The internal structure of oceanic plateaus: inferences from obducted Cretaceous terranes in western Colombia and the Caribbean

Although the structure of mantle plume-derived oceanic plateaus has recently been assessed using remote geophysical techniques combined with petrological modelling, it is nevertheless desirable to test whether these (theoretical) rock types (dunites, gabbros and basalts) actually exist, and to establish their geochemical nature. Oceanic plateaus may have initially formed above or near sea level during a short vigorous pulse, and thereafter commonly subside to abyssal depths as the lithosphere cools, thus making sampling of their deeper levels extremely difficult. However, the Cretaceous-age Colombian-Caribbean oceanic plateau was partially accreted against the South American continent so making the imbricated segments available for study. During the process of plateau accretion and imbrication it is predominantly the basaltic layers which are obducted, but parts of the sequence down to layered and banded gabbros with associated pyroxenites and dunites (sometimes foliated) can be exposed where the imbricate thrusting brings up deeper levels. Most of the upper crustal sequence in western Colombia is composed of basaltic pillowed and massive flows and sills that are chemically uniform and 'undepleted' relative to normal mid-ocean ridge basalts. Komatiites and (more abundant) picrites are found at intervals, and appear to occur near the base of the sequence. In these zones both 'depleted' and moderately 'enriched' basalt and komatiite compositions occur, and may result from dynamic partial melting and mixing processes associated with the high-temperature part of the plume. It is possible to integrate these compositional characteristics into a general model for oceanic plateau structure where the rate of magma supply is in excess of that which can be accommodated by normal spreading processes, thus leading to extrusion of flows and the emplacement of sills and high-level magma chambers (but relatively few dykes). The dense and chemically heterogeneous ultramafic magmas intrude the base of the pile and undergo fractionation to form ultramafic cumulates, whereas the well-mixed basaltic magmas are erupted to form the homogeneous plateau basalts. Based on these observations a possible structure of the Caribbean-Colombian plateau is proposed which is compatible with geophysical models for other less well exposed oceanic plateaus. (C) 1998 Elsevier Science B.V. All rights reserved.