The Exotic Igneous Clasts Attributed to the Cuman Cordillera: Insights into the Makeup of a Cadomian/Pan-African Basement Covered by the Moldavides of the Eastern Carpathians, Romania

Abstract

The Eastern Carpathians are thrust to the east and north over their Eastern European foreland, tectonically covering it over an area several hundred kilometers across. Information about the nature of the underthrust part of the Carpathian foreland can be obtained from the rock fragments preserved in the sedimentary successions of the Carpathian fold and thrust belt, specifically in the Outer Dacides and the Moldavides. Fragments of felsic rocks occurring within the sedimentary units of the Upper Cretaceous successions of the Moldavides have long been attributed to the Cuman Cordillera—an intrabasinal ridge in the Eastern Outer Carpathians. This work is the first complex geochemical and geochronological study on the exotic igneous clasts of the Cuman Cordillera. Igneous clasts from the southern part of the Moldavides (Variegated clay nappe/formation) are investigated here. They include mainly granites and rhyolites. Phaneritic rocks are composed of cumulus plagioclase, albite, amphibole and biotite, and intercumulus quartz and potassium feldspar, with apatite, magnetite, sphene, and zircon as main accessories, while the porphyritic rocks have a mineral assemblage similar to that mentioned above, displayed in a porphyritic texture with a usually crystallized groundmass. SHRIMP U-Pb zircon dating indicated the 583–597 Ma age interval for magma crystallization. Based on calcareous nannofossils, the depositional age of the investigated igneous clasts is Cenomanian to Maastrichtian, implying that the Cuman Cordillera was an emerged piece of land, herein an active source of sediments in the flysch basin for at least 40 Ma, from the Early Cretaceous (Aptian) to the Late Cretaceous (Maastrichtian). The intrusive and subvolcanic rocks show similar trends for trace and major elements, evincing their comagmatic nature. The enrichment in LILE and LREE relative to HFSE and HREE, as well as the element anomalies (e.g., negative Nb, Ta, and Eu and positive Rb, Ba, K, and Pb) suggest a convergent continental plate margin tectonic setting. Mineral chemistry suggests magma crystallization in relatively oxic conditions (magnetite series), during ascent within a depth of 15 km to 5 km. The igneous rocks attributed to the Cuman ridge display compositional and geochronological features similar to Brno and Thaya batholiths in the Brunovistulian terrane, which could be a piece of the Carpathian foreland not covered by the Tertiary thrusts. Our data confirm the non-Carpathian origin of the igneous clasts, revealing a Neoproterozoic history of the Carpathian foreland units, which include a Cadomian/Pan-African continental arc, exposed mainly during the Late Cretaceous as an intrabasinal island of the Alpine Tethys, traditionally known as the Cuman Cordillera.

1. Introduction

The Eastern Carpathians represent the only zone where Alpine or “mobile” Europe comes in contact with Precambrian “stable” Europe (Baltica paleocontinent). This geotectonic configuration is the result of the Tertiary–Quaternary tectonic cover of a portion of continental crust wider than 200 km from the Carpathian foreland. The covered European crust, deduced by Balla in 1986 [1] and 1987 [2], is sometimes designated as the “Carpathian Embayment”, e.g., by Ustaszewski et al. in 2008 [3]. The crustal shortening, which started in the Eastern Carpathians since the Early Cretaceous period, obscured the make-up of a large part of pre-Alpine Europe, as suggested by Bleahu in 1974 [4]. The assumptions on the composition of the buried parts of the European margin are based mostly on the exotic rock fragments deposited in the sedimentary successions of the Carpathian nappes.

This work aims to provide new data on a group of felsic rock fragments, i.e., the exotic clasts attributed to the Cuman Cordillera, which occurred within the Cretaceous sediments of the Moldavides nappe system, as defined by Săndulescu in 1984 [5] and 1994 [6], and thrust during the Tertiary times. Although remarked on since the interwar period, these clasts of igneous rocks have mostly been studied regarding their stratigraphic significance and their origin in terms of being “Carpathian” or “not Carpathian/exotic”, by comparison with the igneous rocks in the Carpathian units with a crystalline basement, later named Dacides by Săndulescu in 1984 [5] and 1994 [6]. Moreover, the knowledge on the silicic igneous rocks has been developed since in terms of petrographic classification criteria and the interpretation of their geochemical features, e.g., by Barbarin in 1990 [7] and Bonin et al. in 2020 [8] and references therein.

Clasts of felsic igneous rocks from the Lower Cretaceous successions have been documented regarding the crystallization age (600 Ma), by Roban et al. in 2020 [9], and depositional age (Aptian–Albian), by Filipescu & Alexandrescu in 1962 [10], Grigorescu & Anastasiu in 1976 [11], Balteş et al. in 1983 [12], Melinte & Roban in 2011 [13], Roban & Melinte in 2012 [14], and Roban et al. in 2020 [9]. The occurrence of similar fragments in Late Cretaceous successions is less constrained, with the mention of the “Senonian” age in the old publications cited in the next section, Cenomanian by Filipescu & Alexandrescu in 1962 [10] and Maastrichtian by Ştefănescu in 1995 [15].

Given this context, we directed our research towards the felsic igneous clasts of the Cuman Cordillera within the Late Cretaceous successions of the Moldavides, to reveal the age of magma crystallization, provide information on magma composition and evolution, and, using the geochemical and geochronological results, to validate the foreland origin of these rocks and explore their affinities with other rocks within the Carpathian foreland.

2. Geological Setting

The Eastern Carpathians are mainly made up of several assemblages of tectonic units (Figure 1) thrust during successive geotectonic events, after Săndulescu 1984 [5], 1994 [6]: (1) the Median Dacides (innermost), thrust during the Aptian–Albian, forming a belt of terranes composed of pre-Alpine (Proterozoic to Paleozoic) metamorphic basement and Alpine (post-Carboniferous) sedimentary cover, carrying with them remnants of the East Vardar Ocean; (2) the Outer Dacides, enclosing mafic and turbidite successions related to the opening of the Middle Jurassic–Early Cretaceous Ceahlău-Severin ocean (a branch of the Alpine Tethys, after Schmid et al. 2008 [16]), thrust during the Late Cretaceous; (3) the Moldavides, that are the outermost (eastern) nappes, which contain Lower Cretaceous up to Miocene sedimentary successions, thrust in the Miocene.

Fragments of igneous rocks, commonly containing reddish feldspar, have been noticed in the Upper Cretaceous sediments of the Moldavides, especially in the southern part of the Eastern Carpathians since the first half of the 20th century and reported by Preda in 1925 [17], Protescu & Murgeanu in 1927 [18], Popescu-Voiteşti in 1929 [19], Murgeanu in 1930 [20] and 1937 [21], or by Filipescu in 1933 [22] and several others, as they were conspicuous in the sedimentary succession due to their color and grainsize, which contrast with those of their host rocks.

