Neoproterozoic–Paleozoic evolution of the Drina Formation (Drina–Ivanjica Entity)

This paper addresses a Drina-Ivanjica basement member, Drina Formation, characterized by ŕ controversial Neoproterozoic to Carboniferous age. The Drina Formation is also informally referred to as the “Lower Drina Formation” and the “Upper Drina Formation” including the Golija Formation as a conditional analog unit of the latter. A review of the biostratigraphic, sedimentary and paleogeographic constraints identified Drina Formation (Inner Dinarides) as a migrated crustal segment derived from a marginal section of northern Gondwana, being, however, of Neoproterozoic–Early Paleozoic age. The presence of arenites, pelites, conglomerates, scarce limestones, basic (sub)volcanics and tuffs of the volcano-sedimentary Drina Formation metamorphosed up to greenschist and locally up to amphibolite facies, coupled with the absence of felsic volcanism implies a passive margin setting. Considering the age, such environment was probably associated with the perplexed Lower Paleozoic Avalonian-Cadomian arc, situated along the former north Gondwanan active margin. More precisely, the Drina Formation originated from a depositional junction between the Gondwana sediment supplier (Sahara metacraton) and Cadomian arc. A comparison with the regional Early Paleozoic succession of the “Kučaj Unit” (eastern Serbia) yields the absence of typical anchimetamorphic Silurian to Lower Devonian deep-marine fossil-bearing succession. The volcano-sedimentary passive margin system of Drina Formation is overlain by a late Variscan convergencerelated voluminous clastic sequence allocated as the Golija Formation.


Introduction
Positioned south of the European Variscides, basement units of Southeast Europe (SEE) -Dinarides and their lateral extension into Eastern Alps and Carpathian-Balkan-Hellenic arc represent a highly composite aggregation of displaced contrasting continental fragments derived from ancient plate boundaries.The Variscan configuration of these units was entirely reactivated during early and late Alpine tectonic processes.Opposite to considerably well-explored Alpine configuration, Variscan and pre-Variscan evolution of Dinarides, i.e.Adria and Apulia microplates, just recently draw certain attention by applying means of paleo-continental dating or ancient paleo-positioning (SIEGESMUND et al., 2018).However, differentiating the Variscan-and pre-Variscan imprints across SEE basement units requires much more effort in comparison to the European Variscides, predominantеly because of the several thermal episodes which obliterated early magmato-sedimentary record, contemporaneous rapid exhumation and erosion of the Variscan peripheral thickened crust (NEUBAUER & HANDLER, 1999), in addition to the terminal late-Alpine thrusting and nappe stacking.
The Drina-Ivanjica Entity (DIE) of western Serbia and eastern Bosnia and Herzegovina represent a highly complex tectonometamorphic amalgamation of Paleozoic and Mesozoic age units (sensu ĐOKOVIĆ, 1985;JURKOVIĆ, 2006).A recent discovery of Neoproterozoic palynomorphs within the Drina Formation (ĐAJIĆ, 2010;ĐAJIĆ et al., 2012) indicated that juvenile DIE is in fact a pre-Phanerozoic system which probably underwent the peri-Gondwanan "rift-drift" stage further implying Lower Paleozoic tectonic transport and Variscan accretion.Considering the polydeformational, protracted plate-tectonic evolution of the SEE basement inliers, in this contribution we rather focus on the multi-disciplinary factual base, in particular we follow a limited set of Early and Late Paleozoic paleogeography and plate-tectonic models.By using the Drina Formation as the key spatio-temporal marker, this review study provides a pioneering, yet a crude reconstruction of the juvenile Neoproterozoic -Early Paleozoic DIE.
The perplexing DIE is an elongated ribbon-shaped basement segment between the Vardar Zone on the East, the Dinaride Ophiolite belt and External Dinarides on the West-Southwest.In the North, the Jadar unit is situated within the Vardar Zone, along the southern Pannonian basin margin (Fig. 2).The DIE is comprised of a northern flank or the Drina block and a southern limb often referred to as the Ivanjica block (Fig. 3).The Drina Formation is exposed in the vicinity of the Drina River from where it extends towards Southeast to Golija Mt. slopes and farther towards the Ibar River where it reaches a thickness of 600-700 m.The overlying Golija Formation has the thickness of 1000 m (ĐOKOVIĆ, 1985).

