Tracing Pre-Mesozoic Tectonic Sutures in the Crystalline Basement of the Protocarpathians: Evidence from the Exotic Blocks from Subsilesian Nappe, Outer Western Carpathians, Poland

: Pre-Mesozoic exotic crystalline blocks within the Outer Carpathian ﬂysch have potential to unravel the nature of their eroded basement source(s) and to reconstruct the Paleozoic–Precambrian history of the Protocarpathians. Strongly tectonized Campanian–Maastrichtian grey marls in the Subsilesian Nappe of the Outer Western Carpathians in Poland contain a variety of different lithology types, including granitoids and andesites. Petrological investigations coupled with zircon and apatite U-Pb dating were performed on crystalline (subvolcanic) exotic blocks from a locality in the Subsilesian Nappe. U-Pb zircon dating yields magmatic crystallization ages of c. 293 Ma for the microgranitoid and c. 310 Ma for the andesite block, with inherited zircon cores yielding Archean, Paleoproterozoic, Mesoproterozoic and Cadomian ages. Whole rock trace element and Nd isotope data imply that the melt source was composed of a signiﬁcant Neoproterozoic crustal component in both the microgranite and andesite. The Late Carboniferous–Permian magmatic activity likely continues outside the Carpathian Belt and can be linked to a Late Paleozoic transtensional zone, which is a continuation of the Lubliniec–Krak ó w Zone that extends under the Carpathians to Moesia. This Late Paleozoic transtensional zone was probably reactivated during the Late Cretaceous under a transpressional regime within the ˙Zegocina tectonic zone, which caused the uplift of the Subsilesian Ridge and intensive erosion. Results are based on ion chromatography TIMS procedures. Sample digestion for Nd–Sr analysis was performed in Savillex ® beakers using an ultrapure 4:1 mixture of HF and HNO 3 for 10 days at 110 ◦ C on a hot plate. For whole rock powders, a minimum dissolution time of 3 weeks was applied to ensure maximum leaching of the REEs from refractory material such as zircon. After evaporating the acids, repeated treatment of the residue using HNO3 and 6.0 N HCl resulted in clear solutions for all samples. The REE fraction was extracted using AG®50W-X8 (200–400 mesh, Bio-Rad) resin and 4.0 N HCl. Nd was separated from the REE fraction using Teﬂon-coated HdEHP, and 0.22 N HCl as the elution media. Maximum total procedural blanks were 50 pg for Nd and were taken as negligible. Nd was run as metal on an Re double ﬁlament, using a ThermoFinnigan®Triton MC TIMS. A 143 Nd/ 144 Nd ratio of 0.511841 ± 0.000005 (n = 5) was determined for the La Jolla (Nd) international standard, during the period of investigation. Within-run mass fractionation for Nd isotope


Introduction
The presence of Pre-Mesozoic basement within the Alpine orogens was first identified c. 200 years ago [1]. In most of the Alpine orogens of Europe, such as the Alps and Carpathians, Pre-Mesozoic basement is present as exotic blocks inside the sedimentary or metasedimentary successions or as uplifted crystalline massifs, termed "core mountains" [2,3]. Investigating the Pre-Alpine basement and constraining the timing of its main tectonic events remain active problems in European geology. The Carpathian Mountains are an orogen of Alpine (Cretaceous-Neogene) age, but their structure reflects a prolonged Neoproterozoic and Phanerozoic history. While the crystalline massifs in the Central Carpathians are relatively well-studied ( [3] and references therein), the presence of pre-Alpine components in the Outer Carpathians are less understood. Pre-Alpine basement is present in the form of exotic blocks and interpreted as remnants of the Protocarpathian crystalline basement, which is inferred to underlay the Mesozoic and Cenozoic basins of the Carpathian orogenic Figure 1. Location of the study area on (a) a simplified geological map of the Carpathian chain within Europe, (b) the general geological structure of Carpathians in Poland (map constructed using information included in [3][4][5]8] and (c) on a detailed geological map of the Żegocina Zone (constructed using information included in [9]).

Figure 1.
Location of the study area on (a) a simplified geological map of the Carpathian chain within Europe, (b) the general geological structure of Carpathians in Poland (map constructed using information included in [3][4][5]8] and (c) on a detailed geological map of theŻegocina Zone (constructed using information included in [9]).

