INTERNATIONAL GEOLOGY REVIEW, 2015 http://dx.doi.org/10.1080/00206814.2015.1092097

Origin and evolution of the South Carpathians basement (Romania): a zircon and monazite geochronologic study of its Alpine sedimentary cover Adriana M. Stoicaa, Mihai N. Duceaa,b, Relu D. Robana and Denisa Jianua

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a Faculty of Geology and Geophysics, University of Bucharest, Bucharest, Romania; bDepartment of Geosciences, University of Arizona, Tucson, AZ, USA

ABSTRACT

ARTICLE HISTORY

This study investigates the advantages of a multi-mineral approach in detrital mineral geochronology, as tracers of regional tectonic events. We present new detrital zircon and monazite ages on six sands and sandstones collected from the South Carpathians, Romania. They represent clastic sediments derived from the pre-Alpine basement and related sedimentary cover, which accumulated in distinct palaeogeographic and geotectonic environments, during the multiphase Alpine tectonic evolution. Three samples are mid-Cretaceous sandstones from different depositional settings of the syn-tectonic clastic wedge that activated during the intra-Albian thrusting phase. One is Upper Cretaceous sandstone from the South Carpathian foredeep, associated with the intra-Maastrichtian thrusting phase. Two additional samples are Quaternary fluvial deposits reworking the Upper Cretaceous hinterland basin siliciclastic deposits. Detrital zircon U–Pb ages confirm periods of zircon-producing magmatism in the Neoproterozoic (ca. 590–850 Ma), Cambrian–Ordovician (ca. 540–450 Ma), and, in one sample, Late Cretaceous (ca. 76–81 Ma). Precambrian tectonics is documented by inherited zircons (ca. 0.9–1.2 Ga, 1.8–2.2 Ga, 2.6– 2.8 Ga), most likely recycled from metasedimentary rock units from the Getic basement. Zircon age distribution patterns from all samples are consistent with derivation from eroded equivalents to basement rocks of the Getic–Supragetic thrust sheets. In contrast, chemical ages on all detrital monazites document a single metamorphic event of Late Devonian to Carboniferous ages (ca. 300–400 Ma), coincident with the Variscan orogeny in central Europe. A small proportion of the zircon population is also of the same age range (ca. 380–320 Ma) – those zircons typically have high U–Th ratios, characteristic of metamorphic zircons. Detrital monazite ages are consistent with previous limited geochronological data on high-pressure metamorphic rocks from the Getic– Supragetic basement. In addition to the timing of tectonic events in the Carpathian basement, geochronology of detrital minerals brings new constraints regarding the duration of these events.

Received 16 May 2015 Accepted 6 September 2015

1. Introduction With the refinement of analytical methods that can be applied to detrital material, particularly single-grain isotopic analyses, the sedimentary record has become an invaluable tool in understanding the tectonic evolution of their source terrains (Gehrels 2014). U–Pb zircon geochronology is a well-established technique, being extensively used in provenance analyses, and palaeogeographic and tectonic reconstructions (e.g. Rainbird et al. 1992; Gehrels and Dickinson 1995; Gehrels et al. 1995; Sircombe and Freeman 1999; Cawood and Nemchin 2000; Fedo and Farmer 2001). Furthermore, laser ablation ICP-MS dating on zircons yields precise and accurate U–Pb ages, while considerably reducing the time for data acquisition compared with conventional thermal ionization mass spectrometry (TIMS) techniques, becoming the

Detrital; zircon; monazite; provenance; palaeotectonic evolution; geochronology; South Carpathians

preferred method for studies requiring large numbers of analyses (Kosler and Sylvester 2003). The majority of provenance studies rely solely on age information provided by detrital zircon geochronology (more recently augmented by petrogenetic information using in situ Hf isotopes on the same zircon grains), to trace back igneous and high-temperature metamorphic events of basement units that contributed to the sediment load. Combined detrital zircon–monazite chronology approaches in provenance studies have demonstrated the utility of monazite as a reliable complementary tracer of orogenic processes that were otherwise under-represented or even entirely missed by zircon ages alone (e.g. Adachi and Suzuki 1994; Suzuki and Adachi 1994; Fergusson et al. 2001; Moecher et al. 2011; Hietpas et al. 2010, 2011). Monazite is a common accessory mineral in a

CONTACT Adriana M. Stoica [email protected] Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00206814.2015.1092097 © 2015 Taylor & Francis

KEYWORDS

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wide range of magmatic and metamorphic rocks, including granites, pegmatites, felsic volcanic rocks, mediumand high-grade metamorphic rocks, and low-grade metasedimentary rocks (Parrish 1990). Owing to its moderate resistance to chemical and mechanical weathering, detrital monazite is a widespread occurrence in the heavy mineral suite of clastic sediments and sedimentary rocks. This study investigates the advantages of combining two high-temperature chronometers (U–Pb on zircons and U–Th/Pb on monazites) in deciphering the major tectonic events that resulted in the magmatic and metamorphic making of the South Carpathians basement in central Romania. We report 521 U–Pb ages on detrital zircons and 178 Th–U–total Pb ages on detrital monazites collected from the siliciclastic sedimentary archive derived from the South Carpathian basement and preserved in adjacent, tectonically controlled basins, during the Cretaceous thrusting events. Integrating all the ages obtained for this study allows additional interpretations regarding the timescales of processes that were involved in the genesis of the Carpathian pre-Alpine terranes. We provide first-order constraints on the timing of magmatism and metamorphism and show that the major episode of regional metamorphism that shaped the South Carpathians would have been almost entirely missed in the zircon record, whereas it dominates the monazite archive. We also provide constraints on the duration and changes in the rates of magmatic activity, from episodes of increased magmatic pulses to periods of little to no magma production.

