The «Venice Granodiorite»: constraints on the «Caledonian» and Variscan events in the Alpine domain

Abstract

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.