Microscopy
Petrographic analyses of thin sections were undertaken at the Institute of Earth Sciences in the University of Silesia using an Olympus BX-51 microscope to constrain textural and microstructural relationships and to determine the presence of zircon.

Electron Probe Micro-Analyses (EPMA)
Microprobe analyses of the main rock-forming and accessory minerals were carried out at the Inter-Institutional Laboratory of Microanalyses of Minerals and Synthetic Substances, Warsaw, using a CAMECA SX-100 electron microprobe. The analytical conditions employed an accelerating voltage of 15 kV, a beam current of 20 nA, counting times of 4 s for peak and background and a beam diameter of 1-5 µm. Reference materials, analytical lines, diffracting crystals, mean detection limits (in wt%) and uncertainties were as follows: rutile-Ti (Kα, PET, 0.03, 0.05), diopside-Mg (Kα, TAP, 0.02, 0.11), Si-(Kα, TAP, 0.02, 0.21), Ca-(Kα, PET, 0.03, 0.16), orthoclase-Al (Kα, TAP, 0.02, 0.08), and K (Kα, PET, 0.03, 0.02), albite-Na (Kα, TAP, 0.01, 0.08), hematite-Fe The study area is located within the Subsilesian Nappe. Originally occupying a structural position between the Silesian and the Skole nappes, the Subsilesian Nappe exposures south of Kraków are limited to small tectonic windows forming belts within the Silesian Nappe and along the northern boundary of the Magura Nappe (Figure 1c). The sampled area is located in a belt stretching over a distance of several kilometers along the boundary of the Magura Nappe and is known as theŻegocina Zone sensu Skoczylas-Ciszewska [16]. In theŻegocina Zone, a series of E-W trending structures are associated with the exposure of the Subsilesian nappe [9,16], and the zone is interpreted to represent a tectonic mélange. The Subsilesian Nappe is composed mainly of Upper Cretaceous and Lower Paleogene carbonate deposits, which were deposited in the Subsilesian Sedimentary Zone, an uplifted high that separates the Silesian and Skole basins [4,17,18]. The central part of this high is inferred to have been composed of crystalline basement rocks and is termed the Subsilesian Ridge. This paleogeographic entity, which is now destroyed, supplied crystalline basement blocks to the slope and basinal areas. The crystalline blocks occur within marly slope deposits. These strongly tectonized Campanian-Maastrichtian grey marls contain different types of lithology, including limestones, gneisses, granitoids and andesites [19]. Two samples of c. 10 kg each were extracted from large rounded blocks of grey andesite (3 m in diameter) and a pale, fine-grained granitoid (4 m in diameter), in the Pluskawka Stream valley near the villages of Nowe Rybie and Kamionna in the Beskid Wyspowy Range (Figure 1c, N 49 • 47 15,8", E 20 • 21 03,4").

Microscopy
Petrographic analyses of thin sections were undertaken at the Institute of Earth Sciences in the University of Silesia using an Olympus BX-51 microscope to constrain textural and microstructural relationships and to determine the presence of zircon.

Electron Probe Micro-Analyses (EPMA)
Microprobe analyses of the main rock-forming and accessory minerals were carried out at the Inter-Institutional Laboratory of Microanalyses of Minerals and Synthetic Substances, Warsaw, using a CAMECA SX-100 electron microprobe. The analytical conditions employed an accelerating voltage of 15 kV, a beam current of 20 nA, counting times of 4 s for peak and background and a beam diameter of 1-5 µm. Reference materials, analytical lines, diffracting crystals, mean detection limits (in wt%) and uncertainties were as follows

