# Contrasting sources of Late Paleozoic rhyolite magma in the Polish Lowlands: evidence from U-Pb ages and Hf and O isotope composition in zircon

Elzbieta Slodczyk, Anna Pietranik, Sarah Glynn, Michael Wiedenbeck, Christoph Breitkreuz, Bruno Dhuime

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Elzbieta Slodczyk, Anna Pietranik, Sarah Glynn, Michael Wiedenbeck, Christoph Breitkreuz, et al. Contrasting sources of Late Paleozoic rhyolite magma in the Polish Lowlands: evidence from U-Pb ages and Hf and O isotope composition in zircon. International Journal of Earth Sciences, 2018, 107 (6), pp.2065-2081. 10.1007/s00531-018-1588-8. hal-01974826

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# Contrasting sources of Late Paleozoic rhyolite magma in the Polish Lowlands: evidence from U–Pb ages and Hf and O isotope composition in zircon

Elżbieta Słodczyk<fnref n="1" /> · Anna Pietranik<fnref n="1" /> · Sarah Glynn<fnref n="2" />,<fnref n="3" /> · Michael Wiedenbeck<fnref n="3" /> · Christoph Breitkreuz<fnref n="4" /> · Bruno Dhuime<fnref n="5" />

Received: 7 September 2016 / Accepted: 28 January 2018 / Published online: 23 February 2018
© The Author(s) 2018. This article is an open access publication

## Abstract

The Polish Lowlands, located southwest of the Teisseyre–Tornquist Zone, within Trans-European Suture Zone, were affected by bimodal, but dominantly rhyolitic, magmatism during the Late Paleozoic. Thanks to the inherited zircon they contain, these rhyolitic rocks provide a direct source of information about the pre-Permian rocks underlying the Polish Lowland. This paper presents zircon U–Pb geochronology and Hf and O isotopic results from five drill core samples representing four rhyolites and one granite. Based on the ratio of inherited vs. autocrystic zircon, the rhyolites can be divided into two groups: northern rhyolites, where autocrystic zircon is more abundant and southern rhyolites, where inherited zircon dominates. We suggest that the magma sources and the processes responsible for generating high silica magmas differ between the northern and southern rhyolites. Isotopically distinct sources were available during formation of northern rhyolites, as the Hf and O isotopes in magmatic zircon differ between the two analysed localities of northern rhyolites. A mixing between magmas formed from Baltica-derived mudstone–siltstone sediments and Avalonian basement or mantle can explain the diversity between the zircon compositions from the northern localities Daszewo and Wysoka Kamieńska. Conversely, the southern rhyolites from our two localities contain zircon with similar compositions, and these units can be further correlated with results from the North East German Basin, suggesting uniform source rocks over this larger region. Based on the ages of inherited zircon and the isotopic composition of magmatic ones, we suggest that the dominant source of the southern rhyolites is Variscan foreland sediments mixed with Baltica/Avalonia-derived sediments.

**Keywords** Zircon isotopic composition · Rhyolite petrogenesis · Trans-European Suture Zone · Crustal basement diversity

## Introduction

The Central European Basin was affected by extensive magmatism with the emplacement of c. 80,000 km<fnref n="3" /> of magma during the Late Carboniferous to Early Permian (Paulick and Breitkreuz 2005). This resulted in a bimodal volcanism which is observed across central Europe (Benek et al. 1996; Breitkreuz and Kennedy 1999; Geißler et al. 2008; Hoffmann et al. 2012; Kryza and Awdankiewicz 2012; Awdankiewicz et al. 2013; Turniak et al. 2014; Fig. 1a). In Poland, volcanism extended from the Fore-Sudetic Monocline northwards up to Western Pomerania. The resulting Permo-Carboniferous volcanic rocks are today covered by younger sediments with thickness varying from approximately 1000 m in the Fore-Sudetic Monocline to over 5000 m close to the Teisseyre–Tornquist Zone (TTZ; Fig. 1); (Jackowicz 1994; Maliszewska et al. 2003; Dadlez 2006). Hence, in Poland, samples of such volcanic rocks are provided exclusively

**Electronic supplementary material** The online version of this article (https://doi.org/10.1007/s00531-018-1588-8) contains supplementary material, which is available to authorized users.

<figure><img src="image_3.png" /></figure>

Elżbieta Słodczyk
elzbieta.slodczyk@uwr.edu.pl

<footnote n="1">¹ Institute of Geological Sciences, University of Wrocław, ul. Cybulskiego 30, 50-205 Wrocław, Poland</footnote>

<footnote n="2">² School of Geosciences, University of the Witwatersrand, Witwatersrand, South Africa</footnote>

<footnote n="3">³ Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany</footnote>

<footnote n="4">⁴ Institute for Geology and Palaeontology, TU Bergakademie Freiberg, Bernhard-von-Cotta-Str. 2, Freiberg, Germany</footnote>

<footnote n="5">⁵ University of Bristol, Bristol, UK</footnote>
**Fig. 1** a Plate tectonic sketch of West and Central Europe after Smit et al. 2016; TTZ Teisseyre–Tornquist Zone, L Laurentia; b map of northwestern Poland indicating the location of the sampling sites of Permo-Carboniferous volcanic rocks. Numbers indicate the thickness of drilled volcanic rocks in meters (data from Dadlez 2006; Geißler et al. 2008). Map modified after Dadlez (2006), and Breitkreuz et al. (2007). Teisseyre–Tornquist Zone based on compilation map from Grad and Polkowski (2016)

<figure><img src="image_4.png" /></figure>

from drill cores obtained during an intense hydrocarbon exploration program carried out from the 1970s through until the 1990s. The volcanic rocks constitute less than 12% of the material in the drill cores (Breitkreuz et al. 2007), but in many places, they provide crucial information about

the underlying pre-Permian basement. The composition of the pre-Permian basement is also constrained by geophysical studies (Grad et al. 2003; Dadlez et al. 2005; Guterch and Grad 2010) and drill core data showing the presence of Ordovician-to-Carboniferous sediments (Podhalańska

<figure><img src="image_5.png" /></figure>
and Modliński 2006; Matyja 2006). The pre-Permian base-
ment is probably composed of several Precambrian-to-lower
Paleozoic units that have been interpreted as having formed
through a juxtaposition of several fragments of continental
crust along the margin of Baltica prior to being overlain by
sedimentary and low-grade metasedimentary cover (Mazur
et al. 2010, 2015; Żelaźniewicz et al. 2016).

An interpretation whereby some of the underlying crust
may be composed of Baltica-derived sediments is supported
by recent studies on detrital and inherited zircons of the
North East German Basin (NE GB; Pietranik et al. 2013a)
and of the Mid-German Crystalline Rise in central Germany
(Zeh and Gerdes 2010). It is, therefore, reasonable to expect
that data from zircons from volcanic rocks from the Pol-
ish Lowlands will provide additional constraints on crustal
structures and their arrangement during the Late Paleozoic.

Here, we present new analyses of zircon from four rhy-
olitic (boreholes Daszewo, Wysoka Kamieńska, Chrzypsko,
and Pniewy) and one granitic (borehole Chrzypsko; Fig. 1b)
units. Our data include Secondary Ion Mass Spectrometry
(SIMS) U-Pb ages and O isotope analyses followed by Hf
isotope analyses using Laser Ablation Inductively Coupled
Mass Spectrometry (LA-ICP-MS). The goal of the present
contribution is to constrain the sources of rhyolite magma in
the Polish Lowlands and to fit this information into current
models of continental crust and plate tectonic evolution in
the region (Mazur et al. 2015; Smit et al. 2016; Żelaźniewicz
et al. 2016). In addition, we address the extent to which
rhyolite magmas were the product of remelted crustal gra-
nitic bodies.

