U-Pb Zircon Age Constraints on the Paleozoic Sedimentation, Magmatism and Metamorphism of the Sredogriv Metamorphics, Western Balkan Zone, NW Bulgaria

Abstract

The Sredogriv greenschist facies rocks belong to the Western Balkan Zone in northwestern Bulgaria. The low-grade rocks consist of clastic-tuffaceous precursors and presumably olistostromic magmatic bodies. We present U-Pb LA-ICP-MS zircon age constraints for the Sredogriv metaconglomerate, intruding metaalbitophyre and a breccia-conglomerate of the sedimentary cover. Detrital zircons in the Sredogriv metaconglomerate yield a maximum depositional age of 523 Ma, with a prominent NeoproterozoicEarly Cambrian detrital zircon age clusters derived from igneous sources. The metaalbitophyre crystallized at 308 Ma and contains the same age clusters of inherited zircons. A 263 Ma maximum age of deposition is defined for a breccia-conglomerate of the Smolyanovtsi Formation from the sedimentary cover that recycled material from the Sredogriv metamorphics and Carboniferous–Permian magmatic rocks. The depositional setting of the Sredogriv sedimentary succession is characterized by proximity to Cadomian island arc sources and provenance from the northern periphery of Gondwana. The timing of the Variscan greenschist facies metamorphism of the Sredogriv metamorphics is bracketed between 308 Ma and the depositional age of 272 Ma of another adjacent clastic formation. These results constrain the timing of the Cadomian sedimentary history and the Variscan magmatic and tectono-metamorphic evolution in this part of the Western Balkan Zone.

1. Introduction

The Balkan Zone forms a crucial part of the Alpine fold-and-thrust belt exposed in northwestern Bulgaria. The origin of the Balkan, Moesian and Thracian large-scale terranes on the territory of Bulgaria has been linked to the geodynamic evolution of Gondwana [1,2,3,4] (Figure 1). This evolution involves oceanic crust formation, island arc development and collision of Gondwana-derived crustal blocks with the Laurasia continental margin during the Late Neoproterozoic to Early Paleozoic times [2,3,4,5,6], evidenced by the presence of shared tectono-magmatic elements in the Avalonian–Cadomian orogeny [3,6,7,8,9], and the Variscan orogeny [6,10,11] in the Balkan Zone. The Late Carboniferous–Early Permian crystallization ages of Variscan magmatism in the Balkan Zone are relatively well known, e.g., [7,12,13]. However, reliable age constraints for pre-Variscan and Variscan sedimentation and Variscan metamorphism are generally scarce or absent for most parts of the Balkan Zone. Specifically, the clastic sedimentation of the Permian red beds is inferred only from their stratigraphic position relative to the underlying Lower Paleozoic strata, for some of which biostratigraphic ages exist [10,14]. Similarly, the age of the Carboniferous high-grade metamorphism of 336.5 ± 5.4 Ma is based only on a single zircon age [6] and its cooling derived by ⁴⁰Ar/³⁹Ar mica ages in the range of 306–317 Ma [15] in the Sredna Gora Zone to the south of the Balkan Zone.

In this contribution, we present detrital zircon U-Pb geochronology for the depositional timing of the precursors of the Sredogriv low-grade metamorphic rocks, as well as of the clastic sedimentary rocks of its immediate cover (Figure 2). This is also complemented by U-Pb geochronology of the felsic magmatic rocks contained in the low-grade metamorphic rocks and whole-rock geochemistry of the latter. The aim of this study is to provide age constraints for the magmatic protolith and the depositional history of the Sredogriv metamorphics as an important crustal element of the Western Balkan Zone, as well as to bracket the temporal frame of the greenschist facies metamorphism using also the depositional history of the Late Paleozoic cover rocks.

2. Geological Setting and Field Observations

In northwestern Bulgaria, the Balkan Zone consists of the Forebalkan and Western Balkan units, which were both involved in the Alpine fold-and-thrust belt [18] (Figure 1). Further subdivision within the Western Balkan Zone define the Alpine thrust-imbricated Berkovitsa, Vratsa, Montana and Belogradchik units [19].

