Building an Orogen: Review of U-Pb Zircon Ages from the Calabria–Peloritani Terrane to Constrain the Timing of the Southern Variscan Belt

: The application of zircon dating to the reconstruction of orogenic systems is invaluable since time constraints of the geological evolution of orogens are crucial for the proposal of geodynamic and paleogeographic models. Zircon is one of the most promising accessory minerals in geochronology of crystalline basements because of its high-closure temperature. Moreover, U-Pb data of relict and recrystallized grains indicate the maximum sedimentation age as well as the timing of metamorphism in metasediments. In addition, the U-Pb ages of magmatic zircons constrain the timescale of magmatism. The Calabria–Peloritani terrane (CPT) represents a key area in the Southern Variscan Belt, whose reconstruction is still unresolved. Therefore, a review of literature zircon age data accompanied with new data from six samples of orthogneisses, paragneisses, amphibolites, and actinolite schists, helps to constrain the evolution of this Cadomian fragment, a ﬀ ected by metamorphic and magmatic Variscan events. A revisiting of the timing of the geological events from Paleo-proterozoic to Permian is revealed by comparing the internal textures of zircons and their U-Pb age clusters. The detected age peaks at 2500 Ma, 1600 Ma, and 1000 Ma in the CPT were related to a provenance from West and East Gondwana realms. A sedimentation age around 630 Ma emerges for the middle-deep crust terranes of the CPT, a ﬀ ected by Ediacaran (579–540 Ma) intrusions, accompanied by metamorphism dated at 556–509 Ma in the host metasediments. In the following, during Ordovician–Silurian extensional tectonics, the former Cadomian terranes were at least locally a ﬀ ected by ﬂuid-assisted metamorphism (around 450 Ma) whereas the upper extensional basins that formed, were inﬁlled by sediments along with interspersed volcanic to subvolcanic products. All these pre-Silurian terranes were involved in the subduction process of the Palaeotethys–Gondwana margin beneath Laurussia. The compressive phase began around 347 Ma, with under-thrusting of the formerly Gondwana substrate that was subjected to middle-high grade metamorphism, while the Ordovician–Silurian sediments were scraped o ﬀ along the front of the Southern Variscan Belt and a ﬀ ected by low-grade metamorphism. Decompression of the whole Variscan orogenic system started around 320 Ma, together with uplifting of the chain and emplacement of widespread granitic intrusions which ended around 280 Ma and completed the Variscan orogenic cycle in the CPT.


