Polyphase Permo-Carboniferous magmatism adjacent to the Intra-Sudetic Fault: constraints from U–Pb SHRIMP zircon study of felsic subvolcanic intrusions in the Intra-Sudetic Basin, SW Poland

Abstract

The SHRIMP U–Pb dating of zircons from felsic subvolcanic rocks in the Carboniferous formations of the Intra-Sudetic Basin in SW Poland verifies previous views on the timing of Late Palaeozoic magmatic activity in this area and constrains the links between regional tectonics and intra-basinal volcanism in the mid-European Variscides. Two main stages of magmatism are identified: the Late Carboniferous late orogenic stage, and the Early Permian post-orogenic stage. The Carboniferous late orogenic subvolcanic to volcanic activity was contemporaneous with the formation of the Karkonosze granite pluton to the west and comprised episodic emplacement of predominantly felsic laccoliths and sills within the basin fill during the Westphalian–Stephanian in two sub-stages at ca. 313–310 Ma and 306–305 Ma. Local folding of the Carboniferous succession peaked at ca. 303–300 Ma adjacent to the Intra-Sudetic Fault, in relation to late-stage sinistral movements along this regional wrench fault. The emplacement of minor subvolcanic intrusions at ca. 293 Ma completed the activity in the northern Intra-Sudetic Basin; this Permian post-orogenic volcanism, however, developed fully in areas further south. The predominance of subvolcanic intrusions over extrusions in Carboniferous can be linked to the trapping of rising magmas within the several kilometres thick sedimentary basin fill. Zircon xenocrysts in the Carboniferous subvolcanic rocks indicate that older crustal lithologies, possibly metamagmatic and metasedimentary rocks of dominantly Neoproterozoic–Palaeozoic ages, were involved in petrogenesis as magma sources and/or contaminants. The crustal influence on magma formation was stronger during the earlier magmatic events.

Introduction

The Intra-Sudetic Basin (ISB), situated in the northern part of the Bohemian Massif, is one of the largest Late Palaeozoic intramontane troughs in the mid-European Variscan Belt (e.g. Wojewoda and Mastalerz 1989; Dziedzic and Teisseyre 1990; Mazur et al. 2006, 2020; Opluštil et al. 2016; Botor et al. 2019, and references cited there). The volcano-sedimentary basin fill accumulated over a prolonged period of time of at least 50 Ma, from the Early Carboniferous to the Late Permian, spanning the transition from the Variscan collision to post-collisional extension. Compared to similar, neighbouring basins in the Varsicides such as the Saale Basin or the Saar-Nahe Basin (McCann et al. 2006), the succession of the ISB is the oldest and thickest and thus provides the longest and most complete record of the geological evolution of the region. The ISB developed in a complex segment of the Variscides affected by wrench tectonics along NW-trending faults (Mazur et al. 2020, and references therein), like the Intra-Sudetic Fault, which was especially influential in the earlier phases of the basin evolution in the Carboniferous. However, the complex interplay of the regional and local tectonics, sedimentation and volcanism are not constrained in detail.

Volcanism in the ISB occurred in few phases starting in the Carboniferous and culminating in the Permian (Awdankiewicz 1999a, b; Ulrych et al. 2004, and references therein). However, the onset of the volcanic activity (in the early, or in the late Carboniferous?) and its timing (two or three main stages?) have remained controversial (Kryza and Awdankiewicz 2012). This is partly due to the abundance of subvolcanic bodies and equivocal geological constraints on their emplacement ages (especially of those in the Carboniferous formations). In recent years, Opluštil et al. (2016) dated several intercalations of tonstein, tuff and rhyolite in the southwestern part of the ISB and in other basins in northern Bohemia—this enabled a significant improvement and correlation of lithostratigraphic schemes of the Permo-Carboniferous successions in these basins. However, the majority of the subvolcanic to volcanic complexes of the ISB remained undated.

The general problem addressed in this paper if the chronology and timing of Late Palaeozoic magmatism in the ISB in the regional tectonic context. New U–Pb SHRIMP zircons ages of six rock samples representative of all major subvolcanic complexes outcropping in Carboniferous deposits of the Intra-Sudetic Basin are provided. These results solve the problem of the onset of (sub)volcanic activity in the basin. Distinct volcano–tectonic episodes can be identified and placed in the regional tectonic framework of the eastern segment of the Variscan Belt. The links between subvolcanic activity and the deformation of the basin fill adjacent to the Intra-Sudetic Fault are discussed. The inherited zircons in the volcanic rocks provide also clues on the crustal materials involved in petrogenesis as magma sources and/or contaminants.

The Intra-Sudetic Basin—geological setting, evolution, lithostratigraphy and volcanism

The Sudetes are situated in Central Europe in the NE part of the Bohemian Massif (Fig. 1). The pre-Mesozoic basement of this region comprises a “mosaic” of several relatively small tectonostratigraphic units, built up of Neoproterozoic and Palaeozoic sedimentary and igneous rocks, that were variably deformed, metamorphosed and tectonically juxtaposed during the Variscan orogeny in late Devonian—early Carboniferous times (overview in Mazur et al. 2006). An important structural feature of the region are NW-trending Elbe, Intra-Sudetic and Odra fault zones, along which the Variscan orogenic pile has been affected by strike-slip displacements (Mazur et al. 2020). During the waning stages of the Variscan orogeny, in the Carboniferous and later in the Permian, granitic plutons intruded the basement complexes (Kryza et al. 2014a, b; Turniak et al. 2014; Oberc-Dziedzic et al. 2015), whereas intracontinental sedimentary basins successively developed on top of the decaying orogen, accommodating thick siliciclastic successions with volcanic intercalations (Baranowski et al. 1990; Dziedzic and Teisseyre, 1990; Awdankiewicz et al. 2014a).

Fig. 1

Tectonic scheme of the European Variscan Belt (modified from Mazur et al. 2006) with the location of the study area. Zones of the Variscan orogeny: RH Rhenohercynian, ST Saxothuringian, TB Tepla–Barrandian, MO Moldanubian, MS Moravo–Silesian, OFZ Odra Fault Zone, EFZ Elbe Fault Zone

The Intra-Sudetic Basin (Fig. 2), known also as the Intra-Sudetic Synclinorium, is a NW–SE aligned depression, nearly 70 km long and 35 km wide, fault-bounded and framed by uplifted basement blocks (Dziedzic and Teisseyre 1990; Mastalerz and Prouza 1995). It is the earliest of the late- to post-orogenic intracontinental basins in the mid-European part of the Variscan Belt and presumably represents a pull-apart basin related to the Intra-Sudetic Fault (Aleksandrowski et al. 1997). There, sedimentation started at 335–333 Ma (middle to early late Viséan; Turnau et al. 2002) and the basin fill comprises the Permo-Carboniferous late to post-orogenic succession, followed by Upper Triassic and Upper Cretaceous epi-platform cover (Dziedzic and Teisseyre 1990). This succession is weakly deformed with predominantly moderate to gentle inward dips, which developed during the polyphase tectonic inversion of the basin (e.g. Mazur et al. 2006; Botor et al. 2019, and references cited there), except local zones of more intense folding, e.g. adjacent to the Intra-Sudetic Fault.

