Abstract: The mechanisms of magma crust accretion at large igneous provinces (LIPs) are questioned using arguments based on the north Atlantic case. Published and new data on the calculated flow vectors within dike swarms feeding the early traps and subsequent seaward-dipping reflector lavas suggest that most of the mafic magmas forming the north Atlantic LIP transited through a small number of igneous centers. The magma was injected centrifugally in dike swarms at some distance away from individual igneous centers along the trend of the maximum horizontal stress acting in the crust, feeding lava piles via dikes intersecting the ground surface. This mechanism is similar to that observed in present-day Iceland and, more generally, in mafic volcano-tectonic systems. The absence of generalized vertical magma transit in a LIP has major geodynamic consequences. We cannot link the surface extent of LIP magmas to the dimensions of the mantle melting zone as proposed in former plume head models. The distribution of LIP magmas at the surface is primarily controlled by the regional stress field acting within the upper crust, but is also affected by magma viscosity. The igneous centers feeding LIPs most likely represent the crustal expression of small-scale convective cells of the buoyant mantle naturally located beneath the mechanical lithosphere.

Abstract: The Kerguelen Archipelago, constructed on the submarine Northern Kerguelen Plateau, is attributed to Cenozoic volcanism arising from the Kerguelen hot spot. Geochemical studies of 325 to 1000 m thick lava sections of the ?30 to 25 Ma flood basalt forming the bulk of the archipelago show a temporal change from older tholeiitic basalt to younger slightly alkalic basalt. This compositional transition is expressed in a 630 m lava section at Mt. Capitole where the lava sequence is lowermost tholeiitic basalt overlain by slightly alkalic basalt overlain by plagioclase-rich cumulates that are mixtures of plagioclase-phyric basalt and more evolved magmas. During growth of the archipelago, magma supply from the hot spot was variable and at times sufficiently low to enable extensive crystal fractionation; e.g., at Mt. Capitole and nearby Mt. Tourmente only 10 of 120 lava flows have >6 wt% MgO. On the basis of this study and previous isotopic data for the ?34 Ma submarine lavas erupted on the Northern Kerguelen Plateau, other flood basalt sections in the Kerguelen Archipelago, and younger lavas erupted in the archipelago and at Heard Island, there is significant Sr, Nd, Hf, and Pb isotopic heterogeneity that can be explained by two stages of mixing. The first mixing event, best shown by the submarine lavas, is between components that are related to Indian Ocean mid-ocean ridge basalt (MORB) and the Kerguelen hot spot. From ?34 Ma to <1 Ma, on average the proportion of the MORB-related component decreased. Subsequently, a second mixing process involved addition of a component with relatively high 87Sr/86Sr (>0.7060) and low 143Nd/144Nd (<0.5125) and 176Hf/177Hf (<0.2827) and nonradiogenic Pb isotope ratios (<17.9 for 206Pb/204Pb). We infer that this component was lower continental crust.