Silvio GIuseppe Rotolo* and Igor Maria Villa ³⁹Ar-⁴⁰Ar dating of an alkali-granite enclave from Pantelleria island

ABSTRACT - A peralkaline microgranitic enclave was found enclosed in the Green Tuff ignimbrite on Pantelleria island. This enclave represents the intrusive equivalent of pantelleritic lava flows. Its K-Na-feldspar has been dated by ³⁹Ar-⁴⁰Ar and gave a plateau age of 517 ± 19 ka. This age is considerably older than all outcropping rocks dated so far (< 324 ka), and requires to shift the onset of magmatc activity on Pantelleria back in time.

* Corresponding author

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Alfredo Caggianelli* and Giacomo Prosser An exposed cross-section of late Hercynian upper and intermediate continental crust in the Sila nappe (Calabria, southern Italy)

ABSTRACT - Upper to intermediate levels of continental crust, as shaped by Late Hercynian metamorphism and magmatism, are exposed in the Sila nappe (Calabria, S. Italy) after Tertiary tectonics. Direct observation of the crustal section reveals a composite metamorphic block dominated by low- to high-grade metapelites and meta-arenites, intruded centrally by granitoids. The average thickness of granitoids, estimated on cross-sections and by geobarometry, reaches 9 ± 2 km. Their composition changes with depth from leucogranite to tonalite with increasing colour index. The estimated bulk composition of the Sila nappe is characterised by higher contents of Al₂O₃, REE and Th and lower contents of CaO and Sr than is typical of upper continental crust, reflecting abundant metapelite in the Sila nappe. Comparisons between metasedimentary rocks from the upper and intermediate levels indicate that intracrustal differentiation was determined by anatexis, which favoured Al₂O₃, REE and Th enrichment in medium- to high-grade metapelites. Distribution of REE, Th and Zr was connected to the fate of monazite and zircon during crustal anatexis. Melting reactions involved mainly muscovite whereas biotite, enclosing most accessory phases, often remained stable in the residue. Consequent passive enrichment of Th in medium- to high-grade restitic metapelite determined an unusual increase in radiogenic heat production with depth. Compositional data suggest a connection between leucogranite genesis and anatexis of intermediate crust. The composition of leucogranites is similar to that of crustal melts generated by muscovite dehydration melting reactions. The extremely low concentrations of Sr and Ba and high Rb contents are comparable with those of Himalayan leucogranites.

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Francesco Secchi*, Francesco Cincotti, Gian Piero Cherchi and Edoardo Sarria Geological and petrographical aspects of late Hercynian intrusive sequences from southern Ogliastra area (SE Sardinia, Italy)

ABSTRACT - The granitoids of southern Ogliastra (central-eastern Sardinia, Italy) were emplaced at shallow levels in a nappe edifice (the so-called "thrust zone") at the end of the Hercynian orogenesis. Field relationships and textural characters indicate at least three pulses of hornblende-biotite granodioritic magmas (GD1, GD2, GD3), preceded by smaller amounts of tonalitic magma (TO). The leucogranitic magma of Monte Arista is late in the sequence.
The granodiorite is crosscut by N-S-trending lamprophyre and mainly NW-SE-trending felsic dikes. Geological and petrographical observations indicate that most of the acidic dikes were injected before leucogranite was emplaced.
The Altot-in hornblende barometer indicates pressure values of crystallization in the range from 1 to 3.5 Kb, in agreement with the common occurrence of andalusite-cordierite in the associated hornfels.
Temperature estimates by the plagioclase/hornblende thermometer on tonalitic and granodioritic rocks gives Tmax of 1000°C. Petrographical and petrochemical data do not exclude a possible genetic link between TO tonalite and GD3 granodiorite. Instead, fluorite-bearing leucogranite should be considered genetically independent of the rest of the rock sequence.

* Corresponding author.

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Costanza Bonadiman*, Massimo Coltorti, Lorenzo Milani, Leonardo Salvini, Franca Siena and Renzo Tassinari Metasomatism in the lithospheric mantle and its relationships to magmatism in the Veneto Volcanic Province, Italy

