Assunta 1 borehole, Veneto, Italyi

Regional Level Types
Assunta 1 borehole	Borehole
Veneto	Region
Italy	- not defined -

The borehole Assunta 1, located in the Northern Adriatic Sea close to the Venice Lagoon, is a 4747-m-deep exploratory well drilled in 1976 by the ENI-AGIP oil company.

An Ordovician granodiorite body, buried under a sedimentary cover (dolostones, limestones, marls, clays, and sandstones) spanning from Triassic to Quaternary, was sampled starting from 4711 m below the sea floor.

The granodiorite hand specimens show medium-grained, holocrystalline equigranular structure, without evident mineral orientations. Their colour ranges from grey (in its mass) to light pink-red (in micro-patches). Several dark enclaves (4-25 m) are present. Some light-pink calcite-clinozoisite veins cross the sampled material, without any preferred orientation. The granodiorite displays a hypidiomorphic-granular texture. K feldspar and plagioclase form subhedral and euhedral crystals respectively. Both feldspars are sericitized. Quartz displays a weak post-crystalline deformation. Euhedral biotite is variably altered into chlorite + opaques. Accessory apatite and zircon are present.

Three different groups of enclaves occur in the granodiorite:
a) sub-rounded microgranitoid enclaves (ME), whose mineralogy is similar to that of the enclosing granodiorite; b) sillimanite-garnet bearing metamorphic xenoliths, characterized by angular shape and sharp boundaries. Large skeletal sillimanite occurs, surrounding sericite + hercynite + biotite + plagioclase ± cordierite aggregates, together with fibrolite. Garnet (almandine) is partially replaced by biotite + plagioclase symplectites; c) patchy metamorphic xenoliths, displaying rounded shapes and sharp boundaries, with a patchy texture in which biotite-rich clots are separated by colourless minerals. Large, poikilitic, pinitized cordierite crystals, which include biotite, ilmenite and hercynite, are present. Small apatite crystals form trails.

The Ordovician age for granodiorite crystallisation is inferred by U/Pb single-grain conventional dating of zircons (461-463 Ma; Meli and Sassi, 2003). This granodiorite is therefore approximately coeval with the granitoids occurring in the Austridic domain (Anterselva and Casies Orthogneisses), which underwent Variscan metamorphism. Meli and Sassi (2000) suggested the contribution of at least one older crustal component to the genesis of this Pre-Variscan granodiorite, the minimum age of which is constrained by the ²⁰⁷Pb/²⁰⁶Pb datum of a discordant zircon crystal at 1435 ± 26 Ma.

References

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