Geological setting

The investigated zircons were extracted from: (1) drill
cores of rhyolitic lavas and ignimbrites in the western part
of the Polish Lowlands and (2) a granite underlying Per-
mian volcanics near Poznań city. All our samples lie within
the Polish Basin, in the easternmost part of the South-
ern Permian Basin, which is part of the central European

Permo-Mesozoic basin system (Kiersnowski et al. 1995;
VanWees et al. 2000; Littke et al. 2005). The Polish Basin
formed along the NW–SE trending TTZ, which constitutes
the southwestern margin of the East European Platform
(Fig. 1b; Dadlez et al. 2005). The TTZ is one of the most
important crustal-scale boundaries in Europe (Zielhuis and
Nolet 1994; Schweitzer 1995; Pharaoh 1999), and has been
investigated in detail using high-quality, deep refraction seis-
mics (Fig. 2; Grad et al. 2003, 2006, 2012; Guterch and Grad
2010). Mazur et al. (2016) have merged these results with
potential field data. The filling of the Polish Basin has also
been constrained by numerous boreholes, which show suc-
cessions of variable thicknesses of Ordovician to Cenozoic
deposits containing several unconformities (Jackowicz 1994;
Protas et al. 1995; Maliszewska et al. 2003; Breitkreuz et al.
2007; Modliński and Podhalańska 2010).

The volcanic rocks in drill cores are commonly Late Car-
boniferous to Early Permian in age (Breitkreuz et al. 2007).
Facies analysis has revealed the presence of lava flows and
pyroclastic deposits (Jackowicz 1994; Breitkreuz et al.
2007). These volcanics are best preserved in areas of sub-
sidence, where the thickness of volcanic rocks can exceed
1550 m, commonly decreasing eastwards (Fig. 1b; Jacko-
wicz 1994; Maliszewska et al. 2003; Dadlez 2006). In this
study, samples were taken from drill cores in (1) Wysoka
Kamieńska, a 480-m-thick porphyritic rhyolite lava flow
with feldspar phenocrysts <0.5 cm (down hole sampling
depth: 3500 m); (2) a 230-m-thick, weakly porphyritic dac-
ite/rhyolite lava flow in Daszewo (sampling depth: 3450 m);
and (3) a 150-m-thick ignimbrite in Pniewy 3 (sampling
depth: 3875 m; Breitkreuz et al. 2007). The granite and one
rhyolitic sample come from the Chrzypsko well (sampling
depth: 4218 m), which comprises an upper sedimentary
succession with 470 m of rhyolites and ignimbrites under-
lain by a granitic body with a minimum thickness of 254 m
(Żelaźniewicz et al. 2016). The rhyolite drill core samples
analysed in this study were previously dated by Breitkreuz
et al. (2007) using SHRIMP (sensitive high resolution
ion micro-probe) and yielded concordant ages between
293.0±1.2 and 298.5±1.7 Ma (1 SD). The zircon separated

<figure><img src="image_6.png" /><figcaption>**Fig. 2** Geological and seismic section through Poland across A–A’ line marked in the inset (after Grad and Polkowski 2016); TTZ Teisseyre–Torn-quist Zone</figcaption></figure>
from the Chrzypsko granite gave a slightly older SHRIMP age of ca. 302 Ma (Żelaźniewicz et al. 2016).

Not much is known about the pre-Permian basement at the sampling sites, apart from seismic data from the Daszewo and the Pniewy/Chrzypsko regions and several boreholes that sampled Carboniferous and pre-Carboniferous sediments. In the northern part of the Polish Lowland rocks older than Carboniferous were drilled in several localities, particularly, noteworthy were drilling profiles from the Koszalin–Chojnice Zone (Ordovician–Silurian, Poprawa et al. 2006; Modliński and Podhalańska 2010). From the localities analysed in this study Daszewo is located in the Koszalin–Chojnice Zone (Dadlez et al. 2005), so the seismic and drilling data can provide information on the source of Daszewo rhyolites. Refraction seismic data reveal that within this segment the crust has three layers: an upper layer of consolidated crust from 11 to 20 km, a middle layer at depths between 20 and 26 km, and a lower layer at depths of 26–36 km (Dadlez 2006). The nature of the upper-most part of the crust is also constrained by electromagnetic data, which indicate that it is dominated by Paleozoic metasediments (Ernst et al. 2008; Jóźwiak 2013). This upper crust was penetrated by several boreholes and the deepest deposits were a clay to mudstone sequence of Ordovician age as well as Silurian shales and siltstones. This upper crust may also be composed of coal facies and products of crustal brines transported through fault systems (Ernst et al. 2008). The sedimentary cover within the investigated area is composed of lower Rotliegend volcanic and subordinate sedimentary rocks, which extends over the paleotopography of the top Carboniferous surface, including the covering of grabens, horsts and tilted fault blocks (Pożaryski et al. 1992; Dadlez 2006). The U–Pb ages of the detrital zircons supplied to the Koszalin Chojnice zone (boreholes Słupsk IG and Nowa Karczma 1) cluster at 1756–2050, 1485–1510, 974–1227, 739–805, 854–856, 559–623, and 442–495 Ma (Poprawa et al. 2006). These zircon come from clastics delivered to the Koszalin–Chojnice zone from variable sources such as basement elements that were detached from the margin of Baltica and from the Avalonia Terrane, subduction-related volcanic island arc rocks and sediments of the accretionary prism (see Nawrocki and Poprawa 2006; Poprawa 2006).

The Pniewy and Chrzypsko sampling sites are located within the Fore Sudetic Monocline, which deep refraction data reveal a typical crustal profile for the Trans European Suture Zone (TESZ): thick (12 km) units consisting of folded Early Paleozoic sediments, underlain by intermed diary crystalline units with a thickness of ~5 km, and a lower crystalline crust (Dadlez 2006). These Carboniferous sediments were penetrated by the Września IG-1 borehole and include monotonous turbidities deposited after 320 Ma (Mazur et al. 2010). Geochemical and detrital zircon age data from these sediments point to either

a continental island arc or an active continental margin provenance for the precursor clastic materials (see Mazur et al. 2010). In addition, Early Paleozoic phyllites were drilled over the Wolsztyn–Leszno High (metamorphic age: 358.6 ± 1.8 Ma; see Mazur et al. 2006; protolith age: Early Paleozoic–Devonian; see Haydukiewicz et al. 1999). Mazur et al. (2010) studied zircons from Carboniferous turbidites that were located close to the Pniewy borehole, which contributes to this current study. They obtained the zircon ages in the ranges of 2081 ± 18, 566 ± 10, 544 ± 8, 516 ± 6, 426 ± 5, 380 ± 5 Ma.