The Western Balkan Zone consists of Neoproterozoic–Cambrian crystalline basement and Late Paleozoic–Mesozoic cover rocks [3,4,8,18,19,20]. In the Bulgarian geological literature, the low-grade crystalline basement of the Western Balkan Zone has been referred to as to the Diabase–Phyllitoid formation [21] or complex [22], in which basalt lenses and blocks were interpreted as olistoliths [23]. The latter complex is associated with the latest Neoproterozoic–Early Paleozoic age based on finds of Lower and Upper Ordovician acritarchs [24] and the stratigraphic relationships with the underlying, presumably older magmatic units (see below). However, in the Diabase–Phyllitoid complex, a recent U-Pb zircon geochronology revealed crystallization of a pillow basalt at 519.9 ± 3.6 Ma [25] and a maximum depositional age of a metaconglomerate at 510 ± 6 Ma and a sandstone at 540 ± 5.9 Ma [4,20], which clearly define a Cambrian age of this complex.

In the Diabase–Phyllitoid complex, three lithostratigraphic groups have been subdivided, namely Cherni Vrah, Berkovitsa and Dalgi Djal groups [26,27]. These groups differ among each other in their lithologic character and tectono-stratigraphic relationships.

The Cherni Vrah Group is ophiolitic, and it is traditionally regarded as representing an element of the Balkan–Carpathian complete ophiolite that includes N-MORB-type cumulate, mafic inrusive and volcanic sections of an oceanic lithosphere [5,28]. A Neoproterozoic age of the Cherni Vrah Group is based on a single zircon U-Pb crystallization age of a gabbro at 563 ± 5 Ma [29]. This age was later interpreted as detrital by Kiselinov et al. [30], who dated the Cherni Vrah gabbro as Devonian at 391.2 ± 1.3 Ma by the LA-ICP-MS method using multiple zircon grains. The counterparts of the Balkan–Carpathian ophiolite in Eastern Serbia and Romania also have Devonian U-Pb zircon crystallization ages: Deli Jovan ophiolite gabbro at 405 ± 2.6 Ma [31], Zaglavac ophiolite gabbro at 388.1 ± 5.1 Ma [32] and Tishovitsa-Jutz ophiolite gabbro at 380–390 Ma (Sm-Nd isochron ages) [33]. Figure 1 shows the location of the ophiolitic bodies.

The Berkovitsa Group is island arc-related, and consists of Ca-alkaline volcanic (metabasalt, metaandesite and metatrachyandesite) and plutonic counterparts and associated sedimentary successions of an intra-oceanic arc system. These are considered as pre-Ordovician (latest Neoproterozoic–Cambrian), based on previously reported acritarchs in the Diabase–Phyllitoid complex [27]. An angular unconformity of the Berkovitsa Group island arc onto the Cherni Vrah Group ophiolite is reported by Haydoutov [27]. A Cambrian U-Pb zircon age of 493.0 ± 6.6 Ma was defined for the Berkovitsa Group Pilatovo gabbro [34]. Similarly, a tuffitic metasiltstone and a metasandstone from the Berkovitsa Group yielded maximum depositional ages of 494.5 ± 5 Ma and 521 ± 5 Ma, respectively [4].

The Dalgi Djal Group is olistostromic, comprising metasedimentary lithologies that host olistoliths from the Cherni Vrah ophiolite and magmatic rocks of the Berkovitsa island arc system [27]. The inferred early Ordovician age of the Dalgi Djal Group is based on acritarchs found in rock types similar to this group from other exposed areas of the Diabase–Phyllitoid complex in the Balkan terrane [27]. The Dalgi Djal Group unconformably overlies the Berkovitsa Group and it is interpreted as a sedimentary assemblage formed during the destruction and obduction of the Balkan–Carpathian ophiolite.

The object of this study, the Sredogriv low-grade metasedimentary rocks, has been correlated with the Dalgi Djal Group [27]. Hovewer, critical age constraints on their sedimentary depositional history and their magmatic components, as well as the timing of the low-grade metamorphism they experienced, are still lacking. This hampers the establishment of the earlier Paleozoic sedimentary and tectono-metamorphic evolution of some of the low-grade metamorphic units that constitute the important Cadomian and Variscan crustal elements subsequently implicated in the Alpine Balkan fold-and-thrust belt.

