Three age ranges of Cenozoic basaltic rocks from Lower Silesia (SW Poland) based on 40Ar/39Ar step-heating data

The precise ages of Cenozoic basaltic rocks from 20 localities (24 samples) in south-western Poland were studied by means of the 40Ar/39Ar isotope method. Three phases of volcanic activity were identified in this area. The older, Ruphelian phase took place 31–29 Ma ago and the younger, Aquitanian phase occurred 22–21 Ma ago. Significantly younger ages (4.8–4.6 Ma: Zanclean phase) were obtained for the basalts cropping out in the vicinity of Lądek Zdrój only. Most of the analyzed basalts from the Ruphelian phase are classified as nephelinites. They occur in the area of Lausitz Masif, south of Sudetic Marginal Fault and in the Opole Depression. The volcanic rocks from the Aquitanian phase crop out in the wide area north of the Sudetic Marginal Fault. They are composed of alkali basalts. The youngest phase is represented by the basanites only. The deep discontinuous tectonic structures intersecting the areas of volcanic activity in SW Poland, parallel or oblique to the tectonic stress directions previously reconstructed for the Oligocene and Miocene in the Bohemian Massif, were most probably reactivated by an extension approximately parallel or slightly oblique to them, thus opening pathways for the migration of basaltic magma in the Sudetes and their foreland.


Introduction
Cenozoic volcanic rocks are common in Central Europe and crop out in many places in France, Germany, the Czech Republic, Hungary, and Poland. The alkaline magmatism of this age in the Bohemian Massif and its northern foreland forms the easternmost part of the so-called Central European Volcanic Province (CEVP) of Wimmenauer (1974). The volcanic fields are grouped here in the basement of lithospheric terranes that were affected by tectono-thermal events 300-400 Ma ago (e.g., Franke and Żelaźniewicz 2000). They are characterized by higher heat flow and thinner lithosphere than the surrounding larger cratons (Meier et al. 2016). According to Wilson and Downes (2006), the Cenozoic reactivation of Permian/Carboniferous fault systems in the area of CEVP occurred, thanks to the superposition of tectonic stress related to the collision of Iberia with the rest of Europe and tectonic stress transmitted from the front of the Alpine collision. Changes of regional tectonic stress were of key significance for opening the pathways of magma migration (Wilson and Downes 2006;Ulrych et al. 2011). The direction of maximum horizontal stress in Central Europe rotated from the NNE-SSW to the NNW-SSE during the Late Eocene/Early Oligocene, and in the Late Oligocene towards NW-SE (Schreiber and Rotsch 1998).
The magmatic activity in the whole CEVP was associated with a rift system about 1100 km in length. It has been traditionally linked with the modification of lithospheric mantle and existence of mantle plumes (Wilson and Downes 1991;Puziewicz et al. 2011). However, this model was not fully supported by geophysical data. Therefore, it was modified to obtain the so-called "hot fingers" model (Wilson and Patterson 2001), where local passive elevations of the partially melted upper mantle did not have to be linked with 1 3 anomalies of the geothermal field even though high-resolution seismic tomography indicates a positive spatial correlation between the places where Cenozoic volcanic fields occur in Central Europe and the decreasing thickness of the lithosphere (Meier et al. 2016). Piromallo et al. (2001) link the occurrence of the Cenozoic volcanic belt in the foreland of the Alpine orogen with the zone of the deeply buried oceanic lithosphere. On the other hand, Wilson and Patterson (2001) associate phases of magmatic activity with periods of the rising Icelandic mantle plume. The accurate geochronology of the particular volcanic fields of the CEVP is obviously very important for the verification of any of these models.
According to the existing data, the exact time the main phases of volcanic activity ended varies in different parts of the CEVP. Relying on K/Ar isotope ages, Badura et al. (2005) indicate 21 Ma as the lower limit for the main stage of basaltic volcanism in the Lower Silesia. The same time is assumed for this magmatism in the Elbe zone (Ulrych et al. 2008), Vogtland-Mariańskie Łaźnie Fault Zone (Haase and Reno 2008) and the Westerwald (Haase et al. 2004). A slightly younger age, i.e., 19 Ma was estimated for the volcanic field of the Upper Palatinate (Rohrmüller et al. 2005). This age is significantly older in the Hohe Eifel area only, where the main volcanic activity ended 35 Ma ago (Fekiacowa et al. 2007). In many smaller volcanic domains of Germany and Moravia, the extinction of volcanic activity took place 11-18 Ma ago, but in the whole area of the CEVP there are further individual magmatic bodies, which are younger, with an age even below 0.1 Ma (West and East Eifel; Jung et al. 2012).