2. Geologic setting The South Carpathians form a 300 km-long E–W-trending segment of the Romanian Carpathian mountain belt. The South Carpathians have a fold and thrust structure with south vergence (Burchfiel and Bleahu 1976), assembled during the Alpine continental collision that followed the closure of the Ceahlău–Severin ocean, which was a relatively small oceanic basin within the Tethyan realm (Schmid et al. 2008). It consists of several thrust nappes: the Getic–Supragetic and Danubian nappe systems of continental origin separated by the oceanic Severin tectonic melange (Figure 1). More detailed classifications of the thrust units are found in Săndulescu (1984), Balintoni (1997), and Iancu et al. (2005), among others. Their Alpine tectonic evolution involves two Cretaceous and one Late Miocene contraction events, separated by quiescent periods and some characterized by extensional basin formation (Săndulescu 1984; Iancu et al. 2005). The local literature has several conflicting interpretations about the relative timing of Alpine shortening events in the South Carpathians; in part, these interpretations are

hampered by the limited syn-tectonic sedimentary cover in the South Carpathians, which is in contrast with the continuation of these structures in the East Carpathians where Mesozoic and Cenozoic deposits abound (Săndulescu 1984). One view is that the mid-Cretaceous (intra-Aptian) phase emplaced the Supragetic units onto the Getic nappes (Iancu 1985; Hann 1995), whereas the Latest Cretaceous (intra-Maastrichtian) phase overthrust the Getic–Supragetic nappe system and the Severin complex over the Danubian domain (Berza et al. 1994; Iancu et al. 2005). The Late Miocene (intra-Sarmatian) phase is responsible for thrusting of the entire South Carpathian nappe stack onto the Getic Depression foredeep (Mațenco et al. 1997). However, some intra-Oligocene contractional discordances were documented into the foreland area of South Carpathians (Răbăgia et al. 2011), as an effect of transpression processes along the northern Moesian margin. Late Cretaceous times (Early Maastrictian) pre-dating the intra-Maastrichtian shortening event was an extensional period, marked by the development of extensional basins regionally known as Gosau-type basins (Schuller 2004) and the intrusion of a calc-alkaline magmatic suite, locally known as the ‘banatitic suite’, into the Getic– Supragetic nappe complex (Berza et al. 1998; Nicolescu et al. 1999; Ciobanu et al. 2002). The Upper Cretaceous magmatism extends to the south in Serbia and eastwards in Bulgaria, forming a bending ore-bearing igneous belt – The Banatitic Magmatic and Metallogenetic Belt, which is part of the larger Apuseni–Banat–Srednogorie magmatic arc extending into Bulgaria to the south and east (Berza et al. 1998). Previously believed to have spanned a long period (>50 Ma) (Berza et al. 1998, for a review), limited modern geochronology data (e.g. Zimmerman et al. 2008 and references therein) suggest that magmatism was short-lived and limited to the Upper Cretaceous (between 75 and 85 Ma) in Romania.

The pre-Alpine basement of the South Carpathians Metamorphic rocks, here referred to as basement, constitute more than 85% of the exposed rocks in the South Carpathians. The pre-Mesozoic rock assemblages preserved in the South Carpathian basement belong to the Getic–Supragetic and the Danubian domain, and mainly comprise Neoproterozoic and Palaeozoic preWestphalian metaigneous and metasedimentary units with inferred Variscan high-grade metamorphism. Upper Carboniferous to Permian sedimentary strata and small volumes of associated magmatic rocks constrain the episode or episodes of metamorphism to be pre-latest Palaeozoic (Iancu et al. 2005). A few eclogite and garnet amphibolite facies metamorphic rocks from

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Figure 1. (a) Studied area in the context of geotectonic setting of Central and Southeastern Europe and Balcani Mountains. (b) Simplified tectonic map of Southern Carpathian basement units and sedimentary cover, adapted after Săndulescu (1984), Berza et al. (1994), Mațenco et al. (1997), and Schmid et al. (1998), showing detrital zircon and monazite localities (circles) and sedimentary logs positions (squares). See Figure 3 for stratigraphic ages and depositional context of sampled units. (c). Schematic cross section of Southern Carpathian basement, showing nappe structure. Modified from Csontos and Voros (2004).

