts and of Proterozoic dyke swarms are largely split into those who want massive crustal contamination of magmas and those who prefer to contaminate the mantle source with sediment or subduction-derived fluids in order to account for the “continental” trace element characteristics. Because Lewisian country rock gneisses have such an anomalously low-Rb, -U, -Th composition, Weaver and Tmrney (1981) were able to rule out completely, on trace element grounds, any significant country gneiss contamination of any of the four Scourie dyke magmas. This conclusion has been fully sustained by the detailed Nd and Pb isotopic studies of Waters et al. (1990), and the oxygen isotopic data of Cartwright and Valley (1991). Similarly, Hall and Hughes (1990a) have argued that to achieve the composition of the Greenland norite dykes through contamination of a simple MORB-like magma would require impossibly large amounts (ca. 70%) of gneiss contaminant. Sheraton et al. (1990) have forcefully argued that crustal contamination is not an important factor in the petrogenesis of East Antarctic Proterozoic dykes. The argument then resolves itself if the wide range of Scourie dyke compositions have been generated without crustal contamination of magmas, there must be other processes which can produce such coexisting diverse magma types on a global scale. (4) From the limited Sr isotopic data then available, Weaver and Brney (1981) suggested a lithosphere mantle source for the Scourie dyke suite, and that its continental signature had developed at the same time as the Lewisian crust (i.e. at about 2.9 Ga). The recent Sm-Nd and U-Pb whole rock isotopic data for the Scourie dykes (Waters et al., 1990) now confirm that the trace element characteristics were established in the lithosphere source at about 3.0 Ga, some 0.6 to 1.0 Ga before the dykes were emplaced. ( 5 ) The oxygen isotope data of Cartwright and Valley (1991) provide an important new key to the whole problem (Fig. 5). They show that the wholerock S ” 0 values for the Scourie dykes are rather uniform at ca. 2%0, which is significantly below the “upper mantle” values of 6%0 normally seen in basalts. Because high-temperature magmatic 6l80 distributions are preserved in the primary minerals, and because the Sl8O values for the enclosing gneisses or adjacent shear zones are not anomalous, secondary effects cannot be responsible.

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I SCOURIE DYKES Fig. 5. Summary of oxygen isotope data for nine Scourie dolerite dykes (after Cartwright and Valley, 1991). Seven fresh d kes have very low whole-rock 6"O values of ca. 2%0, and these dykes preserve high temperature 6l 0 mineral distributions (see inset), implying whole-rock values are primary. The two sheared dykes show partial re-equilibration of oxygen isotopic compositions with enclosing gneisses. SMOW = Standard Mean Ocean Water.

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Hence these low SlSO values characterize the dyke magmas and must have been inherited from the source. Further, it is argued that the only way of achieving such low 6l80 values in the source is to subduct hydrothermally altered oceanic crust into the source regions of the Scourie dyke magmas. There are some further consequences arising from these data which are explored below, but the immediate implication is that the volume component that was being added to the lithosphere to become the major material contributor to the Scourie tholeiitic magma, was subducted hydrothermally altered'mafic material (?amphibolite). This new information begins to offer a solution to the intractable problem of contemporaneous tholeiitic and noritic dyke magmas. For instance, it permits the dominant Fe-rich quartz tholeiites to be generated from mafic material (or mantle highly permeated by subducted mafic material), and the Cr- and Ni-rich noritic magmas to be derived through melting of more refractory lithosphere.

Later Proterozoic dykes It is not possible in this short review to synthesise all the information on Middle to Late Proterozoic dyke swarms, but better to focus on the more important petrogenetic features. For instance, in the Southern Superior Province five dyke swarms (Condie et al., 1987), were emplaced over a 1.5 Ga period from the Matachewan plagioclase-phyric dykes at 2.45 Ga to the thick 700 km long Abitibi Swarm at 1.1 Ga. They show a systematic change in chemistry with time from typical Fe-tholeiites, like those described above, to distinctly alkaline dolerites with high levels of Ba and Sr, much more fractionated REE and positive rather than negative Nb anomalies. This distinctly alkaline characteristic at about 1.3-1.1 Ga is even more evident in the Gardar Province in southern Greenland (Nielsen, 1987). Just how complex the dyke chemistry can get, even in a small area, in the

