A. Rottura

Relationships between intermediate and acidic rocks in orogenic granitoid suites: petrological, geochemical and isotopic (Sr, Nd, Pb) data from Capo Vaticano (southern Calabria, Italy)

Abstract Major, trace element and isotopic data are reported for Hercynian granodiorites and tonalites from Capo Vaticano, Calabria, with the aim of clarifying the relationships between acidic and intermediate lithologies in orogenic intrusive suites. These granodiorites (SiO 2 = 70.5%–73.5%) are peraluminous, relatively depleted in Rb, U, Th, and Pb, and display a large variation in many geochemical and isotopic parameters. REE show fractionated, cross-cutting chondrite-normalized patterns with small but significant negative Eu anomalies. Initial (290 Ma) Sr and Nd, and present-day Pb isotope ratios are: ( 87 Sr 86 Sr ) 290 = 0.7105–0.7110 ; ( 143 Nd 144 Nd ) 290 = 0.51175–0.51221 ; 206 Pb 204 Pb = 18.35–18.50 ; 207 Pb 204 Pb = 15.64–15.76 ; 208 Pb 204 Pb = 38.51–39.03 . Pb isotope ratios determined for separated feldspars show similar 207 Pb 204 Pb and 208 Pb 204 Pb and higher 207 Pb 204 Pb ratios compared to the present-day values for the whole rock. The tonalites (SiO 2 = 57.6%–67.3%) are typically calc-alkaline in composition and most of them are weakly peraluminous. They display large heterogeneities in trace-element abundances, REE patterns and Nd and Pb isotope ratios, both between and within individual intrusive units. Sr, Nd and Pb isotopic compositions display a range of values close to that of the associated granodiorites ( 87 Sr 86 Sr ) 290 = 0.7099–0.7111 ; ( 143 Nd 144 Nd ) 290 = 0.51185–0.51224 ; 206 Pb 204 Pb = 18.31–18.56 ; 207 Pb 204 Pb = 15.65–15.72 ; 208 Pb 204 Pb = 38.41–39.59 . The obtained data indicate that the tonalites and granodiorites share many compositional features which point to a genetic linkage. Likewise, many petrological, geochemical and field data exclude the possibility that all the granodiorites derive from intermediate magmas by any common evolutionary process, such as crystal-liquid fractionation or assimilation-fractional crystallization (AFC), and point to a different genesis for the two rock types. An origin of the granodiorites by the mixing of two separate acidic melts, one of crustal anatectic origin and one probably derived from an intermediate magma by crystal/liquid fractionation or AFC, is suggested by the data presented. The tonalites appear to have been generated by interaction of a mafic magma with crustal end-member(s) with isotopic and geochemical signatures similar to those of the granodiorites. The large involvement of crustal material in the genesis of the Capo Vaticano granitoids is responsible for the many common compositional characteristics observed in the intermediate and acidic rocks. However, a contribution from mantle components is also indicated by petrological and geochemical data, as well as by the occurrence of mafic enclaves commonly present in the tonalites. Nevertheless, the geochemical and isotopic features of such a mantle component appear to be greatly diluted by a large quantity of crustal material involved in the genesis of the studied granitoids.

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).

Dike magmatism in the Sila Grande (Calabria, southern Italy): Evidence of Pennsylvanian–Early Permian exhumation

A dike network transecting a basement of intrusive and metamorphic rocks related to the Hercynian orogeny is exposed in the Sila Grande (southern Italy). Dike magmatism, similarly to other regions of the western Mediterranean, such as Sardinia, Corsica, and Catalonia, is of calc-alkaline to alkali-calcic affinity. Zircon U-Pb geochronology indicates that dike magmatism took place between 295 ± 1 to 277 ± 1 Ma, after the main late Hercynian emplacement of granitoids (306 ± 1 Ma). Barometry indicates that the basement underwent exhumation of 8 ± 3 km before dike injection. The dike network has a geometrical arrangement consistent with a transtensional stress regime that resulted in ductile thinning of the lower crust during the late stage of the Hercynian orogeny and concurrent fracturing of the upper crust that made possible magma ascent through dikes. The proposed tectonic evolution is related to dismemberment of the southern Hercynian belt in the central Mediterranean area as a result of dextral transtension of Gondwana in relation to Laurasia during the Pennsylvanian–Early Permian.