Introduction
Zircon is one of the most widespread accessory minerals in magmatic, metamorphic, and sedimentary rocks. Its excellent capacity to record the geological events makes it an indispensable tool for petrogenetic studies.
The ability of zircon to incorporate and retain trace elements such as Hf, U, Th, Y and rare earth elements (REE) as direct substitution into the Zr site [1] permits to evaluate the precise process of its formation and the timing of growth, using the U-Pb decay schemes. However, the U-Pb zircon ages must be accurately interpreted because several processes can disturb the U-Th-Pb systematics of crystals, such that the obtained ages can be without geological significance. The observations of habitus and internal zoning textures through Cathodoluminescence (CL), variable pressure secondary electron (VPSE), and back-scattered electron (BSE) images of zircon grains are essential for this issue, allowing the distinction among domains having igneous, metamorphic, detrital, or hydrothermal origin [2][3][4][5][6].
In the magmatic context, the regular oscillatory zoning is indicative of slow phase of growth from melt, while the euhedral crystals with development of 010 prism faces without internal structures, are generated by rapid growth in volcanic contexts ( Figure 1a).
Obviously, these peculiarities may not be conclusive; therefore, only a large number of U-Pb spot analyses can constrain the timing and the context of the crystal growth and permit reliable dating of magmatic rock. Experimental data established the increase of the distribution coefficient of Heavy Rare Earth Elements (HREE) between zircon and melt (Kd HREE Zrn/Melt ) with the decrease of temperature in the range 800-1050 • C at 20 kb in felsic rocks [6]; consequently, a HREE fractionated pattern of zircon can be assumed as typical of magmatic growth in acidic melts. Moreover, signatures of magmatic growth can also be derived from Th/U ratios being higher in magmatic zircon (i.e., Th/U > 1.0) [7]. Many exceptions are proposed for the chemical parameters defined earlier. One single parameter is not conclusive, but all the petrological, chemical, and structural observations contribute to the definition of the crystal origin. More complex interpretations regard the ages of metamorphic zircons (Figure 1b). Homogeneous luminescent zircon rims without internal textures or perturbed crystals were related to metamorphic growth ( Figure 1b) [2]. The challenge in metamorphic petrology is the dating of the single metamorphic phase in the pressure-temperature (P-T) evolution, hence, the partition of REE and the Th/U ratios of zircons become crucial. In granulitic terranes, garnet can be considered as a valuable "time marker" when associated with zircon [6,8]. Indeed, zircons with flat HREE pattern and Th/U ratio < 0.1 could be indicative of metamorphic growth synchronous with HREE-rich garnet competitor. However, a careful assessment of mineral parageneses (such as garnet and amphibole) coexisting with zircon at P-T conditions of amphibolite-granulite and eclogite facies [9,10], may help to constrain the P-T-t trajectories and to delineate the evolution of poly-metamorphic terranes [6,8,[11][12][13][14][15][16]. Apart the growth from melt, also the interaction with a fluid phase under low grade metamorphic conditions might produce a resetting of U-Pb system of zircons [7]. Therefore, incoherent data ages (rim older than core) can be related to the opening of the U-Pb zircon system in which fluids played a role. Finally, hydrothermal fluids connected to magmatic intrusion can lead, in the surrounding rocks, to new zircon growth (Figure 1b, ZrnE), dating the magmatic emplacement event [17].
Zircons from poly-metamorphic terranes usually contain signatures of inherited grains (Figure 1b, Tur32-Zrn68); the careful dating and interpretation of relict grains contribute to define the origin of protoliths and, in favorable cases, the geological evolution of basement rocks from the depositional stage to the last metamorphic event. The opportunity to establish the sedimentation age of protoliths of metamorphic rocks was also used in the study of detrital zircons in sedimentary rocks (Figure 1c). Common applications of detrital zircon U-Pb ages include determination of sediment provenance, constrains of depositional age, reconstructions of dispersal sediment pathways [5,18], which are extremely useful in paleogeographic studies. The application of U-Pb dating of detrital zircons in volcanoclastic sandstones helps to define the age of deposition, synchronously with volcanic activity in sin-orogenic deposits where the remnants of fossils fail the precise dating [19][20][21].  [17], CL images, and rapid growth in quite homogeneous luminescent volcanic zircons (Zrn45, Zrn56A), modified from [21], VPSE images. (b) Older cores of zircons showing younger and homogeneous overgrowths indicating multiple metamorphic crystallization, VPSE images: Zrn47 and Zrn24 modified from [8]; Zrn68, original; ZrnE modified from [7]. (c) Rounded, detrital zircons from sandstones, VPSE images: Zrn12 and Zrn20; Zrn34 modified from [22].
The brief summary of the applications and possible ambiguities of the U-Pb zircon dating highlights the greater complexity of age interpretations in metamorphic rocks since they may contain zircons inherited from metamorphic, magmatic, and sedimentary rocks and recrystallized crystals in one or more metamorphic events. A great challenge is to bring together, on a regional scale, the time evolution of crystalline rocks forming an orogenetic system.
A well-selected set of concordant zircon ages collected in different rock types forming the orogenic systems, allows the reconstruction of geological events of an orogen. The rock types of younger orogens may contain records that cover a wider age range than the orogenic cycle times, precisely these characteristics favor the paleogeographic reconstruction of the tectonic phases that lead to the formation of orogenic chains.
The power of zircon dating is used in this paper to present a review of spot U-Pb zircon data collected in the last 15 years from the metamorphic Calabria-Peloritani terrane (CPT) (southern Italy), in order to synthesize and link the large number of dating dispersed in several research papers. In particular, Ordovician ages are more and more frequent, and need to be interpreted in the framework of the peri-Gondwana terranes involved in the Southern Variscan Belt growth. The literature data  [17], CL images, and rapid growth in quite homogeneous luminescent volcanic zircons (Zrn45, Zrn56A), modified from [21], VPSE images. (b) Older cores of zircons showing younger and homogeneous overgrowths indicating multiple metamorphic crystallization, VPSE images: Zrn47 and Zrn24 modified from [8]; Zrn68, original; ZrnE modified from [7]. (c) Rounded, detrital zircons from sandstones, VPSE images: Zrn12 and Zrn20; Zrn34 modified from [22].
The brief summary of the applications and possible ambiguities of the U-Pb zircon dating highlights the greater complexity of age interpretations in metamorphic rocks since they may contain zircons inherited from metamorphic, magmatic, and sedimentary rocks and recrystallized crystals in one or more metamorphic events. A great challenge is to bring together, on a regional scale, the time evolution of crystalline rocks forming an orogenetic system.
A well-selected set of concordant zircon ages collected in different rock types forming the orogenic systems, allows the reconstruction of geological events of an orogen. The rock types of younger orogens may contain records that cover a wider age range than the orogenic cycle times, precisely these characteristics favor the paleogeographic reconstruction of the tectonic phases that lead to the formation of orogenic chains.
The power of zircon dating is used in this paper to present a review of spot U-Pb zircon data collected in the last 15 years from the metamorphic Calabria-Peloritani terrane (CPT) (southern Italy), in order to synthesize and link the large number of dating dispersed in several research papers. In particular, Ordovician ages are more and more frequent, and need to be interpreted in the framework of the peri-Gondwana terranes involved in the Southern Variscan Belt growth. The literature data and new U-Pb zircon ages are presented to add more constraints for the magmatic, metamorphic, and sedimentary history, from Precambrian to Permian, of the CPT.