the Getic have garnet Sm–Nd ages of Variscan age (320–360 Ma, Medaris et al. 2003). Regarding the exact origin of these units prior to their metamorphism and the boundary between them, relatively little is known to date. Balintoni and collaborators have initiated several zircon geochronologic studies of these basement terranes, but focused most of their work on interpreting the palaeogeographic origin of these basement rocks rather than their petrologic origin. Nevertheless, their growing database of U–Pb zircon ages from South Carpathian basement rocks is the only one that provides

basement age distributions here. Of their zircon age data bank, Getic–Supragetic zircons have been published (Balintoni et al. 2014, for a review), whereas Danubian zircons, and particularly zircons from granitic rocks constituting the upper part of the Danubian, are mostly unpublished (Balintoni and Balica, personal communication). The third tectonic unit incorporated into the Alpine South Carpathian belt – the Severin–Ceahlău nappe complex – is entirely Mesozoic, including Upper Jurassic oceanic crust rocks overlain by siliceous

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pelagic strata of Azuga Formation and Lower Cretaceous distal to mid-fan turbiditic deposits – Sinaia and Comarnic Formations (Marunțiu 1983; Seghedi and Oaie 1997).

Basement rocks of the Getic–Supragetic nappes The Getic–Supragetic nappe system has been differentiated into several pre-Alpine basement units (Drăgușanu and Tanaka 1999; Iancu et al. 2005; Balintoni et al. 2009). It is unclear whether all of the lithostratigraphic units within the Getic–Supragetic basement can legitimately be characterized as individual terranes as reviewed in Balintoni et al. (2014). Most Getic–Supragetic units are petrographically made of metamorphosed volcanic and volcano-sedimentary units and calc-alkaline metaigneous rocks suggestive of former arcs mixed with accretionary wedge sediments and some back-arcs dominated by mafic rocks, all of which were metamorphosed. Limited metamorphic geochronology data from the Getic– Supragetic basement units (Sm–Nd garnet-whole-rock isochron ages on Făgăraș and Sebeş–Lotru rocks, Drăgușanu and Tanaka 1999; Medaris et al. 2003) suggest that high-grade metamorphism occurred during the Variscan orogeny (320–360 Ma). U–Pb zircon ages on several basement Getic and Supragetic orthogneisses establish Ordovician intrusive ages for Leaota, Padeș, and Caraș lithostratigraphic units, or terranes (e.g. Balintoni et al. 2009, 2010). The Sebeş– Lotru terrane, the most extensive basement fragment in the South Carpathians, is dominated by calc-alkaline metaigneous rocks, and comprises two units: the structurally higher, Ordovician, Cumpăna unit, and the structurally lower, Neoproterozoic to Cambrian, Lotru unit (Balintoni et al. 2010). The Făgăraș terrane (Drăgușanu and Tanaka 1999) is composed of pre-Variscan amphibolites, paragneisses, and mica-schist-dominated metamorphic units with large carbonate lenses. There is little doubt that all of these terranes are dominated by subduction-related arc magmatic rocks that formed along the margin of Gondwana during the Neoproterozoic and continuing into the Cambro– Ordovician (Stern 1994; Nance et al. 2008) as most basement terranes of central and Eastern Europe are. It is believed that they docked to the Eastern European craton during the Variscan collision (von Raumer et al. 2013) Basement rocks of the Danubian nappes The pre-Alpine basement of the Danubian domain consists of Neoproterozoic metamorphic complexes, intruded by Neoproterozoic granitoid bodies (Liégeois et al. 1996) and post-Variscan Permian igneous rocks

(Balintoni and Balica, unpublished) and unconformably overlain by thin Upper Ordovician–Lower Carboniferous sedimentary strata of local extent. Structurally, the Danubian basement has been differentiated into two metamorphic units: the Lainici–Păiuş metasedimentary rock sequence and the Drăgşan terrane comprising metavolcanic rocks separated by a third terrane – Tișovița with oceanic affinity (Berza and Seghedi 1983; Berza and Iancu 1994; Seghedi et al. 2005; Balintoni et al. 2010). The nappe stacking of these distinct units is considered Variscan, constrained by the Permian– Mesozoic low-grade metasedimentary successions that seal the contact surface (Berza and Seghedi 1983; Berza and Iancu 1994; Seghedi et al. 2005). For the purpose of this study, we compiled a cumulative probability density diagram with all of the previously published zircon U–Pb ages for the Getic– Supragetic and the Danubian basement units, respectively (Figure 2).

3. Sampling areas Six samples were collected from several Cretaceous and Quaternary detrital sedimentary deposits from the South Carpathians: 1-DG, 2-BB, and 3-SP from the midCretaceous Bucegi Formation, 4-TR from the Upper Cretaceous Turnu Formation, and 5-PS and 6-PSR from Quaternary sediments along Sebeșel Valley, Pianu de Sus area (details regarding the sampling locations are provided in Figure 1 and in Supplementary Table 1 (see http://dx.doi.org/10.1080/00206814.2015.1092097 for supplementary tables), and the correlation with lithostratigraphic units is shown in Figure 3). We targeted clastic sediments that accumulated in syn- or posttectonic environments related to the Cretaceous thrusting events: the syn-tectonic clastic wedge associated with the intra-Albian thrusting phase (samples 1-DG, 2-BB, and 3-SP) and the South Carpathian foredeep associated with the intra-Maastrichtian phase (sample 4-TR), as well as modern sediments (samples 5-PS and 6-PSR). Brief descriptions are provided for each sedimentary unit under investigation in this study.

