Late-Proterozoic to Paleozoic history of the peri-Gondwana Calabria–Peloritani Terrane inferred from a review of zircon chronology

Castagna unit

The Castagna Unit underlies the high-grade metamorphic complex; it consists of paragneisses, micaschists, augen gneisses, Variscan granitoids and minor amphibolites, quartzites, Ca-silicate rocks and marbles (Colonna and Piccarreta 1976; Paglionico and Piccarreta 1976). It is exposed in the Sila and Serre Massifs (Fig. 1) and includes rocks equilibrated under greenschist to amphibolite facies conditions in Variscan times and reworked by Alpine tectonics (Colonna and Piccarreta 1976; Langone 2008; Micheletti et al. 2007, 2011). The magmatic protoliths of augen gneisses were intruded in the metasediments of Castagna Unit.

Mandatoriccio complex

The Mandatoriccio Complex is exposed in the Sila Massif (Fig. 1). It is tectonically overimposed on high-grade deep crustal rocks and consists of medium-grade metapelites, meta-arenites, meta-volcanites and metabasites with rare marbles and orthogneisses (Acquafredda et al. 1988, 1991; Langone 2008). Micaschists show a static porphyroblastic growth of biotite, garnet, andalusite, staurolite, muscovite and minor cordierite and fibrolite (Lorenzoni and Zanettin-Lorenzoni 1979; Borghi et al. 1992; Langone 2008). A clockwise P–T–t path with a metamorphic peak at about 590 °C and 0.35 GPa during Variscan post-orogenic extension dated 299 Ma has been defined (U–Th–Pb monazite ages; Langone et al. 2010).

High-grade metamorphic complex in Serre

This complex occupies wide areas in the Sila and Serre Massifs (Fig. 1). In the Serre massif, an about 20 km thick section is exposed (Schenk 1989; Acquafredda et al. 2003). It is formed by 7–8 km thick lower crustal rocks equilibrated under granulite–amphibolite facies conditions underlying low-middle grade metamorphic complex of the upper crust (Fig. 1). Both upper and lower crustal domains were intruded by 10 km-thick “layer” of Variscan granitoids (Fig. 2) emplaced about 300 Ma ago (Schenk 1980; Caggianelli et al. 2000).

The Variscan lower crust of the Serre includes, from the bottom (Fig. 2): (a) felsic granulites, metagabbros, metabasites, rare meta-peridotites, metagreywackes and metapelites, (b) migmatitic metapelites representing the wide portion of the Serre Massif (Schenk 1984; Fornelli et al. 2002), with interleaved metagreywackes, metabasites, rare marbles and augen gneisses. The augen gneisses preserve original intrusive features of their protoliths in the metasediments. The lower crustal rocks have been affected by pervasive partial melting (Maccarrone et al. 1983; Schenk 1984; Caggianelli et al. 1991; Fornelli et al. 2002) mostly during the Late Carboniferous–Permian exhumation (Fornelli et al. 2002). Thermobarometric calculations related to the Serre rock types (Acquafredda et al. 2006, 2008; Fornelli et al. 2011b) give: (1) T-peak of ~700 °C and of ~900 °C and P-peak of 0.9 and ~1.0–1.1 Gpa at the top and bottom of the section respectively, and (2) T-peak followed by quasi-isothermal decompression of about 0.5–0.6 Gpa at the top and by total decompression of about 0.3 Gpa at the bottom during Late Carboniferous—Permian times (Fornelli et al. 2011a). During the crustal thinning, at about 323 ± 5 Ma (Fornelli et al. 2011a) some quartz-monzodioritic dikes were emplaced in the middle part of the metapelites (Schenk 1984), subsequently, at about 300 Ma ago (Schenk 1980; Caggianelli et al. 2000), huge volumes of calc-alkaline granitoids were emplaced between the low-middle and high-grade metamorphic complexes (Fig. 2).

