Antonio Paglionico

Petrogenesis of contrasting hercynian granitoids from the Calabrian Arc, southern Italy

Abstract The granitoids of the southern Calabrian Arc have been investigated for whole-rock and phase chemistry, zircon typology, REE, Sr and Nd isotopes. Two distinct granitoid associations, which are related in time and partly in space, are present: a calc-alkaline one and a peraluminous one. The prevailing calc-alkaline association is compositionally expanded (SiO2=48–70%) and biotite dominated, with tonalites and granodiorites as predominant rock types. The peraluminous association is compositionally restricted (SiO2=67–76%) and contains two-mica ± Al-silicates. Distinct peraluminous typologies occur also as core facies within the calc-alkaline types. All granitoids are ilmenite-bearing. The Cittanova (CN), Villa S. Giovanni (VSG) and Capo Rasocolmo (CR) peraluminous granites display zircon typology, REE patterns, ϵSrt- (+51 to +113) and ϵNdt-values (−8.5 to −4.6) at 290 Ma, suggesting a dominantly quartzofeldspathic metasedimentary source. In more detail, the CN types and the CR-VSG types require heterogeneous and different sources. The calc-alkaline granitoids display very variable REE patterns (CeN=25–227 and YbN=3.5–18.5 in the tonalites-granodiorites) and variable age-corrected ϵNdt-values (−8.5 to −0.25), whereas Sr values vary little (+82 to +93). Thus, in terms of ϵNdtϵSrt covariation, the data points define a vertical array, which is inconsistent with a model involving crustal contamination by mantle derivatives. A more viable mechanism seems to be the melting of hydrous and heterogeneous mafic lower crust (and/or basic underplate), producing distinct magma batches evolving independently. Crustal contamination, mingling and fractionation processes may all have contributed to the observed geochemical variations within the granitoids. The peraluminous granodiorites occurring within the Serre and Capo Vaticano multipulse calc-alkaline plutons exhibit isotopic ratios (ϵNdt = −6.11 to +0.33 and ϵSrt = +93 to +97) which are similar to those of the calc-alkaline host rocks, suggesting a possible genetic link. Geologic and geochemical data indicate a continental collision setting for this plutonic activity. The magmas were produced during late-thickening to exhumation phases following collision and moderate crustal overthickening, and were emplaced subsequently during a short time span. A mantle contribution should have favoured melting, which affected various sectors of the continental crust.

Tectonic structure and post-hercynian evolution of the Serre, Calabrian arc southern Italy: geological, petrological and radiometric evidences

Abstract Conflicting opinions exist concerning the structure and the post-Hercynian evolution of the Serre. The present paper deals with these topics on the basis of new geological, petrological and radiometric evidence. The composition of the so-called Stilo and Polia-Copanello units has been redefined. The above domains—former sections of upper and lower Palaeozoic continental crust respectively—came into contact, due to transcurrent movements 130–140 Ma ago. A significant vertical component during the transcurrent movements, probably, exhumed the former section of lower crust. The above domains, juxtaposed, were successively involved as a single kinematic body in the Alpine orogenesis. The results enable us to make inferences for the Calabrian Arc evolution and call attention to similarities between an Austro-Alpine element (Stilo + Polia-Copanello) of the Calabrian chain and a South-Alpine sector of the Alps (Ivrea + Ceneri zones).

Granulite-amphibolite facies metasediments from the Serre (Calabria, Southern Italy): their protoliths and the processes controlling their chemistry

Abstract In the northwestern Serre, a near continuous section through the Palaeozoic lower continental crust crops out. This paper deals with granulite facies metasedimentary rocks of this section. Fifty-five chemical analyses (major elements, Sc, V, Cr, Co, Ni, Rb, Sr, Y, Nb and Ba) are given. Chemical data indicate that the original sedimentary sequence was made up of arenites in the lower part and mainly Fe and Mg rich pelites in the upper part. The metapelites experienced anatexis and removal of some melt, whereas the meta-arenites suffered only incipient anatexis and became dehydration granulites. The various rock types display an overall relatively low K/Rb ratio due mainly to the stability of K- and Rb-retaining phases. A weighted mean composition of the fragment of the lower continental crust outcropping in the Serre has been established by integrating the chemical data for metasediments (55 analyses), metabasites (92 analyses) and dioritic and tonalitic gneisses (10 analyses) with the field data. This composition compared to the compositions of the bulk continental crust and of other sections considered as belonging to the lower crust, confirms the hypothesis that (1) the lower crust is heterogeneous, and (2) it is more basic than the continental crust.

Petrological evidence for crustal thickening and extension in the Serre granulite terrane (Calabria, southern Italy)

The paper presents the metamorphic trajectory recorded by metapelitic migmatites of the upper part of the Hercynian lower continental crust of the Serre (southern Calabria, Italy). The relict minerals, reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from temperature of about 500 ◦ C and pressure of 4-5 kbar to T < 700 ◦ Ca nd P ∼ 8 kbar stabilized the assemblage Grt + Ky + Bt + Ms(Si/11ox =3.26-3.29) in the uppermost metapelites of the profile. Progressive heating led to H2O-fluxed and dehydration melting first of Ms, then of Bt at T < 700 ◦ C in the stability field of sillimanite. This process was followed by nearly isothermal decompression producing additional melt with a transition from Grt to a Grt + Crd stability field. Further decompression caused the formation of Crd-corona around garnet. Nearly isobaric cooling led to rehydration and retrogression across the stability field of andalusite up to the stability field of kyanite. The lowermost metapelites of the studied profile have lost most of the memory of the prograde P-T path; they record decompression and cooling. High-temperature mylonites occur in which boudinage, elongation and pull-aparts characterize the porphyroclasts. The pull-aparts in the high-T mylonites are filled with low-P minerals (Crd + Spl). The Hercynian metamorphic trajectory and the microtextures are consistent with crustal thickening and subsequent extensional regime. During extension, an important tectonic denudation probably caused the isothermal decompression. Extension also occurred in post-Hercynian times as documented by pull-aparts in sillimanite porphyroclasts filled with chloritoid within a low-grade mylonite.

SEM-EDS microanalysis of microphenocrysts of Mediterranean obsidians a preliminary approach to source discrimination

SEM-EDS non-destructive analysis allows the source discrimination of the various microphenocrysts in obsidian rocks of the Mediterranean. Samples of six Mediterranean sources, Monte Arci (Sardinia), Palmarola, Lipari, Pantelleria, Gyali and Melos were studied using SEM-EDS non-destructive techniques in order to obtain a data base of their microphenocryst content. The results demonstrate that provenance of an obsidian can be distinguished on the basis of its petrography, in particular by quantitative analyses of Fe-Mg microphenocrysts; this information is particularly useful when coupled with data obtained by other non-destructive techniques such as glass microanalysis of artefacts (Acquafredda et al. , 1999). Such investigations, carried out by SEM, an analytical technique relatively accessible to geoarchaeologists, are relatively rapid, effective and above all non-destructive.