LIP Reading: Recognizing Oceanic Plateaux in the Geological Record

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 7 PAGES 1041–1056 2000

ANDREW C. KERR∗, ROSALIND V. WHITE AND ANDREW D. SAUNDERS

DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, UNIVERSITY ROAD, LEICESTER LE1 7RH, UK

RECEIVED SEPTEMBER 28, 1999; REVISED TYPESCRIPT ACCEPTED MARCH 2, 2000

Basaltic oceanic plateaux are important features in the geological association of pillow basalts, sheeted dykes, and gabbroic record. Not only do they record ancient mantle plume activity, but and other ultramafic intrusions represent preserved cross- they also are believed to be important building blocks in the formation sections through oceanic crust and upper mantle (e.g. of the continental crust. In this paper we review the salient features Cann, 1970), which have been obducted and accreted of two Cretaceous oceanic plateaux (the Ontong Java and the onto the continental margins. Soon, however, it became Caribbean–Colombian): thick sequences of predominantly homo- evident that oceanic crust of this type formed not only geneous basalt; the occurrence of high-MgO basalt, including at mid-ocean ridges but also at small spreading centres komatiites; and an apparent absence of sheeted dyke complexes. In in back-arc basins associated with volcanic arc systems addition, pyroclastic deposits may be scarce. We then explore ways (e.g. Karig, 1971; Tarney et al., 1977; Watts et al., 1977). of distinguishing plateaux from basaltic sequences erupted in different The main ways of distinguishing between ophiolites tectonomagmatic settings: continental flood basalt provinces; island formed in these different tectonic environments have arcs; back-arc basins; ocean islands and mid-ocean ridges. Using been based mostly on the geochemical signatures or these criteria, potential Archaean and Proterozoic oceanic plateaux affinities of lava sequences (e.g. Pearce & Cann, 1971, are reviewed and identified. Finally, we explore how these remnant 1973) as well as on the nature of associated rocks. oceanic plateaux became incorporated into the continents, by reviewing Although tectonic discrimination diagrams based on geo- the proposed accretion mechanisms for the Cretaceous Caribbean– chemical composition have been much abused in the Colombian oceanic plateau, on the basis of evidence from South geological literature, they are nevertheless a useful aid to America and the tonalites of the southern Caribbean island of deciphering the tectonic setting of volcanic suites, pro- Aruba. vided that they are used only as part of a framework that is also based solidly on other evidence. The ophiolite model has thus served the geological community well over the last 30 years. However, like all KEY WORDS: oceanic plateau; basalt geochemistry; large igneous provinces; scientific ideas, it must be modified to take account of new plumes discoveries. One such discovery has been the existence of oceanic plateaux. In the mid-1970s it became apparent that, although most of the ocean crust is 6–7 km thick, there are several regions of the sea floor, e.g. the Ontong INTRODUCTION Java Plateau (Kroenke, 1974) and the Caribbean plate One of the most successful paradigms in attempting to (Donnelly, 1973), where the ocean crust has a thickness decipher the origin of mafic igneous rocks in the geological well in excess of 10 km. As first proposed by Kroenke record has been the ophiolite model. This was developed (1974) and later discussed by Burke et al. (1978), Ben- in the early 1960s with the formulation of the theory of Avraham et al. (1981) and Nur & Ben-Avraham (1982), plate tectonics and the realization of the importance of these oceanic plateaux, because of their thickness (and, sea-floor spreading. It was quickly realized that the spatial if <20 my have elapsed between formation and attempted

∗Corresponding author. Telephone: +44-116-2523638. Fax: +44- 116-2523918. e-mail: [email protected]  Oxford University Press 2000 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 7 JULY 2000

subduction, their residual heat) are inherently more buoy- with a diameter of the order of 1000 km, producing ant than oceanic crust of normal thickness. a large igneous province (LIP). Oceanic plateaux, like This buoyancy means that some of these plateaux have continental flood basalts (CFB) (Cox, 1980), are a variety resisted complete subduction and have been accreted of LIP (Coffin & Eldholm, 1994) although, as we shall onto the margins of continents. They have thus been discuss below, there are some fundamental differences implicated by many researchers in the growth of the between them. continental crust (Kroenke, 1974; Ben-Avraham et al., At this point it is important to mention that the crustal 1981; Nur & Ben-Avraham, 1982; Abbott & Mooney, thicknesses of these plateaux vary, both in space and 1995; Saunders et al., 1996; White et al., 1999). As shown time. Higher mantle temperatures in the Archaean imply by Tejada et al. (1996) and Kerr et al. (1998), it is not that mantle plumes would also have been hotter than at only the uppermost basaltic layers of these plateaux that the present day (e.g. Nisbet et al., 1993). This would not obduct but also, if conditions are favourable, the deeper only have resulted in hotter and thicker normal oceanic intrusive sections. The inherent difficulty in subducting crust, but could have also resulted in even thicker (and thick, buoyant plateau crust means that it is more likely so even less subductable) oceanic plateaux. than other oceanic igneous rocks to be accreted onto the continents and preserved in the geological record. Consequently, in the interpretation of accreted igneous CASE STUDIES OF TWO terranes within the continents, oceanic plateaux must CRETACEOUS OCEANIC PLATEAUX now also be regarded as a potential tectonic setting The Cretaceous period was a time of intense plume- alongside back-arc basins, oceanic arcs, continental vol- related igneous activity, with the eruption of at least eight canism, mid-ocean ridges and oceanic islands (‘hotspots’). LIP, of both continental and oceanic affinity (Coffin& The possibility of an oceanic plateau origin for accreted Eldholm, 1994). Most of this igneous activity occurred mafic rock sequences is often one that is not considered. between 130 and 70 Ma, with major peaks of activity Therefore, our aim in this paper is not only to heighten the around 122 Ma and 90–88 Ma (Mahoney et al., 1993a; awareness of oceanic plateaux as potential contributors to Tejada et al., 1996; Kerr et al., 1997a; Sinton et al., the growth of the continental crust, but also to review 1998). The majority of these Cretaceous LIP are oceanic much of our current understanding of the chemical and plateaux and include the Kerguelen Plateau in the Indian geological features of these sequences, to assist in the Ocean; the (now) Caribbean Plateau (formerly in the identification of oceanic plateaux in the geological record. eastern Pacific); and the Ontong Java Plateau, the Our first objective will be to highlight the unique Manihiki Plateau, the Shatsky Rise, the Hess Rise and features of oceanic plateaux, that is, those that distinguish the Mid-Pacific Mountains, in the western Pacific. them from ophiolites formed in other plate tectonic Although deep-sea drilling [via the Deep Sea Drilling settings. Second, we will discuss the mechanisms by which Project (DSDP) and Ocean Drilling Program (ODP)] oceanic plateaux become incorporated into the continents and remote geophysical surveys (e.g. gravity, seismic and and how they ‘ripen’ with time into mature continental magnetic) of Cretaceous plateaux have been carried out, crust. these techniques really give us only a sketchy understanding of their structures. In particular, recovery of material is restricted to the upper 100 m or so of the THE FORMATION OF OCEANIC basement, <0·3% of the total crustal thickness. However, over the past 5 years, our knowledge of the formation, PLATEAUX structure and composition of oceanic plateaux has in- One of the single most distinctive features of oceanic creased dramatically through detailed studies of the tec- plateaux is their crustal thickness. In the case of Iceland tonically uplifted margins of two of these Cretaceous (Staples et al., 1997) and the Ontong Java Plateau (Glad- plateaux, namely the Caribbean Plateau [here termed zenko et al., 1997), this exceeds 30 km—approaching the the Caribbean–Colombian oceanic plateau (CCOP) be- thickness of continental crust. McKenzie & Bickle (1988) cause of its outcrop in western Colombia] and the Ontong have shown that, to produce such crustal thicknesses, an Java Plateau (OJP). Obduction occurred during at- elevated mantle potential temperature (Tp) above that of tempted plateau subduction and has resulted in the ambient mantle (1280°C) is required. One way of raising deeper levels of these two plateaux being exposed. ° mantle Tp is to invoke a hot plume (up to 200–300 C hotter than ambient mantle) ascending from a thermal discontinuity in the mantle (e.g. Campbell & Griffiths, Caribbean–Colombian Oceanic Plateau 1990). Initial decompression melting in the head of one (CCOP) of these plumes as it flattens along the base of the oceanic It is now widely accepted that the CCOP formed in the lithosphere results in the eruption of basalts over an area eastern Pacific on the Farallon plate during the mid- to

