In the Serre massif (southern Calabria, Italy) a Variscan crust section crops out consisting of: i) a middleto low-grade metamorphic rocks in the upper segment, ii) an about 13 km thick “layer” of granitoids and iii) 7-8 km thick lower crust. The deep crust forms the lower part of the section and includes from the bottom to the top: a) layered metagabbros including meta-peridotites; b) felsic and mafic granulites with interleaved metapelites; c) migmatitic metapelites with interleaved metabasites, rare marbles and felsic orthogneisses. The aim of this study is to characterize the geochemistry and the evolution of the Neoproterozoic to Early Cambrian basic magmatism represented by metagabbros and metabasites. The reconstruction of the geodynamic setting in which the basic magmas were emplacement could allow a better understanding of the tectonic evolution of the peri-Gondwana terraines having memory of the West African craton. The effects of the Hercynian metamorphism have modified the mineralogical assemblage and, in some case, have induced partial melting modifying the original compositions. Petrographic and geochemical (major and trace elements) analyses have been performed on 20 samples collected at the base of the deep crust in the gabbroic portion. Two groups of basic rocks have been defined on the basis of the petrographic features. The first group characterized by coarse grained and isotropic texture, consists of gabbros and Qtz-gabbro containing Pl+Opx+Cpx+Amph±Qtz±Grt, rare crystals of biotite can be also present. Frequently, amphibolites with green or brown hornblende form thick layers. On the other hand leucocratic portions showing trondhjemitic composition are interspersed within the main gabbroic body. The second group is characterized by a medium grained and anisotropic texture formed by Pl+Opx+Bt+ k-feld±Cpx±Qtz±Grt. Peculiar characteristics of this rock-type are the abundance of biotite and the presence of pockets of granitic melts having eutectic composition. In both groups, few porphyroblastic garnet crystals occur including amphibole, pyroxene, plagioclase, ± biotite and ± quartz. Frequently, garnet crystals are rimmed by Opx+Pl+Amph or Amph+Pl symplectitic corona in biotite free-rock types or by Bt+Pl±Qtz symplectitic corona in biotite bearing-rock types. The two groups of rocks have variable chemical compositions owing to the variability of their components. The former are subalkaline rocks (Na2O + K2O = 1.17-4.84%) with a content of K2O around 0.47%, whereas the latter are alkaline rocks (Na2O + K2O = 4.88-6.77%) with a content of K2O around 3.9%. The metagabbros show lower Rb, Sr and Nb contents (Rb = 0-12 ppm; Sr = 68-592 ppm; Nb = 0.5-13 ppm) than the Bt-bearing metabasites (Rb = 10-210 ppm; Sr = 389-1044 ppm; Nb = 8-25 ppm). The contents of the other major elements are quite similar despite varying in a large range (SiO2 = 41.53- 60.38% vs. 45.75-51.16%; Al2O3 = 14.71-20.75% vs. 14.72-19.48%; FeO = 6.29-13.99% vs. 8.13-12.54%; MgO = 2.59-9.84% vs. 4.72-7.99%; CaO = 6.82-15.49% vs. 5.73-8.96%) for the presence of cumulitic portions and of differentiated rock type, in addition the Variscan partial melting events complicate the chemical variability. The first data (ACF and A’KF diagrams) indicate a common origin from basic magmas for the two rock types so the alkali enrichment for biotite bearing-rocks can be connected with host rock interaction or due to permeation of melts derived from wall rocks in Variscan times. Different hypotheses about the origin and evolution of metabasic rocks of the Serre, can be proposed.