The Sredogriv metamorphics [16,17] belong to the Montana unit, which is a component of the large-scale Western Balkan Zone [19]. They crop-out as NW-SE elongated strips of low-grade metamorphic rocks without an exposed base, below the unconformable latest Paleozoic–Mesozoic sedimentary cover rocks (Figure 2). Initially, they were grouped into the Sredogriv Formation [35], and later included into the Diabase–Phyllitoid complex [20,36]. The Sredogriv metamorphics, together with the presumably Permian clastic and volcanic rocks, build up the basement of the Montana unit, which is covered by Lower Triassic and Middle Jurassic–Early Cretaceous clastic-carbonate successions showing internal unconformities among the distinct formations [16,17] (Figure 3). The Montana unit is thrusted over the Belogradchik unit along the late Alpine Vedernik thrust. The age of the Sredogriv metamorphics is inferred to be Silurian [37], Devonian–Early Carboniferous [22], pre-Devonian [35] and Ordovician by analogy with the Dalgi Djal Group [27]. According to Moskovski et al. [35], the Sredogriv Formation consists of a lower terrigenous member and an upper terrigenous–tuffaceous member. Haydoutov [38] unifies the Sredogriv Formation together with four other formations into the terrigenous–volcanogenic suite called “Stara Planina flyschoid Formation”. Intense deformation, greenschist facies metamorphism and modified primary stratification led by Angelov et al. [16,17] unify the parametamorphic rocks along with the allochthonous magmatic rocks into a single unit called the Sredogriv metamorphics. Kiselinov [39] distinguished six deformational stages (assigned to Cadomian, Variscan and Alpine orogenies) of the Sredogriv metamorphics and obtained a Neoproterozoic U-Pb zircon protolith age of 618 ± 10 Ma for the Protopopintsi metagranitoid, which is considered as an olistolith within these metamorphics. In addition, Kiselinov [39,40] studied the geochemical composition of several samples of metabasites, metagranites and metavolcanites included in the Sredogriv metamorphites and interpreted them as having formed in different tectonic settings. These results support the notion of an older peri-Gondwanan origin of the igneous olistoliths and olistoplaques that were tectonically included during the late Silurian and Devonian in an epicontinental basin. The Ediacaran age (618 Ma) of the Protopopintsi granite (representing a large olistoplaque in the Sredogriv metamorphites) also testifies to the peri-Gondwanan origin of the basement of the Balkan assemblage [41,42].

Our field observations confirmed previous data that the Sredogriv metamorphics are strongly deformed in greenschist facies metamorphic conditions (Figure 4a). These metamorphics consist of alternating quartz–sericite schist, sericite–chlorite schist, calc-schist, phyllite, metasiltstone, metasandstone and metaconglomerate. Basic and acidic metaigneous bodies within this metasedimentary succession are considered allochthonous, representing various olistostromic bodies [16,17]. In the field, it is difficult to identify the olistostromic nature of the magmatic rocks because some demonstrate features of mafic dykes (thin elongated bodies) or felsic sill (foliation/stratification/parallel thin bodies) within the metamorphic succession that show typical greenschist facies mineral assemblages. Moreover, the mafic dykes show rather consistent E-W to ESE-WNW strike that implies they represent a conjugate network of mafic igneous bodies (see Figure 2).

3. Materials and Methods

Our investigation focuses on the key metamagmatic body and metaclastic rock horizon of the Sredogriv metamorphics and the first sedimentary cover of the unconformably overlying Smolyanovtsi Formation built of clastic rocks. The sample numbers mentioned below refer to those shown in Figure 2 and Figure 3. These samples were used for U-Pb zircon geochronology.

