This study focuses on mafic volcanic rocks from the Bouvet triple junction, which fall into six geochemically distinct groups: 1N-MORB, the most widespread type, encountered throughout the study area. 2Subalkaline volcanics, hawaiites and mugearites, strongly enriched in lithophile elements and radiogenic isotopes and composing the Bouvet volcanic rise, and compositionally similar basalts and basaltic andesites from the Spiess Ridge, generated in a deeper, fertile mantle region. 3Relatively weakly enriched basalts, T-MORB, derived by the mixing of Type 1 and 2 melts and exposed near the axes of the Mid-Atlantic, Southwest Indian, and America-Antarctic ridges. 4Basalts with a degree of trace lithophile element enrichment similar to the Spiess Ridge and BouvetI. rocks, but higher in K, P, Ti, and Cr. These occur within extensional structures: the rift valley of the Southwest Indian Ridge, grabens of the East Dislocation Zone, and the linear rise between the Spiess Ridge and Bouvet volcano. Their parental melts presumably separated from a plume material that spread from the main channels and underwent fluid-involving differentiation in the mantle. 5A volcanic suite ranging from basalt to rhyolite, characterized by low concentrations of lithophile elements, particularly TiO2, and occurring on the Shona Seamount and other compressional features within the Antarctic and South American plates near the Bouvet triple junction. Unlike Types 1 to 4, which display tholeiitic differentiation trends, this suite is calc-alkaline. Its parental melts were presumably related to a plume material as well but, subsequently, they underwent a profound differentiation involving fluids and assimilated surrounding rocks in closed magma chambers in the upper mantle. Alternatively, the Shona Smt. might be a fragment of an ancient oceanic island arc. 6Enriched basalts, distinguished from the other enriched rock types in having very high P and radiogenic isotope abundances and composing a tectonic uplift near the junction of the three rifts. It thus follows that the main factors responsible for the compositional diversity of volcanic rocks in this region include i mantle source heterogeneity, ii plume activity, iii an intricate geodynamic setup at the triple junction giving rise to stresses in adjacent plate areas, and iv the geological prehistory. The slow spreading rate and ensuing inefficient mixing of the heterogeneous mantle material result in strong spatial variations in basaltic compositions.
geodynamic evolution, Bouvet triple junction, basalt chemistry, mafic volcanic rocks.
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