Bucegi Formation The Bucegi Formation outcrops over an area of ca. 200 km, in the easternmost part of South Carpathians, structurally belonging to Severin–Ceahlău nappes. Exceeding 2000 m in thickness, the Bucegi Formation is dominated by bedded polimictic clast-supported conglomerates with sandstone intercalations and carbonate and metamorphic olistoliths (Olariu et al. 2014; Patrulius 1969). Three stratigraphic units were recognized: a

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Pooled detrital monazite and zircon ages for the 6 analyzed samples Total monazite: n=178 Total zircon: n=521

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Age (Ma) Figure 2. Cumulative relative probability density plots for the combined monazite and zircon ages from all sampled locations, and compiled existing U–Pb data for the two basement units: Getic–Supragetic domain (Balintoni et al. 2009; 2010; Profeta et al. 2013; Balintoni et al., unpublished data) and Danubian domain (Balintoni et al. 2011).

Lower Member up to 100 m thick, poorly sorted conglomerates with shale and sandstone intercalations; a Middle Member, about 1500 m thick, poorly sorted, massive conglomerates and coarse sandstones; and an Upper Member about 400 m thick, with frequent channelized and tabular fine conglomerates and sandstones with lateral facies variations – Babele and Scropoasa– Lăptici sandstones. The Albian depositional age of the Bucegi Formation was constrained by palaeontological dating of the underlying Barremian–Aptian turbiditic deposits and the unconformably overlying upper Albian hemipelagic deposits (Murgeanu and Patrulius 1957; Patrulius 1969; Melinte and Jipa 2007). Olariu et al. (2014) interpreted the depositional environment of the Bucegi Formation to be a narrow (10–20 km) shelf margin of a relatively deepwater basin in a syn-tectonic setting. The coarse-grained, sub-rounded sediment flux is explained by its proximity to the actively deforming mountain range during the intra-Albian thrusting phase. The Lower and Middle Members are considered to be of deep-water slope and basin-floor origin, whereas the Upper Member is interpreted as fluvial and shallow-marine deposits. For the purpose of this study, we sampled and analysed coarse-grained sandstones from the Lower

Member (sample 3-SP) and Upper Member (samples 2-BB and 1-DG).

Turnu Formation Turnu Formation is a coarse-grained clastic succession, with variable thickness (from 400 to 2000 m) that crops out along Olt valley, between the Supragetic basement to the north and Palaeogene and younger sediments towards the south. Turnu Formation is part of the South Carpathian foredeep (also known as the Getic Depression) that extends ca. 150–200 km southwards from the orogen to the undeformed Moesian platform. During its Late Cretaceous to late Pliocene sedimentation history, the South Carpathian foredeep accommodated more than 6 km of sediments (Mațenco et al. 1997). Turnu Formation comprises massive or normal graded bedded conglomerates and coarse-grained sandstones with laminated marl and shale intercalation, and it is overlain by Căciulata Formation in a general finning-up sequence. The Campanian–Maastrichtian age for Turnu Formation is attributed to specific Upper Cretaceous ammonite fauna occurrence (Murgeanu et al. 1968).

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Figure 3. Schematic stratigraphic sections of Cretaceous to Quaternary strata from the three areas under investigation, showing the depositional context of sampled units. (a) Lower Cretaceous Bucegi Formation section, drawn using data from Olariu et al. (2014); (b) Upper Cretaceous Turnu and Căciulata formations, drawn using data from Murgeanu et al. (1968); and (c) Quaternary deposits section, drawn using data from Savu et al. (1968).

These Upper Cretaceous detrital deposits lie discordantly on the Supragetic basement and mark the beginning of the latest Cretaceous to Palaeogene sedimentation stage in the South Carpathians foredeep (Săndulescu 1984; Mațenco et al. 1997). Turnu Formation is interpreted as marine slope fanglomerates accumulating in a tectonically controlled setting marked by transtension to extension, during latest Cretaceous to Palaeogene, which generated several E–W-trending basins (Mațenco et al. 1997). We collected and analysed one sample (4-TR) from a coarse-grained sandstone, towards the top of the sedimentary succession of Turnu Formation.

Pianu de Sus The Pianu de Sus area, north from Sebeș Mountains, consists of Upper Cretaceous and Quaternary sedimentary units covering uncomformably the Getic basement. The Cretaceous deposits are a thick (ca. 1500 m) clastic succession comprising mainly poorly sorted conglomerates, breccia, and coarse-grained sandstones, with shale and marl intercalations (Anastasiu et al. 2004). The Pleistocene–Holocene sediments consist of unconsolidated polymictic sands and gravels with quartzite boulders, unconformably overlying the Cretaceous deposits (Tamaș-Bădescu et al. 2004).