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late Cretaceous (Burke et al., 1978; Duncan & Hargraves, 1984; Pindell & Barrett, 1990). Less than 5 my after the major formational phase (>88 Ma), the eastward movement of the Farallon plate brought the young, still hot plateau into collision with the proto-Caribbean arc (White et al., 1999) and the NW margin of South America, although the timing of this latter event is not well con- strained (Kerr et al., 1997b). This emplacement of the CCOP into the proto-Caribbean region, and its obduction onto the Colombian and Ecuadorian coast, resulted in the uplift and exposure of deep sections of the plateau around the Caribbean and along the north- western edge of South America. Only a brief review of the CCOP will be given in this paper. All the pre-1996 work on the exposed sections of the CCOP has been summarized by Donnelly et al. (1990) Fig. 1. Plot of Nb/Zr vs MgO showing: gabbros (fields with dashed and Kerr et al. (1996c, 1997b). More recent work on the outlines) from Gorgona and mainland Colombia (Nivia, 1987; A. C. plateau has focused on 40Ar/39Ar dating (Sinton et al., Kerr, unpublished data, 1996); Gorgona komatiites, picrites and basalts 1998), isotopic characteristics (Walker et al., 1999; Hauff (Kerr et al., 1996a; Arndt et al., 1997); picrites and basalts from Curaçao (Kerr et al., 1996b); picrites and basalts from western Colombia (Kerr et al., 2000), the structure of the plateau (Kerr et al., 1998) et al., 1997a); basalts from the Ontong Java Plateau (Mahoney et al., and the gabbros of the province (Re´villon et al., 2000). 1993b) and the Kerguelen plateau (Storey et al., 1992). The CCOP is arguably one of the best exposed and documented oceanic plateaux in the world. Field ob- servations of accreted oceanic plateaux suggest that when and Lan/Ndn and Smn/Ybn ratios of 0·6–1·1 and 0·8–1·4, they are emplaced onto the continental margins, usually respectively (Fig. 3). Radiogenic isotope ratios for the only the uppermost basaltic pillow lavas and shallow- high-MgO lavas are also much more variable than for level dolerite intrusions are obducted (Kimura & Ludden, the basalts (Fig. 4), with (for example) the high-MgO + + 1995). However, the exposed sections of the CCOP lavas having Ndi values ranging between 6 and 12, comprise not only pillow basalts and dolerite sheets, but whereas the basalts possess a much smaller range of Ndi also high-MgO lavas (picrites and komatiites), gabbros values. and ultramafic rocks (Fig. 1) that have been interpreted The increased heterogeneity and higher MgO content as representing the deeper crustal levels of the plateau of the lavas at deeper levels within the plateau are (Nivia, 1996; Kerr et al., 1998). These assemblages re- believed to be related. This is because, as noted by Kerr semble parts of the standard ophiolite model. It has been et al. (1995b), the mantle plume source region of oceanic suggested that the reason for the obduction of these plateaux is heterogeneous. As the magmatic system above deeper sections is that the CCOP commenced its collision a plume evolves, it is likely that large magma chambers with the Proto-Caribbean arc <5 my after its formation, will develop, which can ‘trap’ the more primitive, dense while it was still relatively hot and buoyant (Kerr et al., and heterogeneous magmas. This entrapment results in 1998). This excess buoyancy may have meant that the mixing and crystal fractionation of the heterogeneous CCOP was even less subductable, and so greater uplift picritic magmas and the eruption of the relatively homo- occurred during emplacement. Alternatively, the differ- geneous basalts of oceanic plateaux. ence may be related to the level at which the detachment One notable feature of the obducted sequences of the from the rest of the plateau occurred (see Kimura & CCOP (and indeed the OJP) is the lack of a sheeted dyke Ludden, 1995). complex, which often characterizes ophiolites formed in The high-MgO picrites and komatiites are more back-arc basins and at mid-ocean ridges. This may be heterogeneous in terms of their incompatible trace ele- due to several factors (see, e.g. Saunders et al., 1996). It ment contents and Sr–Nd–Pb isotope ratios than the may simply reflect the tectonic setting of plateaux: if they basalts of the CCOP (Kerr et al., 1996a, 1996b), or the formed away from a spreading axis, on top of pre-existing basalts from the OJP (e.g. Babbs, 1997). Examples of crust, then there is no a priori reason for a sheeted dyke this heterogeneity can be seen in Fig. 2, with the high- complex. Alternatively, a plateau may form at a spreading MgO lavas having Nb contents that range from 2 to 25 centre, like Iceland, where any near-surface dyke systems ppm, and Lan/Ndn and Smn/Ybn ratios of 0·3–1·4 and are likely to be buried several kilometres beneath suc- 0·5–3·5, respectively (Fig. 3) (Smn/Ybn denotes chondrite- cessive accumulations of magma (Pa´lmason, 1986). The normalized Sm/Yb). In contrast, the basalts of the prov- observation that oceanic plateaux do not seem to possess ince possess Nb contents that range from 3 to 7 ppm, sheeted dyke complexes is a useful aid to identifying