Discussion
The Phanerozoic evolution of the former Yugoslavia and SEE can beyond question be qualified as one of the most intricate in global scale, as it goes back to (i) the Late Neoproterozoic Cadomian orogeny (e.g., ANTIĆ et al., 2016;AVIGAD et al., 2017), (ii) subsequent Neoproterozoic-Early Paleozoic peri-Gondwanan wrenching (e.g., MERDITH et al., 2017) coupled with the (iii) subduction-related back-arc extension (NANCE & LINNEMANN, 2008).These Early Paleozoic events are superseded by the two major orogenic imprints -Variscan and Alpine.(iv) The Variscan imprint was confirmed in earlier exploration stages (ĐOKOVIĆ, 1985).This event aggregated previously detached arc crustal segments of North Gondwana margin with the already assembled Laurasia.

Coupling the Drina Formation with the peri-Gondwanan framework
Further complications in reconstructing perplexing history of the Drina Formation are associated with the ambiguity whether this basement system was involved within the Late Neoproterozoic Cadomian and Late Paleozoic Variscan orogenic cycles.A structural record of the Cadomian event (Late Proterozoic to Early Cambrian, sensu D' LEMOS et al., 1990) is poorly preserved outside the northern Armorican Massif, Bohemian Massif, and southwestern Iberian Massif (EGUÍLUZ et al., 2000 and references therein).Nevertheless, certain basement segments of the Central European Variscides contain deformations of a minor intensity governed by the Cadomian orogenic cycle (BEHR, 1983).However, the eventual imprints of Cadomian event have not been investigated within SEE yet.The published paleo-tectonic models of the Variscan orogenic cycle emphasize that the central domain of North Africa was not involved in the Variscan collision (e.g., NANCE & LINNEMANN, 2008;STAMPFLI et al., 2013).Instead, a limited number of recent reconstructions propose that the former North African segments of modern-day SEE originated from the large-scale Hun arc complex (e.g., STAMPFLI et al., 2002;STAMPFLI & KOZUR, 2006) including the Apulia/Adria microplate, a former member of the North African promontory (FRANKE et al., 2017;CA-SAS & MURPHY, 2018).A member of this Early Cambrian E-W-oriented Cadomian aggregation is the Drina-Ivanjica entity, recently recognized as a subsided passive margin devoid of the Variscan imprint (EBNER et al., 2008) which is contrary to the previous observations (ĐOKOVIĆ, 1985).