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shallow-water fanglomerates, and shelf siliciclastic deposits in the latest Cretaceous. Quaternary deposits lay unconformably over the Upper Cretaceous strata. They formed in a fluvial depositional environment, and are primarily reworking the underlying Cretaceous sediments. Two samples were collected and analysed from Pianu de Sus area, one sample (5-PS) from a poorly consolidated Quaternary terrace deposit and the other from modern alluvial sediment from the Sebeșel riverbed – right tributary of Pianu river (sample 6-PSR).

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4. Analytical methods Approximately 10 kg of sample was collected at each location. The samples were crushed, sieved through a 300 μm sieve, and washed. Heavy mineral concentrates in <300 μm size fractions were separated with a heavy liquid (diiodomethane with 3.3 g cm−3 density) and magnetically using a Frantz Magnetic Barrier Separator at 20° and 10° slopes. We further used the nonmagnetic fraction that concentrates the zircon crystals and the 0.7A fraction concentrating monazites. Zircon and monazite crystals were mounted en masse, along with several age standard crystals, in epoxy resin and polished using 2500 grit sandpaper, until their surface was sufficiently exposed. The sample mounts were then carbon coated to ensure good conductivity. All SEM investigations, U–Pb isotopic analyses for zircons, and chemical analyses for monazites were performed at the University of Arizona.

Zircon analysis

Figure 4. (a) Cathodoluminescence image of a selected area of sample 4-TR zircon grain mount, revealing complex internal zonation of most zircons. Owing to the small size of zircon crystals (<150 μm), ablation pits (30 μm) were placed within a single domain, or overlapping two domains within the crystal’s core, in some cases. (b) Backscatter electron image of a selected area of sample 4-TR monazite grain mount. The mineral assemblage includes: 1, monazite; 2, zircon; 3, rutile; and 4, garnet.

All these sediments were accumulated on the southern rim of the Transylvanian Basin, developed in the Carpathian hinterland. The Transylvanian Basin is a post-Cenomanian to Pliocene intra-Carpathian sedimentary basin bordered by the Eastern and South Carpathians and delimited from the adjacent Pannonian Basin by the Apuseni Mountains (Kreszek and Filipescu 2005). The sedimentary fill, in our sampling area, evolved from non-marine to marine,

Following mounting, zircon crystals were first identified using a SEM equipped with an energy-dispersive spectrometry (EDS) detector. We also used cathodoluminescence (CL) imaging to identify potential defects or inclusions in crystals (Figure 4a), as well as other CL evidence for inherited zircon cores or zonal overlaps to be avoided during laser ablation. U–Pb isotopic analyses were performed in situ on single zircon grains using a 193 nm Excimer laser ablation system with a laser beam diameter of 30 μm attached to a Nu Plasma multicollector inductively coupled plasma-mass spectrometer (LAMC-ICP-MS) at the Arizona LaserChron Center (Gehrels et al. 2006, 2008). About 100 grains were measured from each sample (by random selection), using the quantitative approach of detrital zircon analysis, indicating that the analysed sample was to be as representative of the overall detrital zircon population as possible, thus capturing all potential zircon donor terranes (Fedo et al.

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2003; Andersen 2005). In this sense, we systematically analysed an array of the mounted zircons, irrespective of their morphology or colour. The only selection made was in terms of avoiding obvious imperfections, such as inclusions, fractures, or domain boundaries (core–rim interface or zone overlaps highlighted in CL). A zircon age standard was analysed every 10 unknowns. We used the University of Arizona internal standard, a Sri Lanka zircon crystal that yields an IDTIMS age of 563.5 ± 3.2 Ma (Gehrels et al. 2008).

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Monazite analysis EDS identification and BSE imaging of the monazite grains were carried out first using a Cameca 2 SEM (Figure 4b). In situ chemical analyses of monazite in all six rocks were carried out in polished mounts using the Cameca SX-100 electron microprobe. About 30 monazite grains were analysed for each sample. Five spectrometers were employed using the crystal arrangement thallium acid phthalate (TAP), large pentaerythritol crystal (LPET), lithium fluoride (LIF), and large lithium fluoride crystal (LLIF). An initial wavelength-dispersive spectrometry scan was performed on a representative monazite grain to determine the appropriate background, pulse height analyser settings, overlap corrections, and X-ray lines to use for the analyses. During each analysis, Ca, P, S, and Si were measured at 15 kV, 40 nA using a focused beam and counting for 30 seconds on peak and 30 seconds on background. Beam conditions were then changed to 15 kV and 299 nA, and U, Pb, Y, La, Ce, Pr, Nd, Sm, Gd, and Dy concentrations were measured, again using a focused beam. The Pb Ma line was measured simultaneously using two LPET crystals counting for 120 seconds on peak and 120 seconds on background, and the results were combined to provide greater accuracy. Pb Ma X-ray counts were adjusted for overlap from the Y Lc2 X-ray line using adjustment factors determined by measurement on a YAG garnet interference standard. U Mb was measured on an LPET crystal for 60 seconds on peak and 60 seconds on background, and the U Mb X-ray counts were adjusted for overlap from the ThM3N4 X-ray line using adjustment factors determined by measurement on a ThO2 interference standard. Th Ma was measured on an LPET crystal for 60 seconds on peak and 60 seconds on background, and Y La was measured on a TAP crystal for 240 seconds on peak and 240 seconds on background. La La, Ce La, and Nd Lb were measured on an LIF crystal for 60 seconds peak and 60 seconds background each. The Lb lines of the remaining REE elements were measured using an LLIF crystal for 60 seconds peak and 60 seconds background each.