There is an ongoing debate about the crystalline basement under the central part of the European continent including the Polish Lowlands. Three different models have been proposed (see Mazur et al. 2015 for summary): (1) far East Avalonia is trapped between the East European Craton and the Variscan orogen; (2) the margin of the Teisseyre–Tornquist Zone is formed by the amalgamation of two lithospheric blocks that originated from Baltica and which had differing thicknesses; and (3) Baltica extended to the southwest from the Teisseyre–Tornquist Zone, forming an Early Paleozoic passive margin. This last scenario is supported by recent studies of Smit et al. (2016), who observed a 50–100-km-wide low P-wave velocity zone (see also Fig. 1a) forming a distinctive crustal domain between Avalonia and Baltica.

## Analytical methods

Zircon grains were extracted from samples taken from the four drill cores Daszewo, Wysoka Kamieńska, Pniewy 3 and Chrzypsko (Fig. 1b). The first three samples consist of Permian rhyolites that were previously dated using U–Pb zircon method by Breitkreuz et al. (2007). The Chrzypsko drill core contains a rhyolite in unconformable contact with an underlying granite, both of which we investigated. Zircon grains were recovered from 4-cm diameter half-cores that had been carefully cleaned. All samples were without cracks and were ~3 kg in mass. Zircon grains were separated by standard crushing, panning and magnetic separation techniques. The five concentrates were prepared as two 25.4-mm diameter, round epoxy grain mounts in which the zircon grains were cast and then sectioned by polishing. Both the 91500 and Plešovice zircon reference materials (Wiedenbeck et al. 1995; Sláma et al. 2008) were also embedded in each mount. In order to identify zonation and other internal structures within the grains in advance of data acquisition, our mounts were investigated optically using a petrographic microscope as well as with BSE (Back Scattered Electron) images using a Jeol Hyperprobe JXA-8500F microprobe at the Helmholtz Zentrum Potsdam (GFZ; see Appendix 1 for images).

<figure><img src="image_7.png" /></figure>
## Major elemental determinations

Chemical analyses of the polished mounts were obtained using a Jeol Hyperprobe JXA-8500F in Potsdam with a wavelength-dispersive detector. An accelerating voltage of 20 kV was used in order to generate sufficiently intense signals for the required elements. The electron beam current was 20 nA and the probe diameter was 1 µm. The following calibrants were used: Zircon for Si (Kα) and Zr (Lα), Hf alloy for Hf (Lβ), ScPO4 for Sc (Kα) and U alloy for U (Mβ). Our Hf alloy calibrant is 99.97% pure Hf and the U alloy is 99.7% pure U. The acquisition times were 30 s for Si and Zr and 40 s for Hf, Sc and U. The acquisition time to collect the background level on both sides of the corresponding peak was half of the peak acquisition time. Detection limits were 100 µg/g for Si and Sc, 150 µg/g for U, 300 µg/g for Zr and 600 µg/g for Hf. The Hf (LL) line has an intensity of 3% and can be neglected with regard to the high amount of Si detected.

To check the accuracy of the major element measurements, grains of Plešovice and 91500 were also analysed. Although isotopically homogenous, 91500 has been reported to have a range of Hf concentrations. For example, Gooláerts et al. (2004) report values from 5610 to 6360 µg/g for a single grain. This range was in agreement with the measurements done over the course of this session (average of 6297 µg/g). Similarly the Hf abundance values in Plešovice zircon also show large variations between 8000 and 14,000 µg/g, which is similar to the range of 9000 to 12,000 µg/g reported by Schoene et al. (2010), and also in agreement with the Hf values of 8000–14,000 µg/g reported by Sláma et al. (2008).

## Secondary ion mass spectrometry (SIMS)

We used the CAMECA 1280-HR instrument housed in Potsdam to conduct U–Pb geochronology determinations on zircon. Due to technical problems with the instrument’s chilled water supply, it was necessary to split the data collection into two sessions.

Our analytical method followed that of Ashwal et al. (2017). Briefly, our SIMS U–Pb analyses employed a 6 nA, $^{16}$O$_2^-$ primary ion beam operated in Köhler mode and with a total impact energy of 23 keV, resulting in a beam diameter of approximately 25 µm on the sample’s surface. Oxygen flooding was used to enhance lead sensitivity, with the total pressure in the sample chamber maintained at ~2×10$^{-3}$ Pa throughout both analytical sessions. Each analysis was preceded by a 6 nA, 70 s pre-sputtering of the target domain employing a 25 µm raster, which locally removed the gold coating, suppressed any surface contamination and helped establish equilibrium sputtering conditions. After completing the pre-sputtering, centring routines were applied to the

secondary beam that automatically scanned the sample high voltage, the field aperture in both the X and Y directions and, finally, corrected for any secondary magnetic field drift using the $^{90}$Zr$_2$ $^{16}$O mass station. Data were acquired using an ETP electron multiplier operating in mono-collection mode. A single analysis consisted of 12 cycles of the peak stepping sequence: $^{90}$Zr$_2$ $^{16}$O (1 s integration time per cycle), $^{92}$Zr$_2$ $^{16}$O (1 s), 200.5 (4 s), $^{94}$Zr$_2$ $^{16}$O (1 s), $^{204}$Pb (6 s), $^{206}$Pb (4 s), $^{207}$Pb (6 s), $^{208}$Pb (2 s), $^{177}$Hf$^{16}$O$_2$ (1 s), $^{232}$Th (2 s), $^{238}$U (2 s), $^{232}$Th$^{16}$O (2 s), $^{238}$U$^{16}$O (2 s), and $^{238}$U$^{16}$O$_2$ (2 s). Thus, including pre-sputtering, a single such analysis lasted approximately 12 min. Data were filtered for gross outliers at the three standard deviation level within each run. During both U–Pb analytical sessions, the entrance slits of the SIMS were set at 75-µm width, equivalent to a mass resolution of M/ΔM ≈ 4200 at 10% maximum peak height. The typical count rate for $^{90}$Zr$_2$ $^{16}$O under these conditions was 1–2×10$^4$ counts per second, to which a 46.2-ns dead time correction was applied to our ion counting system, as defined by a delay line in the electron multiplier’s pre-amplifier circuit. The U–Pb age calibration for the machine was based on the zircon reference material 91500 ($^{206}$Pb/$^{238}$U age: 1,062.4 ± 0.4 Ma; $^{207}$Pb/$^{206}$Pb age: 1,065.4 ± 0.3; Wiedenbeck et al. 1995) with the Plešovice reference material ($^{206}$Pb/$^{238}$U age: 337.13 ± 0.37 Ma; Sláma et al. 2008) used as a quality control material to evaluate the accuracy and stability of the U/Pb calibration measurements. Over the two sessions, a total of 90 measurements were made on the reference materials and a further 86 on the unknowns.

Data reduction employed the Excel-based program “NordAge” (M. Whitehouse, NORDSIM facility, Stockholm). The measurements done on the 91500 primary reference material were used to establish the inter-element fractionation using a Pb/UO vs. UO$_2$/UO calibration fitted with a power law function, from which individual best-fit curves were determined for both sessions. As an additional means of verifying the robustness of our calibration approach, $^{206}$Pb/$^{238}$U ages of 1061.7 ± 2.0 Ma (1 SD, N=41) and 1062.3 ± 2.1 Ma (1 SD, N=10) were calculated for the 91500 reference material in sessions 1 and 2, respectively. The Plešovice quality control material gave mean $^{206}$Pb/$^{238}$U ages of 341.4 ± 1.4 Ma (1 SD, N=11) and 340.6 ± 1.9 Ma (1 SD, N=5) for the two data collection sessions, respectively. Hence, as expected 91500 is within uncertainty of its assigned $^{206}$Pb/$^{238}$U age of 1062.4 Ma, whereas our mean Plešovice results are slightly older than this material’s published $^{206}$Pb/$^{238}$U age of 337.13 Ma. We have checked in detail both our common Pb correction method as well as the UO$_2$/UO power law calibration scheme, but we have been unable to find any unusual features to these parts of our data reduction process. Thus, we are unable to explain the circa 4-Ma offset between the assigned age of our quality control material and the results of our measurements. We
have no reason to suppose the age results for the five samples investigated here also suffer from a circa 1% bias in their $^{206}$Pb/$^{238}$U ages, but this possibility cannot be totally discounted.