Aspromonte–Peloritani unit

In the Aspromonte Massif (Southern Calabria), this Unit is sandwiched between the lower “Madonna di Polsi” Unit (Pezzino et al. 2008) not mapped in Fig. 1 and the super-imposed low-grade metamorphic complex, whereas, in the Peloritani massif, it is the highest tectonic Unit. The prevalent rock types are middle-grade biotite paragneisses and augen gneisses (derived from intruded granitoids) with minor amphibolites, micaschists and marbles (Fig. 1). The metamorphic rocks are extensively intruded by late-Variscan peraluminous granitoids (D’Amico et al. 1982; Rottura et al. 1990, 1993; Fiannacca et al. 2005, 2008). The Aspromonte–Peloritani Unit appears as the product of processes of crustal thickening during early- and middle-Variscan collisional stages, followed by crustal thinning, granitoid intrusion and unroofing during late-Variscan extensional stages (Festa et al. 2004; Caggianelli et al. 2007).

U–Pb monazite ages from paragneisses of the Aspromonte Massif dated ~300 Ma the metamorphic peak under T max of 620 °C and P max of ca. 0.25 Gpa (Graeßner et al. 2000; Appel et al. 2011). This metamorphic peak was nearly synchronous with the granitoid intrusions at 303–290 Ma (Appel et al. 2011 and references therein).

Augen gneisses

The augen gneisses from Calabria contain zircon inherited grains giving ages ranging from 2502 to 1760 Ma as clusters or single spots; six ages in the range 748–917 Ma (Table 1) and a lot of the concordant ages ranging from 633 to 532 Ma (Micheletti et al. 2007, 2011). The last age group defines statistically significant clusters (Table 1; Fig. 3): (a) at 619 ± 8 Ma (mean concordia age of nine ages ranging from 633 to 597 Ma) relative to cores having variable U (63–659 ppm) and Th (11–234 ppm) contents and Th/U ratios significantly higher than 0.1 (0.1–0.7), (b) from 575 to 565 Ma including three rims (1 in Fig. 3b) with high U contents (928–1832 ppm) and two core domains and one rim (2 in Fig. 3b) with low U (102–388 ppm), Th ranging from 6 to 71 ppm and Th/U in the range 0.1–0.2 and (c) at 543 ± 4 Ma (20 ages 562-532 Ma, Fig. 3c) mainly related to rims with high Th/U ratio (0.1 to 0.8).

The first age cluster (619 ± 8 Ma) includes rounded or fractured cores appearing as detrital domains (Fig. 3a). Accordingly, the zircon domains averaging 619 ± 8 Ma and the older ones are to be considered as inherited from the source material (Fig. 3a). The three rims dated from 575 to 571 Ma (mean concordia age 573 Ma in Fig. 3b) with high U contents and low and quite similar Th/U ratios (≤0.1) seem to imply that Th and U contents at the time of the zircon growth were probably controlled by the same reactions and suggest compatibility with a metamorphic origin (Rubatto and Hermann 2007; Xia et al. 2009), as well as the cluster at 567 Ma with lower Th/U ratio (Fig. 3b). The cluster peaking at 543 Ma (n = 20) includes many euhedral crystals showing continuity between core and rim having high U contents (14 spot ranging from 659 to 241 ppm) and Th/U ratios mostly between 0.2 and 0.5 (Fig. 3c); one domain analysed for REEs produces a highly fractionated pattern and a distinct negative Eu anomaly (Fig. 3c). The characteristics of this population are common to magmatic zircons (Rubatto and Hermann 2007) or to recrystallised domains preserving memory of parental magmatic zircons (Xia et al. 2009). The moderate variability and the high values of Th/U seem to be consistent with precipitation from a hybrid magma precursor of the augen gneisses (Fornelli et al. 2007).

The augen gneisses from Peloritani contain zircon grains giving ages ranging from 3200 to 520 Ma as clusters or single spots (Williams et al. 2012; Fiannacca et al. 2013).

