**5.5 Formation of a magmatic barrier at the base of the overriding plate**

Seismic tomography shows evidence of a deep reflector at a depth of 60 km (∼18 kbar) beneath the west coast of New Caledonia, and this structure was found to dip northward by at least 30° [115]. This deep seismic discontinuity was attributed to a remnant of a sinking slab of a subduction zone that formed in response to the blocking of an older subduction east of the Norfolk Ridge in the early Oligocene (ca. 32 My) [116]. However, this depth estimate for the subducted slab is not consistent with the fractionated REE and high Sr/Y content of the La Conception lava [20], suggesting the presence of residual garnet in the melting source (i.e., melting within the garnet stability field >25 kbar) [128]. Therefore, we believe that the seismic mirror beneath New Caledonia is the solidification front of accumulated mantle-derived melts at the base of the overriding plate. The incipient melt slowly

**Figure 4.**

*The percentage of the subduction component (%sz) of the La Conception lavas is estimated from the displacement of the average mid-oceanic-ridge basalt (MORB) at a given Nb [20].*

percolates upward (i.e., porous flow), mainly in a vertical direction due to the buoyancy of magma and the high permeability of partially molten systems [129, 130], and eventually, it enters the cooler lithosphere where it begins to solidify [131]. This would cause of the development of a low-permeability zone or permeability barrier [131, 132], under which the subsequent pluses of magma accumulate, forming a melt-rich zone [133–135].
