The opening of the Southern Ocean gateways allowed the emergence of the Antarctic Circumpolar Current (ACC), crucial for the onset of global Cenozoic cooling (e.g., Sijp et al., 2014, Glob. Planet. Change 119; Voigt et al., 2013, EPSL 369/370). South of Africa, the opening was associated with the formation of several large igneous provinces (LIPs) including the Mozambique Ridge, Agulhas Plateau and the smaller Northeast Georgia Rise and Maud Rise. Plate tectonic reconstructions imply that the latter two were once part of the much greater Agulhas Plateau and were separated by subsequent rifting (Parsaglia et al., 2008, Geophys. J. Int. 174). It is debated whether and to what extent the emplacement of these large volcanic features obstructed the exchange of water masses between the Atlantic and the Indian Ocean thereby delaying the onset of the ACC.
The Agulhas Plateau was drilled during recent IODP Expedition 392 (Uenzelmann-Neben, Bohaty, Childress, et al., IODP Exp. 392 Preliminary Report, 2022). Igneous rocks were recovered at two sites on the southern part of the plateau (Sites U1579, U1580) and at one site near its northern edge (U1582). Preliminary data indicate that all sites returned tholeiitic or transitional basalts, formed by low pressure (shallow magma chamber) fractionation of mainly olivine and plagioclase (as typical for mid-ocean ridge basalts and many LIP lavas).
The volcanic glass composition from extrusive (pillow) lavas cored at Site U1582 possesses a clearly tholeiitic composition with narrow compositional range ((La/Sm)n and (Sm/Yb)n ratios spanning from ~0.69-0.77 and 0.73-0.81, respectively (primitive-mantle normalized)). Their H2O/Ce ratios range from 422 to 629, which is obviously higher than found in normal mid-ocean ridge basalts or ocean island basalts (usually <250). Their δ11B composition indicate that these high H2O/Ce ratios are not caused by seawater assimilation.
Whole rock samples from Sites U1579 and U1580 have a transitional composition. Accordingly, they show a more widespread geochemical composition with (La/Sm)n and (Sm/Yb)n ratios ranging from 0.69 to 0.75 and 0.81 to 1.19 (U1579) and 0.89 to 1.63 and 1.38 to 1.97 (U1580).
Upcoming geochemical investigations, including Sr, Nd, Hf, Pb isotope analyses, will further reveal the nature and source of the magmatism