Fig. 2

Schematic geological map of the Intra-Sudetic Basin and adjacent area (modified from Bossowski et al. 1981; Sawicki, 1988; Milewicz et al. 1989; Kodym et al. 1967) showing the distribution of Permo-Carboniferous volcanic rocks and the location of the samples used in this study. SBR Stara Białka rhyolite, NA Nagórnik andesite, TR Trójgarb rhyolite, ChRd Chełmiec rhyodacite, SLRd Stary Lesieniec rhyodacite, RGR Rusinowa-Grzmiąca rhyolite. Sample SG from the Sady Górne rhyodacite (SGRd)—details and dating published in Kryza and Awdankiewicz (2012). Other abbreviations used: ISB Intra-Sudetic Basin, ISF Intra-Sudetic Fault, JA Jabłów Anticline, KPB Krkonose Piedmont Basin, MSF Marginal Sudetic Fault, WWBVA Western Wałbrzych Basin Volcanic Association, EWBVA Eastern Wałbrzych Basin Volcanic Association

The Intra-Sudetic Fault is considered one of the most significant strike-slip dislocations along the NE margin of the Bohemian Massif (Aleksandrowski et al. 1997; Mazur et al. 2006, 2020, and references therein). The fault strikes WNW–ESE, extends for c. 300 km parallel to other well recognized regional dislocations (the Elbe and Odra fault zones) and separates several structural units of the Sudetes. The timing and kinematics of displacements along the Intra-Sudetic Fault remain partly controversial (op. cit.). However, Aleksandrowski et al. (1997) and Mazur et al. (2020) consider that dextral displacements of tens to hundreds kilometres during late Devonian(?) to early Carboniferous times were followed by late-stage sinistral offset of several kilometres in the late Carboniferous. These were the late movements that affected the development of the Intra-Sudetic Basin.

The stratigraphic thickness of the largely continental, siliciclastic Permo-Carboniferous formations of the Intra-Sudetic Basin exceeds 10 km, including 6.5 km of lower Carboniferous, 2 km of upper Carboniferous and 1.5 km of Permian. However, the real thickness does not exceed 4–5 km as the distribution of these deposits is asymmetric, with thickest accumulations of the older deposits in the NW and thinner, younger deposits to the SE (Nemec et al. 1982; Mastalerz and Prouza 1995). The basin fill, accumulated under a strong tectonic control (Wojewoda and Mastalerz 1989 and references therein), comprises several fining-upwards megacyclothemes dominated by conglomerates, sandstones and mudstones of alluvial origin. Marine intercalations occur in the Upper Viséan; coal measures are typical of the Upper Pennsylvanian; and lacustrine shales and limestones terminate cyclothems of the uppermost Pennsylvanian–lower Permian. The transition from greenish-grey coal-bearing Carboniferous to Permian red beds documents a gradual drying of climate (Mastalerz and Prouza 1995 and references therein).

Based on the megacyclic structure of the basin fill and biostratigraphic evidence (scarce in several rock units), an informal lithostratigraphic subdivision of the succession has been proposed (Nemec et al. 1982; Wojewoda and Mastalerz 1989; Mastalerz and Prouza 1995; Wagner 2008). In recent years precise U–Pb dating of zircons from tonstein and tuff intercalations in the southwestern part of the ISB and in other smaller basins further west, enabled verification of this lithostratigraphic scheme (Opluštil et al. 2016). This included correlation of formations from various parts of the ISB (for which different local names are in use), correlation with international chronostratigraphic scale as well as identification of several hiatus within the sequence (Opluštil et al. op. cit.). This revised lithostratigraphic scheme provides the framework for the discussion of dating results presented in this paper (details further in the text).

Sedimentation in the ISB has been accompanied by volcanic and subvolcanic activity that began in the Carboniferous and culminated in the Permian (Dziedzic 1998; Awdankiewicz 1999a, b; Ulrych et al. 2004, and references therein). Scattered outcrops of rhyodacites with minor andesites in the northern and western parts of the ISB are mainly laccoliths and sills that reflect episodic, low-volume, dominantly subvolcanic Carboniferous activity. The Permian climax of volcanism occurred across the basin and resulted in an extensive outcrop of interstratified trachyandesites, rhyolites and rhyolitic tuffs up to ca. 700 m in total thickness. Several intra-basinal centres of volcanic activity were identified, such as a maar/diatreme belt, small lava shields, a tuff ring, as well as larger structures such as an ignimbrite-related caldera (Awdankiewicz 1999a, b; Awdankiewicz et al. 2003, 2014b, and references cited there). The effusive rocks are distinguished from intrusives by associated pyroclastic deposits, by evidence of partial erosion often with related epiclastic deposits, as well as a greater lithological-petrographic variation. In contrast, the sills and laccoliths often show intrusive breccias, peperites, clastic dikes and hydrothermal alteration (e.g. silicification) along their contact zones, and the core facies of some thickest mafic intrusions petrographically grade towards diabase-type rocks. However, the interpretation of some igneous bodies as lavas or shallow-level intrusions is ambiguous due to transitional field and petrographic characteristics, suggestive of the emplacement as, e.g. cryptodomes, as well as the lack of exposures at contact zones(Awdankiewicz, op. cit.). Although volcanism in the ISB is sometimes referred to as “bimodal”, the volcanic suites are in fact polymodal, dominated by silica-rich compositions, with several distinct groups and gaps in the acidic-intermediate compositional range; basalts and related rocks are lacking. In addition, geochemical characteristics of the volcanic rocks changed with time, from the older, calc-alkaline suite of rhyodacites, andesites and basaltic andesites bearing orogenic, arc-like geochemical signatures in the Carboniferous, to the younger, mildly alkaline suite of rhyolites trachyandesites and basaltic trachyandesites with orogenic to within-plate geochemical signatures in the Permian (Awdankiewicz 1999a, b).

Based on previous work (e.g. Kozłowski 1963; Grocholski 1965; Teisseyre 1966; Nowakowski 1968; Nemec 1979) and new geological evidence, Awdankiewicz (1999a, b) distinguished three main complexes of volcanic rocks considered to reflect three stages of volcanism in the early, late and early Permian times. However, geochronological data on the volcanic and subvolcanic rocks of the ISB are scarce (Table 1). Among these results, the U–Pb SHRIMP zircon dating from rhyodacites cropping out near the base of the basin fill (Table 1) showed that these rocks are late Carboniferous sills emplaced at ca. 306 Ma, not early Carboniferous lavas. This raised doubts on the early Carboniferous phase of volcanism within the basin (Kryza and Awdankiewicz 2012). The other preliminary SHRIMP ages of selected Carboniferous and Permian volcanic rocks (Awdankiewicz and Kryza 2010a, b) together with more recent precise U–Pb zircon datings (Opluštil et al. 2016) confirm the late Carboniferous and early Permian volcanic phases, although the majority of volcanic and subvolcanic bodies remains undated until now.

Table 1 Summary of geochronological data on magmatic rocks of the Intra-Sudetic Basin published to date