These rock fragments were mentioned in the literature as exotic clasts, because most geologists considered that they a were not of Carpathian origin, but belonged to the foreland. Murgeanu in 1937 [21], based on the spatial distribution of the unusual clasts in the southern part of the Eastern Carpathian flysch zone (between the Dâmbovița and Buzău rivers), attributed them to a Cretaceous ridge within the basin where the sedimentary successions of the Moldavides accumulated. The author named the provenance area of these exotic rocks “Cuman Cordillera”, after the Cuman nomadic population form Central Asia, who invaded the Carpathian area in the early Middle Ages.

Filipescu & Alexandrescu in 1962 [10] documented the Eastern Carpathian occurrence areas of the exotic clasts further to the north, extending the Cuman Cordillera as far as the Moldova valley. Their extent corresponds, taking into account the up-to-date terminology, largely to the areas occupied by the sediments of the Audia and Macla nappes defined by Săndulescu in 1984 [5]. Cretaceous stratigraphic successions containing similar exotic igneous clasts were also reported from the Tarcău nappe by Săndulescu in 1984 [5], from the Variegated clay nappe by Ştefănescu in 1970 [23], 1976 [24], and 1995 [15], by Ştefănescu et al. in 1978 [25] and 1988 [26], respectively, and from the Teleajen nappe by Roban et al. in 2020 [9] in the outcrops from the southern part of the Eastern Carpathians.

It should be mentioned that the only previous petrographic study on the clasts of igneous rocks attributed to the Cuman Cordillera was carried out by Codarcea in 1937 [27], who investigated samples collected near Breaza town (Prahova County), from an outcrop of the Variegated clay formation.

Our research focused on the fragments of igneous rocks located in the Variegated clay formation of the nappe with the same name. Ştefănescu in 1970 [23] and 1995 [15] defined the Variegated clay formation (“Series”) as “a stripe 1.2 km wide between the Cretaceous flysch area and the Paleogene flysch zone”, with two lithologically distinct parts: (1) The lower one is dominated by red, grey, and green clay ± sand (stone) and gravel lenses consisting almost exclusively of granodiorite fragments up to 10 cm in size. The age of the basal successions of the Variegated clay formation was inferred to be Latest Albian (Vraconian—after Ştefănescu 1995 [15]) based on lithological similarities with a sedimentary succession in the Buzău valley, which was dated based on belemnite fauna; (2) the upper part, made by a unit named “Mălăiştea Sandstone”, contains muscovite and biotite along with sparse granodiorite fragments, and with subordinated bands of grey and green shales. Microfossils from a greenish clay intercalation in the uppermost part of the Mălăiştea Sandstone in Cheia valley (Vulcana de Sus) indicated a Maastrichtian age, reported by Ştefănescu in 1995 [15]. Calcareous nannofossil assemblages identified in the shale intercalations of the lower part of Mălăiştea Sandstone north of Slon village indicated an Early Maastrichtian age, as determined by Melinte & Băceanu in 1996 [28]. Tectonically, the Variegated clay formation makes the body of the Variegated clay nappe, outlined between the Macla nappe towards NW and the Tarcău nappe to the SE, and traced discontinuously from the western end of the Moldavides as far as East of Drăjnuţa valley (Slon village), as shown in Figure 1.

Figure 1. (A) Geotectonic map of the Eastern Carpathians, after Săndulescu 1984 [5] and Bădescu 2005 [29]. The area marked by quadrangle is magnified in (C); (B) position of (A) in the Alpine belt; (C) the outcropping locations of the Variegated clay nappe, sampled in this work at the following localities: 1,2—Vulcana de Sus; 3—Cucuteni; 4—Breaza; 5—Slănic Prahova; 6—Schiuleşti; 7—Slon; (D) legend of the maps in (A,C) [10].

Figure 1. (A) Geotectonic map of the Eastern Carpathians, after Săndulescu 1984 [5] and Bădescu 2005 [29]. The area marked by quadrangle is magnified in (C); (B) position of (A) in the Alpine belt; (C) the outcropping locations of the Variegated clay nappe, sampled in this work at the following localities: 1,2—Vulcana de Sus; 3—Cucuteni; 4—Breaza; 5—Slănic Prahova; 6—Schiuleşti; 7—Slon; (D) legend of the maps in (A,C) [10].

3. Samples and Analytical Methods

Twenty-five igneous rock samples weighing 150–1600 g each were selected for petrographic and bulk rock geochemical analyses. Two of these samples were taken from the vicinity of Slon village, and twenty-three samples were taken from an outcrop of Variegated clays near Breaza town (Figure 2), the location, which was remarked by Murgeanu in 1937 [21] due to the size of the igneous rock fragments, which could reach “the size of a fist”. We selected the samples taking care to cover all petrographic and chromatic varieties: red, pink, and whitish intrusive rocks, as well as red, pink, and whitish porphyritic rocks.

3.1. Petrographic Analysis

The petrographic investigations were effectuated at the Geological Institute of Romania, using a Zeiss AXIO Imager A2m polarizing microscope and a Zeiss STEMI 508 stereo microscope, both equipped with digital cameras, connected to a desktop computer, where the image acquisition was performed using ZEN software v. 2; the petrographic analysis was supplemented by imaging and elemental analysis with a Hitachi TM3030 Tabletop scanning electron microscope, with a magnification of ×15–30,000 (digital zoom: ×2, ×4), equipped with an energy-dispersive X-ray spectrometer (EDS), at observation conditions of 15 kV accelerating voltage and 3 nA beam current and with a chromium sputter coater. This allowed the selection of samples to be further investigated for mineral chemistry and the achievement of composition maps in order to estimate modal compositions.

3.2. Geochemical Analysis

Mineral chemistry was investigated with a Zeiss Merlin Gemini II column field emission scanning electron microscope (FE-SEM), manufactured by Carl Zeiss Microscopy GmbH, in Jena, Germany, equipped with an Oxford X-max50 EDS detector, Oxford, UK, at operating conditions of 20 kV accelerating voltage and a beam current of 2 nA. Some mineral compositions published by Lőrincz et al. in 2024 [30] have been referred to here to make inferences regarding the composition and evolution of the parental magmas.

The bulk rock analyses for major and trace elements were performed at ALS Laboratories. Sample preparation, such as fine crushing (<2 mm) and pulverizing (<75 μm), was performed at ALS Roșia Montană, Romania. Major elements were determined by XRF, on fused disks, while trace elements were determined by ICP-MS, after lithium borate fusion, and by ICP-AES, following four-acid digestion, at ALS Loughrea, Ireland. Calibrations were against international certified reference materials (AMIS0055 and GIOP−96 for major and minor components, MRGeo08 and REE−1 for trace elements). Diagrams representing major and trace elements were created using IGPET software, version 2 January 2010, created by Terra Softa Inc., in Somerset, NJ 08873, USA.

