PLATEAUS ANCIENT AND MODERN - GEOCHEMICAL AND SEDIMENTOLOGICAL PERSPECTIVES ON ARCHEAN OCEANIC MAGMATISM

Unequivocal examples of oceanic plateaux older than about 2.1 Ga have yet to be identified from the geological record. The most likely expla nation for this is the partial dismemberment of ancient plateaux al co nvergent plate margins, and their subsequent juxtaposition in accretio nary complexes with are volcanic, sedimentary and plutonic rocks. A re view of Cretaceous plateaux in the eastern Indian Ocean suggests that certain geochemical-isotopic criteria (e.g. positive Nb anomalies rela tive to primitive mantle) employed by petrologists to identify modem o ceanic plateau lavas are of little practical use in distinguishing anc ient plateau lavas from are and volcanic rifted margin basalts. The ex treme chemical and isotopic heterogeneity of Indian Ocean plateaux, to gether with large variations in area and volume, suggest that these ed ifices represent one extreme of a spectrum of oceanic plateau composit ions. The other extreme, represented by Iceland and Ontong Java, is ch aracterised by relatively homogeneous melt compositions, reflecting st eady-state melting conditions and the absence of a continental lithosp here contaminant to plateau magmas. Crustal thickness and compositiona l estimates for Archaean oceanic plateaux using the McKenzie-Bickle me thod give a mean thickness of similar to 43 km, and mean MgO content o f similar to 19 wt.%. Archaean plateaux were thus compositionally dist inct from surrounding ocean floor (basaltic kornatiite?), and notably more magnesian than plateaux such as Iceland (mean for Tertiary basalt s = 6.3 wt.% MgO) and Ontong Java (mean for core samples = 6.9 wt.% Mg O). By analogy with Cretaceous-Tertiary ultramafic complexes in wester n Colombia, the deeper portions of these plateaux may have consisted o f noritic rocks, underlain by Iherzolite, pyroxenite, gabbronorite and dunite. The general absence of such rocks in Archaean terrains sugges ts that these portions of plateaux are only rarely presented. The sear ch for remnants of ancient oceanic plateaux has identified examples of Archaean submarine lavas in the Canadian Shield (greater than or equa l to 2.7 Ga: southern Abitibi belt, Superior Province) and Kaapvaal cr aton of southern Africa (3.5-3.3 Ga: Barberton and Pietersberg belts). Chemical and isotopic data suggest that komatiite-tholeiite units fro m these areas were derived by partial melting of a plume or asthenosph eric mantle source, but do not distinguish adequately the tectonic env ironment (rifted continental margin, ocean floor, oceanic plateau) in which these rocks were erupted. However, the uplift histories of these putative plateau fragments, as recorded in sediments bounding and int ercalated with the komatiite-basalt sequences, appear to be broadly co mparable to those of the best-documented Phanerozoic oceanic plateaux. Although further stratigraphical investigations of presumed plateaux (ancient and modern) are required to test this hypothesis, we believe that the sedimentary record offers the best evidence yet for the forma tion of oceanic plateaux as far back as the Mid-Archaean (similar to 3 .5 Ga).