Volcanic and subvolcanic rocks in the Carboniferous of the Intra-Sudetic Basin

This paper deals with (sub)volcanic rocks cropping out in the Carboniferous deposits in the northern part of the ISB (Fig. 2). Most of these rocks occur in the area of the Wałbrzych trough, a late Carboniferous intra-basinal depositional centre (e.g. Grocholski 1965; Nemec et al. 1982), adjacent to the presumed strike of the Intra-Sudetic Fault. Geologically distinct volcanic belts along the western and eastern margins of the Wałbrzych trough were distinguished as the Western Wałbrzych Basin Volcanic Association, and the Eastern Wałbrzych Basin Volcanic Association, respectively (Awdankiewicz 1999a). The western group of outcrops is considered as a syntectonic, subvolcanic to volcanic silicic complex, emplaced in the late Carboniferous times (Westphalian–Stephanian; Moscovian to Gzhelian), in relation to local deformation at the western margin of the Wałbrzych trough (Grocholski 1965; Awdankiewicz 1999a, 2004). The main tectonic structure in this area is the SW-verging Jabłów anticline with locally overturned strata on its SW limb (Grocholski 1965; Bossowski and Czerski 1987, 1988). Less intense folding is recognized adjacent to volcanic bodies S-SE of the Jabłów anticline. The main magmatic units in this group (Figs. 2 and 3) are the Trójgarb rhyolite laccolith, the Chełmiec rhyodacite laccolith and the Stary Lesieniec rhyodacite lava (cryptodome ?). The Trójgarb rhyolite is entirely emplaced within lower Carboniferous deposits. The Chełmiec laccolith, higher in the succession, extends up into the upper Carboniferous Żacler Formation but its top is eroded. Further up-sequence, the Stary Lesieniec rhyodacite stratigraphically represents the north-westernmost termination of the Glinik Formation; the siliciclastic members of this formation wedge out in the considered area. In contrast with the lithologically monotonous, massive and clearly intrusive Trójgarb and Chełmiec massifs, the Stary Lesieniec rhyodacite shows locally brecciated top, and felsic volcanic rocks fragments occur in laterally equivalent sedimentary rocks of the Glinik Formation; these features support interpretation of the Stary Lesieniec rhyodacite as an extrusive body (Grocholski 1965; Awdankiewicz 1999a). In addition, the Stary Lesieniec rhyodacite is overlain, with a hiatus as well as with erosional and angular unconformities, by the Westphalian–Stephanian (Moscovian–Gzhelian) Ludwikowice Formation (cf. Grocholski 1965; Bossowski and Czerski 1987; Awdankiewicz 1999a, b, 2004). Samples for dating reported in this study were collected from the three largest subvolcanic to extrusive units in this group, i.e. the Trójgarb rhyolite, the Chełmiec rhyodacite and the Stary Lesieniec rhyodacite (Fig. 1). The various forms and stratigraphic position of the discussed volcanic units are summarized in a schematic log (Fig. 3).

Fig. 3

Schematic log, showing the distribution and various geological forms of volcanic/subvolcanic rocks in the Permo-Carboniferous succession of the northern part of the Intra-Sudetic Basin, together with geological position of the samples collected for SHRIMP dating. BBA Borówno basaltic andesite, ChRd Chełmiec rhyodacite, NA Nagórnik andesite, RGR Rusinowa–Grzmiąca rhyolite, SBR Stara Białka rhyolite, SGRd Sady Górne rhyodacite, SLRd Stary Lesieniec rhyodacite, TR Trójgarb rhyolits. The scheme includes also sample SG, dated in a previous study (Kryza and Awdankiewicz 2012)

A minor component of the WWBVA are mafic intrusive veins; their temporal relationship to the dominant felsic subvolcanic rocks is unclear. Altered lamprophyre dykes crop out near the Chełmiec and Trójgrab intrusions (Awdankiewicz 2007) and basaltic andesites were recognized in drill cores in Žacléř Formation further west (the Borówno basaltic andesites; Awdankiewicz 1999a, b). This group of rocks includes also the geochemically related Nagórnik andesites cropping out further north, near the northern margin of the ISB (Fig. 2). These andesites comprise three sills, 0.5–3.5 m thick, in the conglomerates of the Nagórnik Formation, at the base of the ISB fill (Fig. 3). The andesites were considered so far syn-sedimentary intrusions, contemporaneous with their host deposits, mainly based on textural evidence of magma-sediment interaction in the contact zones(Nowakowski and Teisseyre 1971). Sample for dating was collected from the thickest sill. Noteworthy, the presumed lower Carboniferous lavas reinterpreted as upper Carboniferous sills (Kryza and Awdankiewicz 2012) crop out at a very similar geological position a few kilometres east.

A spatially separate outcrop, although also near the base of the ISB fill, is the Stara Białka rhyolite at the western limb of the ISB (Figs. 2 and 3). This rhyolite occurs within the lower Carboniferous (Viséan) Lubomin and Szczawno Formations (Szałamacha 1961; Szałamacha and Szałamacha 1991). Although detailed studies are lacking and contact zones are not exposed, the outcrop pattern and lithological monotony of the rhyolite suggest that it is a 100–200(?) m thick, semi-conformable, lensoidal intrusive body, possibly a laccolith. Sample for SHRIMP dating was collected from the southern part of the outcrop.

The Eastern Wałbrzych Basin Volcanic Association comprises two lithological units, the Rusinowa–Grzmiąca rhyolites and the Rusinowa–Grzmiąca trachyandesites (Nemec 1979; Awdankiewicz 1999a). These rocks occur in a NNW-trending belt of ca. ten partly overlapping diatremes, known as the Rusinowa–Grzmiąca belt. The belt obliquely transects the late Carboniferous sedimentary formations along the eastern margin of the Wałbrzych trough. The diatremes are filled mainly with rhyolitic tuff of pyroclastic fall, surge and flow origin, with intercalations of reworked volcanogenic and non-volcanic deposits (Grocholski 1965; Nemec 1979, 1981; Awdankiewicz 1999a, 2004; Awdankiewicz et al. 2010). The tuffs are cut by rhyolite and trachyandesite intrusive veins and plugs, but lava domes or cryptodomes can also be identified. The Rusinowa–Grzmiąca belt is interpreted as eroded remnants of maar-type volcanoes (Nemec 1979, 1981), of controversial early Permian age (e.g. Bossowski et al. 1994) or late Carboniferous age (Grocholski 1965; Nemec 1979, 1981; Awdankiewicz 1999a, 2004). The sample for dating reported in this paper was collected in the southern part of the Rusinowa–Grzmiąca belt, from a rhyolite plug culminating at Jałowiec Hill which, from cross-cutting relationships, may be considered as one of the youngest intrusions in the belt.

Methods

Rock samples for zircon separation were studied in thin sections using the petrographic microscope and the identification of their usually fine-grained and variably altered mineral components was confirmed using the electron microprobe (however, data on mineral chemistry, not essential for the present study, are not reported here in detail). Zircon separates were prepared at the University of Wrocław, Institute of Geological Sciences. The samples, 4–5 kg in weight, were crushed, sieved and the 0.06–0.25 mm heavy fraction separated using a standard procedure with heavy liquid (sodium polytungstate) and magnetic separation. Zircons were handpicked under the microscope, mounted in epoxy and polished. Transmitted and reflected light photomicrographs were made along with CL images to select grains and choose sites for analysis.

The Sensitive High Resolution Ion Microprobe (SHRIMP II) in the Beijing SHRIMP Center, Chinese Academy of Geological Sciences, was used for zircon analysis. The U–Pb analyses were performed applying a secondary electron multiplier in peak-jumping mode following the procedure described in Williams (1998). A primary beam of molecular oxygen was employed to bombard zircons to sputter secondary ions. The elliptical analytical spots had a size of 25 × 30 μm, and the corresponding ion current was 4 nA. The sputtered secondary ions were extracted at 10 kV. A 80 μm wide slit of the secondary ion source, in combination with a 100 μm multiplier slit, allowed mass-resolution of M/ΔM ≥ 5000 (1% valley) so that all the possible isobaric interferences were resolved. Two-minute rastering was employed before each analysis to remove the gold coating and possible surface common Pb contamination. The following ion species were measured in sequence: ¹⁹⁶(Zr₂O)–²⁰⁴Pb–background (ca. 204 AMU)–²⁰⁶Pb–²⁰⁷Pb–²⁰⁸Pb–²³⁸U–²⁴⁸ThO–²⁵⁴UO with integration time ranging from 2 to 20 s. Four cycles for each spot analyzed were acquired. Each fifth measurement was carried out on the zircon Pb/U standard TEMORA 1 (Black et al. 2003) with an accepted ²⁰⁶Pb/²³⁸U age of 416.75 ± 0.24 Ma. The 91,500 zircon with a U concentration of 81.2 ppm and a ²⁰⁶Pb/²³⁸U age of 1062.4 ± 0.4 Ma (Wiedenbeck et al. 1995) was applied as a “U-concentration” standard.