Many zircon domains are younger than 630 Ma and form a significant cluster at ≈545 Ma (Fig. 4a) including two kinds of zircon domains having U contents of 320–940 and 40–470 ppm interpreted as suggestive of magmatic and detrital origin, respectively (Williams et al. 2012). The age spectrum, on the whole, overlaps that related to Calabria augen gneisses (Fig. 4b) apart from (1) the absence, except for one domain (917 Ma), of ages 900–1000 Ma in augen gneisses from Calabria (Fig. 4b) and (2) the lack of Ordovician–Silurian ages in augen gneisses from Peloritani (Fig. 4a) and (3) the presence of a relevant peak at 453 Ma in augen gneisses of Calabria (Fig. 4b).

Metabasic rocks from the lower crust

Zircons from metagabbros and metabasites interbedded with felsic granulites and migmatitic metapelites in the Serre have been considered (Micheletti et al. 2008; Fornelli et al. 2011a). The most of analyses on separates and on thin section produce ages (Table 1; Fig. 5a) mostly between 373 and 277 Ma (Fig. 5a, b), single Ordovician–Silurian spots (446 ± 12 Ma; Fig. 5a–c;) and Neoproterozoic ages (in average 579 Ma, Fig. 5a and from 564 to 593 Ma Fig. 5d). A first hand interpretation of this data would lead to propose that this basic magmatism occurred in the Variscan times. However:

1. 1.

   many Variscan domains of zircon (Fig. 5b) from garnet-bearing rock types show evidence of the “garnet effect” in a closed system, such as flat HREE patterns (Fornelli et al. 2011a), in contrast with domains precipitated from a melt (Rubatto 2002). Thus the above domains formed or recrystallised in presence of garnet, which is metamorphic in origin (Fornelli et al. 2011a);

    
2. 2.

   many Ordovician–Silurian domains (Fig. 5c) form rims around older cores (Micheletti et al. 2007). Eleven ages form a cluster at 446 Ma in the range 483–413 Ma (Fig. 5a);

    
3. 3.

   Eight ages in the range 505–593 Ma include four domains dated 593–564 Ma (in average 579 ± 15 Ma Fig. 5a–d) showing oscillatory zoning, high Th/U ratios (0.16–0.19) and fractionated REE patterns (Fig. 5d; Fornelli et al. 2011a), these features are compatible with a magmatic origin (Rubatto 2002).

    

On this basis, it seems that the mafic magmatism occurred in Neoproterozoic time (579 Ma), some tens of million years earlier than the felsic magmatic precursor of the augen gneisses at 543–545 Ma (Micheletti et al. 2008; Fornelli et al. 2011a, Fornelli et al. 2012; Williams et al. 2012). It is noteworthy that Neoproterozoic-Lower Cambrian felsic and mafic magmatism is recorded in many of the so-called “Cadomian blocks” present from the Iberia, Pyrenees, Western Alps to Turkey (e.g. Neubauer 2002; Stedra et al. 2002; Castiñeiras et al. 2008; Fernàndez-Suarez et al. 2013).

Metasedimentary rocks

U–Pb zircon ages of garnet–biotite gneiss from Pollino massif were considered. This gneiss shows features of high-grade metamorphism and represents a sliver of Calabria lower continental crust in the Alpine mélange (Laurita et al. 2015). The zircon age data span from 1789 to 255 Ma (Table 1) with age cluster (Fig. 6a) in the range 836–1111 Ma (n = 12) and some data in each cluster at 610–696, 513–586 Ma (in average 557 ± 7 Ma), 457–475 and 296 ± 8 Ma, (Laurita et al. 2015). The Ordovician to pre-Cambrian ages were related to heritages whereas Carboniferous–Permian ages were related to the Variscan metamorphism.

