The western Val Malenco region (northern Italy) provides an opportunity
to study directly the transition from subcontinental upper mantle into
the lower continental crust in an Alpine nappe. Recent field work
resulted in the discovery of fresh spinel peridotites, replacive
dunites and garnet clinopyroxenites in the Malenco ultramafic body and
of kyanite garnet gneisses with associated pegmatites in the overlying
lower crust. Both upper mantle and lower crustal rocks were intruded by
the Upper Permian Fedoz gabbro demonstrating that the subcontinental
mantle was welded to the lower continental crust, at least since that
time. The gabbroic rocks reveal a tholeiitic magmatic evolution from
more primitive gabbronorites to highly differentiated quartz diorites
and pyroxene hornblende gabbros. Crosscutting relationships between the
different members of the intrusion demonstrate that the Fedoz gabbro
cannot be regarded as a layered intrusion. Underplating of the Fedoz
gabbro and concomitant heat supply has caused granulite facies
metamorphism which is well developed in the lower crustal recks but has
left scarce imprints in the peridotites. In the granulitic pelites,
anatectic melts have been generated which intruded the already
solidified gabbroic rocks as dikes thus demonstrating the close
relationships between deep gabbro intrusions and granulite facies
metamorphism.
The well preserved field relations between lower crustal rocks,
subcontinental mantle and gabbroic rocks allow to reconstruct a Permian
lower crust-upper mantle interface and correlate the Late Permian -
Early Mesozoic evolution of the subcontinental upper mantle to that of
the lower crust.