The collected results were then processed with the SQUID v1.12 (Ludwig 2005a) and ISOPLOT/Ex 3.22 (Ludwig 2005b) software, using the decay constants of Steiger and Jäger (1977). The common lead correction was done using measured ²⁰⁴Pb according to the model of Stacey and Kramers (1975). The ages given in text, if not additionally specified, are ²⁰⁶Pb/²³⁸U dates. The errors in the text and tables are at 1σ level for individual points, and at 2σ level in Concordia diagrams.

Petrographic characteristics of the rock samples

Six samples were selected for the SHRIMP dating reported in this study (Table 2). The location of sampling sites and the geological position of samples are illustrated in Figs. 2 and 3. All samples were collected from internal parts of shallow-level intrusive to extrusive bodies. Most samples are massive, coherent rocks typical of core facies except sample 773 which shows a welded fragmental texture (cf. McPhie et al. 1993). A pronounced alteration is typical of all samples, as evidenced by the replacement of primary igneous phases by post-magmatic, low-T minerals. The characteristic petrographic features are illustrated in Fig. 4.

Table 2 Samples dated in the present study

Fig. 4

Photomicrographs of the studied volcanic rocks. a Sample 783, Stara Białka rhyolite; crossed polarizers. Phenocrysts of biotite, partly chloritized, and plagioclase, strongly sericitized, in a microcrystalline groundmass dominated by anhedral quartz and alkali feldspar. b Sample 710, Stary Lesieniec rhyodacite; plane-polarized light. The photo shows phenocrysts of albitized plagioclase chloritized hornblende(?) are set in microcrystalline groundmass of albitized plagioclase, quartz and alkali feldspar with minor chlorite, calcite and opaques. c Sample 709; Chełmiec rhyodacite, crossed polarizers. Phenocrysts of variably altered plagioclase and smaller chlorite pseudomorphs after pyroxene and/or hornblende in a microcrystalline groundmass of, mainly, albitized plagioclase, quartz and alkali feldspar. d Sample 778, Nagórnik andesite; plane-polarized light. Pseudomorphs after small phenocrysts of olivine and/or pyroxene in a microcrystalline groundmass dominated by albitized plagioclase laths. e Sample TR1, Trójgarb rhyolite; crossed polarizers. This sample consists mainly of fine intergrowths of alkali feldspars and quartz. Indistinct spherulites (arrows) as well as granophyric intergrowths (encircled) can be discerned in places. f Sample 773, Rusinowa–Grzmiąca rhyolite; plane-polarized light. K-feldspar and quartz phenocrysts in a heterogeneous, finely laminated and folded groundmass of alkali feldspar and quartz with abundant Fe-oxide staining. Mineral abbreviations: Ab albite, Bt biotite, Cc calcite, Chl chlorite, Dol dolomite, Kfs K-feldspar, Kln kaolinite, M white mica, Qtz quartz, Pl plagioclase, Src sericite

Four samples represent the older, calc-alkaline suite of the ISB (cf. Awdankiewicz 1999a, b): an andesite (sample 778), rhyodacites (709, 710) and a rhyolite (783). These samples are porphyritic, with phenocrysts typically less than 2 mm in size set in a microcrystalline groundmass. The andesite 778 contains chlorite–dolomite pseudomorphs after pyroxene and/or olivine phenocrysts with Cr-spinel inclusions, in a groundmass of albitized plagioclase laths and aggregates of kaolinite, chlorite, quartz, dolomite, calcite and Fe-oxides. Sample 709 is a phenocryst-rich rhyodacite. The phenocrysts comprise albitized plagioclase, pseudomorphs after pyroxene and/or hornblende composed mainly of chlorite and opaques, and less abundant chloritized biotite. The groundmass consists of albitized plagioclase, alkali feldspar, quartz and smaller amounts of chlorites, carbonates and Fe-oxides. Sample 710 is a phenocryst-rich rhyodacite from the westernmost part of the Stary Lesieniec extrusion. The petrographic features of this sample—with phenocrysts of albitized plagioclase and mafic pseudomorphs—are similar to sample 709, although it is distinguished by (1) a finer-grained groundmass, and (2) alignment of phenocrysts and groundmass plagioclase laths, defining a trachytic texture. Sample 783 from the Stara Białka laccolith is a porphyritic rhyolite characterized by phenocrysts of partly albitized plagioclase and partly chloritized biotite. The groundmass is microcrystalline and composed of albitized plagioclase, alkali feldspar, quartz, chlorite and opaques, locally with clots of white mica.

Samples TR1 and 773 represent rhyolites of the younger, mildly alkaline suite of the ISB (cf. Awdankiewicz 1999a, b). The rhyolite TR1 is an aphyric, felsitic rock composed of microcrystalline, poikilitic to granophyric intergrowths of alkali feldspars and quartz, with minor biotite flakes, patches of kaolinite and Fe–Ti oxides. Sample 773 is a strongly porphyritic, laminated rhyolite, which contains abundant phenocrysts of quartz and alkali feldspars up to 4 mm in size and smaller, less abundant Fe–Ti opaque microphenocrysts. The microcrystalline, felsitic groundmass shows a folded, discontinuous lamination. The texture in places resembles the eutaxitic texture of welded tuffs, with strongly stretched, recrystallized glass shards warped around phenocrysts. The specific texture of this sample may be the result of shearing of vesiculated domains in the rising rhyolitic magma, or localised fragmentation of magma and welding of fragments inside a volcanic conduit.

SHRIMP zircon study

During the SHRIMP analysis, analytical spots were mainly placed near the margins of the crystals to determine the emplacement age of the igneous rocks studied. Some spots were also located in the central parts of the crystals to check the presence of xenocrysts/inherited grains. The CL images of the zircons together with location of analytical spots are shown in Fig. 5. The SHRIMP data are given in Tables 3, 4, 5, 6, 7, and 8 and the Concordia diagrams are shown in Fig. 6. The samples are characterized below, in order of decreasing age.

Fig. 5

Cathodoluminescence (CL) images of the studied zircon crystals with the location of analytical spots and dates obtained: a sample 783, b sample 710, c sample 709, d sample 778, e sample TR1, f sample 773

Table 3 SHRIMP data for sample 783

Table 4 SHRIMP data for sample 710

Table 5 SHRIMP data for sample 709

Table 6 SHRIMP data for sample 778

Table 7 SHRIMP data for sample TR1

Table 8 SHRIMP data for sample 773

Fig. 6

Concordia diagrams for the dated samples: a sample 783 (Stara Białka rhyolite, SBR), b sample 710 (Stary Lesieniec rhyodacite, SLRd), c sample 709 (Chełmiec rhyodacite, ChRd), d sample 778 (Nagórnik andesite, NA), e sample TR1 (Trójgarb rhyolite, TR), f sample 773 (Rusinowa–Grzmiąca rhyolite, RGR)

Sample 783

Zircons in this sample are variable in morphology and textures (Fig. 5a). A characteristic population (referred to below as the main population) comprises a number of euhedral, short to normal-prismatic crystals with well-developed pyramidal faces; some of them are broken. Crystals of the main population are transparent and clear, without inclusions in transmitted light and, in CL images, are moderately bright to dark with sector zonation as well as recurrent concentric zonation. Several other crystals are subhedral-prismatic to anhedral, darker in transmitted light, with some cracks and inclusions. In CL, these crystals are dark to bright, with various styles of zonation (recurrent, or broad bands, sometimes well-defined cores, and others).