U–Pb data on zircon grains are available (Table 1) for migmatitic metapelites (sample GO 182), restitic metagreywacke (Tur 3) and felsic granulites (sample Tur 17) from the Serre, paragneisses from the Peloritani (sample FIU-7) and micaschists from the Mandatoriccio Complex in Sila (sample LL61b2) (Micheletti et al. 2008; Langone 2008; Fornelli et al. 2011a; Williams et al. 2012). Micheletti et al. (2008) report a few Neoproterozoic inherited zircon ages (²⁰⁶Pb/²³⁸U ages 585, 595, 654 and 1688 Ma) from the granulite facies metasediments of the Serre, and many ages in the range 325–270 Ma (Table 1; Fig. 6b). Three of the inherited ages are discordant evidencing Pb loss during the long geological history. In addition, an upper intercept at 1113 ± 100 Ma (Table 1) from discordant data was calculated for the migmatitic metapelite (Micheletti et al. 2008). The age data distribution in the high-grade metasediments from the Serre shows significant age peaks (Fig. 6b) interpreted as: (1) Variscan metamorphism (284 Ma), (2) memory of Ordovician–Silurian activity (489 Ma) and signatures of mafic magmatism (589 Ma) as discussed before (Figs. 5a, 6b). In garnet–biotite gneiss from Pollino Massif similar age peak distribution can be observed apart from the significant age cluster at 1111–836 Ma (Fig. 6a).

Zircons from the amphibolite facies metasediments of Aspromonte–Peloritani Unit in Sicily (Williams et al. 2012) and Mandatoriccio complex in Calabria (Langone 2008) preserve significant pre-Variscan (>400 Ma) memory (Fig. 7; Table 1). The Neoproterozoic ages of the Mandatoriccio Complex (Fig. 7a) show strong similarities with the ages from the paragneisses of Aspromonte–Peloritani Unit (Langone 2008; Williams et al. 2012): significant age peaks between 700 and 500 Ma, some ages comprised between 1000 and 750 Ma (Fig. 7) and minor older age data (>1600 Ma). The Mandatoriccio micaschists, however, preserve also a lot of detrital zircon grains (Langone 2008) having Ordovician–Silurian ages (451 ± 6 Ma; Fig. 7a) absent in paragneisses of Aspromonte–Peloritani Unit (Fig. 7b). In the low-grade metasediments from both Calabria and Sicily, Ordovician–Silurian ages for sedimentation and magmatism have been indicated (Trombetta et al. 2004; Martín-Algarra et al. 2014) and zircon separates dated around 500 and 2000 Ma were recorded in Serre (Schenk and Todt 1989).

Deposition age of crustal source of the augen gneisses

The Calabria augen gneisses relate to hybrid magmas (Fornelli et al. 2007) emplaced into metasediments around 543 ± 4 Ma (Fig. 4b; Micheletti et al. 2007, 2011). They contain Neoproterozoic to Archean inheritance represented by rounded and fractured zircon cores interpreted as detritic (Micheletti et al. 2007). A representative population (13 %) forms a statistically significant cluster peaking at 619 ± 8 Ma (Figs. 3a, 4b) of domains having variable Th/U ratios (0.1–0.7), which is followed by another significant cluster at 573 ± 3 Ma (Figs. 3b, 4b) calculated on rims having low and homogeneous Th/U ratios (≤0.1, Fig. 3b). The latter cluster is interpreted as indicative of the time of metamorphic zircon growth and imposes an absolute limit to the sedimentation that should be older than 573 ± 3 Ma. Considering that the older mean concordia age is 619 ± 8 Ma (Fig. 4b) than this age could approximate the sedimentation age of the protoliths.

The span of time between the presumed age of the sedimentation (619 ± 8 Ma) and the magmatic crystallization ages (in average 543 ± 4 Ma in Fig. 4b) of protoliths of augen gneisses might account for the evolution from sedimentation, metamorphism to partial melting stages during the Cadomian orogenesis.