A single sample L4 (N43°52945° E22°80330°) was collected from a metaconglomerate from the Sredogriv low-grade metamorphics, which form discontinuous distinct horizons of variable thickness. This polymictic metaconglomerate consists of quartz, gneiss, phyllite, schist and granitoid clasts that range in size from 2 to 6 cm, all set in a medium- to coarse-grained supporting matrix of similar composition to that of the clasts (Figure 4c). The metaconglomerate demonstrates a well-developed greenschist facies foliation delineated by chlorite–muscovite (sericite) aggregates and recrystallized quartz grains (Figure 4d).

Another sample L1a (N43°533514° E22°784586°) from the Sredogriv low-grade metamorphics was taken from a felsic sill-like and foliation-parallel metaigneous body reaching a thickness up to 20 m that was intercalated within the metamorphic succession (Figure 4e). In thin sections, this felsic body represents metaalbitophyre consisting of plagioclase and quartz phenocrysts set in a fine-grained recrystallized groundmass that contains foliation-delineating quartz and sericite, together with magnetite transformed to hematite (Figure 4f).

Sample L15 (N43°534928° E22°850475°) was collected from a red-brown clast-supported breccia-conglomerate that belonged to the inferred Lower Permian in age Smolyanovtsi Formation from the first unconformable sedimentary cover of the Sredogriv metamorphics (see Figure 3). The clast-supported breccia-conglomerate consists of quartz, gneiss, phyllite, schist, granitoid and mostly of angular volcanic clasts that range in size up to 20 cm, all set in a coarse-grained matrix of similar composition to that of the clasts (Figure 4b).

Zircons for U-Pb dating were recovered from jaw-crushed and milled-rock samples using a Wilfley shaker table, followed by magnetic and heavy liquid (CHBr₃ and CH₂I₂) separation at the Geological Institute of the Bulgarian Academy of Sciences. Zircon grains were hand-picked under a binocular microscope and thermally annealed at 900° C for 48 h in a muffle and afterwards mounted in epoxy resin and polished. Optical cathodoluminescence (CL) imaging was carried out for identifying inherited cores, cracks and inclusions inside the crystals using a motorized optical system (Cathodyne NewTec Scientific) attached to a microscope (Leica 2700) at the Geological Institute of the Bulgarian Academy of Sciences. U-Pb in situ zircon dating was performed at the laser ablation mass spectrometry (LA-ICP-MS) laboratory of the Geological Institute using New Wave UP193FX LA coupled to a Perkin Elmer ELAN DRC-e quadrupole ICP-MS. Ablation parameters were set up as follows: a diameter of 35 μm, frequency of 8 Hz and detection time within 0.002–0.003 s. Analyses were calibrated with the GEMOC-GJ1 zircon [43] as an external standard for fractionation correction. Plešovice [44] and 91,500 [45] zircons were used as unknowns and used to correct systematic errors. Data were processed using the Iolite v. 2.5 software [46], applying down-hole fractionation correction. Diagram plots and concordia ages were obtained using Isoplot 4.15 [47]. Analytical data derived from U-Pb zircon geochronology are provided in Table S1 of the Supplementary Material.

Samples from the Sredogiv metamorphites include metaconglomerate, quartz–chlorite, sericite–chlorite and black schists. The five samples selected for whole-rock geochemistry were crushed to a fine powder following standard procedures. Whole-rock major element analyses by X-ray fluorescence (XRF) were performed on a PANalytical (EDXRF, Epsilon 3XLE, Omnian 3SW) instrument at the University of Sofia St. Kliment Ohridski, Bulgaria. XRF analyses were conducted on fused beads of powdered material from the samples of mafic rocks. The fused beads were prepared by mixing approximately 1 g of sample with 3 g of lithium metaborate (LiBO₂) and 6 g of lithium tetraborate (Li₂B₄O₇) flux. The mixture was melted in a Claisse LeNeo Fused Bead maker, using 5% Au–95% Pt casting bowls at 1065 °C. The loss of ignition (LOI) at 1000 °C was expressed as a percentage of sample weight dried in an oven at 110 °C overnight. Analytical errors for major oxides were within the range of 1%. The trace elements and the rare-earth elements (REEs) were measured in the fused beads by LA-ICP-MS at the Geological Institute of the Bulgarian Academy of Sciences, using the whole-rock SiO₂ content as an internal standard and NIST 610 as an external standard. Whole-rock chemical analyses are given in Table S2 of the Supplementary Material.