Fifteen spots in 13 crystals were analyzed (Table 3, Fig. 6a). The majority of crystals show low to moderate U contents of 130–660 ppm. One U-rich crystal (spot 12.1, Table 3), with nearly 2900 ppm of U, has been excluded from further interpretation due to possible radiation-related crystal lattice damage and disturbance of the U–Pb system in such zircon grains (e.g. Tichomirowa et al. 2019 and references cited there). ²³²Th/²³⁸U ratios range from 0.13 to 0.75, but in crystals of the main population this ratio is more restricted, from 0.21 to 0.34. Eight analyzed crystals of the main population range in ²⁰⁶Pb/²³⁸U age from ca. 299 to 318 Ma and the mean Concordia age of this group is 313.0 ± 5.4 Ma. The other crystals, texturally and chemically more variable, include three zircons with ages of 1.8–1.9 Ga and one dated at 665–679 Ma at core and 607 Ma at rim.

The age of 313.0 ± 5.4 Ma, defined by the main population of zircons, is interpreted as the crystallization and emplacement age of the Stara Białka rhyodacite magma. This age corresponds to the middle upper Carboniferous (Pennsylvanian–Moscovian). The older zircons with Palaeo- and Neoproterozoic ages, apparently are xenocrysts either derived from crustal magma sources/contaminants or picked up from country rocks upon the emplacement of the rhyolite magma.

Sample 710

The majority of zircon crystals in sample 710 (Fig. 5b) are euhedral to subhedral, normal to short-prismatic (elongation 2–3). Long-prismatic crystal are also found and most of them are broken. There are also rounded and anhedral grains. In transmitted light, most crystals are clear and transparent. However, many other contain oval to aligned inclusions, diffuse rusty zones or patches and cracks. Some larger inclusions are parallel to the c axis. In the CL images most crystals are moderately bright, with fine-scale, recurrent, concentric zonation. Some exhibit recurrent dark and bright zones or bands parallel to c axis. Sector zoning is rare. Some crystals show darker cores and brighter rims, or the reverse. However, well-defined cores are rather exceptional.

Sixteen points in 16 grains were analyzed (Table 4, Fig. 6b). U and Th contents are rather low and range from 101 to 360 ppm and from 41 to 146, respectively. One grain is enriched in U (575 ppm) and another one in Th (720 ppm). ²³²Th/²³⁸U ratios are mostly low to moderate, between 0.25 and 0.5 and slightly higher, 0.8–0.9, in two U-poor crystals. The Th-enriched grain shows distinctly higher ²³²Th/²³⁸U ratio of 2.9.

Four of the analyzed grains, including two crystals with ages of ca. 580 Ma and two other of 1.8–2 Ga, are distinctly older from the rest and apparently represent xenocrysts. These crystals tend to be subhedral, short-prismatic to rounded and CL bright. Two other crystals, 2.1 and 15.1, are excluded from further interpretation due to the highest common Pb contents of 5.5% and 2.3%, respectively. The other 10 crystals analyzed represent the largest population and show ages ranging from 322 to 302 Ma. Their habit and zonation style (mostly euhedral to subhedral, prismatic crystals with fine-scale oscillatory zonation) and geochemical characteristics (e.g. Th/U ratios) are consistent with magmatic origin. The mean Concordia age of these crystals is 310.9 ± 4.9 Ma and the weighted mean age of 310.6 ± 4.5 Ma is practically the same.

The age of 310.9 ± 4.9 Ma of the largest population of zircons is interpreted here as the magmatic crystallization and emplacement age of the Stary Lesieniec rhyodacite. This age corresponds to the middle part of the late Carboniferous (Pennsylvanian–Moscovian). The much older, xenocrystic grains of Paleo- and Neoproterozoic ages may either be derived from crustal source rocks or crustal contaminants of the rhyodacite magma, or may represent accidental crystals picked up by the lava during its emplacement, e.g. from the country rocks of the volcanic conduit.

Sample 709

In transmitted light the zircons of sample 709 are colourless and transparent. Most are euhedral, rarely subhedral and irregular. Their habit varies from short- to long-prismatic, the latter are often broken. Small, prismatic and elongated, oval-shaped inclusions parallel to the C-axis are fairly common. Several crystals are cracked and display local rusty colouration. In CL images (Fig. 5c) most crystals are distinctly zoned, with recurrent thin, magmatic-type zonation. Some crystals display sector, hourglass zonation. No distinct cores are discernable.

Nineteen spots in 18 crystals were analyzed (Table 5, Fig. 6c). The zircons contain rather low amounts of U (126–679 ppm, max. 814 ppm), and Th (42–30 ppm, max. 996 ppm). The ²⁰⁶Pb/²³⁸U ages vary between 299 ± 7 and 327 ± 7 Ma, with the weighted mean of 308 ± 3 Ma (2σ). Excluding the oldest outlier (spot 14.1, 327 Ma, Table 5) and five youngest points with relatively high common lead (points 1.2, 5.1, 10.1, 11.1, 12.1, Table 5), the mean age for remaining 13 points is 310.0 ± 3.8 Ma (Fig. 6c). The mean Concordia age for these 13 points is exactly the same. This age is interpreted as corresponding to the main magmatic crystallization event in the Chełmiec rhyodacite intrusion. The older concordant age of 327 ± 7 Ma may reflect an earlier, unspecified magmatic stage, whereas the younger ages around 302 Ma apparently indicate subsequent disturbances and Pb loss. The magmatic age of 310.0 ± 3.8 Ma for the Chełmiec intrusion corresponds to the middle part of the late Carboniferous (Moscovian).

Sample 778

Zircons in sample 778 range from subhedral, prismatic crystals with sharp or slightly rounded edges, to ellipsoidal grains, significantly rounded. In transmitted light, many crystals are colourless and clear, without inclusions, whereas others are partly cracked, with dark inclusions or rusty coloration. In the CL images (Fig. 5d) most crystals show relatively broad, brighter and darker bands, often aligned parallel to c axis. Some crystals show more patchy zonation, whereas others are rather homogeneous. More rarely crystals with thin, concentric, recurrent zonation can be discerned.

Sixteen points in thirteen grains were analyzed (Table 6, Fig. 6d). U contents are low to moderate and range from 75 to 714 ppm (1228 ppm in one grain). ²³²Th/²³⁸U ratios typically vary from 0.32 to 0.88 (around 1.1–1.2 in two crystals). Except one grain dated at 327 Ma, the other 15 spots analyzed form a coherent group with ages ranging from 316 to 293 Ma. The mean Concordia age of this group is 304.5 ± 3.4 Ma. The zonation patterns and chemical characteristics of these crystals are consistent with their magmatic origin and derivation form relatively mafic magma, which corresponds well with the andesitic composition of the host rock. In addition, the variable rounding of the crystals may reflect some magmatic resorption due to mixing with a hotter, silica-poor melt before the emplacement of the andesitic magma. The mean age of the zircons of 304.5 ± 3.4 Ma is interpreted as the crystallization and emplacement age of the andesitic magma in the late Carboniferous (Late Pennsylvanian–Kasimovian).

Sample TR1

Zircons in this sample are subhedral to euhedral, short- to normal-prismatic, rarely long-prismatic. Several crystals are broken and some have rounded edges. In transmitted light, the crystals are transparent to cloudy, with some inclusions, cracks and brownish coloration. CL images of many crystals are dark, without discernible details. These high-uranium crystals were avoided in analysis. The other zircons (Fig. 5e) are usually characterized by distinctive, recurrent zonation with several dark and light, concentric zones.