3.3. U-Pb Dating

The rejects of 4 rock samples (3 phaneritic rocks and 1 porphyritic rock) out of the 25 rock samples, which showed high Zr contents in bulk rock composition, were chosen for U-Pb dating. Zircon separation was performed at the Geological Institute of Romania, from the 63–250 μm grain size extraction, using SPT (sodium polytungstate), an eco-friendly heavy liquid. Then, zircon crystals were handpicked at the stereo microscope. Several tens of crystals were selected from each sample.

Zircon U/Pb analyses were performed at the SHRIMP ion-microprobe laboratory of the University of Granada. The zircon mineral concentrates were embedded in epoxy resin and polished. The measurement points were selected on the CL, TEM, and SEM images of the zircon crystals. Most of the measurement points were marked in the marginal areas of the crystals, to find out the age of the last magmatic event, and a few points were marked in the middle of the crystals that showed cores different from the zonation of rims, in order to obtain information about inherited crystals. Fifteen points (approx. 10–15 zircon crystals) per sample were measured, with the Shrimp IIe/mc instrument, following analytical procedures described by Williams & Claesson in 1987 [31]. TREMORA zircon was used as the isotope ratio standard after Black et al. 2003 [32], and SL13 zircon as the U concentration standard from Claoué-Long et al. 1995 [33]. Data reduction was made with SHRIMPTOOLS software. The ages corrected for ²⁰⁴Pb, showing discordance less than 10% between U/Pb and ²⁰⁷Pb/²⁰⁶Pb, were plotted on Wetherhill concordia diagrams, generated with IsoplotR v. 5.1 software, created by Vermeesch in 2018 [34].

3.4. Calcareous Nannofossil Analysis

The depositional age of the fragments of igneous rocks attributed to the Cuman Cordillera was determined based on calcareous nannofossil assemblages in 34 samples from the host Variegated clays, collected from outcrops at the Vulcana de Sus (18 samples), Cucuteni (3 samples), Breaza (11 samples), Schiulești (1 sample), and Slon (1 sample) localities (Figure 3).

The Variegated clay formation consists of alternating red and greenish-gray clay layers, usually 1–3 cm thick, with almost vertically inclined, generally W-E oriented stratification (Figure 2A). Out of the 34 samples, the first 6 were collected for preliminary analysis; therefore, we took one sample each from the Breaza, Schiulești, and Slon localities and 3 samples from Cucuteni. Thereafter, 28 samples out of the 34 were taken systematically, along chosen profiles and distances apart. Thus, 18 samples were taken from Vulcana de Sus, along a NE-SW oriented profile, perpendicular to the clay beds, usually 1 m apart, and 10 samples from Breaza, as follows: 6 samples were taken along a NE-SW oriented profile, perpendicular to the beds, a few meters apart; 2 samples were taken 30–40 cm from the sandstone bed intercalation (the bed containing the igneous rock samples studied here), in the southeast and northwest directions from it; and, finally, 2 samples were taken from a clay mound, next to the above-mentioned sandstone bed.

An amount of 5 g of sediment was used for each sample. Smear slides were prepared according to standard techniques described by Bown & Young in 1998 [35]. The analyses were performed with an Olympus optical microscope in parallel and crossed nicols, using an immersion objective, with ×1200 magnification. Taxonomic identification was based on Perch-Nielsen 1985 [36] and Burnett 1998 [37], and the www.mikrotax.org website Biostratigraphy was interpreted following the zonations of Sissingh 1977 [38] and Burnett 1998 [37]. The absolute ages are from Gradstein et al. 2012 [39].

4. Results

4.1. Calcareous Nannofossil Biostratigraphy

We focused our research on the Cretaceous successions of the Variegated clay formation, in the southern part of the Eastern Carpathians (Figure 1), because these contain the largest fragments of igneous rocks, suitable for a comprehensive range of investigations. The largest magmatic rock fragments (centimeter- to decimeter-sized) from the outcrop south of Breaza town are associated with a stratum of carbonate sandstone to sandy limestone, up to 1 m thick, conformable with the banding of the Variegated clays (Figure 2).

The clays in the footwall and hanging wall of the sandy limestone bearing the fragments of igneous rocks (Figure 2B,C) contain several calcareous nannofossils, such as Axopodorhabdus albianus, Braarudosphaera africana, Corollithion kennedyi, Cretarhabdus striatus, Cribrosphaerella ehrenbergii, Eiffellithus turriseiffelii, Eprolithus floralis, Helenea chiastia, Lithraphidites acutus, Manivitella pemmatoidea, Microrhabdulus decoratus, Nannoconus truitii, Prediscosphaera cretacea, Rhagodiscus asper, Rhagodiscus angustus, Rhagodiscus infinitus, Rhagodiscus splendens, Tranolithus orionatus, Watznaueria barnesiae, Zeugrhabdotus ehrenbergii, and Zeugrhabdotus erectus. The age of this assemblage was attributed to the Cenomanian stage, UC3a-UC3e biozones by Burnett 1998 [37], between the first occurrence of Lithraphidites acutus and last occurrence of Corollithion kennedyi, constraining the age of the sandy limestone to the interval of 96.16–94.64 Ma, after Gradstein et al. 2012 [39].

The clays lying on top of the Cenomanian strata (Figure 2A) contain intercalations of coarse arkose with fragments of igneous rocks and carbonate matrix and provided nannofossils that include Arkhangelskiella cymbiformis, Broinsonia parca s.l., Micula staurophora, Micula concava, Eiffellithus turriseiffelii, Eiffellithus eximius, Prediscosphaera cretacea, Zeugrhabdotus ehrenbergii, Lucianorhabdus maleformis, Lucianorhabdus cayeuxii, Microrhabdulus decoratus, Tranolithus orionatus, and Watznaueria barnesiae. This assemblage indicates a Campanian age, UC13-UC17a biozones by Burnett 1998 [37], from the first occurrence of Arkhangelskiella cymbiformis (83.2 Ma) to the last occurrence of Eiffellithus eximius (74.51 Ma).