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later Proterozoic is well shown by the dyke swarms in the Bunger Hills area of East Antarctica (Sheraton et al., 1990) most of which were emplaced at about 1.1 Ga, but the latest at 0.5 Ga. Five different suites of dykes are recognised, which range from quartz and olivine tholeiites through to alkaline dolerites and picrite ankaramites. They display a surprisingly wide range of initial Sr (0.703 - 0.717) and Nd EN^ = +6.3 to -18.6) isotopic compositions, and an equally broad range of spidergram patterns. The latter vary from patterns typical of Early Proterozoic tholeiites (moderately fractionated with small negative Nb, Sr and P anomalies) to highly fractionated patterns with large negative Nb anomalies, to patterns with distinctly positive Ba, Nb, Sr, P and Ti anomalies. A selection is shown in Fig. 6. At least three source components need to be involved to explain the dyke 500 .

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Fig. 6. Multi-element patterns for Late Proterozoic dyke suites from Bunger Hills, after Sheraton et al. (1990), who divided the dykes into 5 groups and several sub-groups. This illustrates much greater diversity of patterns and much higher abundances of incompatible elements in the later Proterozoic compared with the Early Proterozoic (see Fig. 1),arguably a consequence of continual additions of enriched material to the base of the lithosphere.

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trace element and isotopic compositions: a depleted asthenospheric component, a lithosphere component (possibly including subducted sediment) and a Nb-rich alkaline OIB component. As with some of the Early Proterozoic dykes discussed above, there is a requirement for some of these mantle components to be added to the sub-continental lithosphere at the time of continent generation. A recurring theme in this and many recent studies of Proterozoic dykes is the need to develop some chemical characteristics of the mantle sources supplying the dyke magmas at quite an early stage of crustal evolution - often many hundreds of Ma before the dykes themselves were emplaced. This is also the case with Phanerozoic continental flood basalts: for instance the characteristic Parana chemistry can be recognised in Proterozoic mafic suites in Brazil (Oliveira and Tmrney, 1989), and the Karoo source may also have been initiated in the Proterozoic (Ellam and Cox, 1989). The next section attempts to reconcile some of the problems of Proterozoic dyke generation with some general aspects of mantle evolution. MANTLE EVOLUTION

The Earth's mantle is now known to consist of a number of chemically distinct reservoirs which have been isolated from one another for periods in excess of 1.8 Ga, but which are nonetheless contributing to the spectrum of basalt compositions through plume activity, including continental flood basalts and dyke swarms. On the basis of isotopic diagrams such as 87Sr/s6Sr vs. 143Nd/144Nd,87Sr/86Sr vs. 206Pb/204Pb,87Sr/s6Sr vs. 208Pb/204Pbor 207Pb/m4Pbvs. m8Pb/204Pb,a t least four different end-member mantle components have been invoked to explain the variations amongst ocean island basalts (e.g. Hart and Zindler, 1989): DMM

(MORB reservoir): high 143Nd/144Nd,low s7Sr/86Srand 208Pb/204Pb, very low 'OSPb/ '06Pb, and low Ba/Nb, Th/Nb and K/Nb.

HIMU

(e.g. Mangaia, St. Helena): high 'O'Pb, '07Pb and 20sPb,low 207Pb/206Pb,208Pb/206Pb and s7Sr/s6Sr,high U/Pb, low LIL/Nb.

EM1

(e.g. Walvis Ridge): low 143Nd/144/Nd and 87Sr/86Sr,high 207Pb/206Pb and 20sPb/206Pb; generally high LIL/Nb.

EM2

(e.g. Samoa): as EM1 but with high 87Sr/86Srand higher Rb/Nb, K/Nb.

Some of these relationships are illustrated in simplified form in Fig. 7. Many islands have intermediate compositions which can be regarded as mixtures between these different components. The HIMU and EM1 components have low Nd ratios relative to their Sr ratios on the familiar ESr-ENd diagram, and these, along with several islands with similar low Nd characteristics, maintain reasonable consistency on all isotopic plots (the "LoNd array"). However, this consistency does not hold when other components are considered, so it is likely that the assumption