Seventeen points in fourteen crystals were analyzed (Table 7, Fig. 6e). Uranium contents are low to moderate (197–498 ppm) with exception of one U-rich crystal (941 ppm U). ²³²Th/²³⁸U ratios range from 0.18 to 0.44 in 14 spots, and up to 0.61 in 3 grains. Two points show relatively old ages of 348 Ma and 539 Ma and are likely of xenocrystic origin. The other 15 spots were measured in zircons with a very similar habit, zonation style and chemical features, all consistent with crystallization from silica-rich magma. The ages of theses crystals range from 304 to 279 Ma and the mean age is 292.6 ± 3.6 Ma. This age may correspond to the emplacement and crystallization of the Trójgarb rhyolite magma in the early Permian (Sakmarian). However, some points within this group show relatively high common Pb contents and/or variable discordance. Possibly this group includes two sub-populations, one with ages grouped around ca. 299 Ma and the others around 280 Ma (Fig. 6e). It is, therefore, possible that the Trójgarb rhyolite was emplaced earlier, at the turn of Carboniferous and Permian, and the younger points may reflect a younger thermal event and related lead loss by some zircon crystals.

Sample 773

Zircons in sample 773 are euhedral to subhedral and normal-prismatic. Some are broken. In transmitted light crystals range from rusty, with numerous cracks and inclusions, to more clear crystals with few inclusions or cloudy patches. Some grains tend to be slightly rounded on edges. CL images are variable (Fig. 5f). Many crystals show relatively dark central parts but brighter margins. Concentric recurrent zonation, with relatively thick to fine bands, is quite common. Some crystals are also sector-zoned or show more irregular patchy patterns.

Fifteen spots in 10 grains were analyzed (Table 8, Fig. 6f). Uranium contents vary rather widely, from 53 to nearly 1900 ppm, and ²³²Th/²³⁸U ratios range from 0.39 to 1.35. Three spots with the highest U contents (spots 6.2, 7.1, 10.2) and one with the highest Th/U ratio (spot 8.2) were excluded from further interpretation. The remaining 11 spots form a rather coherent group with ages in the range of 286 to 306 Ma. The mean age of this group is 292.8 ± 4.9 Ma. This is considered to represent the emplacement age of the sampled rhyolite subvolcanic body in the early Permian (Sakmarian).

Discussion

Age of magmatic rocks in the in the northern part of the Intra-Sudetic Basin

The results of U–Pb SHRIMP zircon dating are summarized in Fig. 7, together with previously published dating of the Sady Górne rhyodacite (Kryza and Awdankiewicz 2012). These results and other geological and stratigraphic data constrain the age of the magmatic rocks as well as the timing and character of subvolcanic to volcanic events in the northern part of the ISB. The ages fall in two main groups: the older of ca. 313–305 Ma and the younger one of ca. 293 Ma. The older ages define the late Carboniferous stage of activity, whereas the younger ones represent the early Permian stage of activity.

Fig. 7

Summary of the results obtained and comparison to stratigraphic and other geochronological data. a Compilation of currently available U–Pb zircon ages for volcanic rocks of the Intra-Sudetic Basin. SHRIMP ages—after Kryza and Awdankiewicz (2012) (SGRd Sady Górne rhyodacite) and this study (ChRd Chełmiec rhyodacite, NA Nagórnik andesite, RGR Rusinowa-Grzmiąca rhyolite, SBR Stara Białka rhyolite, SLRd Stary Lesieniec rhyodacite, TR Trójgarb rhyolite). TIMS ages after Opluštil et al. (2016) (KI Křenov ignimbrite, KT Křenov tuff, VR Varní Hory rhyolite). TIMS age of the Karkonosze granite (KG) as determined by Kryza et al. (2014a, b) for comparison. The U–Pb ages are shown against: the International Chronostratigraphic Scale for late Carboniferous-early Permian time span (ICS; Cohen et al. 2013, updated), the Western European subdivision (WE) and the lithostratigraphic units of the Intra-Sudetic Basin (ISB, after Opluštilet al. 2016; Fm. Formation, M. Member). b Geological sketch map of the northern part of the Intra-Sudetic Basin (cf. Figure 2) with grouping of volcanic rocks according to their emplacement age as constrained by SHRIMP dating and geological evidence (ages are given in white frames). Rocks dated by Opluštil et al. (2016; samples KI, KT, VH) crop out few kilometres south of the map area; the age of Karkonosze granite (KG, after Kryza et al. 2014b) shown for comparison. c Histograms comparing the new SHRIMP ages of volcanic rocks from the northern part of the Intra-Sudetic Basin with published ages of the Late Carboniferous–Early Permian volcanic rocks from Central Europe: the NE German Basin, the Polish Basin and several eruptive complexes of the Saxo-Thuringian Zone in southeastern Germany (data from: Breitkreuz and Kennedy 1999; Breitkreuz et al. 2007, 2009; Nawrocki et al. 2008; Hoffmann et al. 2013; Kryza and Awdankiewicz 2012; Opluštil et al. 2016)

The late Carboniferous stage of magmatism was contemporaneous with the accumulation of the Glinik Formation and, less likely, also the Žacléř Formation (Fig. 7a). Although the SHRIMP ages partly overlap within errors, their grouping suggests two phases of activity: at c. 313–310 Ma (late Westphalian–Moscovian) and 306–305 Ma (Stephanian–Kasimovian). The largest felsic subvolcanic bodies in the study area, including the Stara Białka rhyolite and the Chełmiec rhyodacite as well as the extrusive(?) Stary Lesieniec rhyodacite, can be linked with the older phase (Fig. 7b). The oldest dated, the Stara Białka rhyolite, was emplaced as a laccolith at around 313 Ma, some 15–20 My after the deposition of its host lower Carboniferous sedimentary rocks. The ages of 311 and 310 Ma of the Chełmiec and Stary Lesieniec rhyodacites, respectively, are identical within errors and suggest the emplacement of these two associated rhyodacite units in the same magmatic episode but at different positions in the sedimentary succession:

• the Chełmiec rhyodacite magma intruded as a laccolith within the Žacléř Formation deposits, rather shortly postdating their accumulation (by a few My?), and thus possibly a few hundred meters below the palaeosurface, whereas

• the Stary Lesieniec rhyodacite magma was emplaced higher in the sequence, as a lava flow or a cryptodome (sub-surface sill) broadly contemporaneous with the laterally equivalent siliciclastic sedimentary rocks of the Glinik Formation.

The Westphalian–Stephanian SHRIMP ages of the Chełmiec and Stary Lesieniec rhyodacites discussed here are consistent with earlier interpretations of Grocholski (1965) and Nemec (1979) of the age of these igneous bodies. Compared to the Stary Lesieniec rhyodacite, the Chełmiec rhyodacite sample 709 contains, however, a more complex spectrum of zircon crystals, possibly including sub-populations of various ages (ca. 327, 311 and 302 Ma). This can be tentatively linked with different crystallization history of the rhyodacite magma that formed the individual igneous bodies. In particular, zircons found within the Chełmiec laccolith, thus in a larger magma mass emplaced at a deeper level and cooling over a more prolonged time (compared to the associated Stary Lesieniec extrusion), possibly preserved a more complex record of cooling/crystallization events. The older zircons can be related to, e.g. recycling of crystals in successive magma batches (possibly coming from various deeper parts of the magmatic system) that formed the laccolith, whereas the younger zircons may record a thermal disturbance due to younger Carboniferous and Permian magmatic events in vicinity. In this context it is noteworthy that the emplacement of the Chełmiec laccolith predated the thermal peak attained by the Carboniferous deposits of the ISB in the latest Carboniferous to Early Permian times (cf. Botor et al. 2019; apatite fission track data).