Well-characterized natural and synthetic standards were used for all calibrations including synthetic UO2 and ThO2 from Cannon Microprobe Company for U and Th, NBS glass K0229 for Pb, and synthetic YAG garnet from C.M. Taylor Company for Y. Monazite 44,069 of Aleinikoff et al. (2006) was used as a test standard. Matrix corrections were performed using the PAP (Pouchou and Pichoir) correction method (Pouchou and Pichoir 1984). The monazite chemical ages were calculated using the elemental concentrations of Pb, Th, and U in an Excel spreadsheet developed by Michael J. Jercinovic that solves the age equation given in Montel et al. (1996). The reported errors on calculated ages are at the 1σ level. The spreadsheet can be downloaded from http://geoinfo.nmt.edu/labs/microprobe/monazite/ home.html.

5. Results Probability density plots and histograms were generated for the detrital zircon and monazite age spectra for each of the six analysed samples (Figure 5). The diagrams were plotted using the Isoplot add-on (Ludwig 2001). Only those grains showing more than 90% concordance were considered for this study. The only zircons that yielded discordant ages are mainly Palaeoproterozoic to Neoarchaean and were not the subject of any interpretation. The isotopic analyses conducted for this study are available in Supplementary Table 2.

Detrital zircon age spectra The detrital zircon age spectrum of sample 1-DG (n = 85) is dominated by Precambrian, mainly Neoproterozoic between 580 and 850 Ma (48%) and Palaeozoic ages peaking at 460 Ma (27%) (Figure 5). The NeoMesoproterozoic limit is represented by 6% of the zircon ages. There is a paucity of detrital ages between 1.2 and 1.8 Ga. The age spectrum records contributions of Palaeoproterozoic to Neoarchaean ages accounting for 15% of the total detrital zircon ages. The youngest recorded age is Carboniferous (358.5 ± 3.3 Ma), represented by one zircon grain with a U–Th ratio of 44.3. In sample 2-BB (Figure 5), the U–Pb ages of 84 detrital zircons reveal a prominent population of Neoproterozoic ages (70% of the entire age population) ranging from 540 Ma to 1 Ga, with two ~100 million year hiatuses between 730–820 Ma and 860–960 Ma. The Mesoproterozoic era is absent from the detrital zircon record, whereas older, Palaeoproterozoic and Archaean ages are present but less abundant, accounting for 11% of the total analysed zircons. The primary signal in the Palaeozoic ages is Cambrian to Ordovician,

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Figure 5. Detrital zircon and monazite age histograms and probability density plots for the six analysed samples. Reported errors are at 1-sigma level, bin size for the histograms is 30 million years. N represents the number of grains analysed for each sample.

represented by 14 zircons (17%), followed by a minor contribution of Silurian ages (n = 2). The U–Pb age distribution of 94 detrital zircons in sample 3-SP is dominated by Palaeozoic (28%) and Neoproterozoic (48%) ages (Figure 5), with subordinate inputs from Palaeoproterozoic to Archaean (24%). The main contributor to Palaeozoic signal is the Cambro– Ordovician population, clustering around two peaks at 460 Ma and 540 Ma. Three zircon crystals record Silurian (444.3 ± 4.6 Ma; 437.3 ± 4.8 Ma) and Carboniferous ages (342 ± 8.2 Ma). The Neoproterozoic ages cover the time

span between 550 and 830 Ma, with a more prominent peak around 640 Ma. Neoarchaean–Palaeoproterozoic ages account for 20% of the total analysed zircons. The detrital zircon record in sample 4-TR (n = 81) (Figure 5) is dominated by a Palaeozoic grain population, mainly Cambro–Ordovician (46%), peaking at ca. 460 Ma. Other minor contributors to the Palaeozoic signal are Silurian (n = 6) and Carboniferous (n = 2) ages. The Carboniferous zircon crystals (347.2 ± 13.6 Ma and 348.6 ± 4.5 Ma) exhibit high U–Th ratios of 158.7 and 64.6, respectively. Ten per cent of the dated detrital

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zircons document a Late Cretaceous 76−81 Ma (Campanian) peak, and represent the youngest recorded age in the six analysed sandstones. The Neoproterozoic signal is less abundant (28%), spanning between 550 and 700 Ma, whereas a small age group clusters around 1 Ga. Older zircons are scarce, accounting for less than 10% of the analysed zircons. The Palaeozoic signal (n = 59) heavily dominates the detrital age spectrum in the sample 5-PS (n = 87) (Figure 5), with a discernible age group at ca. 460 Ma (n = 38). Neoproterozoic ages, mainly Ediacaran, account for 23% of the ages. Mesoproterozoic to Neoarchaean ages (n = 7) are negligible contributors to the total detrital zircon age distribution. Approximately 75% of the analysed zircon grains from sample 6-PSR (n = 90) (Figure 5) record Palaeozoic ages, with the majority of them clustering around 460 Ma (54%). One zircon crystal records a Late Cretaceous age (91.2 ± 1.7 Ma). A smaller U–Pb age population (n = 16) indicates Neoproterozoic origin. Older ages account for only 6% of the detrital zircon age spectrum.