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Fig. 2. (a) Chondrite-normalized REE and (b) primordial mantle normalized multi-element (Sun & McDonough, 1989) plots showing the compositional range for: basalts and picrites from the CCOP (vertical lines); basalts from the OJP (stippled fill); and basalts from the Kerguelen Plateau (data lie between dotted lines). Also shown are Fig. 3. Plot of (a) Lan/Ndn vs Smn/Ybn and (b) Zr vs Nb. Data sources enriched basalts, picrites and komatiites from Gorgona. Data sources as in Fig. 1. Chondrite normalizing values from Sun & McDonough as in Fig. 1. (1989). OJP, Ontong Java Plateau. these plateaux in the geological record. (However, the portion of the CCOP is at least 7 km (Klaver, 1987), identification of the eruption conduits of oceanic plateaux which is much thicker than normal oceanic crust or in remains a problem.) Finally, the geochemistry of the ffi gabbros of the province clearly shows that they are most Phanerozoic ophiolites. It may be more di cult, genetically related to the basalts and picrites (Kerr et al., however, to distinguish early Archaean oceanic crust 1998). from a Proterozoic or Phanerozoic oceanic plateau. The To conclude, we can ask the question: how would we higher potential temperatures in the Archaean (e.g. recognize the CCOP if we observed it, preserved, in 3 Richter, 1988) probably resulted in substantially thicker by time? First, the occurrence of picrites and komatiites oceanic crust, perhaps approaching the thickness (tens of should alert us to the possibility of an oceanic plateau. kilometres) of recent plateaux. The lithologies may also Second, submarine eruptions are indicated by pillow have been similar. Archaean plateaux, by analogy with basalts. Third, the chemistry of the basalts [especially modern examples, and if produced above Archean mantle the flat, almost chondritic rare earth element (REE) plumes, may well have been substantially thicker than patterns] is diagnostic. Fourth, the lack of substantial those we see today, and one of several settings for the tephra layers and sheeted dyke complexes may suggest formation of high-MgO komatiites (e.g. Storey et al., an oceanic plateau. Finally, the thickness of the extruded 1991).

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subduction zone, i.e. the docking of the OJP with the island arc resulted in a reversal in the polarity of subduction from west to east. The OJP has been tectonically uplifted and exposed only in the Solomon Islands (mostly in Malaita and Santa Isabel), and the rest of our geochemical knowledge of the plateau is based on one DSDP and two ODP drill sites, which penetrated to a maximum depth of 149 m into basaltic crust (Mahoney et al., 1993b). Thus we know significantly less about the structure and composition of the OJP than we do about the CCOP. Possibly as a result of the poor exposure of the plateau, and because this exposure appears to be restricted, in Malaita at least, to the uppermost 1–2 km of the original plateau sequence (Babbs, 1997), most of the analysed plateau lavas (both pillowed and massive) possess a very uniform composition (Mahoney et al., 1993b; Tejada et al., 1996; Babbs, 1997). Interestingly, all of the recovered sections suggest that eruption of basalt occurred in the submarine environment, with massive flows and pillow basalts being the predominant lithologies. Volcaniclastic rocks are rare, at least in the 122-my-old sequences, and intercalated sediments within the basalt pile are virtually absent. In the previous section, we noted the trace element heterogeneity and the highly variable MgO contents of the lavas of the CCOP. In stark contrast, however, the exposed and drilled lavas of the OJP are entirely basaltic in nature, with >95% of samples possessing a relatively narrow range in MgO content of between 6 and 8·5 wt % (Fig. 1). Furthermore, the samples from the OJP are 87 86 Fig. 4. Plot of (a) Ndi vs ( Sr/ Sr)i and (b) Ndi vs MgO for basalts, remarkably homogeneous in terms of their trace element picrites and komatiites from the CCOP, and basalts from the OJP and contents (Fig. 2), and like the majority of the basalts from Kerguelen plateau. Data sources as in Fig. 1. the CCOP, possess relatively flat patterns on chondrite- normalized REE plots, and primordial mantle normalized multi-element plots (Fig. 2). This homogeneity is similarly Ontong Java Plateau (OJP) reflected in the isotopic signatures (Fig. 4), which display The OJP is the largest known oceanic plateau, covering a relatively restricted range in values compared with the × 6 2 an area of >1·5 10 km . It consists of two main parts: CCOP. For example, the OJP basalts range in Ndi from the western and northern lobe, believed to have formed +6to+3 (Fig. 4), and in 206Pb/204Pb from 18·2 to 18·7. at >122 Ma, and the smaller eastern lobe, which appears As a result of a detailed petrological study of the basalts to have formed at >90 Ma (Mahoney et al., 1993b; of Malaita, Babbs (1997) proposed that the mantle plume Tejada et al., 1996; Babbs, 1997; Neal et al., 1997). The source region of the Ontong Java Plateau was, like the total crustal thickness of the OJP is thought to be in source region of the CCOP, heterogeneous and that the excess of 30 km, although estimates from seismic and relatively homogeneous nature of the basalts was due to gravity surveys vary between 25 and 43 km thick (Neal mixing either in the melt column or in magma chambers. et al., 1997). From 90 Ma, the OJP moved westwards with Although the deeper levels of the OJP are not exposed, the Pacific plate. However, at >25 Ma, the westward- several recent reinterpretations of seismic velocity data moving plateau collided with the Solomon Islands arc, have resulted in various models of the crustal structure thus clogging the westward-dipping subduction zone of the plateau being proposed (Farnetani et al., 1996; (Petterson et al., 1999). This resulted in tectonic em- Gladczenko et al., 1997). It has been suggested that the placement of the OJP on to the Solomon Islands (Pet- middle crust consists of olivine gabbros that represent terson et al., 1999) and the uplift and exposure of the the magma chambers of the plateau. The lower crust deeper parts of the plateau (Mahoney et al., 1993b; Tejada of the plateau, having compressional P-wave velocities et al., 1996; Babbs, 1997). Continued plate movements >7·1 km/s, has been interpreted as being composed of eventually resulted in the initiation of an eastward-dipping cumulates from the fractionation of the primary picritic