Geological Framework of the Calabria-Peloritani Terrane
Within the Apennine belt, which started to evolve during Oligocene, the CPT, i.e., the 'calabro-peloritano arc' sensu [23], is a stack of Eocene nappes characterized by both metamorphic rocks of the former Jurassic-Cretaceous Neo-Tethys oceanic crust and overlying metamorphic and intrusive rocks of Variscan continental crust derivation [24] (Figure 2).
The largest nappe, both in terms of thickness and areal extent, is represented by the Sila-Serre Unit, and is well exposed in the homonymous massifs [35] (Figure 2). This unit includes a nearly complete but thinned late Variscan continental crust that is approximately 23 km thick [25,36] (Figure 2). The lower crustal level, equilibrated under granulite facies conditions, consists of metagabbros, mafic and felsic granulites, which are topped by migmatitic metapelites and metagreywackes with interbedded metabasite layers [8]. A thick batholith made up of Late Carboniferous-Early Permian granitoids occupies the mid-crustal level [37], while successions dominated by paragneisses or phyllites occur at the top, i.e., the so-called Mandatoriccio Paragneiss Unit and the Bocchigliero Phyllite Unit, in the Sila Massif, and the Mammola Paragneiss Unit and the Stilo-Pazzano Phyllite Unit, in the Serre Massif. The phyllite units and the underlying paragneiss ones were already tectonically juxtaposed, when the Late Carboniferous-Early Permian granitoids were intruding between the deep and the middle-upper crust [38][39][40]. The emplacement of the late Variscan granitoids determined a contact aureole in the middle-upper crustal levels, which was characterized by fluid-saturated or -undersaturated metamorphism near the batholith [41][42][43][44][45].

Pre-Variscan Ages in the CPT
Records of Pre-Cambrian to Devonian magmatism, sedimentation, metamorphism, and anatexis are preserved in many tectonic units of the CPT, which were later subjected to Variscan orogeny [65]. Information on the pre-Variscan history of the CPT are provided by U-Pb zircon ages preserved in Figure 2. Structural sketch map (above) and 2D structural sketch (below), not to scale, of the CPT (modified from [35,56]).

Pre-Variscan Ages in the CPT
Records of Pre-Cambrian to Devonian magmatism, sedimentation, metamorphism, and anatexis are preserved in many tectonic units of the CPT, which were later subjected to Variscan orogeny [65]. Information on the pre-Variscan history of the CPT are provided by U-Pb zircon ages preserved in magmatic and metamorphic rocks belonging to the intermediate (i.e., the Castagna Unit, the Mandatoriccio Unit, the Aspromonte-Peloritani Unit), the upper (i.e., the Lower Complex), and the deep crustal sectors (i.e., the deep crust of the Sila-Serre Unit) ( Figure 2). These ages record the tectonic evolution of peri-Gondwana fragments already from Proterozoic. Records of these geological events are synthetized in the following, accompanied by new U-Pb zircon ages obtained for some crucial lithologies, with the aim of filling some literature gaps. The new U-Pb age data have been obtained on zircons from orthogneisses (AM1, FB1 samples), a paragneiss (GO59 sample), and an actinolite schist (GO176bis sample) of the Castagna Unit cropping out in Catena Costiera and Sila Massif (Figure 2), and from amphibolites (AS53, AS53bis samples) of the Aspromonte-Peloritani Unit cropping out in the Aspromonte Massif ( Figure 2).
The analytical methods for the new U-Pb age data on zircon are detailed in the Appendix S1. The new U-Pb age data of analyzed zircons are reported as a supplementary data file (Table S1). A synthesis of the available literature data of zircon ages from all the CPT units mentioned above, along with the new age data we obtained, is shown in Table 1. In the following, the distribution of age data in the Castagna Unit, the Aspromonte-Peloritani Unit, the Lower Complex, and the Sila-Serre Unit is described.

Age Distribution in the Castagna Unit
According to the several dating carried out in the last 15 years on orthogneisses and muscovite-leucocratic gneisses of the Castagna Unit outcropping in the Sila Massif (Figure 2), their protolith ages have been tightly constrained in the Ediacaran, being 543 Ma and 540 Ma, respectively [17,66,67]. On the other hand, the distribution of zircon ages in their host rocks such as the paragneisses and the actinolite schists, and in the orthogneisses from Catena Costiera (Figure 2), was so far missing. New LA-CP-MS U-Pb zircon data were collected from a paragneiss sample (GO59) coming from the Sila Massif (39 • 01 27" N Lat.; 16 • 33 25" E Long.), an actinolite schist sample (GO176bis) coming from the Catena Costiera (39 • 08 58" N Lat.; 16 • 06 28" E Long.), and, in addition, two samples of orthogneiss intruded in metasediments from Catena Costiera were also analyzed. A complete U-Pb data set on zircons from these samples and relative Concordia diagrams are shown in Table S1 (Supplementary Materials), the most significant clusters are included in Table 1. Twenty-seven zircon grains from GO59 sample, produced 38 concordant ages covering a time span from 2085 ± 34 to 421 ± 6 Ma and 10 discordant ages. In GO176bis sample, 10 zircon grains gave 14 concordant ages from 2700 ± 38 to 596 ± 8 Ma and five discordant data. The new collected data from these samples are shown in Figure 3a.

Age Distribution in the Aspromonte-Peloritani Unit
The protolith age of the orthogneisses of the Aspromonte-Peloritani Unit, was dated around 545 Ma by [56,61], with U-Pb systematics on zircons. Their host rocks are represented by paragneisses, which were dated through the same systematic by [55] in the Peloritani Mountains ( Figure 2). We performed new U-Pb zircon dating (Table S1) from amphibolites (AS53 and AS53 bis samples, 37 • 55 40" N Lat.; 15 • 58 30" E Long.) interbedded within paragneisses in the Aspromonte Massif. Ten analyzed zircon grains gave 19 concordant ages covering a time span from 2463 ± 51 Ma to 368 ± 6 Ma and three discordant ages. The spectrum of new acquired data of the amphibolites is included with that of the paragneiss from [56,61] in Figure 4. Single zircon data of amphibolites are reported in Table S1 with relative Concordia diagrams. Age clusters of the amphibolite and paragneiss correspond to 2668 ± 16 Ma, 2455 ± 40 Ma, 2167 ± 36 Ma, 1573 ± 42 Ma, 1010 ± 16 Ma, 945 ± 13 Ma, 767 ± 10 Ma, 631 ± 7 Ma, 541 ± 6 Ma, and 380 ± 6 Ma. These age data correspond to inherited grains (Figure 4, Zrn13), some of these showing younger recrystallized domains around 541 ± 6 Ma (Figure 4, e.g., Zrn71). Most of the age peaks correspond to those detected in the previous units. Substantial differences are the missing of age peaks around 450 Ma and the presence of Devonian signature (Figure 4, Zrn3 dated at 391 ± 5 Ma).