The Drina Formation: Reassessing the age and primeval deposition
The entire Neoproterozoic-Paleozoic evolution of the DIE remains to be a somewhat underexplored and controversial topic.A pioneering study of the Drina Formation described the DIE basement as a set of the Paleozoic formations ranging in age from the Early Paleozoic to the Carboniferous (ĐOKOVIĆ, 1985).However, the age of the Drina Formation and a set of succeding formations are poorly constrained (DIMI-TRIJEVIĆ, 2001) and even nowadays remains to be a contradiction.Namely, a recent discovery of the Neoproterozoic palinomorph record (ĐAJIĆ, 2010;ĐAJIĆ et al., 2010) ĐOKOVIĆ (1985), however, extended the age of the metamorphic Drina Formation from the confirmed Cambrian up to the Silurian, Devonian, and reaching Early Carboniferous.In this description, the "Upper Drina Formation" is to some extent a synonymous term for the Golija Formation (Fig. 3).Regionally, in SEE (eastern Serbia), typifying Silurian has, however, a distinctive set of graptolite facies (KRSTIĆ et al., 2005) embedded into the Late Ordovician to Early Carboniferous "Kučaj Unit".Furthermore, basement greenschist-facies rocks of the Supragetic basement (Vlasina Complex, adjoining the "Kučaj Unit") contain a set of remarkable fossil remnants of the Ordovician inarticulate brachiopod fauna (PAVLOVIĆ, 1962).An absence of the phytoplankton: Sphaeromorphitae, Mirchystridium sp., Veryhachium sp., Balitesphaeridium sp., rather indicates a missing part of the Ordovician, Silurian and Early Devonian column (ER-CEGOVAC, 1974) within the Drina Formation.In addition to the inconclusive Silurian and Devonian age, another highly ambiguous issue is a continuation of the Drina Formation into the Lower Carboniferous, represented by a clastic sequence with fragments, blocks and autochonous levels of metalimestones with Early Carboniferous conodonts of Gnathodus bilineatus and Lochrieia nodosa zones (DJORDJIJEVSKI-KA-LAMBOKIS et al., 1990; FILIPOVIĆ & SIKOŠEK, 1999 and references therein).These Early Carboniferous rocks are ambiguously considered as the "Upper Drina Formation", a spatio-temporal continuation of the "Lower Drina" (DIMITRIJEVIĆ, 1997; Fig. 3).For a comparison, the Early Carboniferous of the adjacent Jadar block (in Alpine configuration) is marked by the abundance of the provincial ammonoid faunas (KORN et al., 2010) indicating autonomous developments of the DIE and Jadar system (ĐOKOVIĆ & PEŠIĆ, 1985).
The basic/metabasic rocks as conditional markers of crustal extension and the subsequent oceanization are in modern-day Central Europe and SEE, nevertheless, distributed on the both sides of Early Paleozoic Gondwana and formerly juxtaposed Laurasia.These occurrences of (meta)basic rocks are often referred to as "Variscan ophiolites" (e.g., IVAN & MÉRES, 2012;PLISSART et al., 2017).The presence of early Paleozoic (meta)basic rocks in Central Europe and SEE can be recognized either as a remnant of the formerly accreted Cadomian arc which was in turn developed in a back-arc position during Neoproterozoic to Ordovician times (opening of the proto-Rheic Ocean), or as the result of Devonian back-arc plate divergence of internal parts of the orogen.Development of basaltic magmatism along the outskirts of Laurasia (sensu FRANKE, 2006) was probably induced by the shrinking Rheic Ocean and the effect of the descending plate beneath mainland Laurasia or by the interchanging episodes of transpression and transtension between neighboring microcontinents (FRANKE et al., 2017).Here are some of the examples of the recognized Cadomian arc remnants in Central and Southeastern Europe: the Rothstein Formation (LINNEMANN, 2007), the Neukirchen metabasic complex of the Bohemian Massif (BEHR, 1983 and references therein), the Brno Massif (FINGER et al., 2000), the Sandýklý basement metasandstones (Menderes Massif) with flattened clasts of basic volcanic rocks (GÜRSU & GÖNCÜGLU, 2006).Devonian development of a within-plate (meta)basic rock system is documented across Central Europe (e.g., SCHLAEGEL-BLAUT & HEINISCH, 1991).In SEE, in the Western Greywacke zone (Austria), the greenschists are intercalated within calcareous phyllites with a presence of mafic tuff of 384.5±1.9Ma U-Pb zircon age.This age indicates rudimentary development of oceanic systems at the Middle to Upper Devonian boundary (DUM et al., 2012).Metabasites of the Gemeric Superunit (Western Carpathians) display transitional character exhibiting N-MORB, E-MORB and BABB types (IVAN & MÉRES, 2012).
Mid-Edicarian deposition along the extended margin of northern Gondwana (Fig. 5) is dominated by an intensive weathering of the Cadomian orogen and protracted sediment transport into the former (GAR-FUNKEL, 2015).The available Cambrian paleo-conti-nental-and paleogeographic reconstructions place the "paleo-Dinarides" either as a crustal segment being already shifted away of the West African (Saharan) craton (LI et al., 2017) or as a segment still attached to the Ordovician north Gondwana (SIEGESMUND et al., 2018).The Late Ordovician location of the Adria microplate, on the basis of the age of the Cambrian-Ordovician magmatism, the detrital zircon signature and other stratigraphic criteria also suggest a proximity of northern Gondwana (CASIS & MURPHY, 2018 and references therein).Rare detrital zircon age spectra results of modern-day remnants of the "Paleo-Adria", e.   input of Cryogenian, Tonian and Stenian sources and subordinate Paleoproterozoic and Neoarchean ages (SIEGESMUND et al., 2018).Such detrital mixture of sources points to the northeastern Saharan Metacraton and the Northern Arabian-Nubian Shield (Sinai) excluding Baltican inheritance.The volcano-sedimentary protolith record of Neoproterozoic age (ĐAJIĆ, 2010, ĐAJIĆ et al., 2012) was under influence of material eroded and transported from the pre-Pan-African basement (SIEGESMUND et al., 2018).Consequently, we place a juvenile environment in which the Drina Formation originated as a proto-segment of the Apulia/Adria (CASIS & MURPHY, 2018) or the "Paleo-Adria" (FRANKE et al., 2018) as a crustal segment still connected to north Gondwana during Neoproterozoic to Early Paleozoic times (Fig. 4).According to several models, "Paleo-Adria" aggregation was detached from the Gondwana somewhere between Late Silurian and Late Carboniferous (von RAUMER & STAMPFLI, 2008) or was accreted onto Laurasia together with north Gondwana (FRANKE et al., 2017).
The suggested position of juvenile Drina-Ivanjica Entity implies, that during Late Ordovician, Silurian and Early Devonian, this marginal system of North Africa was a continental fragment outboard the modern-day North African coast.Another argument for the passive margin setting is lack of typifying felsic volcanism of the Cadomian age.A tentative paleo-location of the juvenile Drina succession was in a vicinity of the continental Cadomian arc which was affected by the Neoproterozoic-Cambrian epicontinental sedimentation (sensu GARFUNKEL, 2016; Fig. 5).The Drina Formation was probably juxtaposed next to the contemporaneous Cadomian peri-Gondwanan active margin system (VON RAUMER & STAMPFLI, 2008).
According to some recent reconstructions the "Paleo-Adria" travelled over 6500 km between the Silurian (opening of the Palaeotethys) and the Devonian (360 Ma) prior to the Variscan accretion (FRANKE et al., 2017).However, the presence of metabasic rocks intermixed with Neoproterozoic sedimentary succession of the Drina Formation rather indicates a vicinity of precursory proto-Rheic Ocean further implying a pre-Silurian detaching or different paleogeographic location of "Paleo-Adria" in particular Adria/Apulia.Tentatively, the location was more to the west in Ordovician reference relative to the the location reference proposed by FRANKE et al. (2017;Fig. 10a).
The "Paleo-Adria" arrival and Pangea assembly are either interpreted as Late Devonian-Carboniferous dextral shearing (FRANKE et al., 2017)  The subsequent Carboniferous accretion with SE Laurussia should be marked by a set of ductile fabric documented within the Drina Formation (ĐOKOVIĆ, 1985).However, recent dating of samples taken from metamorphic Paleozoic units in Serbia by applying the Illite crystallinity and K/Ar methods, indicate the latest Jurassic-earliest Cretaceous timing of metamorphism (130-145 Ma, PORKOLÁB et al., 2018).Nevertheless, the Early Carboniferous siliciclastic system (FILIPOVIĆ & SIKOŠEK, 1999) should mark a descending plate position of the Drina-Ivanjica during a Late Paleozoic convergence.To summarize, it is obvious that further investigations of the Paleozoic evolution of "Paleo-Adria", particularly of the Drina-Ivanjica are necessary.