The uppermost succession, which consists of red and white clay bands, contains a rich nannofossil assemblage with Arkhangelskiella cymbiformis, Arkhangelskiella maastrichtiana, Braarudosphaera bigelowii, Broinsonia enormis, Calculites obscurus, Calculites ovalis, Ceratolithoides aculeus, Ceratolithoides kamptneri, Cribrocorona gallica, Cribrosphaerella daniae, Cribrosphaerella ehrenbergii, Criborocorona gallica, Cyclagelosphaera alta, Eiffellithus turriseiffelii, Eiffellithus eximius, Eiffellithus gorkae, Eiffellithus parallelus, Lithraphidites quadratus, Lithraphidites carniolensis, Lucianorhabdus maleformis, Lucianorhabdus cayeuxii, Manivitella pemmatoidea, Micula murus, Micula staurophora, Micula concava, Micula decussata, Micula praemurus, Micula cubiformis, Micula prinsii, Micula swastica, Microrhabdulus belgicus, Microrhabdulus decoratus, Prediscosphaera cretacea, Prediscosphaera grandis, Prediscosphaera stoveri, Petrarhabdus copulatus, Placozygus fibuliformis, Rhagodiscus infinitus, Rhagodiscus splendens, Uniplanarius sissinghi, Watznaueria barnesiae, and Watznaueria ovata. Within this succession, two age intervals were separated: Late Maastrichtian (UC20b and UC20c biozones), based on the first occurrence of Ceratolithoides kamptneri (67.84 Ma) and the first occurrence of Micula prinsii (67.3 Ma), and Latest Maastrichtian (UC20d biozone), between the first occurrence of Micula prinsii (67.3 Ma) and the mass extinction of Cretaceous species at the Cretaceous–Tertiary boundary (66 Ma). Microphotographs of the Cenomanian and Maastrichtian nannofossils from the Breaza outcrop are shown in Figure S1.

At Cucuteni (Dâmboviţa County), the Variegated clay formation contains lens-shaped intercalations of detrital fragments of magmatic rocks (mostly of sand- to grain-sized microbreccia), locally cemented with carbonate and showing sharp limits with host clay, which contains nannofossils such as Watznaueria barnesiae, Watznaueria ovata, Micula decussata, Micula concava, Micula swastica, Lithastrinus grilli, Cribrosphaerella ehrenbergii, Aspidolithus parcus parcus, Arhangelskiella cymbiformis, Lucianorhabdus cayeuxii, Lucianorhabdus maleformis, and Nannoconus truitii. Based on the first occurrence of Aspidolithus parcus s.l., CC18 biozone by Sissingh 1977 [38], and, respectively, UC14 biozone by Burnett 1998 [37], the nannoplankton assemblage was ascribed to the Early Campanian.

Eighteen clay samples from Vulcana de Sus were investigated for nannofossils and proved sterile, probably because of diagenesis processes.

4.2. Petrography

The intrusive rocks were classified by Codarcea in 1937 [27] as granodiorites, which contain oligoclase and albitic granodiorites, whose plagioclase was only albite with less than 5% An. This probably followed the classification proposed by Johannsen in 1920 [40], where the albite with less than 5% An was classified as plagioclase, not as alkali feldspar. According to the rock descriptions published by Codarcea in 1937 [27], the rocks he classified as albitic granodiorite would now be classified as alkali feldspar granite, according to the IUGS recommendations after Le Maître 1989 [41]. Figure 4 shows the classification of the investigated rocks using the diagram elaborated by Streckeisen & Le Maître in 1979 [42] based on normative compositions, the results of which are generally very close to those of the IUGS-recommended QAP diagram, e.g., in Whalen & Frost 2013 [43] or Bonin et al. 2020 [8]. In the diagram by Streckeisen & Le Maître in 1979 [42], shown in Figure 4, the intrusive rocks we investigated are mostly alkali-feldspar granites and granites. The porphyritic rocks, classified by Codarcea [27] as granodioritic porphyries based on the presence of the albite phenocrysts, are now classified as rhyolite (Figure 5), using the IUGS recommendations after Le Maître 1989 [41].

The intrusive rocks show cumulate features, with potassium feldspar and quartz as intercumulus phases, and plagioclase, pure albite, biotite, amphibole, and apatite as cumulus phases (Figure 6A–D). Sometimes, quartz can occur as a cumulus phase in relation with potassium feldspar, as described by Lőrincz et al. in 2024 [30]. All cumulus phases are euhedral to subhedral when they occur within the potassium feldspar and quartz (Figure 6A–D).

The composition of plagioclase varies from albite with more than 5% An to oligoclase and andesine. Plagioclase feldspars show polysynthetic twins and various degrees of alterations to sericite and/or clay minerals. Biotite is mostly altered to chlorite ± magnetite and almost always contains apatite inclusions. When not altered, biotite shows Ti- and Fe-rich compositions (ca. 3–5% TiO₂ and 24–29% Fe₂O₃), as disclosed in Table S1. Amphibole is often euhedral, with columnar habit or showing the outlines and cleavage characteristic of basal faces (Figure 6C). Commonly, amphiboles are completely altered to chlorite, calcite, epidote ± sphene, which suggests their initial Ca-rich composition, and can preserve apatite inclusions. The very few amphibole grains that escaped complete alteration show edenite-ferro-edenite-magnesio-hornblende-ferro-hornblende composition. Apatite is included in most cumulus and intercumulus minerals (potassium feldspar, quartz, plagioclase, albite, biotite, amphibole, magnetite, and sphene).

The porphyritic rocks contain phenocrysts of albite, potassium feldspar and quartz, which may exceed 1 cm in size, and smaller (up to 5 mm in size) biotite phenocrysts, commonly completely replaced by chlorite. Some quartz phenocrysts are euhedral to subhedral (± bipyramidal). Resorption shapes of quartz phenocrysts are common (Figure 6E). Albite phenocrysts are euhedral (Figure 6F). The groundmass is usually microcrystalline with quartz-feldspathic intergrowths (graphic, myrmekitic, granophyric) or with allotriomorphic texture, and sometimes cryptocrystalline, even glassy (Figure 6E,F). The granular groundmass contains all the leucocratic and mafic minerals mentioned as phenocrysts and opaque minerals (magnetite + sulfides) ± glass with silica-rich composition (92–98% SiO₂, 1.2–6% Al₂O₃, and 0.7–1.8% Na₂O).

The accessory minerals in both phaneritic and porphyritic rocks are represented mainly by apatite, sphene, magnetite, and zircon (generally less than 100 µm, sometimes forming aggregates). Epidote, zoisite, chlorite, and calcite occur as secondary minerals at the expense of biotite and amphibole. Feldspars (especially the alkaline ones) often show reddish coloration.

4.3. SHRIMP Zircon U-Pb Ages

The measured zircon crystals are transparent prisms with magmatic habit, free of inclusions and exhibiting oscillatory zoning (Figure S2). The measured values of the four investigated samples are shown in Table S3. Concordia diagrams are shown in Figure 7. Sample CC51M (rhyolite) showed ²⁰⁶Pb/²³⁸U ages between 539 ± 5.3 Ma and 646 ± 23.4 Ma, with a weighted mean age of 590.6 ± 1.5 Ma. Sample CC51P (pink granite) showed ²⁰⁶Pb/²³⁸U ages between 544.9 ± 4.7 Ma and 597.8 ± 23.5 Ma, with one measurement of 451.5 ± 4.7 Ma and a weighted mean age of 562.8 ± 1.7 Ma. If the outlier measurement of 451.5 ± 4.7 Ma is not used for the calculation (considering that it might have resulted from Pb loss), the weighted mean age would be 582.2 ± 3.6 Ma. Sample CC51R (reddish alkali feldspar granite) showed ²⁰⁶Pb/²³⁸U ages between 571 ± 11.1 Ma and 605.8 ± 8.4 Ma, with a weighted mean age of 597 ± 2 Ma. Sample CC51U (whitish granodiorite to diorite) showed ²⁰⁶Pb/²³⁸U ages between 520 ± 8.0 Ma and 611.3 ± 9.2 Ma, with a weighted mean age of 592.7 ± 1.7 Ma. These results indicate crystallization of the magma at ca. 600 Ma.

