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 (epsilonNd(160): 7.4-10.6) and
associated mafic crust (epsilonNd(160): 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. (C) 2004 Elsevier B.V. All rights reserved.