Geochemistry and significance of mafic dyke swarms in the Proterozoic I

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Fig. 7. Summary diagrams illustrating the compositional characteristics of main mantle components in Sr-Nd and Sr-Pb isotopic space (simplified after Hart and Zindler, 1989). PUM = primordial uniform mantle; DMM = depleted MORB mantleA and B; HIMU component with high U/Pb ( p ) ratio; “enriched” EM1 and EM2, not strictly defined but with low €Nd but variable esr; PREMA, or prevalent mantle which typifies major hotspots like Iceland and Hawaii, but could be a mix of several components.

of only four discrete components is an oversimplification. It is important to note that there are many subtle trace element differences between these mantle components, which correlate with the isotopic differences (Weaver et al., 1987; Weaver, 1991) and which were therefore established when the isotopic systems were set. There is, in addition, a rather common mantle composition represented by basalts from Iceland, Hawaii, many oceanic flood basalts and oceanic plateaus (and which is therefore volumetrically abundant) that could be regarded as a mixture of perhaps three components. Alternatively it can also be regarded as a discrete compositional entity; this has been termed PREMA (PREvalent MAntle) by Hart and Zindler (1989). This mantle composition in particular has high

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3He/4He ratios, compatible with a deep, rather primitive source, and consistent with deeply anchored hotspot plumes. This mantle component is not so depleted as DMM and could represent material from the lower mantle that has been isolated from the convecting upper mantle for a significant proportion of Earth history. Invoking this origin for PREMA begins to have some interesting consequences for Proterozoic dyke swarms. Providing explanations for the formation and preservation of the various mantle reservoirs (or components) is more difficult. We know that the earliest Archaean mantle products were moderately isotopically depleted, and that the early “planetary” stage of Earth evolution may have left the whole mantle slightly depleted (e.g. PREMA-like). Since then the convecting upper mantle above the 670km discontinuity has become progressively more depleted in incompatible elements to form the DMM reservoir. To a first approximation this correlates with the growth of continental crust, but strictly (cf. Saunders et al., 1988; Sun and McDonough, 1989) it results from basalt extraction at ocean ridges, from which crustal components are extracted at subduction zones, and the residues (which then have many of the compositional characteristics of the HIMU OIB component) are then removed down the subduction zone. So the complement to the continental crust is not just the DMM reservoir, but DMM plus an OIB component. It has been suggested (e.g. Ringwood, 1985, 1990; Ringwood and Irifune, 1988) that the subducted slab residues have been stored a t the 670 km discontinuity, from which they may rise (giga years later) as plumes to feed the ocean island hotspots. Accounting for EM1 and EM2 compositions is not so easy because their Pb isotope compositions require their chemical parameters to be set or acquired in the Late Archaean or Early Proterozoic, or it requires contamination with material of this age. It is possible to account for some of the isotopic characteristics of EM1 or EM2 by metasomatism of, and storage in, the sub-continental lithosphere (e.g. McKenzie and O’Nions, 1983; Hawkesworth et al., 1986). Alternatively, Weaver et al. (1986) and Weaver (1991) have shown that it is possible to account quantitatively for many of the isotopic and trace element characteristics of EM1 and EM2 by contaminating the HIMU residue with abyssal or terrigenous sediment respectively during subduction (in the Precambrian). However as recent models of subduction zones (Peacock, 1991; Davies and Bickle, 1991; Saunders et al., 1991; Davies and Stevenson, 1992) require induced convection of the mantle wedge in order to achieve thermal and material balance in magma genesis, and therefore entrainment and progressive downdrag of the subcontinental lithosphere, then lithosphere fractionation and contamination processes can be employed too, i.e. competing models are not so far apart. So are the enriched characteristics of most Proterozoic dykes and continental flood basalts the result of progressive lithosphere enrichment processes, or of plume additions from the mesosphere?