In Calabria (southern Italy) fragments of south European Variscan chain consisting of ortho- and paraderivates affected by low- medium- high- grade metamorphism crop out. Ortho-derivates involved by green schist to granulite facies metamorphism characterize middle and deep continental crust units. Metagabbros and metabasites affected by granulite facies metamorphism are associated with felsic granulites and migmatitic metapelites of the lower continental crust unit of the Serre. U-Pb spot analyses on zircon separates (performed by SIMS and LA-ICP-MS) have provided constraints on the ages of protoliths and on chronology of Variscan metamorphism. The age of magmatic protoliths of the orthogneisses and of the metabasic rocks was fixed at ~ 540 and ~ 580 Ma, respectively, representing a bimodal magmatic contribution to Pan-African crust in Early Cambrian-Neoproterozoic times. Mafic and felsic magmas were emplaced in the Pan-African/Cadomian basement preserving memory of older terrains belonging to West African Craton. These peri-Gondwana terrains were reworked by Variscan orogenesis and look like the blocks occurring in the east European chain from the Alps to Turkey. The effects of Variscan metamorphism depend on P-T conditions and rock chemical compositions. A time memory, step by step, of the Variscan evolution was preserved in rock-types where the re-growth or recrystallization of zircon happened. So the U-Pb zircon ages together with the petrological data in granuliteamphibolite facies metamorphic rocks of the Serre allow to depict the Variscan evolution. Ages around 450 Ma were recorded in the metaigneous rocks which coincide with the age derived from Rb-Sr isochron relative to metasedimentary rocks; this age can be related to an Ordovician tectonothermal activity or to Eo-Hercynian events. Cluster ages of 347-340 Ma, 323-318 Ma, 300-294 Ma and 279 Ma were measured in metagabbros and metabasites from the lower portions of the section. Age peaks around 323, 300 and 270 Ma are recorded in overlying migmatitic metapelites. Undated thin luminescent rims of zircon are the only evidence of post- Ordovician events in orthogneisses interleaved with migmatitic metapelites. Conventional thermobarometer, Thermocal calculations and pseudosections indicate clockwise trajectories Ky-Sil-And-Ky and Ky-Sil-Ky for the migmatitic metapelites and metabasites, respectively, from the deep continental crust unit of the Serre. The P-T conditions in the metabasic portion indicate a possible P peak (P ≥ 1.08 GPa and T < 850°C) predating the T-peak (T = 850-900°C and P ~ 1.08 GPa) at 347-340 Ma, probably followed by quite isothermal decompression (P = 0.8 GPa) at 323-318 Ma and successive decrease of T (~ 750°C); a new decompression at P about 0.65 GPa around 300 and 280 Ma ago, occurs. During the decompression stages pervasive partial melting episodes, producing leucogranitic and trondhjemitic melts, interested these rocks in which luminescent overgrowths mantle primary zircon cores. The re-constructed P-T path shows even for the upper migmatitic metapelites, P-peak at 0.9 GPa and T = 650°C pre-dating T-peak (750°C) under quite isobaric conditions 323 Ma ago and a multistage decompression at 300, 280 Ma up to 270 Ma with incremental partial melting episodes. An integrate approach involving U-Pb analyses on zircon and REE distribution in garnet orthopyroxene and zircon on thin section, relatively to the mafic granulites better constrain the Variscan evolution.

In situ U-Pb dating combined with SEM images on zircon crystals represent a powerful tool to reconstruct metamorphic and magmatic evolution of basements recording a long and complex geological history [1-3]. The development of high spatial and mass resolution microprobes (e.g., LA-ICP-MS, SIMS, SHRIMP) allows in-situ measurements of U–Pb ages in micro domains smaller than 30-50 microns [4, 5]. The growth of zircon crystals, evidenced by their internal microtextures, can be easily revealed by SEM imaging by Cathodoluminescence (CL) and Variable Pressure Secondary Electrons (VPSE) detectors on separated grains or in situ within a polished thin rock section [6,4,7]. Therefore it is possible to date different domains of single crystals, which may record magmatic or metamorphic events of the rock’s geological history [8,4]. In acidic magmatic rocks abundant zircon crystals provide precise age data about magma emplacement and origin of source indicating the geodynamic context and the pertinence of terranes forming the continental crust.

The mineralogy of ductilely sheared rocks is controlled by the bulk rock composition of the protolith, together with the P-T conditions of shearing. However, the mineral assemblages of shear zones acting as open system may be strongly influenced by the occurrence of mass transfer processes induced by channeling H2O-rich fluids and mobilizing major elements. Major element mobility is also related to the fluid chemistry, which can be affected by the fluid source location, i.e. the shear zone host-rocks or the shear zone far-field. Recent case studies suggest that significant whole rock compositional changes occurred within ductile shear zones in response to fluid infiltration from the host-rocks, whereas other case studies show that whole rock compositional changes within ductile shear zones occurred due to infiltration of fluids from far-field sources. To investigate the presence of common features regarding the gain and loss of mobilized major elements with respect to the thermobaric conditions of shearing and the fluid source, a review of literature case studies dealing with felsic sheared protoliths has been undertaken. Qualitative results suggest high mobility of major elements under greenschist facies conditions whatever the tectonic context. Under compressive tectonics, qualitative outcomes show that Si has the highest mobility whatever the fluid source location and that sheared felsic rocks are always enriched in Mg relative to Fe. Moreover, a preferential gain of Al and Fe with respect to the fluid source is shown.