In the Bohemian Massif, the following three main periods of Cenozoic volcanic activity were distinguished and linked with the geochemical and mineralogical signature of volcanic rocks, and with the succession of paleostress (Ulrych et al. 2011): • pre-rift period (79-49 Ma) with a compressional stress field; • syn-rift period (42-16 Ma) with a tensional stress field; • late-rift period (16-0.26 Ma), subdivided into Mid-Late Miocene episode (16-6 Ma) with a compressional stress field, Late Miocene-Early Pleistocene episode (6-0.9 Ma) with a tensional stress field, and Early-Late Pleistocene episode (0.9-0.26 Ma) with a compressional stress field.
The most extensive volcanic activity took place here during the syn-rift period when a tectonic graben formed under a N-S tensional stress field (34-24 Ma ago). This activity ended under a NW-SE tensional stress field in the early to mid-Miocene (Adamovic and Coubal 1999;Rajchl et al. 2009;Ulrych et al. 2011Ulrych et al. , 2016. In the Lausitz Volcanic Field, 18 out of 22 Ar/Ar-dated eruptions took place between 34.9 and 28.0 Ma, and 13 of them are tightly clustered between 31.2 and 30.1 Ma (Büchner et al. 2015). This area stretches up to the westernmost part of the region of this study.
The Cenozoic volcanic rocks in Lower Silesia are mainly associated with the deep-seated faults of the Labe-Odra fault system. The westernmost domain of volcanic activity in the Polish part of the Sudetes is situated at the extension of the NE-oriented Ohře Rift Graben that was dissected by the Lusatian Fault (Ulrych et al. 2011). The intrusions of basaltoids are also located in the Opole Depression filled with Cretaceous strata. The basaltic and trachybasaltic volcanoes of Lower Silesia display a diversity of eruptive styles, including effusive and variably explosive eruptions (Awdankiewicz et al. 2016). Most of the volcanic intrusions from SW Poland were dated using the K/Ar method (Birkenmajer et al. 2002a(Birkenmajer et al. , b, 2004Badura et al. 2005;Kasiński et al. 2015). The obtained isotope ages were juxtaposed with the magnetic polarity record (Birkenmajer et al. 2002b(Birkenmajer et al. , 2004. Most of the reversed and normal polarities linked with the K/Ar ages defined for the same samples do not fit the Global Polarity-Time Scale (Gradstein et al. 2012). It means that most of the K/Ar ages cannot be correlated with the time of emplacement of these rocks. Nearly, all the K/Ar ages from SW Poland are distributed evenly between twenty and thirty Ma. Such a distribution of ages does not support the thesis about two phases of volcanic activity (21-24.5 Ma and 31.3-33.7 Ma) as postulated by Birkenmajer and Pecskay (2002) and Birkenmajer et al. (2002aBirkenmajer et al. ( , b, 2004. According to Badura et al. (2005) the K/Ar ages of individual basalts older than Oligocene are not reliable.
The aim of this paper is to present and discuss the newly obtained 40 Ar/ 39 Ar isotope ages of basalts from SW Poland. The tectonic frames that could determine the magma emplacement will also be shown.
The lithology of the volcanic rocks from the Zgorzelec area is diverse. Basanites (Sulików quarry), nephelinites (Opolno Zdrój and Bukowa Góra quarry) as well as trachytes can be distinguished (Jasna Góra). Trachytes are fine-grained with fluidal textures emphasized by the orientation of the elongated sanidine crystals (Fig. 3A). The alkali feldspar dominates in the microlithic groundmass of these rocks. The basanites are porphyritic as they contain phenocrysts of olivine and clinopyroxene. Their microlithic groundmass comprises of olivine, clinopyroxene, plagioclases and titanomagnetite (Fig. 3B). The olivine crystals are slightly altered. The olivine nephelinites are porphyritic as they contain olivine and clinopyroxene phenocrysts exceeding 2 mm in length. Nepheline, olivine and clinopyroxene crystals, titanomagnetite and, rarely, Cr-spinels and apatite occur within the groundmass. All samples from the Opole area are porphyritic olivine nephelinites (Fig. 3C, D). The olivine and clinopyroxene crystals are in the form of phenocrysts, which exceed 4 mm in length, as well as microcrystals. Additionally, nepheline and titanomagnetite crystals are also observed within groundmass.