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PREMA mantle and Proterozoic dykes At this stage it is necessary to consider further the status of PREMA (1) If predominantly residing in the lower mantle, the PREMA reservoir has escaped the depletion processes that have affected the convecting DMM upper mantle since the early Archaean. When it rises as plumes to form ridge-centred hotspot islands like Iceland, it not only has more potential thermal energy, but is also more fertile in terms of major elements (cf. Brooks et al., 1991), hence it is able to generate ocean crust two or three times thicker than normal (cf. White and McKenzie, 1989); indeed ocean crust that may become sub-aerial. (2) Larson (1991) has recently shown that there was a major spurt in oceanic crust production between 120 and 80 Ma, but this resulted not in a significant increase in global spreading rate, but was manifest in production of oceanic plateaus (e.g. Ontong Java, Manihiki Rise) with considerably over-thickened ocean crust like Iceland. Moreover, because this period coincides almost exactly with the Cretaceous magnetic quiet zone (normal polarity), he argues that this represented a major release of material j?om the lower mantle that actually affected the convective behaviour of the outer core and inhibited magnetic reversals for 41 Ma. ( 3 ) Many of these ocean plateaus and oceanic flood basalt provinces still exist in the western Pacific, where some have been sampled via ocean drilling. However, in the eastern Pacific, where their counterparts suffered attempted subduction along the Cordillera of South America, this was clearly difficult because large volumes were obducted along the coastal belt of Colombia. Indeed some refused to be subducted and were carried through to form the floor of the Caribbean. The implication is that these oceanic plateaus are dificult to subduct. (4) Geochemical studies of these Colombian volcanics (Millward et al., 1984; Guevara, 1987) and the drilled ocean plateaus (Saunders, 1986) show that their closest geochemical counterparts (both trace element and isotopic) are with Icelandic/Reykjanes Ridge basalts. As stressed above, the closer geochemical counterparts of Proterozoic tholeiitic dykes amongst oceanic basalts are Icelandic basalts. Interestingly, some of the few modern high-Mg counterparts of Archaean komatiites occur within the Colombian and Caribbean obducted volcanic sequences (Gorgona, Curacao, Romeral), as emphasised by Storey et al. (1991). There are two very important points here which bear upon the problem of accounting for the oxygen isotope data for the Scourie dykes, reported above, which required that their source be hydrothermally altered ocean crust injected into the Lewisian sub-continental mantle. The first difficulty with the model is in explaining why oceanic crust should be injected into the lithosphere rather than being subducted in the normal fashion. The observations above, on Colombian/Caribbean volcanic sequences, suggest that thick PREMA-type crust may be rather resistant to subduction, simply because it is warmer (McKenzie and Bickle, 1988), less dense and less likely to transform to eclogite to provide the “slab-pull” force; hence it is more likely to underplate into a rheologically weak young lithosphere, as required.

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The second problem is in accounting for the low S1'0 values in the proposed underplated ocean crust, because low values of the order of ~ 2 % 0in equilibrium with seawater can only occur in the deeper sections of ocean crust - therefore it is necessary to strip off, and dispose elsewhere, the whole upper section of ocean crust during underplating to maintain these low values. However, if the PREMA ocean crust was sub-aerial, like Iceland, this problem disappears because the isotopic exchange is with meteoric water which (particularly in polar climates) can result in very low exchange S1'0 values. In the event it would not be necessary to strip off all but the low S1'0 value rocks, but simply homogenize them. Note that with a smaller continental crust volume in the Archaean, and hence an average shallower ocean, it is more likely that elevated plateaus could become subaerial (cf. Abbott, 1954; Schubert and Reymer, 1985; Galer 1991). However, because of the latitude dependence of the oxygen isotopic composition of meteoric water, we should not expect all underplated altered ocean crust to have low S " 0 values. There is a fairly wide range in S1'0 reported from pyroxenite veins and eclogites thought to be derived from the lithosphere (Pearson et al., 1991), and which are interpreted as subduction components, and which reflect the diverse nature of material being subducted. This model is useful in quite a number of respects: (1) Because the Earth's upper mantle has evolved from PREMA composition to DMM with time, there is more likelihood of thick sub-aerial crust like Iceland being generated in the late Archaean, and therefore potentially more thick crust that would underplate rather than subduct. If dolerite dykes are linked to this underplate, is this why there is an abundance of dykes near old potential sites of plate subduction like the Nagssugtoqidian Belt (= Kangamiut dykes)? (2) The material is already a low-melting component when underplated. It is also hydrous. It would therefore be very vulnerable to flushing out in huge volumes with the development of any major thermal anomaly. This would explain the high primary hornblende contents of some Proterozoic dykes. It also accounts for the fact that they are Fe-rich since during melting they would not necessarily be in equilibrium with Mg-rich mantle. It obviates the need for sub-crustal magma chambers to hold the mafic magmas while they fractionate to Fe-rich compositions before emplacement. Some of the observed compositional variation in Proterozoic dolerites (which is always difficult to account for by fractional crystallisation) could, in fact, have been inherited from the underplated oceanic crust. (3) The isotopic data for the Scourie dykes (Waters et al., 1990) which give Sm-Nd whole rock "isochrons" of ca. 3.0 Ga, and indicate that the U-Pb systems were disturbed perhaps as much as 3.0 Ga ago, are perfectly consistent with the model as these would indicate the age of processes associated with the formation and emplacement (underplating) of the source. (4) If Iceland can be regarded as a typical product of a PREMA source, it should be noted that, despite the source being moderately 'depleted' with respect to Sm-Nd and Rb-Sr isotopic systems, the erupted lavas are dominantly LREE-enriched (see compilation in Walker, 1991, fig. 13.11). Hence an underplate