Age Distribution in the Lower Complex
Available U-Pb zircon ages from the literature were obtained from felsic porphyroids and metaandesites interbedded with very low-grade metasediments of the Lower Complex cropping out in the Peloritani Mountains (Figure 2). The crystallization age of their protoliths was in the range of 461-

Age Distribution in the Lower Complex
Available U-Pb zircon ages from the literature were obtained from felsic porphyroids and meta-andesites interbedded with very low-grade metasediments of the Lower Complex cropping out in the Peloritani Mountains ( Figure 2). The crystallization age of their protoliths was in the range of 461-432 Ma [62], which was interpreted as volcanic-subvolcanic activity, synchronous with the deposition of the protoliths of the metasediments. However, inherited zircon ages corresponding to Paleo-proterozoic (2013 ± 1 Ma) and Meso-proterozoic (1140 ± 10 Ma) were also collected, together with younger Devonian ages of 401 ± 20 Ma and 367 ± 13 Ma [62] (Table 1).

Age Distribution in the Sila-Serre Unit
Many U-Pb zircon ages are available for the lower continental crust of the Sila-Serre Unit (i.e., mafic and felsic granulites, metagabbros, migmatitic paragneisses, metabasites, and orthogneisses) cropping out in the Serre Massif [8,66,69] (Figure 2) and in a garnet-biotite gneiss belonging to a Cenozoic tectonic melange near the northern border of the CPT [70], as well as for the paragneisses of the Mandatoriccio Paragneiss Unit [71]. U-Pb zircon data for the Mammola Paragneiss Unit and from low-grade metamorphic rocks of the Sila-Serre Unit (i.e., the Stilo-Pazzano Phyllite and the Bocchigliero Phyllite units) are missing.
As regards the Mandatoriccio Paragneiss Unit, many U-Pb age data on zircon were collected on paragneiss of this unit by [71]. The age clusters shown in Figure 6 [70] as indicative of magmatic and metamorphic origin, respectively. The age clusters in these paragneisses were interpreted as indicative of the ages of the source rocks, and the youngest ages (about 446 ± 12 Ma) were considered as the maximum sedimentation age [72,73]. However, a network of bright veins characterizes crystals dating around 450 Ma ( Figure 6, Zrn28A), suggesting that such zircons may have been perturbed.    [71]). Spot ages in the selected zircons (CL images) reflect the main age clusters. Note the network of bright veins affecting Zrn28A. Zircons 28A, 92B, and 43B are modified after [70]. Symbols as in Figure 3.

A Synthesis of the Variscan Ages
The Carboniferous-Permian ages of zircons from metasediments, orthogneisses, metabasites, and granitoids regard the timing of Variscan metamorphism and magmatism. However, because the ability of zircon to record the metamorphic events depends mostly on temperature (e.g., [74,75]), the correlation between zircon ages and the timing of the main metamorphic events was only possible for high-grade metamorphic rocks of the lower continental crust of the Sila-Serre Unit. More in detail, dating was largely carried out on felsic and mafic granulites, including layered metagabbros, along with interspersed metaperidotites, restitic metasediments and the overlying migmatitic metapelites, metagreywackes, metabasites, and orthogneisses, as all these rocks experienced different degrees of partial melting during the Variscan orogeny [76,77]. However, also some muscovite-leucocratic gneisses of the Castagna Unit record some Variscan ages, the latter related to intense, late-Variscan hydrothermal fluid activity [17].
The Variscan age spectrum of zircons from the lower continental crust of the CPT covers a range from 357 ± 11 to 231 ± 5 Ma (Figure 7).
Moving from older ages (Figure 7), a first consistent cluster of U-Pb ages (n = 10) occurs at 347 ± 10 Ma in four samples of mafic granulites (cf. Table 1), which is related either to poorly zoned crystals or structureless poorly luminescent cores (Figure 7, e.g., Tur49-Zrn29, Tur49-Zrn7) whereas, a second, more pronounced age cluster, occurs at 321 ± 9 Ma, obtained on perturbed and poorly luminescent domains (Figure 7, e.g., Tur49-Zrn60). According to [8], the Variscan zircon domains dated at about 347 Ma show flat HREE patterns and low Eu/Eu* ratios, indicating a synchronous growth with strong competitors such as garnet and plagioclase, respectively [6]. According to [77], ages around 347 Ma date the stage of prograde metamorphism during crustal thickening. The age group at about 321 Ma Figure 6. Histogram and probability density plot of zircon ages in paragneiss from the Mandatoriccio Unit (data from [71]). Spot ages in the selected zircons (CL images) reflect the main age clusters. Note the network of bright veins affecting Zrn28A. Zircons 28A, 92B, and 43B are modified after [70]. Symbols as in Figure 3.