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melts of the mantle plume (Farnetani et al., 1996; Glad- spatially localized occurrences of volcaniclastic-dom- czenko et al., 1997). Furthermore, Gladczenko et al. (1997) inated sequences. proposed that deformation and hydrothermal fluid in- vasion may have recrystallized the lower-crustal cumulates into garnet granulite, which would have a similar Oceanic plateaux and Phanerozoic oceanic compressional P-wave velocity to olivine cumulates. crust On first appearance, the observation that normal thickness ocean crust appears to be readily subductable at the present day suggests that only in exceptional cir- LIP READING THE GEOLOGICAL cumstances will it become accreted to the continents and RECORD: RECOGNIZING OCEANIC so become preserved in the geological record (e.g. Cloos, 1993). However, in the Archaean, the Earth’s mantle PLATEAUX was substantially hotter than at the present day (Nisbet Although chemical discriminants are useful in the assess- et al., 1993), and a corollary of this is that more melt ment of original tectonic setting of a suite of igneous was generated at mid-ocean ridges. ‘Normal’ Archaean rocks, they cannot, and must not, be the only means. In oceanic crust would therefore have been thicker, and so this section we will, therefore, focus not only on chemical potentially less subductable than Phanerozoic oceanic discriminants but also on geological means of identifying crust. Thus, although more recently formed oceanic crust oceanic plateaux. We will look at how to distinguish is unlikely to be preserved in the geological record, the oceanic plateau sequences from those of volcanic arcs, more buoyant Precambrian oceanic crust may have been mid-ocean ridges, marginal (or back-arc) basins, ocean obducted, and so when considering these older sequences islands, continental flood basalt provinces and seaward- this possibility should be borne in mind. dipping reflector sequences. Table 1 summarizes the Present-day mid-ocean ridge and oceanic plateau basalt chemical and geological features that can be used to help sequences have much in common, and there is a con- in the identification of oceanic plateaux in the geological tinuum between them. Both types of lava can be pillowed record, and these will be amplified in the following but because of uplift (both thermal and dynamic: e.g. sections. Ito & Clift, 1998) oceanic plateau volcanoes can erupt subaerially, in a manner analogous to Tertiary to Recent plume volcanism on Iceland. Therefore a basaltic sequence in the geological record that preserves evidence Oceanic plateaux and volcanic arcs of emergent to subaerial eruption (conglomerates, fossil Arc rocks are relatively easy to distinguish from oceanic soil horizons, vesicles, accretionary lapilli, tuffaceous plateau sequences on the basis of geochemistry. Condie horizons, reworking in shallow water) in combination (1999) has recently reviewed the chemical distinctions with pillow basalts may well represent an oceanic plateau between these two types and has suggested several chem- sequence. However, many of these lithological as- ff ical discriminants. One of the most e ective is Lapmn/ sociations are also found in an arc settings and so it Nbpmn (where Nbpmn is primordial mantle normalized Nb is vital to consider these features in conjunction with abundance). In Fig. 5a it can be seen that the Lapmn/ geochemistry (see above). Nbpmn ratio of Cretaceous oceanic plateaux (CCOP and In contrast, eruption at mid-ocean ridges will occur in OJP) is always <1·1, whereas modern and ancient con- deep water, and never subaerially, and so any intercalated tinental and oceanic arcs all possess Lapmn/Nbpmn ratios sediments in such sequences will be deep-water sediments. that are >1·1. Some basalts recovered from the Kerguelen However, basalts forming the OJP were clearly erupted Plateau, especially those from the southernmost parts of in a marine, and possibly deep marine environment the plateau, have slightly higher Lapmn/Nbpmn than do (deeper than the Aptian calcium carbonate compensation basalts from the OJP or CCOP, possibly because of depth), so the absence of shallow water criteria does involvement of subcontinental lithospheric mantle, or not preclude an oceanic plateau origin. Furthermore, even continental crust (e.g. Storey et al., 1989). This obducted sequences of Cretaceous oceanic plateaux do diagnostic feature is perhaps one that is best observed not possess sheeted dyke complexes, which are believed on a primordial mantle normalized multi-element plot, to be typical of oceanic crust formed at mid-ocean ridges where the high Lapmn/Nbpmn ratio of arc rocks is graph- (Saunders et al., 1996; Kerr et al., 1997b, 1998). ically represented as a dip in the relative abundance of From the above discussion it is clear that the geological Nb (Fig. 5b). In addition to this, arc rocks are generally discriminants of oceanic plateaux may be ambiguous, more evolved and contain more volcaniclastic layers especially in ancient terranes. It is for this reason that than oceanic plateaux, although subaerial emergence of we must turn to geochemical criteria to distinguish mid- oceanic plateau volcanoes can result in temporally and ocean ridge from plateau sequences.

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Table 1: Diagnostic geochemical and geological characteristics of volcanic sequences from diﬀerent tectonic settings

Tectonic setting High-MgO Lapmn/Nbpmn (REE)cn pattern Pillow lavas Tephra Subaerial lavas (>14%) layers eruption

Oceanic plateau yes Ζ1 predominantly flat may be common very few occasionally (e.g. OJP) or absent (e.g. Kerguelen) Mid-ocean ridge rare Ζ1 LREE depleted common very few no Marginal basin rare Ζ1 predominantly flat common very common no Oceanic island rare Ζ1 predominantly LREE present very few frequently enriched Volcanic rifted yes contain flat to LREE enriched not all lavas are occasional common margin sequences pillowed with Ζ1 and q1 Arc (continental and rare q1 LREE enriched not all lavas are very common frequently oceanic) pillowed Continental flood yes mostly q1; flat to LREE enriched usually absent occasional always basalt <10% of flows Ζ1 pmn, primitive mantle normalized; cn, chondrite normalized.