4.4. Geochemistry

The chemical composition of the investigated rocks is shown in Table S2. Silica is lower in the intrusive rocks (58–70.32%) than in rhyolites (70.6–74.85%). All the other major components, as well as the minor components, are higher in the intrusive rocks and lower in the rhyolites, with clear differences for TiO₂ (0.31–0.83% in intrusive rocks and 0.13–0.26% in rhyolites), Al₂O₃ (14.54–17.9% in intrusive rocks and 13.04–13.92% in rhyolites), MgO (0.82–2.24% in intrusive rocks and 0.29–0.59 in rhyolites), and P₂O₅ (0.09–0.27% in intrusive rocks and 0.04–0.09% in rhyolites), with some overlapping for Fe₂O₃ (3.12–5.94% in intrusive rocks and 2.82–3.78% in rhyolites), MnO (0.06–0.32% in intrusive rocks and 0.03–0.07% in rhyolites), and CaO (0.46–3.88% in intrusive rocks and 0.26–0.96% in rhyolites) and more equilibrated for alkalies (3.54–5.79% Na₂O in intrusive rocks and 3.2–4.5% Na₂O in rhyolites, 2.58–4.69% K₂O in intrusive rocks, and 1.78–4.4% K₂O in rhyolites).

According to the three-tiered geochemical classification proposed by Frost et al. in 2001 [44], all granitoid samples are magnesian (FeO* = 0.68–0.78) and all rhyolite samples are ferroan (FeO* = 0.85–0.90), as shown in Figure 8; the granitoid compositions straddle the limits between alkalic, alkali-calcic, and calc-alkalic compositions, while the rhyolites plot in the alkali-calcic and calc-alkalic fields (Figure 9). Mg# is 34–45 in the granitoids and 16.5–23.7 in rhyolites. All samples show compositions with normative quartz, orthoclase, albite, anorthite, corundum (except for one sample), hypersthene, magnetite, ilmenite, and apatite.

On the plots of the mantle-normalized values, all investigated rocks show Rb, Ba, K, Th, U, and Pb enrichment and Nb, Ta, Ti, and P depletion patterns (Figure 10). The rhyolites show marked P, Ti, and Sr negative anomalies on the mantle-normalized values plots, while the granitoids show small positive Sr anomalies and moderate P and Ti negative anomalies, as compared with rhyolites.

The chondrite-normalized rare earth element (REE) patterns (Figure 11) show steeper decreasing trends of LREE ratios (La_N/Sm_N = 2.0–7.1 for granitoids and 4.4–6.2 for rhyolites) and flatter heavy REE patterns (Gd_N/Yb_N = 1.3–2.6 for granitoids and 1–1.3 for rhyolites). Almost all the samples show negative Eu anomalies (Figure 10), which are small for granitoids (Eu/Eu* = 0.76–1.01) and marked for the rhyolites (Eu/Eu* = 0.26–0.71).

5. Discussion

The Cuman Cordillera is a concept intended to explain the presence of the igneous clasts within the Cretaceous successions of the Moldavides. In the interwar period, when the “Cuman Cordillera” term was used for the first time by Murgeanu in 1937 [21], it did not bear the geotectonic significance it might deliver today, at that time mainly referring to an elongated, emersed relief within the depositional basin of the Moldavides. Because of the Miocene tectonic coverage of a very large area in the Carpathian foreland, it is difficult to reconstitute the original emplacement of the different igneous rock types we studied and the spatial relations among them or between these rocks and other lithostratigraphic units, as well as the geotectonic setting of the strip of land named Cuman Cordillera. Considering these circumstances, our only tool remains the use of the petrological, geochemical, and geochronological data on the igneous clasts attributed to the Cuman Cordillera to obtain insights into magma properties, its evolution during crystallization, and the geotectonic setting of magma generation and emplacement. Based on these data, we attempt to identify rocks with similar characteristics that occur in areas accessible to direct observation, which could be remnants of a more extended land that comprised the Cuman Cordillera.

5.1. Magma Crystallization

The similar trends of trace element values normalized to primitive mantle and chondrite (Figure 10 and Figure 11) suggest the comagmatic character of the studied granitoids and rhyolites. The crystallized groundmass of the investigated rhyolites indicates their subvolcanic character and their crystallization within the crust, not as lavas.

All the investigated samples plot in the field of I + S granite and outside the field of A-type granite (Figure 12). Some features of the investigated rocks are characteristic of I-type granites, as defined by Chappell & White in 1974 [46] and 2001 [47]. The mineral assemblage includes biotite, sphene, and amphibole (all of these containing apatite inclusions) and lacks muscovite. The sodium contents are relatively high (3.2–5.8% Na₂O). On the other hand, all the investigated samples but one are peraluminous, most of these displaying Al/CNK values greater than 1.1 (Figure 13), which is consistent with their normative corundum >1%, like in S-type granites. The high LILE values and depletion of HFSE, the relative enrichment of LREE to HREE, and element anomalies such as negative Nb and Eu and positive Pb can be considered as a crustal feature, characteristic of subduction-related magmas, like in Rudnick & Gao 2003 [48] or Harangi & Lenkey 2007 [49].

Mineral relations suggest the following order of crystallization: (1) zircon + apatite; (2) plagioclase + amphibole; (3) biotite; (4) quartz + alkali feldspar. Zircon and apatite seem to be the earliest crystallized phases since they occur as inclusions in plagioclase, amphibole, and biotite. Silica shows strong negative correlation with TiO₂, Al₂O₃, MgO, P₂O₅, and Fe₂O₃ (Figure S3), suggesting the fractionation of cumulus minerals, especially of the mafic ones. It is of note that the elements showing the strongest negative correlation with silica are concentrated in biotite (which is Ti- and Fe-rich), either as chemical components or as mineral inclusions (e.g., apatite). Most likely, biotite was the main phase that concentrated Ti from the magma, as sphene is present in accessory amounts and magnetite commonly contains less than 1% TiO₂. Furthermore, Ti shows much stronger positive correlation with Mg than with Ca (Figure S4). Considering the difference in density between the plagioclase showing albite to andesine composition (<2.7 g/cm³) and the Fe-rich biotite (~3 g/cm³), it is reasonable to presume the fractionation of biotite at a higher proportion than plagioclase fractionation, which might explain the very small negative Eu anomaly and positive Sr anomaly in the intrusive rocks concomitantly with the clearly shaped negative anomalies for P and Ti (Figure 10).