The younger phase of the late Carboniferous stage of activity in the study area occurred at 306–305 Ma and comprised the emplacement of thin sills at the northern margin of the ISB: the Sady Górne rhyodacites (cf. Kryza and Awdankiewicz 2012) and the Nagórnik andesites. The SHRIMP age of the Nagórnik andesites as determined in this study shows them to be relatively late intrusives, postdating their host rocks by ca. 25–30 Ma, and thus they are not syn-sedimentary sills as suggested by Nowakowski and Teisseyre (1971). The other mafic intrusive veins occurring west of the Chełmiec laccolith, such as the Borówno basaltic andesites (sills; Awdankiewicz 1999a) and lamprophyres (dykes; Awdankiewicz 2007), although still undated, can be linked with the late Carboniferous stage of volcanism on geological and geochemical grounds. These include geological forms (thin intrusive veins in Carboniferous deposits) and geochemical affinities (intermediate, calc-alkaline compositions) similar to the dated Nagórnik andesites (Awdankiewicz 1999a, b, 2007). However, their emplacement in the younger (or older?) of the late Carboniferous phases remains more equivocal.

The ca. 313–310 Ma SHRIMP ages discussed above overlap the CA-ID-TIMS age of 312.39 ± 0.10 Ma obtained by Opluštil et al. (2016) from a rhyolitic ignimbrite (the Křenov ignimbrite) intercalated in the Žacléř Formation in the southwestern part of the ISB (a similar though slightly older age of 314.44 ± 0.17 Ma was determined in a tonstein from the same area—but this tonstein may possibly be linked to an extrabasinal volcanism). Although the ages determined using SHRIMP are an order of magnitude less precise compared to those obtained using CA-ID-TIMS (cf. Tichomirowa et al. 2019), the similarity of results discussed here highlights the contemporaneous emplacement, during the late Carboniferous volcanic phase, of felsic subvolcanic intrusions and pyroclastic deposits in various parts of the ISB. On the other hand, the late Carboniferous ages of the Sady Górne rhyodacites (Kryza and Awdankiewicz 2012) and of the Nagórnik andesites (this work) negatively verify the idea of early Carboniferous volcanic stage in the ISB. In contrast to earlier opinions (e.g. Teisseyre 1966; Nowakowski and Teisseyre 1971; Awdankiewicz 1999a) the onset of sedimentation of the several km thick lower Carboniferous deposits in the ISB was apparently not associated with intra-basinal volcanic or subvolcanic activity.

The other two dated samples of volcanic rocks are several million years younger and document the Permian phase of magmatism in the study area. As outlined in preceding sections, the age of the Trójgarb rhyolite of 293 Ma (or 299 Ma?), remains partly controversial. Nevertheless, the obtained results show that the Trójgarb rhyolite laccolith was emplaced in the early Permian, postdating the spatially associated rhyodacites west of Wałbrzych by at least 20 My. The age of the Rusinowa-Grzmiąca rhyolite sample was determined as ca. 293 Ma (identical within errors as the preferred age of the Trójgarb rhyolite). However, this result does not solve the problem of the age of the Rusinowa-Grzmiąca diatreme belt along the eastern margin of the Wałbrzych basin. The dated sample represent a discordant rhyolite body which is considered the youngest subvolcanic intrusion within the belt (cf. Awdankiewicz 1999a). Notably, zircon separation from three other rhyolite samples failed, and in two other cases (a rhyolite and a trachyandesite) the separated zircons turned out to be unsuitable for SHRIMP analysis due to very high-uranium contents. Thus, the dated rhyolite sample likely documents the youngest magmatic episode in the Rusinowa-Grzmiąca diatreme belt, and an earlier activity in Carboniferous, inferred from geological evidence (Grocholski 1965; Nemec 1979; Awdankiewicz 1999a), cannot be ruled out.

Stages of magmatism, relation to tectonics and regional context

Opluštil et al. (2016) identified a number of hiatus in the revised lithostratigraphic subdivision of the Carboniferous of the ISB, and linked these stratigraphic breaks to tectonic events/phases of a wide regional influence in the Central European Variscides. Figure 7a shows that all the seven ages of subvolcanic to volcanic rocks discussed in this paper fit very well (despite relatively large errors), to several of the hiatus. Thus, some of the regional tectonic events that affected sedimentation in the intramontane basins of the northern Bohemian Massif were in fact volcano-tectonic events, at least in the study area of this paper. These include the intra-Westphalian, Leonian and Asturian tectonic events in the late Carboniferous, and the Saalian event in the early Permian (cf. Opluštil et al. 2016). Their volcano-tectonic character may be considered as a result of a convergence, in time and space, of regional tectonic activity with local magma availability and its ability to rise to near-surface levels. In addition, the older ages of 313–310 Ma of the felsic subvolcanic rocks determined in this study correlate also very well with the emplacement age of the Karkonosze granite pluton west of the ISB determined at 312.5–312.2 ± 0.3 Ma (Kryza et al. 2014a, b). However, a more prolonged time span and an older age and of the Karkonosze pluton of 320–315 Ma, are considered by Žák et al. (2013). This convergence in age together with close spatial association suggest similar petrogenetic processes involved (e.g. similar magma sources and differentiation processes), although this remains a subject for separate study.

Close links between tectonics, sedimentation and magmatism can be recognized in the NW part of the ISB (Fig. 7), especially in development of the folded volcano-sedimentary association at the western margin of the Wałbrzych basin, an intra-basinal depositional trough (see section “Volcanic rocks in the Carboniferous of the Intra-Sudetic Basin”). There, the megacyclic structure of the Carboniferous sedimentary succession (e.g. Wojewoda and Mastalerz 1989 and references therein) with several hiatus (Opluštil et al. 2016) document prolonged tectonically controlled sedimentation; and the new SHRIMP ages point to associated though episodic magmatic activity expressed by the emplacement of laccoliths and sills starting at 313–310 Ma during the intra-Westphalian and Leonian phases in the Moscovian (contemporaneous with the emplacement of the Karkonosze pluton to the west) and continuing at 306–305 Ma during the Asturian phase in the Kasimovian (Fig. 7a). Similarly, short-lived (1–2 Ma) pulses of magmatism separated by longer breaks (2–8 Ma) were recognized in granitic plutons of Western Erzgebirge using several isotopic dating methods, including CA-ID-TIMS and SHRIMP (Tichomirowa et al. 2019). In the study area of this work the volcano-sedimentary succession adjacent to the Intra-Sudetic Fault, underwent also deformation that peaked in the formation of the SW-verging Jabłów anticline (with associated smaller folds) during the intra-Stephanian tectonic phase at ca. 303–300 Ma corresponding to Gzhelian. The latter age is constrained by the unconformities above the Glinik Formation, on top of the folded succession (see previous sections in this paper). This folding was followed by the final subvolcanic episode at ca. 293 Ma during the Saalian phase (or earlier, at 300 Ma?). These younger ages compare well to those of the youngest granites in the Strzegom Sobótka Massif in the Fore-Sudetic Block to the east, dated at 294.4 ± 2.7 Ma (U–Pb zircon SHRIMP age, Turniak et al. 2014). The predominance of shallow-level subvolcanic bodies in the studied succession of the ISB can be explained by trapping of rising magmas within the thick, low-density sedimentary basin fill, which formed a density barrier (cf. Awdankiewicz 1999a, b and references therein). In a broader context, the discussed late Carboniferous tectonic, magmatic and sedimentary processes can be linked to late-stage sinistral movements along the Intra-Sudetic Fault (Aleksandrowski et al. 1997) in the regime of NE–SW-directed shortening, which terminated the Variscan orogenic processes in SW Poland (Mazur et al. 2020). The SHRIMP ages presented here assign the majority of (sub)volcanic rocks in the Carboniferous formations of the northern ISB to the older, Pennsylvanian phase of volcanism, with a weaker influence of the major, younger, Asselian–Sakmarian phase of late Palaeozoic volcanism recognized in the Central European Variscan Belt (the “magmatic flare-up”, cf. Breitkreuz and Kennedy 1999; Hoffmann et al. 2013; Fig. 7c). However, the Permian ages of volcanic rocks reported here, as well as the precise TIMS age of 297.11 ± 0.04 Ma for the Varní Hory/Góry Krucze rhyolite (Opluštil et al. 2016) are just the few first datings of the extensive and heterogeneous Lower Permian Volcanic Complex (Awdankiewicz 1999a, b) which developed more completely to the south of the present study area; more age determinations are necessary to constrain the timing of the Permian climax of volcanism in the ISB in detail. Summing up, the currently available geochronological data together with regional tectonic constraints define two main stages of volcanic activity in the ISB: the late Carboniferous late orogenic stage, and the early Permian post-orogenic stage, each characterized by specific style of activity as well as geochemistry of related volcanic rocks (cf. Awdankiewicz 1999a, b).