Most Cretaceous oceanic plateau lava sequences pos- is high, because they are on the upper plate and hence sess relatively flat chondrite-normalized REE patterns more likely than normal oceanic crust to be obducted > (Lan/Ndn 1) and can contain picrites and komatiites, during continent–arc or arc–arc collisions. For this whereas most mid-ocean ridge basalt (MORB) suites reason, many ophiolite complexes are considered as have depleted light REE (LREE) and rarely contain high- having formed in a supra-subduction zone setting. MgO lavas. Although it is possible for plume-derived Depending on the nature of the mantle below the lavas to be depleted in the LREE (Kerr et al., 1995b; back-arc basin, two fundamental compositional types of Fitton et al., 1997), LREE-depleted oceanic plateau lavas back-arc basin lavas can form. First, the mantle can be can generally be recognized by their higher MgO content composed of a similar material to the source region of and/or their higher Nb/Y ratios than MORB (Fig. 5c). MORB (e.g. Saunders & Tarney, 1991). Although these As mentioned above, it may be difficult to identify and basalts possess relatively flat chondrite-normalized REE characterize Archaean oceanic crust, which will have patterns they can be distinguished from oceanic plateaux enhanced thickness and an increased amount of high- by virtue of their lower Nb/Y ratios (Fig. 6b). Second, MgO rocks (as a result of higher mantle source tem- more trace element enriched, plume-derived material peratures), and perhaps a less depleted composition. can also make up the mantle source region of back-arc basins (Leat et al., 2000) and, as a result, the erupted basalts appear similar in trace element contents to oceanic Oceanic plateaux and back-arc basins plateau basalts and plot with the plateaux and intra-plate Basalts erupted in back-arc basin (BAB) settings are ocean islands on a Zr/Y–Nb/Y plot (Fig. 6b). We can, potentially the most difficult to distinguish from oceanic however, employ other geological features to distinguish plateau basalts. Often they possess flat chondrite-nor- an oceanic plateau basalt sequence from a back-arc basin malized REE patterns, and frequently are found as pillow sequence. First, the proximity of back-arc basins to an lavas (Saunders & Tarney, 1984). Additionally, like mid- explosively erupting island or continental arc volcano ocean ridges, the small spreading centres related to increases the likelihood that back-arc basin sequences volcanic arc activity may produce crust of a similar will contain more abundant tephra layers than oceanic > ° thickness to oceanic crust, and sheeted dykes may also plateaux. Second, the lower temperature (Tp 1280 C) be present. The preservation potential of back-arc basins of the mantle below a back-arc basin when compared

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° with a mantle plume (Tp >1400 C) means that, as at mid-ocean ridges, the eruption of high-MgO lavas is relatively rare. This higher mantle temperature for oceanic plateaux results in the basalts generally possessing higher Ni and Cr contents (at a given MgO content) than basalts formed in a back-arc basin (Fig. 6a). In Fig. 6a, most of the back-arc basin lavas plot in a separate field parallel to the trend displayed by the oceanic plateau lavas. One of the major differences between MORB and BAB basalts is the presence of a supra-subduction zone signature in some BAB basalts, reflected in increased contents of volatiles and large-ion lithophile elements (e.g. Rb, Ba, Th; Saunders & Tarney, 1991). BAB basalts tend, therefore, to have higher Ba/Nb and Th/Nb ratios than unaltered plateau basalts, although this difference may be obliterated during secondary alteration.

Oceanic plateaux and ocean islands In some respects, ocean islands are the small-scale equivalents of oceanic plateaux. In general, the magmatic flux rates at ocean islands are substantially less than those estimated for plateaux. There is, however, effectively a continuum between plateaux and islands, controlled by mantle flux rates and the thickness of the lithospheric cap. Magmatism at ocean islands tends to be focused, which, on moving plates, forms chains of islands and seamounts, and aseismic ridges. Many oceanic islands exhibit considerable magmatic differentiation, resulting in formation of rhyolites or phonolitic sequences and their pyroclastic equivalents. Not all do, however; Hawaii, for example, erupts very few silicic differentiates and, conversely, some plateaux also contain some differ- entiated lavas (e.g. Iceland, and the Kerguelen Plateau: Frey et al., 2000). Chemical data can be used to distinguish plateaux from ocean islands. Many of the lavas that erupt above hotspots are relatively enriched in incompatible trace elements and depletion of the heavy REE (HREE) suggests deep melting within the garnet stability field (Fig. 6c). Although a few oceanic plateau lavas from the CCOP are enriched in incompatible trace elements and show evidence of melting in the garnet stability field, the vast majority of lavas from oceanic plateaux possess flat REE patterns, indicative of shallow, or, alternatively, high- degree melting. A predominance of lavas, therefore, with Fig. 5. (a) Frequency diagram of Lapmn/Nbpmn (primordial mantle normalized: Sun & McDonough, 1989) showing the distribution of an enriched trace element signature (high values of Smn/ data for the CCOP and OJP (data sources as in Fig. 1), Kerguelen Ybn, Nb/Zr and La/Y) would suggest that the sequence Plateau and oceanic and continental arc rocks [see also Lassiter & is part of an oceanic island rather than a plateau. DePaolo (1997)]. (b) Primordial mantle normalized multi-element plot showing data fields for representative arc rocks and OJP basalts. (c) Plot of Nb/Y vs Zr/Y [after Fitton et al. (1997)] showing data from the CCOP, OJP and Kerguelen Plateau, together with fields for arc Oceanic plateaux and continental flood lavas and MORB (East Pacific Rise: Mahoney et al., 1994). Data sources basalt provinces (including volcanic rifted as in Fig. 1; representative arc data from Marriner & Millward (1984) margins) and Thirlwall et al. (1996). ‘Iceland Array’ represents bounding limits for basalts and picrites from the neovolcanic zones, Iceland (Fitton et The lavas of CFB provinces are susceptible to erosion as al., 1997). a consequence of dynamic uplift from the plume and