The software package Isoplot (Ludwig 2012), was used to plot the U–Pb results using the decay constants recommended by the IUGS sub-commission on geochronology (Steiger and Jäger 1977). The isotope ratios and ages reported in Table 1 and Appendix 2 have an analytical uncertainty of 1 SD, whereas the uncertainty ellipses plotted in Fig. 4 are at the 2 SD level. During data evaluation, some analyses were found to suffer clearly from overcounts on the $^{204}$Pb mass station. We therefore elected for most samples to employ a $^{208}$Pb correction for correcting for the non-radiogenic component. The single exception to this strategy was for sample P (Pniewy), where the Th/U ratios appeared to have been disturbed, thereby preventing the use of a $^{208}$Pb correction. In the case of sample P, we therefore did not apply any common Pb correction, and this approach appears justified based on the raw data which point to a nearly pure radiogenic Pb content in this particular case.

We also used the Potsdam CAMECA 1280-HR instrument to perform $\delta^{18}$O determinations on zircon, largely following the method described in Nasdala et al. (2016). Briefly, the SIMS analyses employed a 2.5 nA, $^{133}$Cs primary ion beam with a Gaussian intensity distribution with a total impact energy of 20 keV which was rastered over a 10

µm area with the dynamic transfer circuitry activated. Negative secondary ions were extracted using a – 10 kV potential as applied to the sample holder, to which no offset voltage was applied, in conjunction with a 30 eV wide energy window, the position of which was mechanically centred at the beginning of the analytical session. Each analysis was preceded by a 2.5 nA, 60 s pre-sputtering of the target domain with a 20 µm raster. Total pressure in the source chamber varied between $1.4 \times 10^{-6}$ and $3.2 \times 10^{-6}$ Pa, throughout the first session, and $8.5 \times 10^{-7}$ to $1.4 \times 10^{-6}$ Pa during the second session. After completing the pre-sputtering, automatic centring routines were applied to the secondary beam in both the X and Y directions on the field and contrast apertures using the $^{16}$O$^-$ mass station. The instrument was operated in FC–FC multi-collection mode with the secondary magnetic field being fixed using the NMR sensor. The L2' ($^{16}$O$^-$) and the H2' ($^{18}$O$^-$) Faraday cups employed $10^{10}$ Ω and $10^{11}$ Ω resistors, respectively. The ion count rate on the $^{16}$O$^-$ peak was typically in the range of 3.0 to 3.5 billion ions per second. A single analysis consisted of 20 independent integrations, each lasting 4 s. Hence, an entire analysis, including pre-sputtering and the centring routines, lasted approximately 150 s. Data were filtered at the 2 SD level for the 20 integrations within the individual run, and the results were found to be consistent with a Gaussian distribution. Machine calibration was based on the 91500 zircon reference material (Wiedenbeck et al. 2004). The 91500 reference material was

<table><caption>**Table 1** Compilation of zircon ages determined during this and previous studies (for details of this study, see also Fig. 4, Appendixes 2a and 2b)</caption><thead><tr><th>Locality</th><th>Magmatic age (Ma)<br>(published<sup>d</sup>)</th><th>Magmatic age (Ma) (this<br>study<sup>d</sup>)</th><th>Inherited ages (Ma)<br>(published<sup>d</sup>)</th><th>Inherited <sup>206</sup>Pb/<sup>238</sup>U ages<br>(Ma) (this study<sup>d</sup>)<sup>e</sup></th></tr></thead><tbody><tr><td>Daszewo<sup>a</sup></td><td>293.0 ± 1.2 (N = 21)</td><td>297 ± 1.1 concordia age<br>(N = 8)<br>MSWD = 2.2</td><td>726; 1260; 2094; 2344;<br>(N = 4)</td><td>Not found</td></tr><tr><td>Wysoka<br>Kamieńska<sup>a</sup></td><td>294.6 ± 1.2 concordant age<br>(N = 18)</td><td>302 ± 1.5 concordia age<br>(N = 16)<br>MSWD = 1.7<sup>c</sup></td><td>Not found</td><td>356 (N = 1)</td></tr><tr><td>Pniewy<sup>a</sup></td><td>298.5 ± 1.7 (N = 5)</td><td>Spread along the Concordia curve <sup>206</sup>Pb/<sup>238</sup>U age<br>between 318–323</td><td>599; 984; 1020; (N = 3)</td><td>II groups<br>I: 381; 368 (core)<br>363 (rim); (N = 2)<br>II: 473; 624; 668; 1544<br>(N = 4)</td></tr><tr><td>Chrzypsko (rhyolite)</td><td>—</td><td><sup>206</sup>Pb/<sup>238</sup>U age ca. 302 (N = 4)</td><td>—</td><td>552; 1935 (core)<br>1724 (rim) (N = 2)</td></tr><tr><td>Chrzypsko (granite)<sup>b</sup></td><td>302 ± 2 mean age<br>(MSWD = 1.17) (N = 18)</td><td>305 ± 0.8 concordia age<br>(N = 8)<br>MSWD = 1.6</td><td>910 (N = 1)</td><td>866 (core)<br>611 (rim) (N = 1)</td></tr></tbody></table>

N number of grains

<sup>a</sup>Published ages from Breitkreuz et al. (2007)

<sup>b</sup>Published ages from Żelaźniewicz et al. (2016)

<sup>c</sup>The uncertainties going up with n in Wysoka Kamieńska is attributed to its being a heterogeneous population, MSWD ranges from 1.6 to 2.2, the probability of concordance for the samples is within the range of 5%

<sup>d</sup>Uncertainty in the age reported at the 1 SD level

<sup>e</sup>Ages have been rounded up and errors not included, for details see text or Appendix 2
measured a total of 29 times in the first session and further
ten times during the second session. The overall repeatabil-
ity of the determined IMF was ±0.18 and ±0.24‰ (1 SD),
respectively, for our two sessions. All together data for 39
RMs (Appendix 3) and 103 unknowns were collected over
4 days of SIMS data acquisition. Three of the days were
continuous; the last day however was not, because it was
necessary to re-polish one mount in order to have a fresh
surface on the piece of 91500 embedded within it. Hence, we
treated the data as two independent sessions. Data reduction
used +9.86 ± 0.11‰ as the value for the δ <sup>18</sup>O<sub>VSMOW</sub> value
of 91500 (Wiedenbeck et al. 2004), for which we applied
an <sup>18</sup>O/<sup>16</sup>O ratio for VSMOW of 2.00520 × 10<sup>-3</sup> (Baertschi
1976).