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of such material would experience retarded growth of 143Ndand evolve towards low 6Nd compositions (and high cSr) depending upon the time stored. Such characteristics are common in Proterozoic dykes (e.g. Waters e t al., 1990) as well as continental flood basalts. (5) The source of the dykes is relatively shallow, which is one of the requirements outlined earlier to account for the apparent lack of co-genetic volcanic suites. (6) It is now possible to explain the norite dykes as partial melting products of the sub-crustal mantle lithosphere. This source may have been harzburgitic because of previous melt extraction; alternatively, as silica is always mobile in the subduction environment, and as excess silica is liberated when low-silica hornblende- or garnet-assemblages develop in subducted mafic rocks, the mantle may have become harzburgitic because of silica metasomatism (olivine > orthopyroxene). THERMAL PROBLEMS IN DYKE GENERATION

With dyke swarms representing volumes of mafic magma of the order of 50000 to 100000 km3, there is a need for a major thermal source which has to be focused to provide the energy for melting. Moreover, if dyke swarms are emplaced over a very short time interval of not much more than 2-3 Ma, as implied by recent U-Pb dating (LeCheminant and Heaman, 1989), then it is necessary to turn the thermal tap on and off very quickly. Within the scenario outlined above, there are two possible ways in which this might be done.

Mantle pluvltes It has been common to appeal to mantle plumes to supply this energy for melting (White and McKenzie, 1989; Campbell and Griffiths, 1990). The difficulty is that hotspots like Iceland, Hawaii, Kerguelen or Cape Verde tend to remain active for many tens of Ma. How then can the intrusive pulse be shortened? First, many of the Earth’s major hotspots that are thought to represent deep mantle plumes, such as Iceland, Kerguelen, and Hawaii, as well as many or all of those which gave rise to the ocean plateaus (Larson, 1991), seem to have been initiated near spreading ridges, though, like Hawaii, they may later migrate off the ridge provided the mechanical boundary representing the base of the lithosphere can be raised and the magma conduit kept open. There is no inherent reason why plumes initiated in the lower mantle should be constrained by shallow-level plate boundaries near the Earth’s surface. Why do they not burn their way through the middle of plates? The reason must be that the mechanical boundary layer (MBL) beneath is too thick, and that it is only plumes that are rising near ridges that can reach the surface and turn their potential thermal energy into extrudable magma. This is not surprising: Watson and McKenzie

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(1991) have calculated that the MBL beneath Hawaii is 72 km thick and the melt-producing region only a further 55 km in vertical extent. The likelihood is that even major plumes rising beneath continents or thick ocean plates with a MBL perhaps well over 100 km thick may be unable to penetrate to the surface and simply contribute to a more fertile asthenosphere, with their energy converted into small degree melts which permeate and fertilise the lithosphere. In fact Storey et al. (1989) have suggested that the major Kerguelen plume has effectively contaminated much of the Indian Ocean asthenosphere in this way, and has contributed to the rather distinctive composition of Indian Ocean basalts. Hill (1991) has similarly argued that plumes cannot provide the ultimate driving force for continental break-up. This may be one reason why many major Proterozoic dyke swarms are closely linked to transtensional/transpressional shear zones: these provide vertical access. Without tectonic assistance, extensive adiabatic melting cannot take place, and the magma cannot penetrate upwards as dykes. The second and rather surprising point, well exemplified by studies of Mesozoic radial dyke swarms around the Cape Verde hotspot (Oliveira et al., 1990) or the development of Kerguelen (Storey et al., 1988), is that the material input to magma from the plume itself is very minor. The magmas, particularly those emplaced in the early stages, carry a strong lithospheric signature. Hence a very large proportion of the available energy is converted to lithosphere melting. There must be some very strong controlling factor here. A possible reason follows from the model discussed above. If the basaltic component in the lower lithosphere or upper asthenosphere is largely held in hydrous minerals, phlogopite, kaersutite or K-richterite, which have probably formed close to their stability limit just below the MBL, then any thermal perturbation may produce dehydration, and the fluids released so alter the rheological properties in the region below or even within the MBL so that advection replaces conduction, the whole zone becomes unstable, and large amounts of melt are available if tectonic conditions (shear zones or stretching) permit rise to the surface. The useful feature of this model is that it relies on hydrous phases to create the instability, and structural control to deliver the magma. Once the fluids are expelled along with the magma, the system becomes anhydrous, stability returns and it would take a great over-supply of thermal energy to de-stabilise it again. The dyke phase is short.

