JOURNAL OF PETROLOGY VOLUME 41 NUMBER 7 PAGES 1041–1056 2000 LIP Reading: Recognizing Oceanic Plateaux in the Geological Record 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 under- standing 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 1042 KERR et al. LIP READING 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 ob- duction onto the Colombian and Ecuadorian coast, res- ulted in the uplift and exposure of deep sections of the plateau around the Caribbean and along the north- western edge of South America.