No Oceanic Plateau— No Caribbean Plate? The Seminal Role of an Oceanic Plateau in Caribbean Plate Evolution

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Oceanic plateaus are areas of elevated and anomalously thick oceanic crust that are believed to form by enhanced partial melting in a mantle plume that is hotter than ambient upper asthenosphere. They are regarded as the oceanic equivalent of continental flood-basalt provinces. Because of the continual subduction of oceanic crust, the oldest known oceanic plateaus occurring in situ are Cretaceous in age. In order for oceanic plateaus to be preserved in the geologic record, they must be accreted onto continental margins. This process, involving their preservation as tectonic slices, depends on the fact that oceanic plateaus are more buoyant than normal ocean floor; thus, they are not easily subducted. If these plateaus encounter an oceanic arc, subduction polarity reversal may occur, and/or the locus of subduction may step back behind the trailing edge of the advancing plateau. At a continental subduction zone, only subduction back-step occurs. Geochemical evidence shows that basaltic and picritic rocks exposed in the thickened part of the Caribbean plate and around its margins (including northern South America) are parts of an accreted oceanic plateau that originated in the Pacific Ocean during the middle-to-late Cretaceous. Cretaceous subduction-related rocks also occur around the Caribbean margins and possess geochemical signatures (e.g., lower Nb and Ti) that are distinct from those of the oceanic plateau rocks. This arc material represents the remnants of the subduction-generated rocks with which the plateau collided at 80–90 Ma. Both island arc tholeiite and calc-alkaline magmatism occurred in these Cretaceous arcs, but the changeover between the two types appears to be gradual and cannot be used to determine the timing of subduction polarity reversal. Many Cretaceous tonalitic batholiths around the Caribbean margins appear to have formed during or shortly after accretion of the plateau rocks. In addition to the arc and oceanic plateau assemblages, Jurassic to Early Cretaceous fragments of the preexisting oceanic crust also occur around the region. The environmental impact of oceanic plateau volcanism around the Cenomanian-Turonian boundary and its link to the formation of organic-rich black shales is discussed in this paper.