A Synthesis of the Variscan Ages
The Carboniferous-Permian ages of zircons from metasediments, orthogneisses, metabasites, and granitoids regard the timing of Variscan metamorphism and magmatism. However, because the ability of zircon to record the metamorphic events depends mostly on temperature (e.g., [74,75]), the correlation between zircon ages and the timing of the main metamorphic events was only possible for high-grade metamorphic rocks of the lower continental crust of the Sila-Serre Unit. More in detail, dating was largely carried out on felsic and mafic granulites, including layered metagabbros, along with interspersed metaperidotites, restitic metasediments and the overlying migmatitic metapelites, metagreywackes, metabasites, and orthogneisses, as all these rocks experienced different degrees of partial melting during the Variscan orogeny [76,77]. However, also some muscovite-leucocratic gneisses of the Castagna Unit record some Variscan ages, the latter related to intense, late-Variscan hydrothermal fluid activity [17].
The Variscan age spectrum of zircons from the lower continental crust of the CPT covers a range from 357 ± 11 to 231 ± 5 Ma (Figure 7).
Moving from older ages (Figure 7), a first consistent cluster of U-Pb ages (n = 10) occurs at 347 ± 10 Ma in four samples of mafic granulites (cf. Table 1), which is related either to poorly zoned crystals or structureless poorly luminescent cores (Figure 7, e.g., Tur49-Zrn29, Tur49-Zrn7) whereas, a second, more pronounced age cluster, occurs at 321 ± 9 Ma, obtained on perturbed and poorly luminescent domains (Figure 7, e.g., Tur49-Zrn60). According to [8], the Variscan zircon domains dated at about 347 Ma show flat HREE patterns and low Eu/Eu* ratios, indicating a synchronous growth with strong competitors such as garnet and plagioclase, respectively [6]. According to [77], ages around 347 Ma date the stage of prograde metamorphism during crustal thickening. The age group at about 321 Ma represents the beginning of decompression of the lower continental crust in which multistage partial melting events occurred [76]. Moreover, the U-Pb age of magmatic zircons from a quartz-monzodioritic dike intruded within the lower continental crust, were dated at 323 ± 5 Ma by [77,78], and interpreted as the beginning of the Variscan magmatism in the Serre Massif [78]. In addition, small trondhjemitic plutons were emplaced at around 314 Ma as evidenced by U-Pb zircon ages in Aspromonte-Peloritani Unit [79,80].
Minerals 2020, 10, x FOR PEER REVIEW 18 of 30 represents the beginning of decompression of the lower continental crust in which multistage partial melting events occurred [76]. Moreover, the U-Pb age of magmatic zircons from a quartzmonzodioritic dike intruded within the lower continental crust, were dated at 323 ± 5 Ma by [77,78], and interpreted as the beginning of the Variscan magmatism in the Serre Massif [78]. In addition, small trondhjemitic plutons were emplaced at around 314 Ma as evidenced by U-Pb zircon ages in Aspromonte-Peloritani Unit [79,80].  Other two consistent clusters of U-Pb zircon ages were determined for the high-grade metamorphic rocks of the lower continental crust of the Sila-Serre Unit (Figure 7): (i) at 299 ± 8 Ma, measured on grey and homogeneous rim domains (Figure 7, e.g., Tur49-Zrn60); (ii) at 282 ± 10 Ma (Figure 7, e.g., Tur32-Zrn49, GO182-Zrn131) obtained on homogeneous rims.
The cluster at around 299 Ma was interpreted by [77] as the main decompression phase under isothermal conditions which culminated with widespread partial melting events, involving the most fertile source rocks, such as the metapelites in the upper portion of the lower continental crust [76]. This age broadly overlaps with the cooling ages of 292-297 Ma and 293 ± 2 Ma measured on the granitoids by [79,81] and [37,82], respectively, signing the intrusions of huge masses of granitoids forming the batholith exposed in the Sila and Serre massifs. Finally, some ages around 282 Ma (Figure 7) were determined on zircons which show fractionated HREE patterns connected to garnet absence [8]; this age was interpreted as the end of the crustal decompression [69].

Discussion
Considering all pre-Carboniferous and Carboniferous-Permian U-Pb zircon dating from the CPT, our review of age data provides new insights for both the evolution of the northern peri-Gondwana, especially during Middle Ordovician-Upper Devonian, and its involvement in the Variscan orogeny. As shown in several probability density plots of zircon ages (Figures 3-6), the pre-Variscan orogeny signatures from the CPT are summarized in the comprehensive Table 2.