The intrusive rocks were subjected to some post-emplacement deformation efforts, as suggested by the undulatory extinction and local granulation of quartz (Figure 6A), as well as by the bent plagioclase twin lamellae (Figure 6B).

5.2. Magmatic Conditions

Based on biotite composition and hornblende–plagioclase thermometry, Lőrincz et al. in 2024 [30] estimated the crystallization temperature to 730–780 °C; based on biotite and amphibole compositions, the pressure of crystallizations was estimated to 1.6–3.7 kbar, with the smallest values (1.6–2.6 kbar) indicated by biotite composition. The zircon saturation temperature estimated from the bulk rock composition data of the present research (Table S2) is within the interval of 767–824 °C. This is relatively close to the temperature values estimated for biotite and amphibole–plagioclase but higher than that and is consistent with zircon presence as inclusions in some plagioclase, amphibole, and biotite crystals. Using the method recommended by Wones & Eugster in 1965 [53], the oxidation state of magma during biotite crystallization (Figure 14) corresponds to the magnetite series described by Ishihara in 1974 [54], i.e., to relatively oxidizing conditions. This is in agreement with the occurrence of magnetite and the absence of ilmenite in the studied rocks.

5.3. Tectonomagmatic Affinities

The rocks attributed to the Cuman Cordillera plot within the field of Cordilleran granites (Figure 15), as defined by Frost et al. in 2001 [44]. In Figure 16, most granitoid samples are included in the continental collision field defined by Maniar & Picoli in 1989 [52]. In the diagrams (Figure 17) perfected by Pearce et al. in 1984 [55], all the investigated samples plot in the fields of volcanic arc granites, showing some affinities with syn-collisional granites.

Figure 14. Fields of magma oxidation state relative to the main oxygen fugacity buffers, diagram after Wones & Eugster 1965 [53] and Shabani et al. 2003 [56], based on biotite composition from Lőrincz et al., 2024 [30].

Figure 14. Fields of magma oxidation state relative to the main oxygen fugacity buffers, diagram after Wones & Eugster 1965 [53] and Shabani et al. 2003 [56], based on biotite composition from Lőrincz et al., 2024 [30].

Figure 15. The plot of investigated rocks within the Cordilleran granites, which includes the composition field of magnesian granites, partially overlapping the field of anorogenic (ferroan) granites. + = granitoid, * = rhyolite.

Figure 15. The plot of investigated rocks within the Cordilleran granites, which includes the composition field of magnesian granites, partially overlapping the field of anorogenic (ferroan) granites. + = granitoid, * = rhyolite.

Figure 16. Plot of the investigated rocks in the tectonomagmatic fields from Maniar & Picoli 1989 [47]. IAG = island arc granitoids, CAG = continental arc granitoids, CCG = continental collision gra-nitoids, POG = post-orogenic granitoids, RRG = rift-related granitoids, CEUG = continental epeirogenic uplift granitoids, OP = oceanic plagiogranites.

Figure 16. Plot of the investigated rocks in the tectonomagmatic fields from Maniar & Picoli 1989 [47]. IAG = island arc granitoids, CAG = continental arc granitoids, CCG = continental collision gra-nitoids, POG = post-orogenic granitoids, RRG = rift-related granitoids, CEUG = continental epeirogenic uplift granitoids, OP = oceanic plagiogranites.

Figure 17. Tectono-magmatic diagrams based on Pearce et al. 1984 [49], with separation between SF and VAG after Whalen & Hildebrand 2019 [57]. ORG = ocean ridge granites; SF = slab failure; syn-COLG = syncollisional granites; VAG = volcanic arc granites; WPG = within-plate granites. + = granitoid, * = rhyolite.

Figure 17. Tectono-magmatic diagrams based on Pearce et al. 1984 [49], with separation between SF and VAG after Whalen & Hildebrand 2019 [57]. ORG = ocean ridge granites; SF = slab failure; syn-COLG = syncollisional granites; VAG = volcanic arc granites; WPG = within-plate granites. + = granitoid, * = rhyolite.

Whalen & Hildebrand in 2019 [57] argued for the importance of slab failure and break-off in most collisional settings, where the detachment of the subducted slab induces the formation of melts with a substantial contribution of mantle material, generating the emplacement of additional magmatic suites next to the arc magmatites. Hildebrand & Whalen in 2017 [58] and Whalen & Hildebrand in 2019 [57] refined the chemo-tectonic discrimination of magmas proposed by Pearce in 1984 [55] and separated a field of granites associated with slab failure, distinct from the field of “typical” volcanic arc granites.

Although many of our samples are more peraluminous than recommended by Whalen & Hildebrand in 2019 [57], it can be noted that almost all our samples are clustered in the slab failure fields (Figure 17). Biotite chemistry in the studied rocks published by Lőrincz et al. in 2024 [30] indicate crystallization from calc-alkaline magmas, consistent with convergent plates settings.

5.4. Provenance of the Igneous Clasts Within the Cretaceous Successions of the Moldavides

Murgeanu in 1937 [21] argued for the extra-Carpathian origin of the igneous clasts attributed to the Cuman Cordillera, based on the petrographic evidence reported by Codarcea in 1937 [27], who suggested that these rocks are distinct from those known in the Carpathian belt. Filipescu in 1937 [59] came to the same conclusion by studying the heavy mineral grains enclosed in the Upper Cretaceous sands. Moreover, since the exotic clasts occur in the Eastern Outer Carpathian nappes, i.e., the Moldavides, and are missing from the inner nappes, namely Outer Dacides, such as the Ceahlău and Bobu units, Filipescu in 1939 [60] suggested that the two aforementioned units (which he named “the inner flysch” and “the outer flysch”) were deposited on different types of basements.

The Late Proterozoic ages of the igneous rock fragments reported from the Lower Cretaceous by Roban et al. in 2020 [9] and Upper Cretaceous (this work) sediments are in agreement with the hypothesis regarding their extra-Carpathian source.

