PETROGRAPHIC AND GEOCHEMICAL FEATURES OF THE
"VENICE GRANODIORITE" (ITALY) (from the borehole Assunta)
Sandro Meli, Raffaele Sassi & Filippo Perfido
In the frame of the ENI-AGIP drilling project for monitoring hydrocarbon occurrence in Italy, several boreholes were drilled (AGIP Mineraria, 1977). One of them (Assunta Borehole, Adriatic Sea, N 45°26'18''8, E 12°33'19''4; sea depth: 15 m; reached depth: 4747 m), stands a few km to the E of Venice. A granodioritic body was found at the depth of 4711 m, representing the only one sample of the crystalline basement underneath the Po Plain. This unicity, jointly to its radiometric age of 446 + 18 Ma (AGIP, internal reports), makes it worth of special attention. This note is aimed at a full petrographic and geochemical characterization of this unique sample of the South-Alpine crystalline basement buried by the Mesozoic cover and the thick pile of Tertiary and Quaternary sediments of the Po Plain.
A 20 cm long granodioritic core ( = 6.5 cm) was available for the present study. It shows a medium grained, holocrystalline equigranular structure. No mineral orientations are evident. The hand specimen colour ranges from grey (in its mass) to light pink-red (in micro-patches). Some light-pink calcite-clinozoisite veins cross the sample without any preferred orientation. Seven dark coloured enclaves occur (4-25 mm).
In thin section, it is possible to see that K-feldspar and plagioclase form anhedral and euhedral crystals (0.4-4 mm) respectively; plagioclase often builds up 1.25 up to 4 mm sized glomerocrysts. Both feldspars are sometimes altered. Quartz shows a weak post-crystalline deformation. Biotite, in euhedral crystals, is always altered into chlorite in variable amount. Opaque minerals mainly result from the biotite alteration. Accessory components are apatite and zircon. The rock modal compositions plot in the field of the granodiorites (Streckeisen, 1967).
Three different groups of enclaves are present in the studied granodiorite sample.
a) Mafic microgranular enclaves (MME) (10-14 mm). Their composition and structure is similar to that of the enclosing granodiorite, the main differences consisting of a significantly smaller grain size and a large amount of biotite.
b) Cordierite + green spinel + biotite metamorphic xenoliths (4-25 mm). Large anhedral altered cordierite aggregates include biotite and green spinel crystals. Apatite occurs in a large number of small crystals grouped into elongated trails.
c) Sillimanite + biotite + green spinel + sericite + biotite + plagioclase + garnet + cordierite surmicaceous enclaves (15mm). Large, partially resorbed, sillimanite skeletal crystals occur within sericite + green spinel + biotite + plagioclase + cordierite aggregates. Garnet is partially replaced by a biotite + plagioclase symplectite. Biotite is partially retrogressed into chlorite.
140 electron probe analyses of the mineral phases were performed, both from the host granodiorite and the enclaves; whole rock major, minor and trace elements analyses of eight different sub-samples of the host rock were carried out.
The major element chemistry indicates a granodioritic composition, concordant with the modal classification, with a well-defined peraluminous character. The HFSE are relatively low, as well as some LILE. The REE chondrite-normalized arrays show slightly fractionated LREE and nearly horizontal HREE patterns, with small Eu anomalies. These features are consistent with a crustal origin of the magmas, even if a unique interpretation is not possible until isotopic data will be available; however, the occurrence of surmicaceous enclaves is a further clue for an anatectic origin of a liquid which has not completely segregated the restite during its formation and emplacement.
In the tectonic discriminant diagrams for granitoid rocks (e.g. Pearce et al., 1984; Whalen et al., 1987), the trace element signatures of the "Venice granodiorite" exclude an anorogenic setting, particularly for what concerns the Ga content. It suggests a late orogenic environment, even if an active margin setting cannot be excluded. The different enclaves found in the sample suggest both crustal interaction and magma mixing with a more mafic, possibly mantle derived, melt.