This reconstruction does not agree with that hypothesized for the equivalent augen gneisses from Peloritani massif. Williams et al. (2012) and Fiannacca et al. (2013) envisage an almost synchronous process from sedimentation to partial melting (at 545 ± 7 Ma Fig. 4a and 546 ± 5 Ma in Fig. 7b) of the paragneisses hosting the augen gneisses because they measured similar zircon ages both in paragneisses and augen gneisses (Figs. 4a, 5, 6, 7b). In the proposed geological model for the Peloritani area, however, the evidences of restitic features of paragneisses compatible with extraction of abundant melt in Neoproterozoic–Cambrian times have not yet been documented. In addition, the ε_Nd values calculated for Calabria augen gneisses (from −3.19 to −5.35 in Micheletti et al. 2007) are higher than ε_Nd of the Peloritani paragneiss (−6.6 in Williams et al. 2012) precluding a direct link. We think that the similar age distribution in augen gneisses and paragneiss of Peloritani (Figs. 4b, 5, 6, 7b; Table 1) could be due to rejuvenation of zircon from paragneisses caused by intruding magmas (protoliths of augen gneisses), or by younger tectono-thermal events (Ordovician and Variscan) well documented in the felsic porphyroids and andesites from Peloritani (Table 1; e.g. Trombetta et al. 2004; Appel et al. 2011). The hypothesized metamorphism in Neoproterozoic–Cambrian times in Calabria at about 573 Ma (this paper) and in Peloritani paragneiss around 535 Ma (Williams et al. 2012; Fiannacca et al. 2013) give information about the evolution of Panafrican/Cadomian orogenesis.

Depositional ages of protoliths of the metasediments

The metasediments of the Serre, Castagna and Aspromonte–Peloritani terrains were intruded in Neoproterozoic times by acidic (543–545 Ma) and basic (579 ± 15 Ma) magmas (Micheletti et al. 2007, 2008). On this basis, the deposition of protoliths of metasediments must have been older than magma emplacements. This agrees with conclusion of Schenk (1990) indicating a sedimentation age from 1000 to 600 Ma for the high-grade metasediments of the Serre, on the basis of Sr isotopic evolution. In Variscan times these crustal domains were affected by medium- high-grade metamorphism. (Micheletti et al. 2008; Fornelli et al. 2011a). However, only the deep crustal metamorphites of the Serre and the garnet–biotite gneiss from Pollino massif evidenced zircon domains formed in Variscan times (Table 1; Fig. 6). The granulite facies conditions as well as the pervasive fluid-present dehydration melting in the Serre (Fornelli et al. 2002) seem to account for generation of new zircon and/or modification of the older ones erasing nearly completely the pre-Cambrian ages (Fornelli et al. 2011a, Fornelli et al. 2012) which, however, are present in garnet–biotite gneiss from Pollino massif (ages 1111–836 Ma Fig. 6a) where the Variscan metamorphism was not able to produce significant annealing/recrystallization processes in zircons (Laurita et al. 2015), probably as effect of lower temperatures of metamorphism. The here deduced deposition age of protoliths of metasediments (>600 Ma) is decidedly older than that suggested by Laurita et al. (2015) indicating 457 Ma as maximum depositional age for the sedimentary protoliths of garnet–biotite gneiss of the Pollino massif, which look like the high-grade metasediments of Calabria.

As concerns the Peloritani paragneisses and the Mandatoriccio micaschists representing medium grade metasediments, must be evidenced that the youngest detrital zircon age in the former was 535 ± 4 Ma (FIU-7 sample in Table 1) whereas in micaschists was in Ordovician–Silurian times 428 ± 10 Ma (LL61b2 sample in Table 1). These facts indicate the Lower Cambrian as minimum sedimentation age for Peloritani paragneisses (Williams et al. 2012) and the Ordovician–Silurian times as maximum sedimentation age for Mandatoriccio micaschists (Langone 2008).

According to our interpretation, the geological evolution of these terrains in pre-Paleozoic times was distinct: lower and intermediate Variscan crust portions (metasediments of Serre, Castagna and Aspromonte–Peloritani) record an older history with respect to Mandatoriccio complex, sliver of garnet–biotite gneiss from Pollino and very low-grade Variscan metasediments. In fact the very low-grade metasediments in southern Calabria and Sicily contain porphyroids and meta-andesites having Ordovician ages (Acquafredda et al. 1991) and Ordovician-Silurian zircon ages were revealed in low-grade metasediments of Serre (e.g. Martìn-Algarra 2014).