The Numidian Flysch shows constant lithological features from the strait of Gibraltar to central Italy. It is characterized by quartzarenites showing grains of monocrystalline, rounded and frosted quartz, and by kaolinitic mudstones. This research has pointed out that in the southern Apennines 1) the Numidian Flysch was deposited exclusively in the Campania-Lucania carbonate platform and in the Lagonegro-Molise basin, both located on the Apulian continental margin, and never is present in tectonic units originated from the oceanic area located west of that margin; 2) in the axial zone of the Lagonegro basin it stratigraphically follows a formation consisting of varicoloured clays (Argille Varicolori Auct.); 3) its age is limited to the early-middle Langhian, that is to say, it begins to sediment about 7 million years later than in the Maghrebian chain and deposited for a time span limited to 1-1.5 Ma. The thickness of the Numidian Flysch gradually decreases towards the north from about 600-1,000 meters to a few tens of meters and in some of the northeastern outcrops it is represented only by some layers of quartzarenites. This is accompanied by a decrease in size of the particles becoming more and more finer. In addition, northwards and frequently in the same section, a lower mineralogical and textural maturity (from quartzarenites to litharenites, and presence of abundant matrix, sub-angular, polycrystalline and deformed quartz grains) is well recognizable. In the Campania-Lucania carbonate platform the Numidian Flysch evolves to pelagic marly-clayey deposits, followed by mineralogically immature turbidite sandstones of Serravallian age. In the Lagonegro basin the Numidian Flysch replaces Cretaceous-lower Miocene turbidite deposits, consisting of limestones and red marls, on the western side of the basin, variegated clays in the axial zone and calcareous turbidites or variegated clays in the eastern side. Since the late Langhian, it evolves to pelagic sediments followed by lower Tortonian immature turbidite sandstones. In the successions of the Molise basin the Numidian Flysch is interbedded in a succession consisting of calcareous turbidites and pelagic limestones and marls, reaching the Messinian. The lithological features and the age of the Numidian Flysch in central-southern Apennines, therefore, point out an evolution different from that of the Numidian Flysch of the Maghrebian chain. During the early Miocene, a paleogeographic barrier or other unknown obstacles prevent Numidian sands from reaching the south-Apenninic domains. In the early Langhian, the disappearance of these obstacles allows sands to reach the deep basins located on the Apulian margin. In the late Langhian the Numidian sedimentation is canceled and replaced by mainly pelagic sediments, which will evolve to foredeep deposits in the Serravallian-Messinian time span. In addition, the significant presence of feldspathic and lithic grains testifies a double detrital supply: polycyclic quartzose sands and kaolinitic mudstones from the African craton and metamorphic and plutonic grains from the Hercynian or older rocks of the internal units of the southern Apennines. The Numidian Flysch of the southern Apennines allows to assign the tectonic units in which is present to the Campania-Lucania carbonate platform or to different zones of the Lagonegro-Molise basin and therefore is of great importance in the reconstruction of both the Mesozoic-Cenozoic paleogeography and a tectono-sedimentary evolution very difficult to decipher, given the convergence of sedimentary facies in the Apenninic deep basins since Cretaceous to Miocene, the presence of several tectonic phases and of out of sequence and back-thrusts.

