The "Venice Granodiorite": constraints on the "Caledonian" and Variscan events in the Alpine domain
Meli
S., Sassi R.
An Ordovician granodiorite body was sampled along an
exploratory well in the Northern Adriatic Sea, close to the Lagoon of Venice.
It stands 4711 m below the sea floor, and is buried under a sedimentary cover
spanning from Triassic to Quaternary. It contains several metasedimentary
xenoliths, as well as some microgranular enclaves. The drilled core is a unique
example of the Southalpine crystalline basement under the Po Plain. The
granodiorite is undeformed, and does not display post-emplacement metamorphic
overprints. It therefore limits southwards the possible extent of the Variscan
orogenic belt, which is well documented in the Southalpine and Austridic
metamorphic basements. The granodiorite is classified as an S-type granitic
rock, on the basis of major and trace element geochemistry and the occurrence
of abundant metasedimentary xenoliths. These features, together with the
isotopic signatures, point to an anatectic origin of the granodiorite. The
metamorphic xenoliths are probably fragments from the wall-rocks of the pluton,
and do not represent restites of the main crustal source of the granodiorite.
The microgranular enclaves probably represent parts of chilled margins
disrupted within the pluton, and do not indicate interaction with more mafic,
mantle-derived melts. The representative points of the granodiorite plot in
post-collisional fields in some discriminant diagrams. The age and many
geochemical features of the Venice granodiorite are similar to those of the
Austridic metamorphosed granitoids occurring in the Eastern Alps, possibly
indicating a unique geodynamic setting for their genesis and emplacement.
Together with the coeval calc-alkaline, mostly mantle-derived metagranitoids
occurring in the “Serie dei Laghi” (Western Southalpine), they define a
magmatic suite which is consistent with a convergent plate boundary tectonic
setting.