An exhumed, undisturbed fossil lower crust to upper mantle section is
preserved in Val Malenco, Italian Alps, and is now exposed along the
boundary between Penninic and Austroalpine nappes. Lower-crustal
metapelitic rocks are welded to upper-mantle ultramafic rocks by a
mid-Permian gabbro intrusion. The underplating of gabbro caused
granulite metamorphism and partial melting of the metapelites. In the
crust-to-mantle transition zone of at least 1 km thickness, gabbros,
large xenoliths of restitic metapelites and ultramafic rocks occur,
with densities of 2.95-3.14, 3.25 and 3.27 g/cm(3), respectively. The
seismic Moho therefore did not coincide with the boundary between
peridotites and crustal rocks but was situated above the upper limit of
the peridotitic mantle. The whole complex underwent cooling with only
moderate decompression within the kyanite field. This process started
at 1 GPa and similar to 800 degrees C and ended at 0.85 GPa and 600
degrees C and is interpreted as thermal relaxation after the gabbro
intrusion. Later, during Jurassic rifting, the crust-to;mantle section
was exhumed at the Adria margin of the Tethys ocean.