Many mantle peridotites exhumed along ancient and present-day magma-poor passive continental margins, along (ultra-) slow spreading ridges and fracture zones are plagioclase-bearing and generally too fertile to be the residue of partial melting processes alone. Likewise, the associated gabbroic and basaltic rocks are not a priori genetically linked to the underlying mantle rocks. Trace element and Nd isotopic studies in the eastern Central Alps peridotites in eastern Switzerland and northern Italy provide evidence for near-fractional melting and depletion at high pressure in Permian time followed by refertilization of subcontinental mantle by ascending melts at low pressure in Jurassic time. These results suggest regional-scale modification of ancient subcontinental mantle by melt infiltration and melt^rock reaction during incipient opening of oceanic basins. The similar Nd isotopic composition of plagioclase peridotite (ONd160 : 7.4^10.6) and associated mafic crust (ONd160 : 7.3^9.6) indicates that the liquids, which reacted with the peridotites derived from similar N-MORB type mantle sources. Plagioclase peridotites in magma-poor passive margins may predominantly form as a consequence of diffuse porous flow of melt in the thermal boundary layer above an upwelling asthenosphere and probably represent modified ancient subcontinental mantle. Thus, plagioclase peridotites exhumed in passive margins and possibly in (ultra-) slow spreading ridges may represent magma-poor periods where liquids stagnate in the thermal boundary layer and react with the surrounding peridotites. Once the effects of conductive heat loss dominate over advection of heat from below, diffuse porous flow of melt becomes less important and might be replaced by the formation of gabbro bodies.