Whole-Rock Chemical and Isotope Analyses
Whole-rock analyses were undertaken by X-ray fluorescence (XRF) for major and large ion lithophile trace elements (LILE), and by fusion and ICP-MS for high field strength elements (HFSE) and rare earth elements (REE) at Bureau Veritas Minerals (Canada). Preparation involved lithium borate fusion and dilute digestions for XRF and lithium borate decomposition or aqua regia digestion for ICP-MS. LOI was determined at 1000 • C. Uncertainties for most of the major elements are 0.01%, except for SiO 2 , which is 0.1%. REE were normalized to C1 chondrite [20].
The Sm-Nd analytical work was performed at the Laboratory of Geochronology, Department of Lithospheric Research, University of Vienna. Results are based on ion chromatography TIMS procedures. Sample digestion for Nd-Sr analysis was performed in Savillex ® beakers using an ultrapure 4:1 mixture of HF and HNO 3 for 10 days at 110 • C on a hot plate. For whole rock powders, a minimum dissolution time of 3 weeks was applied to ensure maximum leaching of the REEs from refractory material such as zircon. After evaporating the acids, repeated treatment of the residue using HNO3 and 6.0 N HCl resulted in clear solutions for all samples. The REE fraction was extracted using AG®50W-X8 (200-400 mesh, Bio-Rad) resin and 4.0 N HCl. Nd was separated from the REE fraction using Teflon-coated HdEHP, and 0.22 N HCl as the elution media. Maximum total procedural blanks were 50 pg for Nd and were taken as negligible. Nd was run as metal on an Re double filament, using a ThermoFinnigan®Triton MC TIMS. A 143 Nd/ 144 Nd ratio of 0.511841 ± 0.000005 (n = 5) was determined for the La Jolla (Nd) international standard, during the period of investigation. Within-run mass fractionation for Nd isotope compositions was corrected to 146 Nd/ 144 Nd = 0.7219. Uncertainties on the Nd isotope ratio are quoted as 2 σm.

Mineral Separation and Imaging
Zircon crystals were separated using standard techniques (crushing, hydro-fracturing, washing, Wilfley shaking table, Frantz magnetic separator and hand picking). Mineral separation was carried out at the Institute of Geological Sciences at the Polish Academy of Sciences, Kraków, Poland. The crystals were cast in 25 mm diameter epoxy resin mounts, ground and polished to half-thickness to expose the grain interiors. Internal mineral textures were then characterized by back-scattered electron (BSE) and cathodoluminescence (CL) imaging, using a FET Philips 30 scanning electron microscope with a 15 kV accelerating voltage and a beam current of 1 nA at the Institute of Earth Sciences, University of Silesia, Sosnowiec, Poland.

LA-ICP-MS U-Pb Dating
LA-ICPMS U-Pb age zircon and apatite data were acquired using a Photon Machines Analyte Excite 193 nm ArF excimer laser-ablation system with a HelEx 2-volume ablation cell coupled to an Agilent 7900 ICPMS at the Department of Geology, Trinity College Dublin, Ireland. The instruments were tuned using NIST612 standard glass to yield Th/U ratios of unity and low oxide production rates (ThO+/Th+ typically <0.15%). A quantity of 0.4 l min −1 He carrier gas was fed into the laser cell, and the aerosol was subsequently mixed with 0.6 l min −1 Ar make-up gas and 11 mL min −1 N 2 . For zircon, data reduction of the raw U-Pb isotope data was performed through the "VizualAge" data reduction scheme [21] in the freeware IOLITE package [22]. For apatite, the "VizualAge_UcomPbine" data reduction scheme [23], which can account for variable common Pb in the apatite standards, was employed. Sample-standard bracketing was applied after the correction of downhole fractionation to account for long-term drift in isotopic or elemental ratios by normalizing all ratios to those of the U-Pb reference standards. Final age calculations were made using the Isoplot add-in for Excel [24].

U-Pb Apatite Dating
Apatite crystals were prepared as separates on polished epoxy mounts. A total of 29 isotopes ( 31 P, 35 Cl, 43 Ca, 51 V, 55 232 Th and 238 U were acquired using a 60 µm laser spot, an 11 Hz laser repetition rate and a fluence of 2.5 J/cm 2 . A~1 cm sized crystal of Madagascar apatite that has yielded a weighted average ID-TIMS concordia age of 473.5 ± 0.7 Ma was used as the primary apatite reference material in this study. McClure Mountain syenite apatite (the rock from which the 40 Ar/ 39 Ar hornblende standard MMhb is derived) and Durango apatite were used as secondary standards. McClure Mountain syenite has moderate but reasonably consistent U and Th contents (~23 ppm and 71 ppm, [29]), and its thermal history, crystallization age (weighted mean 207 Pb/ 235 U age of 523.51 ± 2.09 Ma) and initial Pb isotopic composition ( 206 Pb/ 204 Pb = 17.54 ± 0.24; 207 Pb/ 204 Pb = 15.47 ± 0.04) are known from high-precision ID-TIMS analyses [30]. NIST 612 standard glass was used as the apatite trace-element reference material, and a crushed aliquot of Durango apatite that has been characterized by solution quadrupole-ICP-MS analyses [31], was used as the apatite trace-element secondary standard.

