Alfredo Caggianelli

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.

Modelling the thermal perturbation of the continental crust after intraplating of thick granitoid sheets: a comparison with the crustal sections in Calabria (Italy)

Thick granitoid sheets represent a considerable percentage of Palaeozoic crustal sections exposed in Calabria. High thermal gradients are recorded in upper and lower crustal regional metamorphic rocks lying at the roof and base of the granitoids. Ages of peak metamorphism and emplacement of granitoids are mostly overlapping, suggesting a connection between magma intrusion and low-pressure metamorphism. To analyse this relationship, thermal perturbation following granitoid emplacement has been modelled. The simulation indicates that, in the upper crust, the thermal perturbation is short-lived. In contrast, in the lower crust temperatures greater than 700 °C are maintained for 12 Ma, explaining granulite formation, anatexis and the following nearly isobaric cooling. An even longer perturbation can be achieved introducing the effect of mantle lithosphere thinning into the model.

Cooling and exhumation history of deep-seated and shallow level, late Hercynian granitoids from Calabria

The thermal and exhumation history of late Hercynian granitoids from Calabria (Sila and Serre massifs) has been studied using thermobarometry and radiometric age determinations. The uplift and erosion which followed contractional tectonics of Tertiary age exposed in Calabria a nearly complete section of the Hercynian crust. Field data, constrained by igneous thermobarometrical data, have enabled us to draw simplified crustal profiles. In both the Sila and Serre massifs, granitoids make up the intermediate portions of the crustal sections and are stacked as tabular intrusions for up to 13 km cumulative thickness. Shallow granitoids are characterized by a weak fabric, mostly developed in the magmatic stage, whereas deep-seated granitoids display a strong fabric developed in the magmatic state and, with decreasing temperatures, in the subsolidus state. The intrusive bodies were emplaced at 300–290 Ma, at a time when the Calabrian crust was undergoing extensional tectonics and crustal thinning. The subsequent post-Hercynian evolution is recorded by Rb-Sr dates of micas and fission track ages of zircon and apatite obtained from granitoids emplaced at different depths. A decrease in Rb-Sr and fission track ages is observed as depth of emplacement increases. Data on the post-Hercynian geological evolution of Calabria were used to model in three stages the cooling and exhumation history of deep-seated and shallow granitoids. The first stage, in Permian to Triassic times, was characterized by slow erosion. It was followed by a second stage of extensional tectonics in Jurassic times. The third stage was exhumation during the Apenninic Orogeny. The model has generated two P–T–t arrays, one for deep-seated and the other for shallow granitoids of the Serre massif. The T–t paths suggest that the dates of micas, zircon and apatite are cooling ages. They also show that deep-seated granitoids remained at temperatures above the brittle–plastic transition for a long time, whereas shallow granitoids cooled rapidly. Distinct P–T–t paths explain why deep-seated and shallow granitoids display different fabric and microstructural features. Copyright

Time Constraints on the Building of the Serre Batholith: Consequences for the Thermal Evolution of the Hercynian Continental Crust Exposed in Calabria (Southern Italy)

New laser ablation inductively coupled plasma mass spectrometry U-Pb dating on zircon and monazite was performed to estimate the time required for the building of the Serre batholith in Calabria. Age spectra from the bottom and top of the pluton are characterized by two main peaks at 306 and 295 Ma, resulting from a mutual interference between serial intrusive events. On this basis, the emplacement of the top granodiorite layer postdates by about 10 m.yr. emplacement of the lower tonalite layer. These results have been incorporated into a two-dimensional numerical thermal model, assuming overaccretion of a batholith in an extensional tectonic regime. With this approach it was possible to reproduce pressure-temperature paths for various levels of the continental crust and define timing for low-pressure regional and contact metamorphism. In a unique tectonomagmatic scenario the model reproduces regional low-pressure metamorphic effects in the lower to intermediate continental crust and, with a time lag of about 6 m.yr., contact metamorphism in the upper crust. Finally, we propose a conceptual model for the emplacement of the Serre batholith in an extensional tectonic setting. Space for magma can be created by lower crust thinning and rock uplift at the bottom and top of the batholith, respectively.

Post-emplacement thermo-rheological history of a granite intrusion and surrounding rocks: the Monte Capanne pluton, Elba Island, Italy

Abstract A thermo-rheological model of the Monte Capanne pluton, Elba Island, Italy is proposed as having general relevance for the thermal and tectonic evolution of upper crustal granites and their surrounding rocks in extensional regions. The thermal evolution of the pluton and country rocks is followed for 1 myr after emplacement, which occurred at c. 6.9 Ma. The pluton completely crystallized in c. 210 kyr (±20%). The adjacent rocks reached a thermal peak of 550 °C (±10%), maintaining a temperature higher than 500 °C for c. 100 kyr. The temperature distribution is used to construct a model for the time-dependent rheology of the pluton and surrounding rocks. A series of 2D cross-sections shows an upward migration of the regional brittle−ductile transition, and the formation of a ductile horizon above the pluton. The former is a combined effect of unroofing and middle crust heating; the latter is the result of temperature increase in rheologically weak country rocks. This ductile horizon has a potential role in the tectonic evolution of the region, since it could favour the formation of upper crustal shear zones and listric faults rooting in the transient brittle−ductile transition and playing a major role in further post-emplacement extension.