4. Results

4.1. U-Pb Geochronology

The dated zircons in the metaconglomerate sample L4 vary in size from 150 µm to 350 µm, with an average aspect ratio of 2. They display semi-rounded shapes and mostly oscillatory and rarely sector zoning patterns; both features are characteristic of a magmatic origin (Figure 5A). The Th/U ratios of the dated zircons of this sample range from 0.14 to 1.77, which is also typical for magmatic zircons, e.g., [48,49].

In sample L4, the ²⁰⁶Pb/²³⁸U ages obtained from 105 analyses range from 1996 Ma to 516 Ma (Figure 6A, Table S1). From seventy-seven concordant zircons, a series of clusters were established with different densities and various ages. The main age cluster of twenty-three zircons yielded a concordia age of 597.1 ± 3.3 Ma, followed by a cluster of seventeen zircons that gave a concordia age of 587.5 ± 3.5 Ma, and a cluster of fourteen zircons with a concordia age of 640.8 ± 5.8 Ma (Figure 6A). Eight concordant zircons cluster at 555.5 ± 0.36 Ma, five concordant zircons cluster at 611.3 ± 4.7 Ma and three concordant zircons cluster at 624.6 ± 6.2 Ma. Two zircon pairs gave concordant ages at 687 ± 49 Ma and at 1996 ± 12 Ma, respectively. Additional single zircons yielded concordant ages at 944 ± 21 Ma and at 791 ± 12 Ma. The five youngest concordant zircons provided an age of 523 ± 6.5 Ma, and hence demonstrated an early Cambrian maximum depositional age (Figure 6A).

Zircons from the metaalbitophyre sample L1a show mainly prismatic and rarely pyramidal crystals varying in size from 70 µm to 250 µm, which have magmatic oscillatory and sector zoning patterns (Figure 5B). In sample L1a, the ²⁰⁶Pb/²³⁸U ages obtained from 25 analyses range from 1532 Ma to 302 Ma (Figure 6B, Table S1). Four youngest zircons yielded a weighted mean age of 308 ± 5.8 Ma (Figure 6B). One out of all four was concordant at 307.4 ± 9.7 Ma, confirming the weighted mean age of the youngest population, which was interpreted to date the magmatic crystallization of the albitophyre. Inherited zircons gave concordant ages at 562.6 ± 6.0 Ma by using seven grains (Figure 6B), and single zircon crystals were concordant at 337.5 ± 3.7 Ma, 479 ± 16 Ma, 529 ± 13 Ma, 619 ± 6.0 Ma and 1532 ± 22 Ma, respectively (see Table S1). The Th/U ratios of the dated concordant zircons of sample L1a range from 0.36 to 1.22, which is typical for magmatic zircons.

The dated zircons in the breccia-conglomerate sample L15 vary in size from 80 µm to 300 µm. They display semi-rounded shapes and mostly oscillatory-zoned patterns, which is characteristic for a magmatic origin (Figure 5C). In this sample, the ²⁰⁶Pb/²³⁸U ages obtained from 100 analyses range from 2009 Ma to 261 Ma (Figure 6C). From eighty-three concordant zircons, a series of clusters were established with different densities and various ages. The main age cluster of twenty zircons yielded a concordia age of 279.5 ± 1.5 Ma, followed by a cluster of thirteen zircons that gave a concordia age of 285 ± 1.6 Ma, and a cluster of twelve zircons with a concordia age of 271.5 ± 1.6 Ma. Eleven concordant zircons cluster at 292.5 ± 1.7 Ma, seven concordant zircons cluster at 267.4 ± 1.8 Ma, five concordant zircons cluster at 264 ± 2.5 Ma, four concordant zircons cluster at 590.6 ± 6.6 Ma (Figure 6C) and three concordant zircons cluster at 276.1 ± 2.6 Ma. Single zircons yielded concordant ages at 2009 ± 25 Ma, at 720 ± 12 Ma and at 323.5 ± 5.8 Ma (see Table S1). The five youngest concordant zircons demonstrated an age of 263.3 ± 1.9 Ma, hence their latest middle Permian maximum depositional age (Figure 6C). The Th/U ratios of the dated zircons in this sample range from 0.23 to 0.96, testifying to a magmatic origin.