The presence of igneous rock fragments has been reported in the Moldavide thrust sheets from stratigraphic successions of various age from the Early to the Late Cretaceous: Aptian–Albian by Filipescu & Alexandrescu in 1962 [10], Grigorescu & Anastasiu in 1976 [11], Balteş et al. in 1983 [12], Melinte & Roban in 2011 [13], Roban & Melinte in 2012 [14], and Roban et al. in 2020 [9]; Cenomanian by Filipescu & Alexandrescu in 1962 [10] and this study; Campanian (this study); and Maastrichtian, reported in this study and by Ştefănescu in 1970 [23]. This suggests the presence of the Cuman Cordillera as an emerged piece of land over a period exceeding 40 Ma (Aptian–Maastrichtian). Nevertheless, the presented age distribution might indicate the role of the Cuman ridge as an intermittent source of clastic material during the Cretaceous. Roban et al. in 2020 [9] proposed the formation of the Cuman ridge as “a rift horst (possibly inverted)” or “by flexural bulging of the lower plate in response to Cretaceous thrusting events”. The discontinuous age distribution presented above for Cretaceous deposition of the igneous clasts can also be the effect of their discontinuous outcropping (as lenses), posing difficulties in correlation and highlighting the necessity of further paleontological dating.

The rocks attributed to the Cuman Cordillera plot within the Cordilleran granite field (Figure 15), as defined by Frost et al. in 2001 [44], with the phaneritic rocks on the magnesian side and the porphyritic rocks on the ferroan side. The typical Cordilleran-type granites, as characterized by Frost et al. in 2001 [44], include I-type magnesian rocks (metaluminous or peraluminous), calc-alkaline, or calcic. In the case of the Cordilleran-type batholiths, the alkalinity (reflected by the MALI parameter) seems to be influenced by the source region of the parental magma, which becomes progressively more alkaline as the melting occurs farther from the subduction zone, as suggested by Frost et al. in 2001 [44]. This trend appears to be consistent with the rocks we studied.

5.5. Possible Correlations Involving the Rocks of the Cuman Cordillera

Paleogeographic models that include cordilleras (emersed zones) within the sedimentation basin of the Eastern Carpathian flysch are not limited to this belt, to Romania, or to the Eastern Carpathians but are also developed and better documented for the flysch of the Western Carpathian areas. Intrabasinal ridges/cordilleras have been inferred in Polish and Czechoslovak literature at least since the 1960s, reported, e.g., by Ślaczka in 1961 [61], Ksiazkiewicz in 1962 [62], or Ślaczka & Wieser in 1962 [63], based on the “fragments of exotic crystalline rocks”. The theme of cordilleras and basin segmentation has been a topic of active debate and is well integrated into the regional geological knowledge. The mentioned authors and several others, e.g., Ksiazkiewicz 1962 [62], Ślaczka 1976 [64], or Danysh et al. 1984 [65], infer the occurrence of several emersed zones. These intrabasinal ridges and their relative positions were also approached in more recent paleogeographic reconstitutions proposed by Golonka et al. in 2003 [66], 2005 [67], and 2014 [68], or by Golonka in 2011 [69], Michalik et al. in 2006 [70], Budzyn et al. in 2008 [71], Burda et al. in 2019 [72], Roban et al. in 2020 [9], and Gawęda et al. in 2019 [73] and 2021 [74]. The cordilleras in the Carpathian chain flysch basin presumably resulted from the fragmentation of the East European Craton, suggested by Golonka et al. in 2014 [68], or of the Brunovistulian terrane, suggested by Burda et al. in 2019 [72], as an effect of the crustal extension induced by the development of the Atlantic rift, proposed by Golonka et al. in 2005 [67]. Their destruction occurred when the Carpathian accretionary prism reached them, leaving behind only exotic clasts of varied sizes enclosed within the hemipelagic and/or turbidite successions. Most granitoid pebbles (rounded, up to 10–30 cm in size) in the sedimentary successions of the Silesian basin of the Outer Western Carpathian flysch are attributed to the Silesian ridge. Their make-up is dominated by quartz, potassium feldspar, plagioclase and biotite, with zircon, apatite, sphene, and Fe oxides as main accessory minerals, and frequently show metamorphic deformation. Their composition is peraluminous, with a small negative Eu anomaly, as described by Burda et al. in 2019 [72]. The geochronological investigations of the rocks attributed to the Silesian ridge (U-Pb on zircon rims) indicated Ediacaran ages: 530–570 Ma reported by Michalik et al. in 2006 [70], 592 Ma by Budzyn et al. in 2008 [71], respectively, and 580 and 542 Ma by Burda et al. in 2019 [72]. Numerous Carboniferous–Permian K-Ar and Th-U-Pb monazite ages were interpreted as a Variscan imprint by Poprawa et al. in 2006 [75].

Aside from the lands exposed as ridges within the Carpathian flysch basins, it is reasonable to consider that the sediments deposited on a basement connected with the crustal fragments from the present Carpathian foreland. The continuation of Central Dobrogea as far to the north as the Małopolska Massif in front of the Eastern Carpathians has been known for more than a century, as suggested by Zuber in 1902 [76], Mrazec in 1911 [77], Stille in 1953 [78], Paraschiv & Paraschiv in 1978 [79], Pătruţ et al. in 1995 [80], Kalvoda et al. in 2002 [81], 2003 [82], or Żelaźniewicz et al. in 2009 [83]; it is well documented with observations (in outcrops and drillings) on rock fragments characteristic of Ediacaran turbidites and with geochronological data indicating Ediacaran deposition of the Central Dobrogea and Małopolska turbidites, reported by Żelaźniewicz et al. in 2009 [83]. Balintoni et al. in 2011 [84] reported ages of 600–750 Ma from the Histria formation (detrital zircon U-Pb, LA-ICP-MS) of Central Dobrogea, considering them to indicate a peri-Amazonian provenance, because such ages are not typical, either for Baltica or for Laurentia. The above-mentioned authors also obtained younger ages (584–587 Ma) from the Histria formation of Central Dobrogea. Balintoni & Balica in 2016 [85] obtained Ediacaran ages (ca. 600 Ma) on granites from North Dobrogea (Boclugea formation). Ages of 604–609 Ma were reported by Roban et al. in 2020 [9] from detrital zircon crystals in rocks of the Audia thrust nappe of the Moldavides (inner part) of the Eastern Carpathians, which contained fragments of felsic rocks similar to the ones studied here. The authors also obtained a prominent peak at 605 Ma in conglomerates of the Outer Moldavide nappes of the Eastern Carpathians (i.e., Vrancea nappe), which contained pebbles of rocks similar to the Ediacaran turbidites of Central Dobrogea.

The basement of the Moesian platform extended in the south and west of Central Dobrogea is poorly known. Granitoid intrusions have been intercepted in several drill holes located in the basement of the Wallachian sector of the Moesian platform, which probably is different from the basement of the Dobrogean sector; this was also suggested by Tari et al. in 1997 [86]. Nevertheless, considering the relative position of Central Dobrogea and Małopolska Massif, the basement of South Dobrogea and its northern continuation should correspond with the outcropping Brunovistulian terrane, as suggested, e.g., by Kalvoda et al. in 2008 [87] and references therein, in the northern foreland of the Carpathians (Figure 18), where it contains batholiths of Ediacaran age that share several features with the igneous rocks attributed to the Cuman Cordillera.