Sinking (negative)plumes What comes up must go down, and as it is density contrast that determines whether material will rise or sink, it is perfectly possible that inherently dense, but warm, mantle material might sink (cf. Griffiths and Turner, 1988). McKenzie and O’Nions (1983) suggested that portions of the subcontinental lithospheric keel might detach themselves and sink to provide ultimately an enriched source reservoir for alkali basalts. More particularly, Kroner (1981) has suggested that

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delamination and sinking of subcontinental lithosphere in a n ensiulic environment (“A-subduction”) may account for the apparent differences between Proterozoic and Phanerozoic orogenic styles, the latter being controlled by normal “Bsubduction” of oceanic lithosphere a t Benioff zones. The attractive feature of A-subduction is that the orogenic compression and crustal shortening can be achieved without having to account for the absence of ophiolites, blueschists and continental margin sedimentary sequences that normally characterise the Wilson Cycle. The further attractive feature is that as hot asthenosphere eventually rises to replace the sunken A-subducted lithosphere, a ready supply of thermal energy is provided to generate voluminous post-orogenic Proterozoic granites through melting of lower crust (cf. Houseman et al., 1981). The additionally useful factor as far as dykes are concerned is that there is a ready parallel between the boninites and island arc tholeiites commonly associated with the initial stages of B-subduction and the norites and tholeiites associated with A-subduction. However, a major problem with the A-subduction concept is that subcontinental lithosphere beneath A ch a e a n cratons is thought to be very refractory (Boyd, 1989) and inherently buoyant. Ellis (1992) has demonstrated that such lithosphere would need to be some 700°C cooler than the underlying asthenosphere for it to sink spontaneously. This effectively rules out A-subduction in normal circumstances; and as it is also argued that transformation to eclogite is unlikely to occur in any mafic material in the lower crust, there is little potential help from a mafic underplate either. However, if mafic material is emplaced into a rheologically weak lithosphere during the crustal growth phase, as implied by the Scourie dyke S ” 0 data, then transformation to eclogite is potentially possible at any later date (perhaps several hundred m.y. later), thus providing the enhanced density contrast necessary to initiate A-subduction (or a sinking plume). With A-subduction, any hornblende present in the assemblage would suffer pressureinduced breakdown, with release of fluids (as opposed to the temperature-induced breakdown caused by an uprising plume). It is this fluid, in combination with the hot uprising asthenosphere replacing the A-subducted material, which initiates melting. In physical terms the conditions are not unlike that of melting with induced convection in the mantle wedge of a modern subduction zone (Saunders et al., 1991), though the fluids would be less oxidised, and hence the magmas tholeiitic rather than calc-alkaline. Recent work by Foley (1991) has demonstrated that fluorine can substantially enhance the depth range over which hydrous minerals like pargasite, K-richterite and phlogopite are stable in the mantle, hence greatly increasing the amount of fluid that could be available for melting. With this model, the suggested correspondence between Proterozoic continental boninites (Hall and Hughes, tholeiite + norite dykes and modern arc tholeiite 1987) becomes rather more plausible, as is the fact that Proterozoic dykes share many chemical features (high LIL, low-Ti02, negative Nb anomalies) with subduction zone magmas. Equally importantly, because the amount of lithosphere that could be consumed by A-subduction is limited, this would constrain the time period of associated mafic dyke magmatism. The comparable situation in

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a modern arc system like the Marianas (cf. Tirney et al., 1981) is the period following initiation of subduction, where the rate of magma production is very high, and the volume of magma produced (Kyushu-Palau Remnant Arc) similar to that of a typical dyke swarm. COMPARISON WITH CONTINENTAL FLOOD BASALTS