4.2. Whole-Rock Geochemistry

The five samples selected for whole-rock geochemistry represent different types of clastic metasedimentary rocks that include metaconglomerate, metasandstone, metasiltstone and phyllite.

The content of major oxides SiO₂ (55.36–73.27 wt%), Al₂O₃ (13.12–24.03 wt%), Fe₂O₃^t (2.66–6.57 wt%) and MgO (0.64–2.28 wt%) is similar to the average upper continental crust composition reported by Taylor and MacLennan [50]. The higher value of Na₂O/Al₂O₃ (0.06–0.34) compared to that of K₂O/Al₂O₃ (0.07–0.19) confirms the plagioclase predominance over the K-feldspar. The strong negative correlation between SiO₂ and Al₂O₃ (−0.98) and the contents of TiO₂ (−0.93) and Fe₂O₃^t (−0.82) could be related to quartz predominance and sorting of the precursory sedimentary rocks. The major minerals (muscovite, chlorite, and plagioclase) induce a pronounced positive dependence between the oxides of Al with K (0.70) and Fe_t (0.71). The logarithmic values of SiO₂/Al₂O₃ and Na₂O/K₂O on the classification diagram according to [51] comprise greywacke protolith composition (Figure 7a).

The dominance of intermediate igneous input is determined by discrimination functions (DFs) used in the diagram of Roser and Korsch [52] (Figure 7b). The moderate degree of alteration corresponds to a Chemical Index of Alteration (CIA) of 69–80 according to [54].

The trace element composition indicates the provenance origin, depositional settings, and heavy sorting because of their low mobility during diagenesis and metamorphism. Most trace elements (e.g., V, Rb, Ta, U, Th, and REE) are incorporated into rock-forming minerals (micas), evidenced by their positive correlation with major oxides (Al₂O₃, Fe₂O₃, K₂O, and MgO). The Th/Sc (0.24–0.43) and Zr/Sc (6.65–12.98) ratios indicate compositional variation consistent with the upper continental crust [55]. The chondrite-normalized REE patterns (Figure 7c) confirm continental crust composition with light REE enrichment (La_N/Sm_N = 2.37–3.71) and heavy REE depletion (Gd_N/Lu_N = 1.11–1.60) and negative Eu-anomalies (0.66–0.83). The La/Th vs. Hf classification diagram [53] based on the composition of low-mobility elements (La, Th, Hf) shows a predominance of andesitic arc sources (La/Th~5, Hf < 5 ppm, Figure 7d) for the studied samples. The high-field strength elements (HFSEs) (La, Th, Sc, Zr, Ti) used for discriminating tectonic regimes [56] suggest a continental island arc tectonic setting.

5. Discussion

The Sredogriv metaconglomerate has an early Cambrian maximum depositional age of 523 Ma. The high Th/U ratios of the detrital zircons reflect a magmatic source. The predominant age clusters of detrital zircons around 520 Ma and 670 Ma are similar to the detrital zircon age of 563 Ma for the Cherni Vrah gabbro from the Berkovitsa Group [29] and the age of 618 Ma for the Protopopintsi metagranite [41]. The latter is not hosted by the Sredogriv sedimentary succession, but underlies this succession, and the Protopopintsi metagranite obviously supplied detrital crustal material for the sedimentation of the younger Sredogriv siliciclastic succession. This interpretation is further supported by the U-Pb zircon ages of nearby volcanic arc affinity Neoproterozoic gneisses of ca. 651–601 Ma. These gneisses host the 517 Ma-old felsic intrusion [8] in the Stakevtsi metamorphic complex located immediately southwest of the study area, and this complex belongs to the Berkovitsa Group of the Vratsa unit (see Figure 2 for location). The proximity to the volcanic arc edifice of the basin that accumulated Sredogriv sedimentary rocks is indicated by established tuffaceous material in the upper part of the succession [35] that resulted in omnipresent chlorite within this succession (see Figure 4a) and supported by the geochemical data of the metasedimentary rocks (Figure 7).