ABSTRACT - A detailed textural and geochemical study was carried out on a suite of anhydrous, spinel-bearing peridotitic xenoliths hosted in alkaline basic lavas from the Veneto Volcanic Province (VVP). Lherzolites and harzburgites show metasomatic textures superimposed on primary, protogranular textures. On the basis of textural relationships, relative modal percentages and grain size between primary and secondary parageneses, three metasomatic textures were distinguished: Type A characterized by reaction areas around orthopyroxene and spinel, generating olivine, clinopyroxene and spinel secondary crystals; Type B by the abundant presence of glassy patches closely intergrown with olivine, clinopyroxene, spinel and feldspar; and Type C by reaction areas around clinopyroxene, giving rise to spongy or completely recrystallized clinopyroxenes.
Unmetasomatized lherzolites display flat REE patterns at about 2-3 ¥ chondrite, while metasomatized lherzolites and harzburgites show a classic spoon-shaped light (L) REE-enriched patterns. Major and incompatible element analyses of primary and secondary minerals (including glass), together with textural features and whole-rock data, allow the definition of at least two distinct events which affected the VVP lithospheric mantle. Geochemical features (including trace elements) of primary clinopyroxenes and spinels, as well as whole-rock geochemistry, indicate that the initial process was of partial melting, followed by enrichment due to infiltration of a strong undersaturated Na-alkali silicate melt as suggested by secondary clinopyroxenes and coexisting glass. The measured Nd and Sr isotopic compositions of whole-rock and separated clinopyroxenes plot in a distinctive array originating from isotopically depleted compositions in the ⁸⁷Sr/⁸⁶Sr vs ¹⁴³Nd/¹⁴⁴Nd diagram (⁸⁷Sr/⁸⁶Sr = 0.702214-0.702543; ¹⁴³Nd/¹⁴⁴Nd = 0.51325-0.51329), close to the DM component, and trending towards the enriched low Nd - high Sr components (HIMU and EMI) recorded by the metasomatized peridotites (⁸⁷Sr/⁸⁶Sr = 0.702951-0.70365, ¹⁴³Nd/¹⁴⁴Nd = 0.51297-0.51299). Late Cretaceous lamprophyric dykes, cutting the South Alpine Basement at Calceranica (Trento) and well reproducing the geochemical features of the inferred metasomatic melt, plot at the end of the enrichment trend of the VVP peridotites. These isotopic features, coupled with the other geochemical parameters, strongly suggest that a metasomatic agent interacted with the primary mantle assemblage, forming glass and feldspar (plus recrystallized olivine, clinopyroxene and spinel) as reaction products.
In the same diagram, VVP basic lavas lie within a field encompassing both the metasomatized xenoliths and the inferred metasomatic agent. On the basis of their incompatible element contents, the presence of amphibole and phlogopite in the VVP magma sources could be invoked. The incompatible element budget related to these minerals was compared with the metasomatic contribution in the mantle xenoliths, calculating the elemental distribution between primary and secondary phases. The strict analogy between incompatible element contents in VVP magma sources and VVP mantle xenoliths, together with their similar isotopic ratios, strongly suggest that the metasomatic agent responsible for the enrichment of the VVP magma sources was very similar to that affecting the VVP xenoliths. It is conceivable that a continuum metasomatic enrichment, starting at least from Late Cretaceous, progressively affected the upper portion of the VVP lithospheric mantle.

* Corresponding author.

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Antal Embey- Isztin* and Heinz G. Scharbert A tentative pre-Glasses in peridotite xenoliths from the western Pannonian Basin

ABSTRACT - Mantle peridotite xenoliths, hosted in alkali basalts of the Western Pannonian Basin, contain small amount of glass. These glasses are highly enriched in alumina and have lower Mg and Fe contents than host alkali basalts. Amphibole is present in some xenoliths, but its role in mantle glass formation was dismissed on the basis of petrographic and geochemical constraints. Mass balance calculation, in fact, require the involvement of metasomatic fluids, particularly for alkali elements. Metasomatism may also have triggered partial melting process at mantle depth. Similar glass pockets occurring in lower crustal, granulite xenoliths may, on the other hand , be related to the combined effect of sudden decompression and heating in rapidly ascending host lavas.

* Corresponding author.

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Giovanna Sapienza, Vittorio Scribano* and Sonia Calvari Kelyphitic breakdown of garnets from pyroxenite xenoliths, south-eastern Sicily, Italy.

ABSTRACT - Upper-mantle garnet-pyroxenite xenoliths rarely occur in Miocene basaltic diatremes from the Hyblean Plateau, SE Sicily (Italy). Three representative samples from Valle Guffari tuff-breccia deposits are examined here. A subsolidus origin for garnet may be invoked for two of these samples (one spinel-garnet websterite, UL-c17; one garnet-spinel-orthopyroxene clinopyroxenite, UL-d47). In the third sample (a garnet clinopyroxenite, UL-d21), which has mineral compositions similar to those of sample UL-d47, garnet is a cumulus product.
Garnet appears partially replaced by an optically unresolvable mineral intergrowth (kelyphite) which, under SEM (EDS), turns out to be composed of Ca-poor pyroxene, spinel and Ca-plagioclase. Kelyphite bulk compositions, obtained by automatic raster analyses, closely match their relative garnet mineral chemistry. This fact and mass balance calculations suggest a near-isochemical garnet breakdown process, primarily controlled by an increase in temperature and decompression, as a consequence of xenolith entrainment into the host magma and ascent to the surface, although a catalysing effect by fluids was very probable at any stage of the process.

* Corresponding author.

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