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remnant uplift from magmatic underplating (Cox, 1989). Therefore, unless buried by sediment, CFB may not survive in the geological record. What do survive, however, are the dykes and vents through which these lavas erupted, and several Precambrian CFB provinces have been identified on the basis of their remnant dyke swarms (e.g. LeCheminant & Heaman, 1989; Cadman et al., 1994). In contrast to oceanic plateaux, continental flood basalts are usually not pillowed, and may show extensive evidence of subaerial eruption, in the form of intercalated terrestrial sediments, weathered horizons and initial eruption onto an eroded land surface. Furthermore, because many continental flood basalts erupt through continental lithosphere, they may become contaminated en route to the surface (e.g. Fodor et al., 1985; Thompson et al., 1986; Kerr et al., 1995a; Baker et al., 1996). This contamination results in the enrichment of the large ion lithophile elements leading to elevated Ba/Nb (primordial mantle normalized Ba/Nb >10). Mantle plumes have been implicated both as a cause and as a consequence of continental break-up (e.g. Hill, 1991; Anderson et al., 1992; Saunders et al., 1992; Barton & White, 1995). In either case, the associated eruptives form thick lava sequences on the margins of the rifting continents, forming seaward-dipping reflector sequences (SDRS). The SDRS lavas may spill over onto the adjacent continent to form a CFB province. The opening of the North Atlantic at >58 Ma and the break-up of Gondwana to form the Indian Ocean from >120 Ma were both intimately associated with a vigorous mantle plume (White & McKenzie, 1989; Kent, 1991; Kent et al., 1992; Saunders et al., 1997). These lavas commonly display chemical signatures indicative of significant interaction with the continental lithosphere, and so can be distinguished easily from oceanic plateau basalts. How- ever, it is also perfectly possible for an oceanic plateau to merge seamlessly with SDRS and CFB (e.g. Iceland–East Greenland Margin–East Greenland basalts: Larsen & Jacobsdo´ttir, 1988) as the magmatism straddles the continent–ocean divide. In places, where fragments of continental lithosphere become isolated in the middle of oceans during break-up, complications can occur. The Nd isotope geochemistry of lavas from the Kerguelen Plateau in the Indian Ocean (negative Nd) implies that they have interacted with ancient lithosphere, and suggests that rafts of this old Fig. 6. (a) Plot of Ni vs mg-number for back-arc basins and oceanic plateaux. (b) Plot of Nb/Y vs Zr/Y [after Fitton et al. (1997)] for back- continental lithosphere occur within this region of the arc basins and oceanic plateaux. ‘Iceland array’ represents bounding Indian Ocean basin (Frey et al., 2000). limits for basalts and picrites from the Icelandic neovolcanic zones

(Fitton et al., 1997). (c) Plot of Lan/Ndn vs Smn/Ybn for the CCOP, OJP and Kerguelen Plateau, together with representative basalt data Examples of oceanic plateaux in the from ocean islands (OIB). Plateau basalt sources are as in Fig. 1; OIB geological record data from Palacz & Saunders (1986), Dupuy et al. (1988, 1989) and Chaﬀey et al. (1989); back-arc basalts data from Wood et al. (1980), Although Condie (1999) has argued that accreted oceanic Woodhead et al. (1998) and Leat et al. (2000); Paciﬁc MORB data from plateaux are relatively uncommon in the geological re- Mahoney et al. (1994). cord, we do not accept that this is necessarily the case.

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As demonstrated in the following section, we believe that et al., 1990; Boher et al., 1992), with pillowed basalts oceanic plateaux may, in fact, be widespread in the characterized by flat REE patterns and low positive Ndi. geological record. Table 2 shows a summary of proposed Subsequent to this discovery, oceanic plateaux have oceanic plateaux and continental–oceanic margin plat- been identified in the Arabian–Nubian Shield (Stein & eaux from the Archaean to the late Jurassic. Furthermore, Goldstein, 1996) and the Flin Flon Belt in Canada (Stern we hope that this discussion of the characteristics of et al., 1995; Lucas et al., 1996). As in the Archaean, oceanic plateaux will allow the identification of other examples of Proterozoic continental–oceanic plateaux examples throughout the world. have also been found in northeastern Finland (Peltonen Archaean greenstone belts in the Baltic Shield, such et al., 1996) and in other parts of the Baltic Shield (Puchtel as the Kostomuksha belt (Puchtel et al., 1998b) and the et al., 1997, 1998a). Lava sequences in the Proterozoic Olondo belt in the Aldan Shield, Siberia (Puchtel & Cape Smith Fold Belt of northern Que´bec were proposed Zhuravlev, 1993), have been inferred to consist of oceanic by Francis et al. (1983) and Dunphy et al. (1995) to have plateau sequences. This interpretation is based on the formed in a continental rifting event. The chemical occurrence of uncontaminated pillowed basalts and composition of these basalts is consistent with their de- komatiites with no terrestrial sedimentary intercalations. rivation from a mantle plume (Dunphy et al., 1995), and + Furthermore, the chemistry of the lavas ( Ndi 2·8 to we agree that the stratigraphy of the Cape Smith Fold + > 3·4; Lapmn/Nbpmn 1) is consistent with an oceanic Belt represents the transition from a continental to an plateau origin (Puchtel & Zhuravlev, 1993; Puchtel et oceanic magmatic event. al., 1998b). Archaean greenstone belts of the Canadian Basalts from the Wrangellia terrane of western North Superior Province have also yielded lava sequences that America have been identified as an accreted oceanic contain remnants of oceanic plateaux (Desrochers et al., plateau of Triassic age (Lassiter et al., 1995a, 1995b). 1993; Kimura et al., 1993; Skulski & Percival, 1996; Fan This interpretation is based on both geological and & Kerrich, 1997; Polat et al., 1998; Hollings & Wyman, geochemical evidence. First, the lava sequences contain 1999; Kerrich et al., 1999). The argument for an oceanic both subaerial and subaqueous units, with no evidence plateau origin is based on the occurrence of komatiites of nearby arc volcanism or terrigenous sediment. Second, + and pillow basalts with no intercalations of terrestrial the lavas possess generally flat REE patterns, Ndi 4·8 + > sediments or sheeted dyke sequences. Chemically, these to 7·3 and Lapmn/Nbpmn 1. Other postulated ex- Ζ lavas possess low positive Ndi values, Lapmn/Nbpmn 1 amples of accreted Cambrian to Jurassic oceanic plateaux and flat to LREE-depleted chondrite-normalized REE are less well studied. However, one area that appears to patterns, similar to lavas from recent oceanic plateaux. display considerable promise includes the post-400 Ma De Wit et al. (1992) suggested that the core of the accretionary complexes of Japan. It has been suggested Kaapvaal Shield in southern Africa, particularly the that Permian oceanic plateau derived basalts are pre- >3·5 Ga komatiites and pillow basalts of the southern served in the Yakuno ophiolite in SW Japan (Herzig et Barberton and Pietersberg Belts, may represent the rem- al., 1994) and parts of the Mino Terrane in central Japan nants of one or more oceanic plateaux. This interpretation ( Jones et al., 1993). Kimura et al. (1994) identified the is supported by the geochemistry of the komatiites, which accreted remnants of a Tithonian (150–145 Ma) plateau show no evidence for crustal contamination (Smith & in northern Japan that they named the Sorachi Plateau. Erlank, 1982). Possible Permian oceanic plateau-type basalts have also Several sequences within the Canadian Superior Prov- been proposed to exist within the Cache Creek terrane ince contain basalts and komatiites with both the geo- of the Canadian Cordillera (Mihalynuk et al., 1994), chemical and geological characteristics of oceanic plateau although more detailed work on these sequences is re- lavas. However, these are in stratigraphic contact with quired to verify this proposal. similar basaltic and komatiitic sequences of lavas that Thus, we cannot concur with the view expressed by possess a strong signature of continental lithospheric Condie (1999) that oceanic plateau sequences are rare contamination (negative Ndi and Lapmn/Nbpmn >1: Tom- in the geological record. The evidence outlined above linson et al., 1998, 1999; Hollings & Wyman, 1999). shows that oceanic plateaux are an important, and ar- These sequences are interpreted as continental–oceanic guably a major, component of Archaean greenstone belts, margin plateau sequences, which have erupted in tectonic and strongly supports the oceanic plateau model of crustal settings similar to the North Atlantic Tertiary igneous growth (see Ben-Avraham et al., 1981; Stein & Hofmann, province and portions of the Cretaceous Kerguelen Plat- 1994; Abbott & Mooney, 1995; Saunders et al., 1996; eau. White et al., 1999). We believe that there are many more In addition to these Archaean occurrences, examples accreted oceanic plateaux still to be discovered, and the of accreted Proterozoic oceanic plateau crust have also onus is therefore on Precambrian workers to bring these been discovered. One of the first of these to be identified ‘out of the closet’. It should be noted that the observation was in the Birimian province of West Africa (Abouchami by Bickle et al. (1994) that ‘Archean greenstone belts are