Significance of Proterozoic-Cambrian Signatures
The U-Pb zircon ages in low-to middle-grade metamorphic rocks from the CPT ( Table 2) preserve memory of previous events owing to the low-temperature metamorphism (i.e., T < 680 • C) to which they were subjected, where the zircon isotopic system usually remains closed. In fact, inherited and detrital zircon ages from intermediate to upper crustal levels show age peaks of 2686 ± 34 Ma and 1909 ± 45 Ma (Table 2), and an exceptional very old age of 3242 ± 13 Ma [55], which are missing in high-grade basement rocks ( Figure 5) from the CPT, owing to the high-temperature Variscan metamorphism.
Neo-archean (from 2686 ± 34 Ma to 2506 ± 43 Ma), Paleo-proterozoic (from 2455 ± 40 Ma to 1637 ± 49 Ma), Meso-proterozoic (from 1573 ± 42 Ma to 1005 ± 26 Ma), and Neo-proterozoic (from 992 ± 26 Ma to 622 ± 17 Ma) ages represent inheritances (Figure 8a and Table 2) from ancient basements recognizable in the African Craton that collected the remnants of the pan-African orogen, forming the Gondwana supercontinent. The age peaks between 2686 ± 34 Ma and 1573 ± 42 Ma are connected to west-Gondwana domains (Table 2 and Figures 3, 4, 5, 6, 7 and 8a), whereas the ubiquitous occurrence of zircon ages between 1025 ± 18 Ma and 945 ± 13 Ma (Table 2 and Figures 3, 4, 5, 6, 7 and 8a) constraints potential source regions connected to east-Gondwana domains, which are similar to those present in the early Palaeozoic cover sequence of the Saharan Metacraton [55,65,83].  A more detailed interpretation regards the age peaks between about 649 and 596 Ma, which are present in metasediments of all the considered sectors (Figures 3-6). They could indicate the maximum sedimentation age of the protoliths of the metasediments [65], since they were collected on inherited detrital zircons. This interpretation is coherent with the textural features of the dated A more detailed interpretation regards the age peaks between about 649 and 596 Ma, which are present in metasediments of all the considered sectors (Figures 3-6). They could indicate the maximum sedimentation age of the protoliths of the metasediments [65], since they were collected on inherited detrital zircons. This interpretation is coherent with the textural features of the dated zircons in the metasediments from the Castagna Unit (Figure 3). In fact, some rounded zircon cores in paragneisses, dated at 638 ± 9 Ma and 637 ± 9 Ma, show younger metamorphic overgrowths (Figure 8b GO176 bis Zrn2, Figure 8c; GO59 Zrn7), recording a successive metamorphic history at 596-556 Ma (Figure 8b, e.g., GO6-Zrn39, GO176bis-Zrn2). The orthogneisses hosted by metasediments in which they were intruded at 543-545 Ma [61,66], were affected by a later zircon recrystallization at around 531-527 Ma (Figure 8b, e.g., FB1-Zrn30, Figure 9, e.g., ADR5-Zrn60ar and ADR18-Zrn56) up to about 518 ± 11 Ma (Figure 3b). Accordingly, the metasediments of the Castagna Unit were intruded by Ediacaran granitoids (later metamorphosed during Variscan orogeny), supporting for an age of sedimentation older than 543-545 Ma [61,66], probably around 637 Ma (Figures 3 and 8, and Table 2). Following this interpretation, the few zircon rims dated between 596 and 556 Ma could suitably represent post-depositional metamorphic events. A successive common geological history of metasediments and Ediacaran granitoid intrusions was recorded by zircon overgrowths in these rock types (Figure 8b, e.g., GO176bis-Zrn2, AM1-Zrn8 and FB1-Zrn30), as shown by the age peaks at 519 ± 12 Ma in metasediments (Figure 3a) and at 518 ± 11 Ma in orthogneisses (Figure 3b).

Figure 9.
Histogram and probability density plot of U-Pb zircon ages from orthogneisses of Castagna and Sila-Serre units (data by [66], new data from AM1 and FB1 samples), and Aspromonte-Peloritani Unit (data from [55,61]). Spot ages in the selected zircons (CL and VPSE images) replicate the age clusters; note the younger mimetic metamorphic overgrowths on older relict zircons as in ADR18-Zrn56 and AM1-Zrn7b. Symbols as in Figure 3.