Detrital monazite spectra The chemical age spectra of detrital monazites in all sampled areas are unimodal, revealing a narrow late Palaeozoic age population. More than 80% of monazite ages in each sample are Devonian to Carboniferous (ranging from ca. 300 to ca. 400 Ma). Other minor contributors to the age spectra are Permian (256 ± 27 Ma, 274 ± 22 Ma in sample 1DG; 244 ± 21 Ma in sample 2BB) and Silurian (documented by one or two monazite grains in each sample). Two Ordovician ages were registered: 468 ± 28 Ma in sample 1DG and 484 ± 34 Ma in sample 4-TR; and one Neoproterozoic–Ediacaran age (596 ± 30 Ma) in sample 6PSR. (See Figure 5 for individual probability density plots and number of analyses per sample.)

6. Discussions Monazite versus zircon ages This study underscores the importance of performing multi-mineral age analyses in provenance studies, especially when the potential source terrains are regionally metamorphosed rocks. We show that monazite ages unravel a younger geologic event than the great majority of zircons in the studied area. The overwhelming majority of monazites of the South Carpathians have grown during Barrovian metamorphism associated with the Variscan collision, whereas the U–Pb zircon age range reflects the igneous formation of the Getic– Supragetic terranes as island or transitional continental

arcs during the latest Precambrian to Ordovician. Even older and rarer Precambrian zircons reflect the composition of the nearby terrigenous detritus transported into the Getic–Supragetic arc terranes – their distribution is broadly consistent with a peri-Gondwanan origin of these terranes (Balintoni et al. 2009, 2010). Very few zircons in the South Carpathians formed or partially recrystallized during the metamorphism accompanying Variscan collision, although the metamorphic peak was well into the amphibolite or even granulite facies in most exposed units (Medaris et al. 2003).

Variscan metamorphism The monazite age range of ~60 million years or longer between 380 and 320 Ma suggests that the duration of continental collision was significant, similar to the Cenozoic Himalayan system. This is consistent with the hypothesis that the Variscan collisional event that generated most metamorphic rocks from European basement rocks to the Alpine-Carpathian belt was a major continental collisional event (Menard and Molnar 1988). The monazite age range is consistent with existing Sm–Nd garnet isochron ages performed on metamorphic rocks from the Getic– Supragetic rocks (Drăgușanu and Tanaka 1999; Medaris et al. 2003). Minor zircon formation accompanied this prolonged collision event – less than 1% of the zircon population in our study is of that age. Those zircon grains have typical metamorphic high U–Th ratios, indicating they most likely formed subsolidus. However, Variscan collision was accompanied in the Carpathians by partial melting – in some Getic–Supragetic units (e.g. the Făgăraș unit, Krautner 1997), migmatization is regionally important, whereas others contain numerous bodies of leucogranites (the lower part of the Sebeș–Lotru unit, Hann 1983). Source rocks for materials analysed in this study did not include these crustal-derived melts formed during the Variscan. Another common U–Pb age peak in central Europe, including the South Carpathians (Balintoni and Balica, personal communication), is 290–310 Ma, an immediately postVariscan time of massive granitic magmatism, considered by some to have formed synchronous with the collapse of the Variscan orogen (e.g. Timmerman 2004). Such ages are dominant within the upper half of the Danubian nappes, but have not been reported from the structurally higher Getic–Supragetic units. This suggests that the Danubian thrust sheets did not contribute as source rocks to any of the sedimentary rocks analysed here.

Zircon age ranges; implications for arc magmatism All our samples display a rather wide age range of zircons in the late Precambrian and continuing into

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the early Palaeozoic. Of particular interest is the range starting at 850 Ma and continuing with few interruptions to the massive Ordovician magmatic event, the youngest and perhaps the most important zircon generator in all of the Carpathians (Balintoni et al. 2014). All of these zircons have U–Th <5 and are interpreted to be magmatic. They most likely formed as a series of magmatic arcs as suggested by limited knowledge of the exposed Getic and Supragetic rocks. Although no published study has elucidated the geologic history of these rocks prior to metamorphism, our preliminary mapping and geochemical data (Ducea in press) suggest that the geology of the central and eastern South Carpathians is most consistent with a series of arcs and back-arcs with major element compositions indicative of either mature island arcs or transitional continental arcs. The geology of the Sebeş–Lotru terrane is dominated by calc-alkaline amphibolites to tonalitic and granodioritic gneisses interlayered with metamorphosed volcano-sedimentary rocks, indicating that they formed in a low topography arc environment, similar to island and transitional arcs from the western Pacific today. The proximity of a craton if not the emplacement directly to continental crust is inferred for these arcs by the existence of several Neoarchaean to Palaeoproterozoic grains throughout the South Carpathians (Balintoni et al. 2014 and references therein). Ductile shear zones of unknown but probably pre-Alpine age (e.g. Pană and Erdmer 1994; Profeta et al. 2013) mark the boundaries between these different sub-terranes that, although different in detail, overall share rather similar geologic histories. The prolonged duration of arc magmatism sourcing the Carpathian basement and subsequently the rocks analysed here is indicative of continental margin and not island arc magmatism. Magmatism appears to have been continuous since before the break-up of Rodinia and through the Ordovician, possibly continuing into the Silurian. Jicha and Jagoutz (2015) show that the lifetime of island arcs is typically around 25–50 million years, whereas the continuous nature of magmatism for over 350 million years preserved in these zircon populations is more typical for transitional or continental arcs (Ducea et al. 2015).