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Table 2: Proposed Archaean–Jurassic accreted oceanic plateaux and accreted continental–oceanic margin plateaux

Name Location Age References

Proposed accreted oceanic plateaux Southern Barberton Belt Kaapvaal Craton, 3·5–3·2 Ga De Wit et al. (1992) South Africa Pietersberg Belt Kaapvaal Craton, >3·4 Ga De Wit et al. (1992) South Africa Olondo Belt Aldan Shield, Siberia 3·0 Ga Puchtel & Zhuravlev (1993); Bruguier (1996) South Rim Unit, Superior Province >3·0 Ga Hollings & Wyman (1999) North Caribou Belt Kostomuksha Belt Baltic Shield 2·8 Ga Puchtel et al. (1998b) Vizien Belt Superior Province 2·79 Ga Skulski & Percival (1996) Malartic–Val d’Or area Superior Province 2·7 Ga Desrochers et al. (1993); Kimura et al. (1993) Tisdale Group, Superior Province >2·7 Ga Fan & Kerrich (1997) Abitibi Belt Schreiber–Hemlo– Superior Province 2·8–2·7 Ga Polat et al. (1998) White River Dayohessarah Birimian Province West Africa 2·2 Ga Abouchami et al. (1990); Boher et al. (1992) Flin Flon Belt Central Canada 1·92–1·90 Ga Stern et al. (1995); Lucas et al. (1996) Arabian–Nubian Shield NE Africa–Middle 900–870 Ma Stein & Goldstein 1996) East Yakuno ophiolite SW Japan 285 Ma Herzig et al. (1994) Mino Terrane Central Japan L. Permian Jones et al. (1993) Cache Creek Terrane Canadian Cordillera Permian? Mihalynuk et al. (1994) Wrangellia Terrane Western N America 227 Ma Richards et al. (1991); Lassiter et al. (1995a, 1995b) Sorachi Plateau Northern Japan 152–145 Ma Kimura et al. (1994)

Proposed accreted continental–oceanic margin plateaux Balmer Assemblage, Superior Province 2·99–2·96 Ga Tomlinson et al. (1998) Red Lake Greenstone Belt Steep Rock and Superior Province 3·0–2·9 Ga Tomlinson et al. (1999) Lumby Lake Belts Opapimiskan–Markop Superior Province >3·0 Ga Hollings & Wyman (1999) unit, North Caribou Belt Vetreny Belt Baltic Shield 2·44 Ga Puchtel et al. (1997) Povungnituk and Northern Que´ bec 2·04 Ga Francis et al. (1983); Chukotat Groups, Dunphy et al. (1995) Cape Smith Fold Belt Onega Plateau Baltic Shield 1·98 Ga Puchtel et al. (1998a) Jormua Ophiolite NE Finland 1·95 Ga Peltonen et al. (1996)