Figure 9.
Histogram and probability density plot of U-Pb zircon ages from orthogneisses of Castagna and Sila-Serre units (data by [66], new data from AM1 and FB1 samples), and Aspromonte-Peloritani Unit (data from [55,61]). Spot ages in the selected zircons (CL and VPSE images) replicate the age clusters; note the younger mimetic metamorphic overgrowths on older relict zircons as in ADR18-Zrn56 and AM1-Zrn7b. Symbols as in Figure 3.
As regards the metasediments of the Mandatoriccio Unit [72], the most pronounced age peak occurs at around 622 Ma ( Figure 6). However, perturbed and recrystallized zircons dated around 524 Ma and 446 Ma ( Figure 6) suggest a replacement of zircon textures due to a fluid activity [7], as evidenced by the network of luminescent veins affecting the crystals (Figure 6, e.g., Zrn28A), probably related to post-depositional metamorphic events. Hence, we interpret the age peak at around 622 Ma as representative of the maximum sedimentation age of the protoliths, similar to the sedimentation age of the Castagna Unit metasediments ( Table 2).
Similar interpretation can be also suggested for the metasediments from the Aspromonte-Peloritani Unit. The age peaks at 631 ± 7 Ma from these rocks are numerous, as well as the age peak at 541 ± 6 Ma ( Figure 4). According to [61,84], these younger ages (around 541 Ma) represent an event of fast sedimentation and metamorphism up to anatexis. However, this interpretation does not take into account the age signature of 631 ± 7 Ma ( Figure 4 and Table 2). Considering the zircon ages that we measured from an amphibolite interbedded with paragneisses of a similar portion of the same unit cropping out in the Aspromonte Massif (sample AS53 and AS53bis), few ages between 565 Ma and 503 Ma are recorded, even into a single crystal (Figure 4, e.g., Zrn71). This wide range of ages suggests a U-Pb isotopic system opening, which supports a different interpretation. We interpret the age peak of zircons at 541 ± 6 Ma ( Figure 4, Table 2), in the paragneisses of the Aspromonte-Peloritani Unit, as related to a fluid-assisted metamorphism. In addition, since also these paragneisses were intruded by the Ediacaran granitoids (dated around 545 Ma [55,61]), the sedimentation of their protoliths was likely at around 631 Ma (Figure 4), with the later fluid-assisted metamorphism (at around 541 Ma) likely connected to the emplacement of the granitoids.
Moreover, this interpretation is strongly supported by the similar sedimentation age of the protoliths of the Castagna Unit paragneisses (cf. Table 2).
Considering the U-Pb zircon ages in high-grade metasediments and metabasites belonging to the Sila-Serre Unit, as well as those in a garnet-biotite gneiss derived from a sliver of Alpine tectonic melange at the Calabria-Lucania boundary ( Figure 5), the distribution of pre-Carboniferous zircon ages in these rocks is complex for the relevant Variscan metamorphic imprint. A few ages in metasediments are around 1784 Ma, 992 Ma, 714 Ma, 649 Ma, and 596 Ma ( Figure 5 and Table 2); whereas a higher frequency of ages around 556 Ma, 452 Ma, and 375 Ma are shown in Figure 5. However, even these metasediments were intruded by Ediacaran granitoids and gabbros dated at around 543 Ma and 579 Ma, respectively [17,65,69]. Hence, the age peaks around 556-509 Ma can be related to Ediacaran magmatism that triggered a metamorphic recrystallization opening the U-Pb system of zircon [65]. According to this interpretation, the sedimentation age of high-grade metasediments from the CPT was older than 556 Ma, presumably between 596 and 649 Ma (Table 2), as shown by an inherited zircon age from the orthogneiss interbedded with the high-grade metasediments dated at around 610 Ma with an overgrowth around 537 Ma (Figure 8b, e.g., GO100 Zrn106). Therefore, geochronological and geological aspects suggest that even the protoliths of high-grade metasediments from the CPT were formed in Neo-proterozoic and were later affected by Ediacaran magmatism and metamorphism [65].

Significance of Ordovician-Silurian Signatures
A characteristic large cluster of Ordovician-Silurian ages has been revealed on zircons from paragneisses of the Mandatoriccio Paragneiss Unit (446 ± 12 Ma) and the lower continental crust (452 ± 12 Ma) of the Sila-Serre Unit, and from metasediments of the Castagna Unit (452 ± 10 Ma) ( Table 2). The Ordovician ages of zircons from Castagna and Sila-Serre units were detected both at the core of grains showing Variscan overgrowths (Figure 8c, e.g., Tur 32 Zrn95, Tur 49 Zrn 44b) and at rims of Cambrian zircons as well (Figure 3, e.g., GO59 Zrn4b). These Ordovician-Silurian ages represent an important thermal-metamorphic event in these units, that, however, did not involve the Aspromonte-Peloritani Unit, due to the lack of age peak at around 450 Ma (Figure 4 and Table 2). In contrast, the Lower Complex contains Ordovician-Silurian volcanic rocks (Table 2) emplaced synchronously with the deposition of the protoliths of phyllites and meta-limestones, with the volcanic rocks metamorphosed as porphyroids and metandesites during the Variscan orogeny [62].