The Cryogenian–Early Cambrian major age cluster of detrital zircons (670–520 Ma), together with two Tonian (791 Ma and 944 Ma) zircons, and the two Paleoproterozoic (1996 Ma) zircons in the Sredogriv metaconglomerate (Figure 6A), are indistinguishable from the reported detrital zircon age clusters at 496 Ma, 519 Ma, 533 Ma, 568 Ma, 577 Ma, 586 Ma, 601 Ma, 630 Ma, 1.9 Ga and 2.0 Ga in the clastic metasedimentary rocks from the Berkovitsa Group and the Diabase–Phylitoid complex [4]. Moreover, the maximum depositional age of the Sredogriv metaconglomerate is very close to indistinguishable from the maximum depositional ages of 521 ± 5 Ma and 494 ± 5 Ma of tuffitic metasiltstone and metasandstone of the Berkovitsa Group, respectively, and the maximum depositional age of 510 ± 6 Ma of a metaconglomerate in the Diabase–Phyllitoid complex [4]. Based on the statistical analysis of the detrital zircons from the Moesian and Balkan terranes and other Avalonian–Cadomian terranes worldwide, Žák et al. [4] and other authors referenced in this manuscript defined the provenance of the detrital sedimentary material from the trans-Saharan belt and/or Saharan metacraton igneous sources at the northern periphery of Gondwana for the Berkovitsa Group and Diabase–Phyllitoid complex, and for other units in the western and central Balkan Zone, i.e., the Balkan terrane. These igneous sources and provenance area fully correspond to those recovered by the detrital zircons in the Sredogriv siliciclastic rocks.

In summary, the Sredogriv siliciclastic rocks were deposited in a basin proximal (e.g., fore-arc/back-arc) to the Cadomian arc system (Berkovitsa island arc sensu [27]) during the early Cambrian period, where they received rather unimodal major Cryogenian–Early Cambrian crustal material input from adjacent igneous sources at the northern periphery of Gondwana. We therefore correlate the Sredogriv metasiliciclastic rocks with the sedimentary section of the Berkovitsa Group based on their lithologic context and Early Cambrian depositional age.

The Sredogriv metaalbitophyre demonstrates magmatic crystallization in the Late Carboniferous period at ~308 Ma, when it emplaced into the Sredogriv sedimentary succession. This igneous age is fully comparable to the ages of many others Late Carboniferous magmatic bodies known in the Western Balkan Zone, e.g., [7,13] and references therein. In this sense, the metaalbitophyre represents a manifestation of the region-wide Late Carboniferous magmatism linked to the late Variscan tectono-magmatic evolution recorded in this zone. The inherited Ediacaran–Early Cambrian zircons (620–530 Ma, Figure 6B) in the metaalbitophyre are obviously xenocrysts sampled “en route” to the surface from the host Sredogriv sedimentary succession that demonstrates the same major Ediacaran–Early Cambrian detrital zircon age cluster. The inherited Mesoproterozoic zircons in the metaalbitophyre that cluster at 1.5 Ga might match the zircon peaks at 1.5 Ga and 1.6 Ga in the Ediacaran gneisses of the Stakevtsi Massif, as reported by Žák et al. [8].

In turn, for the Smolyanovtsi Formation, its latest middle Permian maximum depositional age is 263 Ma, as derived from the conglomerate sample L15. This unconformably lying formation contains well-defined crustal components from Paleo- to Neoproterozoic magmatic sources, Early Cambrian Sredogriv metasiliciclastic rocks, as well as a detrital material from the Late Carboniferous to Middle Permian intrusive and extrusive bodies that is well known in the region and the Western Balkan Zone as a whole (see Figure 3). Thus, the red beds of the Smolyanovtsi Formation recycled the crustal material from all underlying units and unequivocally support their detrital and magmatic crystallization zircon ages.