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)

The acquisition of Lu–Hf data was performed at the Uni-
versity of Bristol (Bristol Isotope Group) using a Thermo
Scientific™ Neptune multicollector ICP-MS coupled with
a Photon-Machine Analyte G2 Excimer laser (193 nm
wavelength). The beam diameter ranged from 40 to 50 µm
depending on the grain size, and measurements were made
using single circular spots, a laser frequency of 4 Hz, and an
energy density of the laser beam of ca. 5.4 J/cm². A typical
analysis lasted 90 s, including a 30-s background measure-
ment and a 60-s ablation period. Since 50 µm spots produced
smaller internal uncertainties than 40 µm spots, the former
was preferred whenever the grain size allowed for its use.
The spot sizes and placement within investigated zircons are
presented in Appendix 1.

A procedure similar to those described in Kemp et al. (2009) and Pietranik et al. (2011) was applied to correct for the isobaric interference of <sup>176</sup>Yb on <sup>176</sup>Hf. The Yb mass bias was determined by measuring the <sup>173</sup>Yb/<sup>171</sup>Yb ratio during each run and calculating the <sup>176</sup>Yb/<sup>177</sup>Hf ratio using the natural <sup>176</sup>Yb/<sup>171</sup>Yb ratio of 0.897145 (Segal et al. 2003). The <sup>176</sup>Lu abundance determination was based on measurements of <sup>175</sup>Lu that were then corrected using the natural abundance factor <sup>175</sup>Lu/<sup>176</sup>Lu = 0.02655 (Vervoort et al. 2004). According to a detailed study by Fisher et al. (2011), the Bristol Isotope Group's analytical method for Lu–Hf data acquisition and reduction provides accurate results for both natural and synthetic zircons with a large range of Yb/Hf ratios (typically <sup>176</sup>Yb/<sup>177</sup>Hf from 0.001 to 0.3).

The <sup>176</sup>Hf/<sup>177</sup>Hf values of three zircon reference mate-
rials measured during this study (Plešovice, Mud Tank
and Temora2) are presented in Fig. 3. It appears that the
Plešovice and Mud Tank <sup>176</sup>Hf/<sup>177</sup>Hf values analysed in this
study are in perfect agreement with the literature (for ref-
erences see Fig. 3), whereas Temora2 gives slightly lower
values. However, the lower values appear to be linked to

<figure><img src="image_8.png" /><figcaption>**Fig. 3** LA–ICP–MS Hf isotope data from the Plešovice, Mud Tank and Temora II reference materials (uncertainty estimates on individual analyses are 2 SD)</figcaption></figure>

analyses on newly sampled Temora II grains shipped from
Australia within the last 2 years. Other Neptune users in
Bristol have reported a similar problem, where they were
getting consistently lower values for “new” Temora2 grains
compared to older pieces (Bruand: personal communication
2015). Therefore, we interpret the slightly lower <sup>176</sup>Hf/<sup>177</sup>Hf
values for ‘new’ Temora2 as likely due to genuine heteroge-
neity. Chondritic uniform reservoir (CHUR) values of Bou-
vier et al. (2008) and <sup>176</sup>Lu decay value of Söderlund et al.
(2004) were used to calculate εHf.

**Results**

Age and composition of zircons from the Polish
Lowlands

The zircon grains from all four of our localities are charac-
terized by irregular shapes, with rare oscillatory zonation
(BSE images in Appendix 1).

Daszewo (rhyolite)

Fourteen zircon grains were dated from this sample, all
of which were magmatic, with no inherited grains being
detected. Eight zircons gave a mean concordia age of
297 ± 1.1 Ma (1 SD; Fig. 4a; Table 1). The six remaining
grains were slightly discordant, but together, all 14 grains
give a <sup>206</sup>Pb/<sup>238</sup>U weighted average age of 297 ± 3 Ma.
δ<sup>18</sup>O<sub>SMOW</sub> values measured on 12 of the grains (two of
which were measured for both core and rim δ<sup>18</sup>O) ranged
from 7.3 to 8.8‰, whereas εHf values determined for
seven grains were between -8.5 and -9.8. We found no
correlation between δ<sup>18</sup>O and εHf (Fig. 5). Th/U ratios
<figure><img src="image_9.png" /><figcaption>Fig. 4 U–Pb concordia plots of the Polish Lowlands zircons. For details see Appendix 2</figcaption></figure>

for the analysed grains are between 0.44 and 0.64 and the Hf concentrations varies between 8000 and 12,500 µg/g.

### Wysoka Kamieńska (rhyolite)

Twenty-nine zircon grains from this sample were dated, six of which having been evaluated on both the core and rim.

Of the 29 grains there was a single inherited grain dated at 355.5 ± 5.1 Ma. Not all the grains were concordant. The 16 grains that could be used to calculate a concordia age gave a mean age of 302 ± 1.5 Ma (Fig. 4b), whereas a weighted average $^{206}$Pb/$^{238}$U age of 301 ± 5 Ma was calculated for the full suite of grains (Appendix 2b). $\delta^{18}$O for 19 zircon grains (of those, nine had both core and rim analysed) giving

<figure><img src="image_10.png" /></figure>
**Fig. 5** Variations in δ¹⁸O relative to εHf for zircons from the Polish Lowlands. Only grains <315 Ma, and which have been analysed for both Hf and O isotopes are included in this plot. The ranges shown along the x and y axes represent the grains in which either δ¹⁸O or εHf or the age were not analysed. Closed circles represent zircons where the age is known, whereas open circles are zircons without age determinations

<figure><img src="image_11.png" /></figure>

values ranging from 6.2 to 7.7‰. εHf values for 12 magmatic grains lie between –5.6 and –7.7. Thus in this case, the δ¹⁸O values increase with a corresponding decrease in the εHf values (Fig. 5). The Th/U ratio varies between 0.31 and 0.57 and the Hf concentration varies from 8000 to 13,000 µg/g.

### Pniewy 3 (rhyolite)

The 14 zircon grains dated in this sample can be divided into: (1) those located on the concordia curve between 318 and 323 Ma (N=6; Fig. 4c); (2) those which are inherited (six grains); (3) the remaining two analyses which were not considered in the age calculations as they were too discordant. The inherited grains are further subdivided into two groups: group I with younger ages of ~370 Ma: 381.3 ± 2.4; 367.6 ± 3.6 (core); 362.7 ± 2.4 (rim), and group II with the ²⁰⁶Pb/²³⁸U older ages of: 473 ± 3.7, 624.3 ± 4.4, 668 ± 4.5 and 1543.6 ± 9 Ma. The five most concordant grains in the 318–323 Ma population gave δ¹⁸O ratios ranging from 7.51 to 8.26‰. εHf was measured on two of these ca. 320-Ma grains, and the results range from –6.3 to –7.9. As only two grains from this locality have both δ¹⁸O and εHf results, we cannot assess whether there is any correlation between these isotopic systems. The Hf concentration measured in magmatic rims on inherited varies from 10,500 to 12,500 µg/g.

