The South-Pennine ophiolites of the eastern Central Alps (Graubunden
and northern Italy) are characterized by a predominance of
serpentinized peridotites of subcontinental mantle origin, minor
gabbroic intrusions. and the lack of a sheeted dike complex. Tholeiitic
pillow lavas and flows and Middle Jurassic to Early Cretaceous oceanic
sediments stratigraphically overlie the mantle rocks. The peridotites
record deformation under decreasing temperature during extension
leading to final exposure at the sea floor and their inclusion in
tectono-sedimentary breccias (ophicalcites). Exhumation occurred along
a system of detachment faults. which locally carry stranded klippen of
continental crust and pre-rift sediments. The resulting stratigraphic
and structural architecture is best compared with that of an
ocean-continent transition along present-day magma-poor rifted margins
Such as the west-Iberian margin off Portugal.
Gabbros intruded the partially serpentinized mantle rocks at shallow
depth at 161 Ma. Both gabbros and basalts are characterized by
epsilonNd values typical for an asthenospheric MOR-type source of the
melts. They appear to document the onset of sea-floor spreading across
an exhumed subcontinental mantle.
Day 1 of the excursion (Totalp area) focuses on the interplay of the
tectono-metamorphic and sedimentary processes related to mantle
exhumation: Day 2 (Platta nappe) on the architecture of tire
ocean-continent transition and deformation processes related to
detachment faulting: Day 3 (Platta nappe) on the interrelations of
mantle exhumation and magmatic activity: and Day 4 (Malenco unit)
focuses on the petrology and processes related to a pre-rift
crust-mantle boundary.