Most samples from outcrops located close to the Sudetic Marginal Fault are alkali basalts ( Fig. 3E-G). Only two of them (from Grodziec and Wilcza Góra) are olivine nephelinite. The alkali basalts display a high textural diversity ranging from massive to amygdaloidal. Most of them are porphyritic. The fluidal texture is rarely emphasized by the orientation of plagioclase laths. All types of basalts from this area contain a very similar mineral assemblage. They consist of phenocrysts of olivine and clinopyroxene (up to 3-4 mm in length). The fine-grained or microlithic groundmass contains olivine, clinopyroxene, plagioclase and, as accessory minerals, apatite and titanomagnetite.
All samples from the Zgorzelec, Opolno and Lądek Zdrój area fall within the basanite field in the total alkalis versus silica classification diagram (TAS, Fig. 4A). The sample from Jasna Góra that falls within the trachyte field is the only exception. The samples taken close to the Sudetic Marginal Fault fall within the basanite, trachybasalt and basalt field. According to a diagram by Winchester and Floyd (1977), almost all samples from these exposures can be classified as alkali basalts (Fig. 4B).
The basaltic rocks from Lower Silesia commonly contain mantle and lower crustal xenoliths (e.g., Kukula et al. 2015;Ladenberger et al. 2009;Matusiak-Małek et al. 2021). The least altered pieces of volcanic rocks without xenoliths, megacrystals, and crystal aggregates were chosen for isotopic dating. The selected parts of rock were prepared according to the standard procedure for the whole-rock 40 Ar/ 39 Ar geochronology. Whole-rock analyses have been carried out at the 40 Ar/ 39 Ar Geochronological Laboratory at the University of Lund, Sweden, using a Micromass 5400 mass spectrometer with a Faraday and electron multiplier. The samples were irradiated together with the TCR sanidine standard (28.34 Ma; according to Renne et al. 1994) for 24 h at the Oregon State research reactor. J values were calculated with a precision of < 0.25% and are reported for each sample in the data tables. The decay constants were adopted from Steiger and Jäger (1977). More details of the analytical procedure applied are presented by Dyhr et al. (2013), Dalrymple and Lamphere (1971). The age plateaus were determined using the criteria summarized by Baksi (2006) indicating the presence of at least three continuous incremental heating steps with statistically indistinguishable ages and constituting more than 50% of the total 39 Ar released during the experiment. In some places, where a statistical overlap of steps was not obtained, a forced-fit age is given over a certain percentage of gas. The 40 Ar/ 39 Ar geochronology data were elaborated, plotted and fitted using the Fig. 3 Representative petrographic features of studied rocks. A Trachyte; trachytic texture emphasizes the orientation of sanidine crystals (Jasna Góra outcrop; crossed polarized light), B basanite (Sulików quarry; BSE image), C olivine nephelinite with phenocryst of olivine (Gracze quarry; crossed polarized light), D olivine nephelinite with fine-grained groundmass containing clinopyroxene, nepheline, olivine and titanomagnetite crystals (Gracze quarry; BSE image), E alkali basalt with olivine and clinopyroxene phenocrysts (Żelazów; crossed polarized light), F alkali basalt with olivine and plagioclase phenocrysts within fine-grained groundmass containing plagioclase, olivine, titanomagnetite and apatite crystals (Bazaltowa Góra; BSE image), G alkali basalt with olivine and clinopyroxene phenocrysts within fine-grained groundmass containing plagioclase, olivine and titanomagnetite crystals (BSE image), H basanite with olivine and clinopyroxene phenocrysts within fine-grained groundmass containing plagioclase, nepheline, olivine, clinopyroxene and titanomagnetite crystals (BSE image). Symbols: BSE image-backscatters electron image; Ap-apatite, Cpx-clinopyroxene, Nenefeline, Ol-olivine, Pl-plagioclase, Sa-sanidine, Ti-Mgttitanomagnetite. BSE images were performed at the Microprobe Analysis Laboratory (PGI-NRI) in Warsaw using a LEO 1410 scanning electron microscope ◂ Fig. 4 A Chemical classification of the studied rocks in the total alkalis versus silica (TAS) diagram of Le Maitre et al. (1989). B Chemical classification of mafic intrusions using the Zr/TiO 2 vs. Nb/Y diagram (Winchester and Floyd 1977) 1 3

Age ranges and spatial distribution of volcanics
The 40 Ar/ 39 Ar ages of basaltic rocks from SW Poland are grouped in three ranges with a focus around 9, Table 2). For the second group, a slightly younger plateau age (19.2 ± 0.5 Ma) occurs only at the Męcinka locality (Fig. 6). The Pliocene ages were obtained for the basanites cropping out in the vicinity of Lądek Zdrój only. The new ages compared with the K/Ar ages obtained earlier by Birkenmajer et al. (2002aBirkenmajer et al. ( , b, 2004 and Badura et al. (2005) in the same localities differ from each other up to 6 Ma. Birkenmajer et al. (2004Birkenmajer et al. ( , 2007 checked the credibility of K/Ar ages from the Lower Silesia basalts by their comparison with magnetostratigraphy. They also repeated dating of the same lava flow. Results of these studies indicate that the magnetic polarities of most of dated samples do not correlate with the coeval parts of . It should be also noted that the precision of an 40 Ar/ 39 Ar plateau age generally is better than the precision of K-Ar age because the plateau age is calculated by pooling the ages of several gas increments (e.g., Renne 2006).