Andesite
Two samples (And-J and And-C), weighting approximately 10 kg each, were extracted from a 1.5 m in diameter block of andesite. Two different rock types were recognized in the samples-a light-grey variety with porphyritic texture and a dark-reddish, equigranular, fine-grained facies containing several small fragments of mudstone (1-3 cm in size). The mineral composition of both rock types is similar and consists of albitized plagioclase, chloritized biotite, spinel and sporadic remnants of pyroxene (Figure 2b) [38].     Table 4; Figure 4a). Due to the secondary alteration, the relatively mobile LILE were not used for geochemical discrimination purposes.
The cathodoluminescence images of zircons and apatites are shown in Figure 5.

Andesite
Nine zircon crystals from the andesite facies lacking visible mudstone clasts (And) were analyzed. The zircons are subhedral to anhedral, short-prismatic, with aspect ratios ranging from 1:1 to 2:1. We carried out 17 spot analyses (Table 5). Six analyses from magmatically zoned crystals (Figure 5c) showing moderate luminescence yielded a concordia age of 618.4 ± 8.3 Ma (Figure 6e,f). One zircon crystal with a homogeneous internal structure and weak luminescence yielded an age of 560.2 ± 13.7 Ma. One crystal exhibited homogeneous weak luminescence, typical of metamorphic zircon crystals; 3 analyses located within this grain yielded concordant age of 716.9 ± 7.3 Ma. Cathodoluminescence imaging reveals the presence of two inherited cores, mostly with weak luminescence, and these yield concordant ages of 2958 and 2907 Ma, as well as discordant ages (Figure 6e, Tables S1 and S2). Two CL-bright, magmatically zoned rims which are 20 to 40 µm thick ( Figure 5) yielded an age of 310 ± 4.9 Ma (Table 5).

Petrogenetic Interpretation
The microgranite and andesite from Pluskawka share a volcanic arc affinity and positive Th and negative P and Ti anomalies on a primitive-mantle-normalized multielement diagram (Figures 3b and 4b). These characteristics imply a crustal origin for the parent magma for both rock types. This is supported by the negative εNd 293 value (−5.5, Table 6) and T DM age of 1.45 Ga (Table 6) for the microgranite. This Nd isotope systematics are typical of granitoid rocks of the European Variscan belt and are interpreted to represent the Archean/Neoproterozoic basement that melted during Variscan orogenesis [40,41]. The mixed Paleoproterozoic /Neoproterozoic protolith is inferred from the U-Pb ages of inherited zircon crystals from the microgranite (PL 2) and andesite (AND), which both show Neoproterozoic and Paleoproterozoic (>2000 Ma) age ranges (Table 5). However, it may be unwise to assume that these samples represent Variscan magmatic rocks of volcanic arc affinity, as the geochemical characteristics could also have been inherited from the protolith ( [42] and references therein). The very similar zircon and apatite ages ( Figure 6) in microgranite sample Pl-2 (293.2 ± 4.1 Ma and 290.2 ± 5.6 Ma) imply very rapid cooling from crystallization temperatures to temperatures below the apatite closure temperature window (c. 450 • C) and thus support a hypabyssal origin.

Implications for Paleozoic Paleogeology
The analyzed rocks represent Late Carboniferous-Permian bimodal volcanic-plutonic activity (Figure 3a,b). The samples show lithological, geochemical and age similarities to the magmatic rocks from the Kraków-Lubliniec tectonic zone [36,41,43] as well as to the Carboniferous-Permian intraplate volcanic rocks that are widespread in the Romanian Moesia [43]. As the exotic blocks represent the basement that was eroded during the uplift of the Subsilesian Ridge, we assume that these magmatic rocks continued in the Protocarpathians to both the north-west and south-east (Figure 7).
The lineament connecting Tenczynek, Pluskawka and Moesia is related to the southeastern extension of the Kraków-Lubliniec tectonic lineament as well as to the boundary between the Brunovistulia and Małopolska blocks [41][42][43][44][45]. According to Mazur et al. [46], this zone formed during the latest Carboniferous-Permian transtensional tectonic activity, contemporaneous with the Liplas-Tarnawa pull-apart basin located south of Kraków [47]. This lineament cuts across the Variscan structures within the Neoproterozoic basement of central Pangea (Figure 7). The Protocarpathian region is characterized by the Neoproterozoic basement overlain by Upper Paleozoic sedimentary sequences, similar to the Brunovistulia and Małopolska massifs [5,48,49]. As was previously mentioned, the other analyses of the Subsilesian exotics indicate the prevailing Neoproterozoic (Cadomian) age of the substratum in this area [5,13,14]. The Neoproterozoic rocks of the Protocarpathian basement were the source for the Variscan magmas formation, which subsequently was emplaced within this Late Variscan transpressional zone. The occurrence of the Late Paleozoic magmas is probably limited to this transpressional zone. The Neoproterozoic Outer Protocarpathian basement adjacent to the Laurussian margin was covered by the Paleozoic sedimentary rocks, including