Zircon record Eroded since the Cretaceous, versus currently exposed. All of the analysed samples have the same dominant populations of zircon ages, although their relative abundance is different from sample to sample. From young to old (excluding a few Alpine and Variscan grains) they are: Ordovician (450–480 Ma), Cambrian (510–550 Ma), Neoproterozoic (590–850 Ma), Grenville (900–1100 Ma), and an array of less-common peaks between 2 and

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2.8 Ga. Cretaceous sediments from the SE corner of South Carpathians have twice the proportions of Cambrian and earlier Precambrian (2–2.8 Ga) zircons than the samples from the central South Carpathians. About 6–8 km of bedrock unroofing is documented since the mid-Cretaceous from the South Carpathians, based on various low-temperature thermochronologies (Merten et al. 2010). It appears that the higher nappe units that were the source rocks for the Bucegi and Dragoslavele areas contained a much higher proportion of Precambrian zircons. Data presented here are limited, but we suggest that a pattern of progressively older dominant zircons may exist in the structurally higher units of the Romanian Carpathians due to the higher abundance of metasedimentary rocks in those units. Overall, however, these units were part of the same terrane or were formed very close to each other (Balintoni et al. 2014) and share the subsequent Variscan metamorphism, as evidenced by our monazite age distribution.

Alpine ages The few 76–81 Ma zircons from the Turnu sandstone are coincident with the age range of the banatitic intrusions in southwest Romania (Zimmerman et al. 2008). The main banatitic suite, which is hypabissal and intrusive (Berza et al. 1998), is emplaced through the Getic– Supragetic nappes along two narrow parallel lineaments. In addition to the intrusive banatitic suite, age-equivalent volcanic rocks are found in several Gosau-type extensional continental basins throughout the South Carpathians and Apuseni mountains (Schuller 2004); the amount of volcanic material equivalent to the banatitic intrusions is larger than previously believed (Bârzoi and Șeclăman 2010); over 1 km of primary volcanic material can be found in some of the Gosau basins in Romania, such as the Hațeg basin in the South Carpathians. We believe that the Cretaceous zircons in our study represent reworked Gosau-type materials in the Turnu Formation. No other Alpine ages (zircon or monazite) were found in any of our samples, suggesting that Alpine magmatism other than banatitic or metamorphism was insignificant in the higher structural units of the South Carpathians.

7. Conclusions We show that detrital zircon and monazite record very different geologic events in the South Carpathians (Figure 6): zircon ages correspond to the original formation of crust in the latest Precambrian to Ordovician, whereas all monazites record collision-related regional metamorphism (with minor associated magmatism)

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900

800 Hiatus 700

Cadomian/ Pan-African Orogeny

Island arc and transitional continental magmatism (zircon ages)

Age (Ma)

600

500 Caledonian Orogeny 400

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Variscan Orogeny

Cordilleran arc magmatism (high-flux event at 460 Ma) (zircon ages) Barrovian metamorphism & smallvolume leucogranite bodies (monazite ages) Post-Variscan orogen collapse & wide-spread magmatic pulse*

300

200

100

Alpine Orogeny

Subduction related magmatism “Banatites” (zircon ages)

0

Figure 6. Timeline of major tectonic events that built and shaped the South Carpathian basement rocks. *In the South Carpathian realm, 310–290 Ma igneous intrusions were mapped throughout the Danubian domain (Balintoni et al., unpublished data). This signal is not visible in our zircon or monazite data.

during the Variscan orogeny (Devonian–Carboniferous). The great majority of zircons were not reset during metamorphism. Detrital zircons, which average the age populations eroded from the Getic–Supragetic basement units, suggest that these basement terrains were part of long-lived magmatic arcs formed in a periGondwanan setting along the outer edges of Rodinia. Whereas eroded units of the South Carpathians basement had a higher proportion of meta-sedimentary material in them, reflected by the significantly higher proportion of Precambrian ages, they are clearly of the same lineage as the currently exposed basement units.

Acknowledgements We thank Nicky Giesler and Mark Pecha for help with the Laserchron facility and Ken Domanik for his expert guidance with monazite chronology at the LPL University of Arizona Mike Drake electron probe laboratory. Reviews by Brian K Horton, an anonymous reviewer, and editor Robert Stern have greatly improved the quality of the manuscript.

Disclosure statement No potential conflict of interest was reported by the authors.

Funding This project was financially supported by the Romanian National Sciences Foundation (UEFISCDI) [grant number PHII-ID-PCE-2011-3-0217].

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