Exhumation of subcontinental mantle rocks and its exposure at the
seafloor is known from different magma-poor passive continental
margins. However, the transition from largely amagmatic passive rifting
to seafloor spreading is still poorly documented. In this contribution
we use MOR-type gabbroic and basaltic rocks to characterize the
magmatism associated with the formation of an ancient ocean-continent
transition preserved in the Platta nappe, eastern Switzerland. Gabbros
form individual small intrusions into exhumed serpentinized
subcontinental mantle rocks. Mineral and bulk-rock chemistry and simple
modeling indicate that each gabbro body records different magmatic
processes ranging from predominantly fractional crystallization to
solidification without fractionation. Mg numbers and Ni contents of
equilibrium olivine calculated from basalts and gabbros indicate that
few malic rocks are primary melts but most represent fractionated
compositions ranging from T- to N-MORB. Whereas most mafic rocks may be
explained by low to moderate degrees of melting of an N-MORB-type
mantle, the source of some basalt is enriched in incompatible elements.
This compositional variation seems to correlate with the spatial
distribution of the mafic rocks within the ocean-continent transition
whereby mafic rocks with T-MORB signatures occur close to the
continental margin whereas N-MORB signatures are predominantly found
oceanwards. As in an opening system time and space are closely linked,
the chemical evolution of the mafic rocks along the ocean-continent
transitions suggests continuous thinning of the subcontinental mantle
and associated uplift of the underlying asthenosphere during the time
between the crustal and the lithospheric breakup.