### Chrzypsko (rhyolite)

This sample yielded only a few grains, resulting in just four zircons being dated; however for three of which it was possible to date both the core and the rim (Fig. 4d). Two inherited grains are present; the first has a ²⁰⁶Pb/²³⁸U age of 552.2 ± 10.1 Ma, whereas the core of the second inherited grain gave a date of 1934.6 ± 30.6 Ma, with a younger rim age of 1724.2 ± 35.4 Ma. No concordia age could be calculated from the four dated grains as they did not form a tight enough statistical group; nevertheless, a ²⁰⁶Pb/²³⁸U weighted average age of ca. 302 Ma could be determined, but with so few measurements this age can only be considered as a rough indication (Appendix 2b). Due to the large uncertainty in the true age for Chrzypsko no further isotopic measurements were conducted on this sample.

### Chrzypsko (granite)

Fourteen grains from this sample were dated, only one of which appears to be inherited (G_12; ²⁰⁶Pb/²³⁸U age of core 865.7 ± 11.8 Ma; age of rim 611 ± 8.7 Ma). One grain (G_1; Appendix 2) gave a younger ²⁰⁶Pb/²³⁸U age of 277.7 ± 4.8 Ma. Eight zircon grains give a mean age of 305 ± 0.8 Ma (Fig. 4e), with the data point ellipses plotting just to the left of the Concordia. This shift can be explained by a possible overestimate of the level of common Pb in the sample, which would result in a slight over correction.
A $^{206}$Pb/$^{238}$U weighted average age of 303 ± 4 Ma was calculated for 12 grains (excluding the younger grain as well as the inherited grain; Appendix 2b). The δ$^{18}$O values for these magmatic grains (with two grains analysed for both core and rim) have values between 7.7 and 8.3‰. The εHf values determined for the same zircons give values between –6.3 and –7.9. Thus, we detected no correlation between the εHf and δ$^{18}$O values for this sample (Fig. 5). The magmatic zircons have Th/U ratios between 0.42 and 0.69 and Hf concentrations between 7500 and 13,500 µg/g.

Discussion

Inherited and magmatic zircon ages: insight
into potential magma sources of rhyolitic magmas

Late Paleozoic rhyolites from the Polish Lowlands can
be divided into two groups based on the proportion of
inherited and xenocrystic grains to magmatic zircon
(autocrysts + antocrysts). We define inherited zircons as
those surviving from the protolith melt source, whereas
xenocrystic grains were those derived from external material
during the evolution of magmatic system. Autocrysts and
antocrysts refer to crystals that originate in the same magma
system but antocrysts may represent slightly older episodes
of the system formation and evolution, whereas autocrysts
are related to the youngest magmatic episode (after Miller
et al. 2007; Schmitt 2011). We do not attempt to distinguish
autocrystic from antecrystic grains, because it requires a full
dataset of whole rock and zircon trace element composition
(Siégel et al. 2017), which was not obtained in this study. In
this study the division between the northern and southern
rhyolites is based on the relative proportion between pre-
Permian/Upper Carboniferous and older zircons. Rhyolites
from the northern part of the Polish Lowlands (Daszewo
and Wysoka Kamieńska) contain only a few older zircons,
whereas rhyolites from the southern part (Pniewy and Chr-
zypsko) are dominated by such zircons (Table 1). Similar
proportions between inherited and magmatic grains across
the Polish Lowlands were also reported by Breitkreuz et al.
(2007; Table 1). This suggests differences exist between the
sources for the respective rhyolites and that the pre-Permian
basement in the northern and southern parts may differ. This
is also consistent with the Hf and O isotope record (see “Dis-
cussion” below).

The zircon age distributions in southern and northern
rhyolites shed light on the nature of the magma source.
With regards to older than 300-Ma zircon ages, southern
rhyolites (both the Pniewy and Chrzypsko) contain many zir-
cons with ages 320–380 Ma in addition to individual zircons
with ages ranging from ca. 500 Ma to 2.0 Ga. Interestingly,
even though the ages are Variscan, the range in ages exceeds

that observed in igneous rocks of the Sudetes Mountains,
which are between 300 and 350 Ma (e.g. Turniak et al. 2014;
Oberc-Dziedzic et al. 2015; Kryza et al. 2012; Kusiak et al.
2014; Pietranik et al. 2013b; Białek and Werner 2004). This
abundance of older zircon grains in the southern part of
the Polish Lowlands with Variscan ages indicates that the
source for these rhyolitic magmas was probably a sediment
derived from the Variscan foreland (after 320 Ma). A Vari-
scan-aged sedimentary source is also favoured by both geo-
physical evidence and by the analysis of drill cores showing
monotonous series of turbidites deposited as a flysch during
Tournasian (?) to Westphalian times (358–305 Ma, respec-
tively) (Mazur et al. 2006). Consequently, our data suggest
that the Variscan foreland could extend over the TESZ, in
agreement with previous studies (Mazur et al. 2010, 2015).
In particular, there is excellent agreement between the ages
of inherited zircon in the Pniewy and Chrzypsko rhyolites
and the zircon dated from the Carboniferous sandstones in
the Września IG-1 well, which lies in close proximity to
Pniewy and Chrzypsko (Mazur et al. 2010). Both Variscan
(320–380 Ma) and 500–600 Ma ages are observed in zir-
con from turbidites and as inherited ages of rhyolitic zircon
in this study. Altogether, this suggests that the Carbonifer-
ous foreland sediments were a primary source of rhyolitic
magmas in the southern part of the studied area. However,
scarce pre-Variscan zircon have ages typical for Avalonia or
Baltica (ca. 620, 1350 Ma, two grains with 1490, 1800 Ma)
and not Gondwana, which should not contain inherited ages
between 1000 and 2000 Ma (e.g. Breitkreuz et al. 2007;
Pietranik et al. 2013a; Żelaźniewicz et al. 2016; Franke and
Dulce 2017). Therefore, a mixture of sediments derived from
several crustal blocks is favoured as the source of Pniewy
and Chrzypsko rhyolites, with Variscan foreland sediments
being the dominating source.

The source of the magmas that formed the northern rhyo-
lites may be associated with the drilled lithologies present in
the northern part of the Polish Lowlands. Consolidated pre-
Permian sediments were sampled close to Daszewo locality.
Several drill cores sampled Lower Paleozoic deposits of the
Koszalin–Chojnice zone, including Ordovician fine-grained
siliciclastic rocks (mostly hemipelagic massive or poorly
laminated mudstones and claystones), which are associated
with small amounts of carbonates, and Silurian deep-marine
siliciclastic sediments (Podhalańska and Modliński 2006;
Modliński and Podhalańska 2010; Mazur et al. 2017). These
Paleozoic sediments are mostly fine-grained mudstones to
siltstones; they contain rare sandstones with detrital zircon
U/Pb SHRIMP ages indicative of their provenance from
the East European Craton (clear dominance of zircon ages:
1756–2050, 1485–1510, and 974–1227 Ma over zircons
from peri-Gondwanian terranes: 559–623 Ma and Caledo-
nian U/Pb ages: 442–495 Ma); (Poprawa et al. 2006). Rhyo-
lites from the Daszewo locality sampled during this study do

<figure><img src="image_12.png" /></figure>
not contain any inherited zircon grains; however, inherited/
xenocrystic zircon ages for Daszewo were reported by Bre-
itkreuz et al. (2007) (see Table 1; ages: 726, 1260, 2094,
2344 Ma). These ages are in a very good agreement with
the detrital ages from the sandstones. Neither this study, nor
Breitkreuz et al. (2007) show zircon ages older than 350 Ma
for rhyolites from Wysoka Kamieńska, suggesting that
detrital zircon was scarce to absence in the source of these
rhyolites. It is, therefore, probable that Paleozoic sediments
contributed variable proportions to rhyolitic magmas and
the scarcity of inherited grains in some rhyolites is due to a
dominance of clay-rich sediments being generally impover-
ished in zircon compared to coarser grained sediments such
as Carboniferous turbidites.