Conclusions
The presented results show that the Drina Formation was a segment of the stabilized peripheral Cadomian platform or the pre-Variscan basement of "Paleo-Adria" (Apulia/Adria microplate in Alpine configuration).This subsided continental margin was filled with the mixture of flat-lying sediments.The biostratigraphic comparison with the paleogeographically similar "Kučaj Unit", yields that the Drina Formation is exclusivelly of Neoproterozoic-Early Ordovician age, consequently ruling out any connection with the Lower Carboniferous flysch system of the Drina-Ivanjica basement (informally called "Upper Drina Formation").The overall reconstruction places the juvenile Drina Formation into the, so-called, North African depositional domain, passive margin in the West and North African (Saharan) meta-craton and peri-Gondwanan Cadomian magmatic-arc system.This significantly differs from the older, ca.700 Ma felsic magmatic intrusions of peri-Amazonian Pelagonian micro-fragment.

Fig. 3 .
Fig. 3. a.The geologic map positioning the Drina Formation within the Drina-and Ivanjica segments (after ĐOKOVIĆ, 1985).b.The condensed lithological column indicating the "Lower Drina Formation" and the "Upper Drina Formation" are actually the Drina Formation (column modified after DIMITRIJEVIĆ, 1997).Numbers indicate the raised issues associated with the proposed plate-tectonic solutions: 1, The unconformity represents hiatus between the Neoproterozoic-Cambrian "Lower Drina Formation" and the Carboniferous "Upper Drina Formation".It could actually represent two different formations; 2, Questionable occurrence of meta-basic rocks in both formations, missed in the lower section of the column; 3, There should be an unconformity between the Late Carboniferous and the Early Triassic; 4, The interrelationships between the entire set of Paleozoic formations are ongoing work.
indicated a Neoproterozoic age, somewhat complementary with earlier dating (ERCEGOVAC, 1974) which pointed Cambrian to Ordovician age.