The lower crust of the Serre massif (Calabria, southern Italy) provides a window into the mid- to lower crust of the south European Variscan orogenic belt. Previously, zircon U-Pb ages were employed to date high-temperature processes affecting this portion of the Variscan crust. The present paper reports new LAICP- MS U-Pb data on the zircon of a deformed quartzmonzodiorite dike and of three mafic granulites sampled at the base of the lower crust section. Determination of trace elements on zircon, including rare earth elements (REE), has been also performed. The end of the Variscan exhumation, dated by anatectic zircon from migmatitic metapelites, and the growth-modification of zircon with respect to the growth of Variscan metamorphic garnet have been assumed as “time markers”. The concordant zircon ages of the metamorphic basic rocks cover a range from 744±20 Ma to 231±10 Ma with high age density from 357±11 Ma to 279±10 Ma, a few ages comprised between 418±14 Ma and 483±12 Ma and between 505±11 Ma and 593±14 Ma. Zircon from the deformed quartz-monzodiorite dike evidences a minimum age of emplacement of 323±5 Ma. Most of the analysed zircon domains dated between 357±11 Ma to 279±10 Ma from garnet-bearing metabasic rocks show flat patterns of heavy rare earth elements (HREE), as expected in the case of their simultaneous growth with garnet. This allows to consider (1) zircon domains giving Variscan ages as “metamorphic” with specific geological significance, and (2) zircon domains with ages ranging from 564±17 Ma to 593±14 Ma as dating the emplacement of the magmatic protoliths as shown by internal microtextures, fractionated patterns of HREE and Th/U ratios (0.16– 0.19). The Variscan zircon ages (357–279 Ma) reflect effects of crustal thickening, peak metamorphism and subsequent multistage Variscan decompression documented by the statistically significant clusters of ages around 347–340 Ma, 323–318 Ma, 300–294 Ma and 279 Ma. The U-Pb zircon ages of the metabasic rocks suggest a period of about 60–70 Ma for granulite facies metamorphism and anatectic conditions. Literature data indicate that the migmatitic metapelites of the upper part of the Serre crust section also underwent a long period, about 40 Ma, of granulite facies metamorphism and anatectic conditions. A diachronism emerges through the time comparison of the Variscan evolution between the upper and the lower portions of the Serre deep crust. The duration of the Variscan processes defined in Calabria is comparable to that of other south European Variscan blocks.

U-Pb zircon ages and chemistries of zircon, garnet and orthopyroxene from different structural sites of a thin section of a granulitic rock from the Serre (Calabria, southern Italy) have been considered in order to reconstruct the evolution of a fragment of south European Variscides. The rock consists of Pl+Grt+Bt+Opx+Qtz+K-feld and accessories as zircon, apatite and opaques. The matrix shows layers containing different abundances of biotite and orthopyroxene. One porphyroblastic garnet (~ 3 cm in diameter) rimmed by a Opx+Pl+Bt+opaques+Zrn+Ap symplectitic corona occurs. Ten zircon grains are exposed in the thin section: eight in the Bt-rich matrix, one within the corona and one in the rim of the garnet. Zircon shows generally core-rim structures. Frequently, zircon cores appear chaotic, consistently with modification induced by high-grade metamorphism and some are rimmed by unstructurless lobate rims, invading cores, commonly related to interaction with fluids and/or melts. U-Pb analyses have been performed in situ by LA-ICP-MS (IGG-CNR, Pavia). Two core domains of zircons from the matrix give the oldest concordant ages: 357±11 and 334±12Ma; five concordant data ranging from 324±12 to 320±11Ma give a mean concordia age of 323±2Ma. Two younger concordant ages (305±9 and 300±9Ma) are relative to low luminescent inner rims and similar apparent ages have been determined on luminescent zircon rims from matrix, symplectitic corona and garnet rim. Trace element compositions were collected on specific domains of zircon, garnet and orthopyroxene in different textural sites by LA-ICP-MS. Two trace element profiles relative to porhyroblastic garnet were performed. Garnet is very rich in middle-REE and shows nearly flat profile in the core and fractionated pattern in the rim. The transition zone from core to rim (outer core) is poorer in HREE. Zircon is characterized by a steep pattern with low LREE and high HREE contents both in cores and rims in which, however, the MREE and HREE are lower than in garnet, apart from its outer core region. The orthopyroxenes show fractionated patterns of MREE and HREE. The apparent DHREE between zircon-garnet and orthopyroxene-garnet pairs have been calculated and compared with literature data considered as suggestive of equilibrium relatively to natural samples and experimental results. The calculated DREEzrn/grt and DREEopx/grt define linear and positive trends for zrn or opx/core and outer core combinations of grt evidencing regular, but variable partitioning and reaching even values higher than unity. So the calculated DREE values seem to be indicative of steps of approaching equilibrium. The combination zrn rim/grt rim, instead, indicates disequilibrium suggesting that the zircon rim formed earlier than garnet rim, which subsequently broke down producing corona. The combination with the outer core of garnet gives higher D than the combination with the inner core suggesting, according to experimental results, lower temperature for the former; this is in agreement with the petrology. The calculated DREEopx/grt approach those suggestive of equilibrium from Er to Lu in the combination opx matrix-grt core. The combination opx corona-grt rim is far from the equilibrium. Probably, orthopyroxene from corona is a residue of the matrix. Accordingly, a well constrained contribution to PTt path relatively to fragments of south European Variscides emerges: the Variscan metamorphism of the lower part of the Serre crust section peaked much earlier than previously assumed (at least ~ 340 Ma ago against 300 Ma) and the multistage Variscan decompression after the metamorphic peak lasted tens millions of years, at least locally. The decompression probably protracted after 300 Ma.