Therefore, based on the inherited to magmatic zircon
proportions and zircon inherited ages, we suggest that the
southern rhyolites were derived predominately from the Car-
boniferous turbidites with admixture of Baltica and/or Ava-
lonia component, whereas northern rhyolites were derived
from fine-grained sediments, poor in zircon, such as thick
Ordovician–Sylurian sampled present in the Koszalin–Cho-
jnice zone (Modliński and Podhalańska 2010).

Hafnium and oxygen isotopes: further insight
into the sources of rhyolitic magmas

Magmatic zircons from rhyolites in the Polish Lowlands
have distinctive Hf and O isotopic compositions (e.g.,
Daszewo, Wysoka Kamieńska as well as zircons from the
Chrzypsko granite). Such a range in isotopic compositions
may be surprising both considering that these rhyolites rep-
resent the same magmatic event and considering that their
morphological and geochemical characteristics are roughly
similar (Protas et al. 1995; Żelaźniewicz et al. 2016). This
implies that chemically similar magmas were derived from
isotopically variable sources and that the similar chemical
composition of rhyolites likely reflects rather prolonged
magma differentiation and it is not controlled by the source.
The Hf and O isotope composition in the zircons not only
confirms differences in the sources of the rhyolites from
northern and southern parts of the Polish Lowlands, but
also helps to distinguish additional sources in the northern
part of the Polish Lowlands. Three distinct fields of Hf vs.
O ratios (Fig. 5) suggest that different sources contributed
to the magmatic evolution in these three areas.

Southern rhyolites

The composition of zircon from rhyolites of the southern
part of the Polish Lowlands seems to overlap with that of
zircon from the Chrzypsko granite; however, the southern
rhyolites are represented only by two magmatic grains from
the Pniewy ignimbrite (our sample of Chrzypsko rhyolite

contained only inherited and xenocrystic zircons with no
magmatic ones). Interestingly, the O isotopes for all the
Variscan-aged grains (320–380 Ma) also overlap with
the compositional field defined by the Chrzypsko granite
(Fig. 5). Therefore, it seems that a single source dominated
the production of the southern rhyolites and it was similar
in composition to the Variscan magmas which are repre-
sented by the Chrzypsko granite and the Pniewy rhyolites.
As Variscan-aged zircon grains are clearly found within the
source of the Chrzypsko and Pniewy rhyolites, it is possi-
ble that the crustal component of this rhyolite magma was
primarily derived from a fore-arc deposit, which formed
due to the erosion of various Variscan igneous rocks in a
hypothetical continental arc setting (Mazur et al. 2010). A
derivation from a sedimentary source would explain the
relatively high δ¹⁸O values in the zircons from rhyolites in
the southern part of Polish Lowlands as compared to the
other Late Paleozoic rhyolites (e.g., Wysoka Kamieńska),
but also compared to the Halle Volcanic Complex in east-
ern Germany (Słodczyk et al. 2016). Such high δ¹⁸O values
probably reflect a weathering of an igneous source before
rhyolite magma formation, but because the sediment was
deposited soon after erosion, it did not achieve typical sedi-
mentary δ¹⁸O values (Valley et al. 2005). Also the Hf isotope
values for the granite and rhyolites in the Polish Lowlands
are lower than those observed in most of the zircons derived
from igneous rocks of the Bohemian Massif, which have εHf
values from approximately –5 to 0 (e.g., Siebel and Chen
2009; Pietranik et al. 2011). This reflects the addition of
low εHf materials to the southern rhyolites from the Polish
Lowland, which suggests a larger contribution of sediments
to the rhyolitic magmas compared to that of the mostly plu-
tonic rocks in the Bohemian Massif.

Northern rhyolites

The possible sources for the northern rhyolites are more dif-
ficult to constrain. The lack of inherited grains may indicate
scarce zircon in the source of magma, and such a source
could be the Early Paleozoic, clay-rich sediments described
by Podhalańska and Modliński (2006). However, it is clear
that Daszewo and Wysoka Kamieńska represent two differ-
ent compositional types of rhyolite derived from isotopically
different sources. In detail:

* The high δ¹⁸O and low εHf values for the Daszewo zir-
cons suggest the involvement of (upper) crustal material
of sedimentary origin as the source for these magmas
(Fig. 6). The (meta-) sediments source is supported by
the interpretation of electromagnetic data refraction
within Pomeranian unit (Ernst et al. 2008) and may be
correlated with Early Paleozoic siliciclastic rocks such as
those described in the borehole in the Koszalin–Chojnice
**Fig. 6** Comparison of δ¹⁸O vs. εHf values for the Polish Lowlands samples and magmatic zircons from the NE German Basin (Pietranik et al. 2013a). The typical ranges in the isotopic ratios have been indicated

<figure><img src="image_13.png" /></figure>

area (Podhalańska and Modliński 2006). Generally, high δ¹⁸O values in combination with low εHf values suggest that the magma source might have been an old repeatedly recycled sediment, and as such, fits the characteristics of a typical passive margin deposit. Moreover, high δ¹⁸O may record the influence of crustal brines interacting with the source rocks (Ernst et al. 2008), elevating their δ¹⁸O as evaporates could have δ¹⁸O as high as 20–30 delta units (Valley et al. 2005). The implication for such a source for the Daszewo rhyolites is consistent with the location of the drill hole over the attenuated margin of Baltica (Poprawa 2006; Żelaźniewicz et al. 2016; Mazur et al. 2017, Fig. 7b).

* Wysoka Kamieńska is characterized by lower δ¹⁸O and εHf values compared to Daszewo, which may correspond to more mafic components contributing to the source of these rhyolites (Fig. 6). In fact, both the Daszewo and Wysoka Kamieńska zircon populations plot along the same linear trend in the δ¹⁸O vs. εHf diagram (Fig. 5), suggesting a mixing of sedimentary material with mantle-derived magmas in both areas but with a larger input from the mantle-derived magmas in the Wysoka Kamieńska rhyolite compared with the Daszewo rhyolite. Therefore, we propose that the isotopic difference between compositions of rhyolites from the two locations do not necessarily reflect the difference in the source area/tectonic setting, but rather varying admixtures between volcano-sedimentary successions in the source. However, the two successions may, in fact, reflect the presence of two distinct crustal blocks in the close proximity that contributed material to pre-Permian sediments i.e. the Baltica crustal block (these sediments dominate in Daszewo) and an Avalonian accretionary prism (these sediments dominate in Wysoka Kamieńska).

It is interesting to note that each of the sampling localities plots in its own, rather limited, compositional field. As reported for large eruptable rhyolitic systems such as the Yellowstone Caldera (e.g. Stelten et al. 2015) or the German part of the Central European Basin Large Igneous Province (Pietranik et al. 2013a), the generation of such magmas involves at least two stages of melt evolution, first the extraction from a long-lived, isotopically heterogeneous crystal mush, and second the emplacement of a preeruptive, isotopically homogenized magma chamber. In such evolution scenarios chemical and isotope diversity is expected in zircon grains. The lack of such diversity, as is the case for the Polish Lowland zircons investigated here, may suggest the dominance of a single source in the formation of the rhyolites. Therefore, we stress the importance of the differences in the crustal material available for melting between the Daszewo and Wysoka Kamieńska rhyolites as well as between the northern and southern parts of the Polish Lowlands.