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not oceanic crust’ may be partially resolved if some of (White et al., 1999). The tonalitic magmas have been the belts represent plateaux. The absence of a complete shown to be derived from partial melting of the mafic ophiolite sequence in greenstone belts would, we argue, plateau material, being triggered by the injection of reflect the absence of a full sequence in oceanic plateaux. subduction-related basaltic magmas into the newly obducted plateau crust (White, 1999). The above example illustrates two important points about the maturing of accreted plateau material, and its INCORPORATION OF OCEANIC transformation into continental crust. First, it dem- PLATEAUX INTO THE CONTINENTS onstrates that the addition of silicic magmas can occur In view of the fact that oceanic plateaux are common in extremely quickly after obduction has taken place. In- the modern oceans, it may indeed seem surprising that deed, it is interesting to speculate that this process may they are not an even more obvious constituent of the be instrumental in stabilizing (i.e. adding to the buoyancy accreted collages of the continental crust. A number of of ) a tract of accreted mafic material, increasing its factors may explain this paradox. chances of survival in the geological record. First, it may be that only a small proportion of plateaux Second, the generation of tonalites via remelting of present in the oceans become accreted to the continents. mafic rocks in this setting is considerably assisted if the Cloos (1993) has argued that the minimum crustal thick- accreted material retains residual heat from its formation. ness of an inherently unsubductable oceanic plateau is This is the case for the Caribbean–Colombian oceanic 17 km. As discussed elsewhere in this paper, buoyancy plateau, which was obducted only shortly after its for- is the driving force for the obduction, rather than sub- mation. Thus, the association of a basaltic sequence duction, of an oceanic plateau. This buoyancy derives with voluminous tonalites (having geochemical signatures from the crustal thickness of the plateau and any residual consistent with remelting of basaltic rocks) may be a key heat from its formation. It is logical that the less buoyant factor in recognizing a ‘hot’ accretionary setting. This is plateaux will be less prone to obduction than their thicker, more likely, logically, to be associated with accretion of or hotter, counterparts. The second possibility is that an oceanic plateau (which may be still hot when accretion some plateaux are accreted only temporarily to the takes place), rather than with, say, an island arc or normal continents. As suggested by Saunders et al. (1996), the ocean floor. lower parts of an accreted plateau may be converted Thus, even given that the processes of obduction, into eclogite, in which case, subduction could occur accretion and continuing subduction can obscure the ‘piecemeal’ after accretion to a continental margin has primary characteristics of an accreted terrane, we argue taken place. that the recognition of oceanic plateau successions within Our studies of the Caribbean–Colombian oceanic plat- the geological record should still be possible. The as- eau, however, suggest that the major obstacle to iden- sociation of basalts and tonalites may draw attention to tifying an accreted oceanic plateau is the fact that an accretionary setting having a high thermal energy; secondary processes may obscure the original plateau field and geochemical studies should then be able to characteristics. Whether plateau obduction occurs in an confirm whether the basaltic sequence has oceanic plat- intra-oceanic or continental margin setting, a con- eau affinities. This is still possible, in theory, even if the sequence of the obduction process is that subduction will rocks of interest are metamorphosed, as the geochemical either reverse in polarity or take a step back, leading to discriminants discussed earlier in this paper are based on subduction occurring beneath the plateau. Thus, a re- those elements that remain relatively immobile during cently accreted plateau is likely to become obscured by metamorphism. burial with the volcanic products of continuing subduction beneath the region. It may suffer deformation and metamorphism, making original volcanic and sedimentary features difficult to recognize and interpret. Finally, and importantly, intrusion of silicic magmas into CONCLUSIONS the accreted plateau crust ‘matures’ it into something (1) Oceanic plateaux are more common in the geological resembling continental crust. record than has hitherto been realized. On the basis The island of Aruba, now located in the southern of detailed studies of Cretaceous oceanic plateaux, we Caribbean plate boundary zone, contains a sequence of propose a series of geochemical and geological dis- basalts, dolerites and volcaniclastic rocks belonging to criminants, which should, in most cases, permit the the >88 Ma Caribbean–Colombian oceanic plateau. identification of Archaean and Proterozoic oceanic plat- This sequence is metamorphosed at greenschist (and eau sequences. locally, amphibolite) facies, as a result of the intrusion of (2) These salient features of oceanic plateaux are as a predominantly tonalitic batholith dated at >85–82 Ma follows: the occurrence of high-MgO lavas (picrites and

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komatiites); chemically homogeneous basalts with rel- of Continental Breakup. Geological Society, London, Special Publication 68, atively flat chondrite-normalized REE patterns and low 99–124. La /Nb ratios (<1·1); pillowed lavas; low abundance Arndt, N. T., Kerr, A. C. & Tarney, J. (1997). Dynamic melting in pmn pmn plume heads: the formation of Gorgona komatiites and basalts. Earth of volcaniclastic deposits; lack of sheeted dyke complexes > and Planetary Science Letters 146, 289–301. and a relatively thick ( 5 km) extrusive section. Some Babbs, T. L. (1997). Geochemical and petrological investigations of plateaux show a predominance of subaerial eruptions, the deeper portions of the Ontong Java Plateau: Malaita, Solomon with associated flow morphologies. Islands. Ph.D. Thesis, University of Leicester. (3) We would strongly caution that none of the above Baker, J. A., Thirlwall, M. F. & Menzies, M. A. (1996). Sr–Nd–Pb criteria can be used, on its own, to identify unequivocally isotopic and trace element evidence for crustal contamination of an oceanic plateau. A suite of rocks of unknown tectonic plume-derived flood basalts: Oligocene flood volcanism in western Yemen. Geochimica et Cosmochimica Acta 60, 2559–2581. origin with a significant number of the above features is Barton, A. J. & White, R. S. (1995). The Edoras Bank margin; most likely to be an ancient oceanic plateau. Although continental break-up in the presence of a mantle plume. Journal of this is rather an obvious statement, we feel it is necessary the Geological Society, London 152, 971–974. given the misuse of geochemical discrimination diagrams Ben-Avraham, Z., Nur, A., Jones, D. & Cox, A. (1981). Continental over the past 25 years. accretion: from oceanic plateaus to allochthonous terranes. Science (4) A problem arises when attempting to extrapolate 213, 47–54. Bickle, M. J., Nisbet, E. G. & Martin, A. (1994). Archaean greenstone discrimination analysis to the early Archaean. High belts are not oceanic crust. Journal of Geology 102, 121–138. mantle temperatures at that time would have resulted in Boher, M., Abouchami, W., Michard, A., Albare`de, F. & Arndt, N. thicker oceanic crust, possibly similar to recent oceanic T. (1992). Crustal growth in west Africa at 2·1 Ga. Journal of plateaux. The Archaean equivalents of modern plateaux Geophysical Research—Solid Earth 97, 345–369. may have been even thicker structures, with many tens of Bruguier, O. (1996). U–Pb ages on single detrital zircon grains from kilometres of basalts, high-MgO komatiites and plutonic the Tasmiyele group: implications for the evolution of the Olekma block (Aldan shield, Siberia). Precambrian Research 78, 197–210. equivalents. Burke, K., Fox, P. J. & Sengor, M. C. (1978). Buoyant ocean floor (5) Finally, we would stress that accreted suites of rocks and the origin of the Caribbean. Journal of Geophysical Research 83, cannot be considered in isolation. There must also be 3949–3954. an appreciation of the large-scale structure of the accreted Cadman, A. C., Tarney, J., Baragar, W. R. A. & Wardle, R. J. (1994). terrane under investigation. Relationship between Proterozoic dykes and associated volcanic sequences—evidence from the Harp Swarm and Seal-Lake-Group, Labrador, Canada. Precambrian Research 68, 357–374. Campbell, I. H. & Griffiths, R. W. (1990). Implications of mantle ACKNOWLEDGEMENTS plume structure for the evolution of flood basalts. Earth and Planetary Science Letters 99, 79–93. There are many people whose assistance, advice and Cann, J. R. (1970). New model for the structure of the ocean crust. help has been invaluable during the formulation of the Nature 226, 928–930. ideas contained in this paper; these include John Tarney, Chaffey, D. J., Cliff, R. A. & Wilson, B. M. (1989). 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