From Pre-Variscan to Variscan Geodynamic Frameworks Revealed by Zircon Ages
In the puzzle of pre-Carboniferous geological events recorded in the CPT, the Ediacaran magmatism played a pivoting role, producing the protoliths of the orthogneiss that intruded the metasediments of the Castagna, Aspromonte-Peloritani, and Sila-Serre units, which are dated at around 545 and 543 Ma [55,61,66]. A relevant peak at 623 ± 12 Ma emerges from the age spectrum in the orthogneisses from Calabria (Figure 9). The protoliths of the orthogneisses are derived from mixing of mantle and crustal magmas [86], as they record a crustal signature, with inherited zircons patterns similar to the detrital zircon pattern from the metasediments in which they intruded. In fact, the relict age of 614 ± 10 Ma (Figure 9, e.g., ADR5 Zrn60ar) in orthogneisses nearly coincides with the possible sedimentation age of the metasedimentary protoliths of the Castagna Unit (637 ± 9 Ma), the Aspromonte-Peloritani Unit (631 ± 7 Ma), and the Sila-Serre Unit (596-649 Ma). In addition, a mafic Ediacaran magmatism (around 579 Ma) was also reported at the base of the deep crust sector of the Sila-Serre Unit [65].
Accordingly, the following scenario can be reconstructed: a peri-Gondwana metamorphic basement, formed by the Aspromonte-Peloritani Unit, the Castagna Unit, and the Mandatoriccio Unit and the lower crustal level of the Sila-Serre Unit, was formerly involved in Pan-African orogeny, thus representing a Cadomian fragment. The age clusters between 596 and 649 Ma can be interpreted as the sedimentation ages of the protoliths of the metasediments of these units, which were successively affected by metamorphic events around 556-509 Ma (Table 2), connected to acidic and mafic Ediacaran magmatism.
Successively, the Palaeozoic protoliths of the low-grade phyllite units (i.e., the Lower Complex, the Stilo Unit, the Bocchigliero Phyllite, and Stilo-Pazzano Phyllite units, within the Sila-Serre Unit), together with Ordovician-Silurian volcanic to subvolcanic rocks [48][49][50][51], may have started infilling a basin controlled by extensional tectonics, which affected the northern Gondwana margin during the Middle Ordovician incipient phases of the Rheic rifting [87,88] (Figure 10a). Throughout this extensional event, fluid-assisted metamorphism may have been favored, as observed for similar tectonic contexts [89,90], determining perturbed textures in zircons (dated around 450 Ma) from the Castagna Unit, as well as from the Mandatoriccio Unit and the lower crustal level of the Sila-Serre Unit [65], all forming part of the Gondwana bedrock of the Palaeozoic covers (Figure 10a).
The deposition of the protoliths of the low-grade phyllite units carried on during Early Devonian, when the beginning of a further dismembering of the northern Gondwana, related to the Palaeotethys rifting, occurred [91]. Therefore, the deposition of the low-grade phyllite units protoliths protracted at least up to Upper Devonian [48][49][50][51] in the Palaeotethys basin, as part of the sedimentary level of the upper oceanic crust, and along the passive margin of Gondwana (Figure 10b).
According to [45], the CPT took part in the growth of the southern Variscan belt, due to the subduction of the Palaeotethys-Gondwana beneath Laurussia [91][92][93][94][95][96]. The Variscan zircon U-Pb ages from the CPT indicate that the subduction occurred up to the Upper Mississippian-Lower Pennsylvanian, as also recorded in metamorphic terranes from other sectors of the southern Variscan belt like Sardinia [97,98]. In this broader subduction context, the Gondwana terranes, included in the Mandatoriccio Unit and the lower continental crust of Sila-Serre Unit, Castagna Unit, Aspromonte-Peloritani Unit (and likely in the Fiume Pomo Unit, Mandanici Unit, and Mammola Paragneiss Unit) ( Figure 2) along with the Palaeozoic covers, included in the Lower Complex, Stilo Unit, and Bocchigliero Phyllite and Stilo-Pazzano Phyllite units of the Sila-Serre Unit (Figure 2), were involved in the accretion of the southern Variscan belt. More in detail, the Palaeozoic sediments were scraped off mostly along the front of the southern Variscan belt, whereas the Gondwana terranes (as well as part of the Paleozoic covers) were under-thrusted at depth, and subjected to greenschists-, amphibolite-, and granulite-facies metamorphic conditions (Figure 10c).
Minerals 2020, 10, x FOR PEER REVIEW 24 of 30 part of the Paleozoic covers) were under-thrusted at depth, and subjected to greenschists-, amphibolite-, and granulite-facies metamorphic conditions (Figure 10c).  [96]; cross section, modified from [45]); the area of the belt subjected to Variscan metamorphic imprint is indicated with transparency. In the plan view geodynamic reconstructions the possible location of the CPT, as well as of the Sardina, is indicated; the thick black segment represents the trace of the cross section; meridians and parallel spacing is of 10° (roughly adapted after [96]).

Conclusions
The present review of U-Pb zircon ages collected in different crustal domains of the CPT allowed us to constrain the timing of sedimentation, tectonic, metamorphic, and magmatic events that affected this key sector of the Southern Variscan Belt from Precambrian to Permian. This was possible since several crustal levels stored in the CPT preserve memory of Pan-African orogeny derived from the break-up of peri-Gondwana terranes and Ordovician-Silurian magmatic and sedimentation events. All these domains were later affected by metamorphism and magmatism during Variscan orogeny. A revisiting of the timing of the events revealed by comparing the internal zircon textures and the related U-Pb age clusters, highlights the following scenario: Figure 10. Plan view (left column) and in cross section (right column) geodynamic reconstruction of the northern Gondwana sector, framed in the (a) Middle Ordovician, following extensional tectonics related to incipient Rheic rifting (plan view, modified from [87]); (b) Upper Devonian, following the opening of the Palaeotethys (plan view, modified from [91]); (c) Upper Mississippian-Lower Pennsylvanian, following the subduction of the Palaeotethys-Gondwana beneath Laurussia, that formed the Southern Variscan Belt (plan view, modified from [96]; cross section, modified from [45]); the area of the belt subjected to Variscan metamorphic imprint is indicated with transparency. In the plan view geodynamic reconstructions the possible location of the CPT, as well as of the Sardina, is indicated; the thick black segment represents the trace of the cross section; meridians and parallel spacing is of 10 • (roughly adapted after [96]).

Conclusions
The present review of U-Pb zircon ages collected in different crustal domains of the CPT allowed us to constrain the timing of sedimentation, tectonic, metamorphic, and magmatic events that affected this key sector of the Southern Variscan Belt from Precambrian to Permian. This was possible since several crustal levels stored in the CPT preserve memory of Pan-African orogeny derived from the break-up of peri-Gondwana terranes and Ordovician-Silurian magmatic and sedimentation events. All these domains were later affected by metamorphism and magmatism during Variscan orogeny. A revisiting of the timing of the events revealed by comparing the internal zircon textures and the related U-Pb age clusters, highlights the following scenario: • The age peaks from around 2500 Ma to 1600 Ma and 1000 Ma in the deep-intermediate crustal levels testify that the origin of the CPT was related to fragments of West and East Gondwana realms, respectively.