The discussion above explores a number of different ways in which the compositional, volume and temporal characteristics of Proterozoic dyke swarms might be accounted for. Because of the close compositional similarities between Proterozoic dolerite dykes and Phanerozoic low-Ti02 continental flood basalts (CFB) it is interesting to compare petrogenetic models. There has been a progressive shift over the last decade away from petrogenetic schemes involving crustal contamination of magmas, and two of the most recent papers on the Gondwana CFB provinces (Hergt et al., 1991; Ellam and Cox, 1991) employ subducted sediments and lamproite liquids respectively, as lithosphere contaminants before extracting the CFB magmas. In neither the dominant low-Ti02 (Ferrar, Karoo, Parana, etc.) quartz tholeiites, nor the Karoo picrites can a plume “OIB” component be recognised, though it is apparent in Deccan basalts. The compositions of the predominant uniform low-Ti basalts are consistent with moderately high degrees of melting of a relatively fertile source at moderate water pressures, and a t shallow depths (no garnet), but the consistently low ENd and variably high csr (Hergt et al., 1991) suggest either that the enriched source is old or that the contaminant is old. Contamination of the lithosphere source with subducted sediment is convenient, but difficult to prove, in that where abundant sediment is being subducted beneath arcs, very little appears in the arc magmas (Hole et al. 1984), and subducted sediment can be used equally convincingly to produce other basalt compositions (Weaver, 1991). The fact that these low-Ti basalts have spidergram patterns very similar to average post-Archaean upper crust can be interpreted in two ways: either that their source is contaminated by continental sediment (= granite), or that Proterozoic granitoids were derived from sub-continental lithosphere with a trace element composition close to low-Ti basalts. The latter is a t least consistent with the basaltic underplating model for Australian Proterozoic granitoids of Etheridge et al. (1987). If not a result of sediment subduction, the problem remains how this high Rb/Ba, high Rb/Nb and low Ti/Y component of the low-Ti basalt source is generated. It is not the lamproite component of Ellam and Cox (1991), which has high Sr and Ba, and is a more suitable end-member component for high-Ti Gondwana basalts. However, the high Rb/Ba characteristic is typical of subduction zones, a result of selective transport of LIL elements by subduction fluids (Saunders et al., 1980). If the basaltic components are largely held in hydrous phases such as phlogopite, Krichterite or hornblende (Sudo and Titsumi, 1990), these normally exert a strong control on chemistry, particularly if fluids allow some open system behaviour.

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If slab dehydration continues to much greater depths than previously thought (Ringwood, 1990) then it is possible that hydrous fluids may permeate upwards scavenging the mantle in the back-arc region, ultimately to become trapped in phlogopite or K-richterite in the subcontinental tectosphere. The distinctive negative Nb anomalies may result from a titanite phase being stable in equilibrium with and during the migration of these hydrous fluids, thus sequestering Nb, lh, Ti, etc. No real explanations are yet forthcoming to explain the location, the size and volume, the timing and the thermal causes of Gondwana CFB volcanism. Cox (1978) noted the Parana, the Karoo and the Ferrar CFB provinces reside in the back-arc region along the active margin of the reconstructed Gondwana continent, forming a semi-continuous belt some 10000 km long (see Hergt et al., 1991, fig. 1). It is difficult to envisage how rising mantle plumes could account for this distribution, and indeed a plume signature is not much in evidence in the basalt chemistry, as noted above. However, it is not difficult to imagine large segments of hot over-thickened ocean plateau crust being injected into and beneath the immature lithosphere of that Gondwana margin in perhaps the Late Proterozoic. This material is then available to be mobilised some 0.6 Ga later by plumes, rising or sinking, or during the general disturbances associated with the breakup of Gondwanaland. CONCLUSIONS