Most granitoid rocks in the Brunovistulian terrane are known in two batholiths: Brno, to the north, and Thaya/Dyje, to the south, as reported, e.g., by Finger et al. in 1995 [88] and 2000 [89] or Soejono et al. in 2017 [90]. The granitoids in these batholiths show varied petrology, from granites to granodiorites and diorites, as described by Jelínek & Dudek in 1993 [91] or Finger et al. in 1995 [88]. After Jelínek & Dudek [91], the diorites and quartz diorites in the Brno batholith occur as xenoliths or as relatively small bodies within the granite and granodiorite bodies. The modal composition of the granites, granodiorites, and diorites in the Brno batholith shows plagioclase, quartz, potassium feldspar, and biotite ± amphibole as dominant minerals, while zircon, apatite, sphene, and iron oxides are the main accessory minerals. Muscovite is either absent or in an accessory amount. In the Thaya/Dyje batholith, the granitoid rocks (granite, granodiorite, quartz diorite, tonalite) and their igneous mineral assemblages are roughly similar to those in the Brno batholith, with muscovite, albite, and garnet as metamorphic products, as described by Scharbert & Batík in 1980 [92]. The age of the granitoids in the Brno batholith is Ediacaran (ca. 550–600 Ma), as determined by several authors, such as van Breemen et al. in 1982 [93], Fritz et al. in 1996 [94], or Soejono et al. in 2017 [90]. These are similar to the ages (550–575 Ma) obtained for the Thaya/Dyje batholith by Scharbert & Batík in 1980 [92] and Friedl et al. in 2004 [95].

The igneous mineral assemblages are similar to those identified in the rocks of the Cuman Cordillera. The minor contribution of diorite in the make-up of the Brno batholith is similar to the relative scarcity of the andesine-bearing rocks we investigated. This suggests that the least-alkaline phaneritic rocks studied by us (3 samples out of the 25 studied igneous rock samples) might originate from xenoliths or from small bodies within the dominant granite bodies.

Figure 18. Geotectonic sketch map of the East European craton and the pieces of crust abutting its southwestern margin, based on Winchester et al., 2002 [96], Saintot et al., 2006 [97], and Nikishin et al., 1998 [98]. The orange zone is the Brunovistulian terrane, made up of Moravo-Silesian terrane (MR), which contains the Thaya batholith and Bruno–Silesian massif (BS), which includes the Brno batholith. The light orange quadrangle indicates the area shown in Figure 1A. Other terranes and crustal blocks: CDB = Central Dobrogea; LG = Łysogóry; MP = Małopolska; NDB = North Dobrogea; SDB = South Dobrogea; WMOE = Wallachian sector of the Moesian platform.

Figure 18. Geotectonic sketch map of the East European craton and the pieces of crust abutting its southwestern margin, based on Winchester et al., 2002 [96], Saintot et al., 2006 [97], and Nikishin et al., 1998 [98]. The orange zone is the Brunovistulian terrane, made up of Moravo-Silesian terrane (MR), which contains the Thaya batholith and Bruno–Silesian massif (BS), which includes the Brno batholith. The light orange quadrangle indicates the area shown in Figure 1A. Other terranes and crustal blocks: CDB = Central Dobrogea; LG = Łysogóry; MP = Małopolska; NDB = North Dobrogea; SDB = South Dobrogea; WMOE = Wallachian sector of the Moesian platform.

6. Summary and Conclusions

This research, which focused on the felsic igneous fragments in the Variegated clay formation of the Moldavides thrust nappes, represents a substantial advance in the knowledge on the phaneritic and porphyritic rocks attributed to the Cuman Cordillera, documented here with data of bulk rock chemistry, mineral chemistry, radiometric age (of crystallization), and paleontological age (of deposition in the sedimentary succession—based on biostratigraphy of calcareous nannofossils of the sediments which enclosed the igneous rocks).

The exotic igneous clasts attributed to the Cuman Cordillera were (re)classified as granites (most samples) ± granodiorites/quartz diorites and rhyolites (of subvolcanic type). The similar geochemical trends support the comagmatic origin of the granites and rhyolites and also suggest magma differentiation through fractionation of biotite and plagioclase, which led to depletion of Eu, P, and Ti in the most evolved melts.

Apart from their dominantly peraluminous composition (ASI > 1.1), the granites and rhyolites of the Cuman Cordillera show mineralogical and geochemical features characteristic of I-type granites. Both bulk rock and mineral chemistry indicate magma crystallization in a convergent plate tectonic setting, showing affinity for the continental collision or slab break-off rather than the typical volcanic arc model. The mineral assemblages and the biotite composition suggest relatively oxidizing conditions of magma crystallization (affinity to magnetite-series). The zircon saturation method indicates magma crystallization at 767–824 °C.

The geochronological investigations using the zircon U-Pb SHRIMP method indicate Ediacaran ages (ca. 580–610 Ma) of the rocks attributed to the Cuman Cordillera (from both Early and Late Cretaceous successions), coeval with the Cadomian/Pan-African orogeny. This supports the extra-Carpathian, i.e., the exotic origin of these rocks.

Analysis of calcareous nannofossil assemblages from the Variegated clay formation indicates that the felsic igneous clasts were deposited during the Cenomanian, Campanian, and Maastrichtian. These data add to previous records (Aptian, terminal Albian, and Maastrichtian) and could be completed through additional paleontological investigations. In that respect, the Cuman Cordillera was a source of igneous clasts for the Cretaceous sediments in the Eastern Carpathian flysch basin from the Early Cretaceous (Aptian) to the Late Cretaceous (Maastrichtian), which means a time span longer than 40 Ma.

Thus, products of Ediacaran (Cadomian/Pan-African) magmatism were exposed in the Carpathian foreland during most of the Cretaceous, from Aptian to Maastrichtian times. Their paleogeographic position was likely west of Central Dobrogea–Małopolska terrane (in present-day coordinates), as suggested by the increased abundance of Dobrogean-type Ediacaran turbidite fragments in the easternmost sedimentary formations of the Eastern Carpathians and the occurrence of the Ediacaran igneous rocks attributed to the Cuman Cordillera in the central–inner parts of the Moldavides. The Brunovistulian terrane, which contains Ediacaran felsic igneous rocks and is located west of the Małopolska–Central Dobrogea terrane, is the most likely candidate as a representative fragment of a Cadomian/Pan-African belt extending southward into the Carpathian foreland. This geotectonical correlation is supported by the mineralogical and geochemical similarities between the Ediacaran Brno and Thaya batholiths in the Brunovistulian terrane and the igneous fragments of the Cuman Cordillera. Nevertheless, considering the distance between our study area and the Brunovistulian terrane (more than 1000 km), as well as the geochemical variability typical of magmas associated with convergent plate margin settings, this correlation requires further evidence.