Late Pliocene volcaniclastic products from Southern Apennines: distal witness of early explosive volcanism in the central Tyrrhenian Sea

Two volcaniclastic successions intercalated in Pliocene basinal clays from the Southern Apennines have been analysed to determine their provenance and their relationship with the geodynamic evolution of the Western Mediterranean. The studied deposits are exclusively made up of ashy pyroclasts, dominated by fresh acidic to intermediate glass, mostly in the form of shards, pumice fragments and groundmass fragments with vitrophyric texture. Crystals include Pl, Opx, Cpx, Hbl and rare Bt. Sedimentological features suggest that the volcanic material accumulated near the basin margin by primary fallout processes and was later remobilized by density currents. 40 Ar– 39 Ar geochronology allowed dating of one succession at 2.24 ± 0.06 Ma, corresponding to the Late Pliocene. Composition of the volcaniclastic material is typical of a transitional high-K calc-alkaline series. The age and chemical composition constrain the provenance of the volcaniclastic rocks from the Southern Tyrrhenian domain. Here, volcanic centres were active during Pliocene time, approximately at the northern end of a volcanic arc formed before the opening of the southernmost part of the sea. This paper shows that a detailed study of volcaniclastic products from the southern Apennines and Calabria can be very useful in collecting new pieces of information on the eruption history of the southern Tyrrhenian domain, since they record additional data not available from the study of exposed volcanic edifices.

Late-Hercynian shearing during crystallization of granitoid magmas (Sila massif, southern Italy): regional implications

Shearing of regional extent, involving granitoids and underlying mid-crustal rocks of the Sila massif (Calabria, Italy), is analysed in this paper. The deformed granitoids are affected by a wide NNW-SSE oriented deformation zone, stretching for about 60 km, from the neighbourhood of Cecita Lake to Cropani village. Meso- and micro-structures in granitoids, close to the boundary with underlying migmatitic paragneiss, indicate that deformation developed from melt-present to solid-state conditions. Simultaneous tectonics and magmatism activated a plutonic accretionary process at mid-crustal levels. This took place at about 300 Ma and involved hybrid magmas with a dominat contribution from a mantle source. The deformation regime remained steady for a long time during magma crystallization and cooling in subsolidus conditions. The regional top-to-the-W sense of shear in the present geographic coordinates, recorded in the deformed granitoids, seems geometrically consistent with the coeval direction of maximum extension found in another sector of the southern Hercynian belt, suggesting the original position of the Sila basement in this context. Magmatic ativity ended with the intrusion of mafic and felsic magams affected by a very weak deformation, ongoing during the final strain increments of the late-Hercynian stage.

Time–space relationships among structural and metamorphic aureoles related to granite emplacement: a case study from the Serre Massif (southern Italy)

Tectonic and thermal perturbations, related to emplacement of granodiorite in the upper continental crust, have been investigated in the late-Hercynian basement exposed in southern Calabria (Italy). Here, the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km. Geometrical analysis of the structural aureole and related foliations, lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact. This pattern is compatible with a laccolith-like mode of magma emplacement, related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks. The metamorphic aureole, about 1800 m wide, is characterized by biotite, cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact. The peak assemblage equilibrated between 535 and 590°C at pressures between 175 and 200 MPa, confirmed by Al-in-hornblende barometry on granodiorite. Microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations. With the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations. This estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 × 10 −14 s −1 within the country rocks during granodiorite intrusion.

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.

From the upper to the lower continental crust exposed in Calabria

Outcrops of the pre-Mesozoic basement, representative of the whole Hercynian continental crust are exposed in Calabria. This is the result of Tertiary geological evolution that brought to the surface different crustal levels. This geological field trip aims to provide a general picture of the continental crust that hopefully may represent a reference frame for geochemical, rheological and geophysical models. The itinerary develops in central and southern parts of Calabria, namely in the Serre massif and in the promontories of Capo Vaticano and Monte Sant’Elia. In three days it is possible to examine compositional and structural features across an entire crust section. Thus rocks affected by very low-grade to granulite facies metamorFrom the upper to the lower continental crust exposed in Calabria A. Caggianelli G. Prosser V. Festa A. Langone R. Spiess geogical feld tips 2013 51.2) DOI: 10.3301/GFT.2013.02 in fo rm a io n 5 phism and distinctive features of granitoids emplaced at different structural levels will be examined. The effects of the intense thermal perturbation produced by granitoid emplacement are visible both in the upper and in the lower crust, in a sharp metamorphic aureole and in a migmatitic border zone, respectively. Finally, some cases of Paleozoic rocks with strongly partitioned deformation, produced by Tertiary tectonics in the brittle and the ductile domains, can be observed.