Dyke swarms in the Early Proterozoic include mainly low-Ti quartz dolerites and Mg-rich norites, both of rather consistent composition, which have been generated from two different sources. Crustal contamination does not seem to be an important factor in their petrogenesis, nor can a plume o r asthenospheric source component be recognised except in later Proterozoic dyke swarms. Proterozoic dyke magmas share with Phanerozoic continental flood basalts the severe thermal and tectonic problems of generating huge volumes of uniform and distinctive melts from the mantle system in a relatively short time span. Data for the Scourie dykes suggest that the source for the dolerite magmas may be slivers of warm over-thickened ocean plateau crust that were too buoyant to subduct but were injected into the lithosphere beneath the newly developing continent, and mobilised some 0.6-1.0 Ga later when thermal and tectonic conditions were favourable. The norites are products of melting of silica-metasomatised or refractory harzburgitic mantle. Hydrous minerals in the source (amphibole f phlogopite for the dolerites and phlogopite for the norites) are important in controlling the chemistry and in providing the mechanism to generate large volumes of melt relatively quickly. Later Proterozoic dykes reflect the addition of more alkaline components to the lithosphere. Whereas uprising deep mantle plumes can provide the thermal energy to mobilise the dyke magmas, they must entrain major amounts of lithosphere to satisfy the compositional constraints, and it is not easy to turn the thermal

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tap off. An alternative mechanism of foundering and sinking of stored mafic ocean plateau underplate (as it converts to eclogite) could also provide short-term energy for melting as fluids are released and the sinking mass is replaced by hot asthenosphere.

ACKNOWLEDGEMENTS

The ideas in this paper arose from discussions over many years with colleagues and students, notably Barry Weaver, Andy Saunders, Mike Norry, Elson Oliveira, B l a t Ahmad and Andy Cadman. Very helpful comments on the manuscript were provided by Andy Cadman, Kent Condie, Peter Hall, Ray Kent, Andy Saunders, Shen-su Sun and John Sheraton.

REFERENCES Abbott, D.H., 1984. Archaean plate tectonics revisited, 2. Paleo-sea level changes, continental area, oceanic heat loss and the area - age distribution of the ocean basins. Tectonics, 3: 709-722. Ahmad, T and Tarney, J., 1991. Geochemistry and petrogenesis of Garhwal Volcanics: implications for evolution of the north Indian lithosphere. Precambrian Res., 50: 69-88. Bkdard, J.H., 1989. Disequilibrium mantle melting. Earth Planet. Sci. Lett., 91: 359-366. Boyd, ER., 1989. Compositional distinction between oceanic and cratonic lithosphere. Earth Planet. Sci. Lett., 9 6 1.5-26. Brooks, C.K., Larsen, L.M. and Nielsen, TED., 1991. Importance of iron-rich tholeiitic magmas at divergent plate margins: a reappraisal. Geology, 19: 269-272. Cadman, A., Tarney, J. and Park, R.G., 1990. Intrusion and crystallisation features in Proterozoic dyke swarms. In: A J . Parker, P.C. Rickwood and D.H. Tucker (Editors), Mafic Dykes and Emplacement Mechanisms. A.A. Balkema, Rotterdam, pp. 13-24. Cadman, A.C., Park, R.G., Tarney, J. and Halls, H.C., 1992. Significance of anisotropy of magnetic susceptibility fabrics in Proterozoic mafic dykes, Hopedale Block, Labrador. Tectonophysics, 207: 303-314. Campbell, I.H. and Griffiths, R.W., 1990. Implications of mantle plume structure for the evolution of Rood basalts. Earth Planet. Sci. Lett., 99: 79-93. Cartwright, I. and Valley, J.W., 1991. LOW-'~OScourie dike magmas from the Lewisian complex, northwestern Scotland. Geology, 1 9 578-581. Chapman, H.J., 1979. 2390 Myr Rb-Sr whole rock age for the Scourie dykes of north-west Scotland. Nature, 277: 642-643. Condie, K.C., Bobnow, D.J. and Card, K.D., 1987. Geochemistry of Precambrian mafic dykes from the southern Superior Province. In: H.C. Halls and W.F. Fahrig (Editors), Mafic Dyke Swarms. Geol. Assoc. Can., Spec. Pap., 34: 95-108. Cox, K.G., 1978. Flood basalts, subduction and t h e breakup of Gondwanaland. Nature, 274: 47-49. Cox, K.G., 1988. Numerical modelling of a randomized RTF magma chamber: a comparison with continental Rood basalt sequences. J. Petrol., 2 9 681-697. Crawford, A.J., Falloon, TJ. and Green, D.H., 1989. Classification, petrogenesis and tectonic setting of boninites. In: AJ. Crawford (Editor), Boninites. Unwin Hyman, London, pp. 2-49.

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