**Chrzypsko granite and rhyolite: are they related?**

A large silicic magmatic system may persist within the crust as a long-lived mush with several episodes of rapid remobilization, which finally can crystallize into a granitic body or, alternatively, can feed a large eruption (e.g. Burgisser and Bergantz 2011). It is also possible that such a crystal-line granitic body could be reheated from below, generating “new” portions of rhyolitic magma. The origin of voluminous high-silica ignimbrites by extraction from a shallow, batholith sized granitic magma mush is one of the possible scenarios by which such volcanic systems are generated (see Lipman and Bachmann 2015). However, it seems not to be the case for rhyolitic and granitic samples investigated here. Despite the overall similarities in the Hf and O isotope composition
**Fig. 7** a Map showing the spatial distribution of the rhyolites in the NE German Basin, which were previously analysed by Breitkreuz et al. (2007) and Pietranik et al. (2013a) in addition to the rhyolites from the Polish Lowlands which were analysed during this study. The shaded area indicates the location of the northern and southern rhyolites as discussed in the text, TTZ Teisseyre–Tornquist Zone; b schematic cross section (not to scale) showing the structure of the Trans-European Suture Zone along line A–A’ (model from Żelaźniewicz et al. 2016 modified after; Mazur et al. 2016; Smit et al. 2016). The approximate locations of the analysed samples have been indicated

<figure><img src="image_14.png" /></figure>

between the Chrzypsko granite and the rhyolites from Chrzypsko and Pniewy in the south, the granite and volcanic rocks cannot be directly related. This is in part because the rhyolites contain abundant inherited and very scarce magmatic zircon grains, while in contrast the Chrzypsko granite contains few inherited but quite abundant magmatic zircons (Table 1). The only differentiation process which could enrich an evolving granitic pluton with inherited zircons is by the assimilation of the surrounding sedimentary or metasedimentary rocks. Such assimilation may happen when rhyolitic magma ascents through thick sedimentary sequence, however the process would have been possible for Chrzypsko granite/rhyolite only if the sediment did not contaminate magma itself. Sediments with distinct composition would likely change the isotopic composition of the evolving magma, which then should be recorded in magmatic zircon chemistry; such a process was not observed for the Chrzypsko locality. However, it is possible that the magma composition changed due to assimilation, but we did not sample a new magma composition, because no real rhyolitic magmatic zircon was present. Zircons with young ages could represent antecrystic component derived from granite and, therefore, have the same Hf and O isotope composition as the granitic zircon. Alternatively, it may be that the Pniewy and Chrzypsko rhyolites were not derived by the remelting and/or re-mobilization of the Chrzypsko granite,

but rather that both the granite and rhyolites originate from a common source rich in inherited zircons. The reason for the granite to then contain fewer inherited zircons is a matter of its thermal evolution in time, if the temperature in granite decreased slowly there would be a higher likelihood for the dissolution/melting of the inherited component. In contrast, if the rhyolitic magmas cooled quickly, they may have been more likely to preserve the inherited grains.

## Comparing late paleozoic high silica magmas in the Polish Lowlands with those of the North East German Basin

As has already been established in previous sections, the rhyolites in the south contain a large number of inherited zircons, covering a wide range in ages. This is similar to the Tuchen rhyolite, which is located in the southern part of the North East German Basin (NEGB) (Fig. 7, Breitkreuz et al. 2007). This suggests that the same sedimentary source which was melted to produce the Pniewy and Chrzypsko rhyolites may have extended further to the east, for at least another 100 km. Furthermore, the Hf and O isotopic compositional fields of the southern rhyolites and the rhyolites from Penkun and Salzwedel in the NEGB (Fig. 6, Pietranik et al. 2013a) partially overlap,
suggesting that similar processes could have acted in the evolution over this broad area. Conversely, because there is a small inherited component in the rhyolites found in the northern part of the Polish Lowlands, and their Hf and O isotopic compositions do not overlap with any of the zircons from the NEGB rhyolites, this implies that the rhyolites in the north formed under different conditions to those in the both south and within the NEGB—this might be due to a differences between the sediments that melted to form these rhyolites. One feature common to both the rhyolites in the Polish Lowlands and the NEGB are the magmatic zircons with $\delta^{18}$O values consistently > 6‰. Such relatively high $\delta^{18}$O values are also present in other large rhyolitic eruptions worldwide (e.g., Neogene Central Andean Ignimbrite Province; Folkes et al. 2013). However, many rhyolite localities preserve zircon with $\delta^{18}$O of less than 5‰, which has been interpreted as reflecting extensive remelting/recycling of hydrothermally altered intracaldera rhyolites (e.g., Snake River Plain in Idaho; Bindeman et al. 2008; Watts et al. 2011; Drew et al. 2013). The apparent absence of any hydrothermal alteration in the source of Andean rhyolites may have been related to climatic and geographical factors such as high elevation prior and during the eruption and lack of meteoric water input into lithosphere due to an arid climate. However, the Permian basin developed in an extensional setting and topographically elevated forms, even if they had been present, would not be expected to influence the climate to such a degree as to cause aridity of the region (Geißler et al. 2008). Therefore, we suggest that the lack of low $\delta^{18}$O zircon in the case of both Polish Lowland and NE German Basin could be related to either the nature of the specific sources in conjunction with a large contribution of sedimentary material or to an arid climate specific for Lower Permian period (Gibbs et al. 2002; Roscher and Schneider 2006).

## Conclusions

Late Paleozoic rhyolites from the Polish Lowlands offer insight into the composition and structure of the crust which makes up the Trans-European Suture Zone in Poland. Hf and O isotopic compositions of zircon, as well as the distribution of zircon ages show that the rhyolitic magmas were derived from three different sources: (1) older (Ordovician–Silurian) sediments in the case of Daszewo; these sediments were probably deposited on the passive margin of Baltica predominately as mudstone–siltstone sequences (cf. Modliński and Podhalańska 2010); (2) a mixed mantle or mafic crust-sediment source for the Wysoka Kamieńska rhyolites, perhaps mixing between magmas derived from the mudstone–siltstone

source similar to that implied for Daszewo and mafic arc rocks (an Avalonian accretionary prism); and (3) a younger, Variscan-aged coarser sediments for the Chrzypsko and Pniewy rhyolites, deposited on the Variscan foreland (Fig. 7), which could be related to Carboniferous greywackes (cf. Mazur et al. 2010). The rhyolites from the Polish Lowlands correlate with occurrences of similar rocks within the NE German Basin. Thus, during the Late Paleozoic, there was large scale magmatism in central Europe, and its composition was controlled by regional scale differences in the basement lithologies.

**Acknowledgements** We would like to thank Professor Teresa Oberc-Dziedzic for providing the Chrzypsko samples and Geonafta in Piła for providing the other samples. F. Couffignal was responsible for our SIMS data acquisition; U. Dittmann helped in producing the SIMS mounts; D. Rhede and F. Wilke helped with microprobe analyses. ES acknowledges financial support from the Polish National Science Centre Grant No. UMO-2013/09/N/ST10/00655.

**Open Access** This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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