The aim of this paper was the U-Pb zircon dating in paragneiss of the Castagna Unit outcropping in Catena Costiera (CC) and Sila Piccola and in augen gneiss of the same Unit outcropping in CC to evaluate the similarities or the differences of the geological evolution of the continental crust portions belonging to the same unit and dislocated along the Alpine Chain in Calabria. The measured ages in the paragneiss from Sila Piccola form clusters at 2700 Ma, 2100 Ma, 1700 Ma, 1031-722 Ma, 637 Ma, 559-515 Ma and 499-421 Ma. These ages are comparable with those measured in augen gneisses of Sila Piccola (Fornelli et al., 2014). The magmatic protoliths of augen gneisses intruded an older metamorphic basement (paragneiss) at about 540 Ma and the whole sector was involved by Ordovician-Silurian tectonic. Both metasedimentary rocks and augen gneisses record the same geological events. In the paragneiss from CC the measured age clusters are 2700 Ma, 2100 Ma and 1025-864 Ma and only one age at 638 Ma. In the two samples of augen gneisses from this area, the whole measured ages form clusters at: 1612 Ma, 975- 758 Ma, 583-512 Ma, 508-495 Ma and one age at 327 Ma. The revealed ages in augen gneisses from CC, are comparable with the ages measured in other augen gneisses both from Calabria and Sicily, except the record of Variscan metamorphism (327 Ma) emerged only in CC area. A different geological history, instead, results by paragneisses from CC considered as host basement of Neoproterozoic-Cambrian acidic magmatism (Fornelli et al., 2014). In these paragneisses are missing the records of: 1) Ediacarian magmatism (540 Ma, intrusion ages of protoliths of augen gneisses); 2) Ordovician-Silurian tectono-thermal event (495-420 Ma); 3) Variscan metamorfism (327 Ma). The missing of these ages in the paragneiss of CC represents a problem. Two hypotheses can be made: 1) the data ages are scarce, more determinations must be performed on other samples; 2) a different geological history was followed by the two sectors of continental crust, in particular, the paragneisses from CC could not represent the basement in which the protoliths of augen gneisses were intruded and, in addition, the augen gneisses outcropping in CC were involved by Variscan metamorphism resetting the zircon age, that for the first time emerges in these rock types. In fact the augen gneisses in Calabria and Sicily display a comparable spectrum of zircon ages comprised between 3242 Ma (Williams et al., 2012) and 421 Ma without Variscan ages.

The paper deals with the U–Pb data of zircon separated from three samples representative of mylonitic leucogranites, trondhjemites and pegmatites occurring along the Alpine tectonic zone between the Castagna and Sila Units in northern Calabria. These mylonites are associated to Variscan granitic-granodioritic biotiterich augen gneisses derived from Neo-Proterozoic-Early Cambrian protoliths. Apparent ages ranging from Early Cambrian to post-Variscan have been obtained. Th, U and rare earth elements have been determined in two zircon domains of mylonitic leucogranite and trondhjemite giving different ages in order to get information relative to their geological significance. The pegmatite preserves intrusive contact with the augen gneisses and with the other mylonites; it turns out to be emplaced at 290–300 Ma, like the Variscan plutonites of the Castagna Unit. The deformation masks the original contacts of the mylonitic leucogranite and trondhjemite with the biotite-rich augen gneisses. The age-group averaging 540 Ma is interpreted as indicative of the emplacement of the protoliths and it coincides with the age previously determined for the emplacement of the protoliths of the biotite-rich augen gneisses. Zircon from the mylonitic pegmatite includes domains showing concordant and discordant ages younger than 290 Ma, thus reflecting various degrees of partial resetting and Pb-loss caused by post-Variscan events. Zircon from the mylonitic leucogranite and trondhjemite includes apparent ages between 300 and 280 Ma as well as ages younger than 250 Ma. Perturbation of U–Pb system by Alpine shearing appears evident; however, possibile effects caused by thermal input and hydrothermal fluid infiltration from the Variscan plutonites cannot be excluded.