Implications for Cretaceous-Paleogene Paleogeology
Epicontinental sedimentation continued in the Outer Protocarpathian region during Permian to Jurassic times [53]. The Protosilesian back-arc basin, containing syn-rift, postrift and synorogenic deep-water sedimentary sequences was initiated during Late Jurassic times [3,18,54]. There is no evidence indicating the oceanic crust within this back-arc basin. The northward movement of the Central Carpathian plate, part of the larger ALCAPA (ALpine-CArpathian-PAnnonian) plate, caused progressive inversion and emergence of ridges during the Late Cretaceous. These ridges separated a series of newly formed basins [3,18,56]. The Subsilesian Ridge (Figure 8) separated the Silesian and Skole basins, which were later inverted to form the Silesian and Skole Nappes.

Conclusions
1. Exotic blocks of Late Carboniferous-Permian magmatic rocks are found in Campanian-Maastrichtian grey marls of the Subsilesian Nappe. This magmatic activity is also found outside the Carpathian Belt and can be linked to a Late Paleozoic transtensional zone, which is a continuation of the Lubliniec-Kraków Zone that extends under the Carpathians to Moesia. The Subsilesian Ridge, the source of the exotic clasts, was surrounded by a slope region characterized by mixed pelagic and turbiditic sedimentation that corresponds to the Subsilesian Sedimentary Zone. The large size of these blocks (more than 1 m in diameter, locally up to some tens of meters or even more than 100 m in diameter-see [14])indicates short transport distances and intense tectonic activity during the deposition of the host Campanian-Maastrichtian grey marls [57] on the slope of the Subsilesian Ridge. This tectonic activity may have triggered earthquakes (see e.g., [58]) and produced mélange complexes.
This transpressional tectonic zone (Żegocina Zone) is regarded as a segment of the Kraków-Moesia fault zone in the basement and indicates rejuvenation of an older Paleozoic tectonic lineament. The position of theŻegocina Zone 100 km south of Kraków (see also [51]) and its link to the Kraków-Moesia lineament allow for a more precise palinspastic reconstruction of the Late Cretaceous-Paleocene Outer Carpathian basins (Figure 8). The suggested orientation of the Subsilesian Ridge ( Figure 8) is consistent with the geometry of the Skole Basin that is quite narrow south of Kraków yet wide in eastern Poland, Ukraine and Romania [12,18,52,56]. According to Kováč et al. [59], two units related to the Subsilesian Nappe existed during Paleogene times: the Subsilesian Basin and the Subsilesian Ridge. The orientation of their ridge is similar to our paleogeography ( Figure 8). According to Golonka et al. [18], the Subsilesian Ridge and slope constituted the Subsilesian Sedimentary Area. The Pluskawka exotics were deposited within the marly facies in the Subsilesian Sedimentary Area/Subsilesian Basin located next to the European Platform with Neoproterozoic basement. This area was situated around 100 km south of Kraków. It moved and now is about 30 km south of Kraków.

1.
Exotic blocks of Late Carboniferous-Permian magmatic rocks are found in Campanian-Maastrichtian grey marls of the Subsilesian Nappe. This magmatic activity is also found outside the Carpathian Belt and can be linked to a Late Paleozoic transtensional zone, which is a continuation of the Lubliniec-Kraków Zone that extends under the Carpathians to Moesia.

2.
This Late Paleozoic transtensional zone was probably reactivated during the Late Cretaceous under a transpressional regime within theŻegocina tectonic zone, which caused the uplift of the Subsilesian Ridge and intensive erosion.