The stratigraphic position and the maximum depositional age of the Smolyanovtsi Formation clearly define the upper age limit of the greenschist facies metamorphism and associated deformation experienced by the Sredogriv metamorphics as pre-middle Permian ca. 263 Ma. On the other side, the magmatic crystallization age of the Sredogriv metaalbitophyre unequvocally provides the lower age limit of the greenschist facies metamorphism and deformation postdating intrusion at 308 Ma. However, the inferred Late Carboniferous period of the Zelenigrad Formation of the Belogradchik unit is not supported by our data, as the Zelenigrad Formation demonstrates a maximum depositional age of 272 Ma and recycles the crustal material of the same Neoproterozoic–Cambrian and Carboniferous and Permian igneous sources [57], as shown in Figure 3. These newly obtained U-Pb zircon age constraints imply that the greenschist facies metamorphism and associated deformation experienced by the Sredogriv metamorphics are temporarily bracketed between 308 Ma and 272 Ma. This ca. 36 Ma-lasting time interval corresponds to the Late Carboniferous–Early Permian tectono-metamorphic phase of the late Variscan orogeny, which is well-known in the Variscan belt of Western and Central Europe, e.g., [58,59,60,61,62,63], and to the east in the Black Sea region [64,65]. Thus, the Late Carboniferous–Early Permian tectono-metamorphic time interval recorded by the Sredogriv metamorphics adds a new insight into the Variscan collisional evolution and the accretion of the Balkan terrane to the Moesian terrane as suggested by several authors [3,4,10,25,64].

6. Conclusions

Our study reveals important aspects of the sedimentary and magmatic history and tectono-metamorphic evolution of the Sredogriv low-grade metamorphics as an important crustal element that forms part of the crystalline basement of the Western Balkan Zone.

1. The sedimentation of the Sredogriv siliciclastic succession took place in Early Cambrian period around ca. 523 Ma when crustal materials were recycled from limited Paleoproterozoic and mostly Neoproterozoic–Early Cambrian igneous sources from a nearby located volcanic arc, which have a provenance from the Saharan Metacraton at the northern periphery of Gondwana. In terms of the lithology and depositional age, the Sredogriv sedimentary succession can be correlated to the sedimentary section of the Neoproterozoic–Early Cambrian (Cadomian) island arc system of the Berkovitsa Group in the Western Balkan Zone.

2. Intrusion of acidic sill of metaalbitophyre in the Sredogriv sedimentary succession occurred in the Late Carboniferous period at 308 Ma and the magmatic emplacement of the mafic dykes in this succession possibly occurred at the same time. The metaalbitophyre represents a manifestation of Late Carboniferous magmatism in the Western Balkan Zone. The inherited Neoproterozic–Cambrian zircons recovered in the metaalbitophyre were sampled from the Sredogriv sedimentary succession. The magmatic crystallization age of the albitophyre provides a lower age limit for the greenschist facies metamorphism of the Sredogriv metamorphics.

3. The first unconformable clastic sedimentary cover onto the Sredogriv metamorphics of the Smolyanovtsi Formation was deposited in the latest middle Permian period at 263 Ma. The Smolyanovtsi Formation recycled the crustal material from the underlying Sredogriv metasiliciclastic rocks, Late Carboniferous albitophyre, and the regionally present Late Carboniferous–Middle Permian plutonic and volcanic bodies, as evidenced by the contained detrital zircon populations. The depositional age of the Smolyanovtsi Formation provides an upper age limit for the greenschist facies metamorphism of the Sredogriv siliciclastic rocks.

4. The deposition of the unconformable clastic rocks of the immediately adjacent Zelenigrad Formation at 272 Ma, in turn, further lowered the upper age limit of the greenschist facies metamorphism of the Sredogriv metasiliciclastic rocks. Thus, the greenschist facies metamorphism of the Sredogriv metamorphics is bracketed between 308 Ma and 272 Ma. This metamorphism spans the late phase of the Variscan tectono-metamorphic evolution, and the obtained age data represent the first evidence for the timing of the Variscan low-grade metamorphism in the Western Balkan Zone.