Abstract 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.

The Chilas Complex is a large mafic-ultramafic body closely associated with the Kohistan Arc sequence in the western Himalaya of northern Pakistan. The arc and the Chilas Complex occupy an area of 36,000 km 2 , bounded on the north and south by major sutures. The arc formed close to the margin of Eurasia in response to the northward subduction of neo-Tethyan ocean lithosphere in Late Jurassic to middle Cretaceous time, and consists of intra-arc sediments, calc-alkaline volcanics, and diorite-tonalite-granite plutons. At its base is the Chilas Complex, which extends for more than 300 km and which has a maximum width of 40 km. Most of the complex consists of massive (although locally layered) gabbro-norites, which comprise variable amounts of plagioclase (An 64-40 ), orthopyroxene (En 76-48 ), clinopyroxene (mg = 75-55), magnetite, ilmenite, ±quartz, ±K-feldspar, ±hornblende, ±biotite, ±rare scapolite. In the central part of the complex, near the base, there are minor discordant dikes and intrusive bodies as large as 5 km 2 of a dunite-peridotite-troctolite-gabbronorite-pyroxenite-anorthosite association that displays excellent layering, graded bedding, slump breccias, and syndepositional faults. These rocks contain olivine (Fo 94-71 ), relatively Mg-rich orthopyroxene (En 91-65 ), clinopyroxene (mg = 85-67), and calcic plagioclase (An 98-83 ), ±hornblende, ±chrome spinel, and ±pleonaste, and represent a more primitive magma batch emplaced into the base of the gabbro-norite magma chamber. The mafic complex is not an ophiolite. Rocks of the complex have more petrographic and compositional similarities with plutonic blocks from island arcs and with other major mafic complexes such as the Border Ranges Complex of Alaska and those from the Ivrea Zone in the Alps. Trace-element patterns of the gabbro-norites have marked negative Nb anomalies, positive Sr, Ba, and P anomalies, and high K/Rb ratios, features consistent with melting of a hornblende-bearing sub-arc mantle source. The Chilas Complex either represents the root zone magma chamber of the Kohistan island arc, or magma generated by diapirism in the early stages of intra-arc rifting during formation of a back-arc basin.