Development of passive volcanic margins of the Central Atlantic and initial opening of ocean

Abstract

Geological and geophysical data on the Central Atlantic are discussed in order to elucidate the tectonic setting of the initial magmatic activity, rifting, and breakup resulting in the origination of Mesozoic ocean. The structural, magmatic, and historical aspects of the problem are considered. It has been established that the initial dispersed rifting and low-capacity magmatism at proximal margins was followed by the migration of the process toward the central part of region with the formation of distal zones and the development of vigorous magmmatism, further breakup of the lithosphere and ocean opening. Magmatism, its sources, and the features of newly formed magmatic crust at both the rifting and breakup stages of margin development are discussed and compared with subsequent spreading magmatism. Sr, Nd, and Pb isotopic compositions show that the magmatic evolution of the Central Atlantic proximal margins bears the features of two enriched components, one of which is related to the EM-1 source, developing only at the North American margin. Another enriched component typical of the province as a whole is related to the EM-2 source. To a lesser extent, this component is expressed in igneous rocks of Guyana, which also bear the signature of the MORB-type depleted source typical of spreading tholeiites in the Atlantic Ocean. Similar conditions are assumed for subsequent magmatism at the distal margins and for the early spreading basalts in the adjacent Atlantic belt, which also contain a small admixture of enriched material. A comparison of the magmatism at the margins of Central and North Atlantic reveals their specificity distinctly expressed in isotopic compositions of igneous rocks. In contrast to the typical region of the North Atlantic, the immediate melting of the enriched lithospheric source without the participation of plume-related melts is reconstructed for the proximal margins of the Central Atlantic. At the same time, decompression and melting in the lithosphere could have been determined by the thermal effect of the Central Atlantic hot plume. This is testified by the intense melting in a short time interval over a vast area; the generation of rather homogeneous melts and creation of the thick magmatic crust; the radial arrangement of the dikes intruded in the course of plume ascent; as well as by the data of seismic tomography indicating the occurrence of the hot upper mantle material, which has kept the “memory” of the action of a shallow-seated Mesozoic plume. The formation of the Central Atlantic plume can be presumably regarded as a result of upwelling remobilisation adove the ancient African superplume and the rearrangement of the deep structure in Triassic and Jurassic times.