Bearing in mind the geologic time scale (Gradstein et al. 2012), three phases of Cenozoic volcanic activity in SW Poland can be distinguished, namely, in stratigraphic order, the Ruphelian, the Aquitanian and the Zanclean (Fig. 8). The basalts from each phase are almost always linked with a particular area and specific geochemical features. Also the time of emplacement of rocks from an individual phase can be correlated with a characteristic period of tectonic stress field regime identified in this part of Europe (Ulrych et al. 2011). Except for the trachytes from Jasna Góra and basanites from Sulików, all the studied basalts representing the Ruphelian phase are classified as nephelinites. With the exception of the rocks from Grodziec, they occur near Zgorzelec, i.e., at the extension of the NE-SW-trending Ohře Rift Graben (Fig. 1), and close to Opole, where NE-SW-oriented structures of the Moravian Tectonic Zone occur in the basement. This zone separates the Brunovistulian Terrane from the tectonic blocks accreted during the Variscan orogeny (e.g., Franke and Żelaźniewicz 2000). Both basaltic intrusions in Grodziec and Wilcza Góra are located not far from each other and south of the Sudetic Marginal Fault (Fig. 1B). They also have the same composition of nephelinites, but the Wilcza Góra sample yielded a younger, i.e., Aquitanian isotope age. However, the plateau diagram with the wholerock 40 Ar/ 39 Ar age presented for this sample is complicated due to the excess of 40 Ar in the high steps, and 39 Ar loss in the lower steps cannot be definitely excluded (Fig. 6, Online Appendix 1).
Outcrops of volcanic rocks from the Aquitanian phase occur in the vast area near Strzelin, Strzegom and Złotoryja, north of the Sudetic Marginal Fault (Fig. 9A) which is approximately perpendicular to the Ohře Rift Graben. Except for the above-mentioned nephelinite from Wilcza Góra, located south of this fault, they are composed of alkali basalts. Basalts from the Zanclean phase cropping out near Lądek Zdrój are composed of basanites only. Assuming that all stages of evolution of tectonic stress distinguished by Ulrych et al. (2011) for Bohemia are also valid in SW Poland, we can conclude that the Oligocene basanites were emplaced under a N-S tensional stress field and the intrusions of the Miocene alkali basalts were set up under a NW-SE tensional stress field (Fig. 9B). The NW-SE direction of stress corresponds well to the course of the main tectonic faults occurring in the area of Miocene volcanic activity (Fig. 1). The N-S directed stress is oblique with respect to the NNE-SSW-trending discontinuous tectonic structures in the area where the Oligocene basanites occur. It is possible that some of these discontinuities in both areas were reactivated by an extension approximately parallel or slightly oblique to them, opening pathways for magma migration. The ages of basalts from the westernmost part of the area under study (the Lausitz Massif) and the Opole Depression fit very well with the 40 Ar/ 39 Ar age spectrum obtained for the basaltoids from the German part of the Lausitz Volcanic Field (Büchner et al. 2015). The volcanic rocks from the Aquitanian phase are common in many areas of the CEVP (Haase et al. 2004;Haase and Reno 2008;Ulrych et al. 2008;Büchner et al. 2015). Slightly younger ages of basaltoids (18-19 Ma), for example in the Marcinka locality, were obtained only in a few locations in the CEVP only (Rohrmüller et al. 2005).