Xenocrystic zircons and selected petrogenetic aspects

Xenocrystic zircons were identified in 6 out of 7 samples discussed in this paper (including sample SG, the results for which were published before in Kryza and Awdankiewicz 2012). Overall, 17 xenocrystic grains were analyzed. The age of these xenocrysts is plotted against the emplacement age of their host rocks in Fig. 8. As far as this relatively small amount of data shows, the xenocrysts tend to be more common and more diversified in age in the older rocks; consistently, one of two youngest rocks dated (sample 773) did not reveal xenocrystic zircon. The analyzed crystals can be subdivided into the following groups:

• 1. five ones dated at 1.99–1.78 Ga (late Palaeoproterozoic),

• 2. nine other dated at ca. 680–465 Ma, including two subgroups: 2a—seven crystals with ages of 680–540 Ma (Neoproterozoic to earliest Cambrian) and 2b—two grains with ages of 475 and 465 Ma (Ordovician),

• 3. three crystals dated at 350–327 Ma (early Carboniferous).

Fig. 8

The age of xenocrysts plotted against the emplacement age of their host rocks in the studied samples. a All analyzed xenocrysts; b enlarged part of a for xenocrysts younger than 800 Ma

The discussed zircons represent ‘foreign’ crystals, which can be genetically linked to either:

• partial melting and differentiation processes that formed their host magmas, and/or

• transport, emplacement and eruption processes of their host magmas, including mechanical interaction with the Carboniferous sedimentary rocks at the final emplacement sites.

However, the mechanical contamination at the late stages of emplacement seems less likely given the relatively quiet nature of the subvolcanic processes involved, compared to, e.g. much more violent character of effusive of explosive volcanic eruptions. Thus, a significant part of the xenocrysts may be of deeper origin, i.e. inherited from crustal magma sources or contaminants of the discussed volcanic rocks. Open-system differentiation including some contamination (Awdankiewicz 1999b) or a significant crustal contribution due to assimilation fractional crystallization (Dziedzic 1998; Ulrych et al. 2004, 2006) in the petrogenesis of the evolved Permo-Carboniferous magmas in the Intra-Sudetic Basin is suggested on petrological grounds.

The majority of zircon xenocrysts identified in the rocks studied (groups 1 and 2 above, with a distinct lack of Mesoproterozoic zircon ages), fit very well zircon age spectra in metasedimentary and metaigneous rocks of the crystalline complexes surrounding the Intra-Sudetic Basin: the Góry Sowie, Orlica-Śnieżnik and Karkonosze-Izera massifs (e.g. Oberc-Dziedzic et al. 2009; Jastrzębski et al. 2010; Mazur et al. 2012, 2015; Szczepański et al. 2020; Tabaud et al. 2021). The parental rocks of the group 2 zircons could have been orthogneisses related to the Cadomian and Cambro-Ordovician granitic plutonism which are widespread in the Sudetes (e.g. Pin et al. 2007 and references therein) or younger (meta)sedimentary rock derived from such lithologies. An ultimate magmatic origin of most of the zircon xenocrysts studied is supported by their textural and chemical features, such as zoning patterns or U and Th ratios. In addition, the youngest xenocrysts (group 3) of Early Carboniferous age can possibly be linked to syn/late orogenic Variscan granitoids. The relatively small age difference between these xenocrysts and their host volcanic rocks may indicate recycling of crustal materials and partial remelting of Lower Carboniferous granitoids in younger phases of magmatism. It can also be noted that relatively rare inherited zircons dated at 650–345 Ma were also recognized in the Karkonosze granite to the west (Žák et al. 2013).

Summing up, the xenocrystic zircons identified in the late Carboniferous, late orogenic calc-alkaline intermediate and felsic rocks of the ISB suggest that the crustal materials involved in their petrogenesis predominantly comprised Neoproterozoic–Palaeozoic metaigneous and/or metasedimentary rocks similar to those exposed in adjacent crystalline basement blocks, with weaker influence of or much younger, Carboniferous crustal rocks (e.g. orogenic, early Carboniferous granitoids). The crustal influence in magma genesis was likely stronger during the earlier magmatic events.

Conclusion

The SHRIMP zircon study of six samples of felsic igneous rocks presented in this paper determine the emplacement age of subvolcanic rocks in the northern part of the Intra-Sudetic Basin. These ages, together with other published results, provide constraints on the timing of Carboniferous to Permian volcanism in the Intra-Sudetic Basin in a regional tectonic context of mid-European Variscides.

The felsic igneous rocks cropping out in the Carboniferous sedimentary formations in the northern part of the ISB were emplaced as shallow-level subvolcanic intrusions in the late Carboniferous, mainly at ca. 313–310 Ma (Stara Białka rhyolite laccolith, Chełmiec rhyodacite laccolith, Stary Lesieniec cryptodome) and partly at ca. 306–305 Ma (Sady Górne rhyodacite and Nagórnik andesite sills). The youngest subvolcanic bodies intruded in the Early Permian and these include the ca. 293 Ma Trójgarb rhyolite laccolith as well as Jałowiec rhyolite plug in the Rusinowa-Grzmiąca belt. The Jałowiec plug is amongst the youngest intrusions in the Rusinowa-Grzmiąca diatreme belt and an earlier, Carboniferous formation of the diatremes cannot be excluded. Zircon xenocrysts identified in the studied subvolcanic rocks constrain the crustal materials involved in their petrogenesis as magma sources and/or contaminants. These were, predominantly, Neoproterozoic–Palaeozoic rocks metaigneous and/or metasedimentary rocks, with weaker influence of much younger, lower Carboniferous components, e.g. Variscan orogenic granitoids. The crustal influence on magma formation was stronger during the earlier magmatic events.

The late Carboniferous igneous activity in the ISB reveals close links with regional and local tectonics and magmatism. Laccoliths and sills were emplaced episodically within the basin sedimentary fill during intra-Westphalian, Leonian and Asturian volcano-tectonic events between 313 and 305 Ma (mainly around 312 Ma, contemporaneous with the emplacement of the Karkonosze granitic pluton to the west). Local deformation of the volcano-sedimentary succession adjacent to the Intra-Sudetic Fault, possibly linked to late-stage sinistral displacements, peaked at ca. 303–300 Ma during the intra-Stephanian tectonic phase. The final subvolcanic episode occurred at 293 Ma (? 300 Ma), during the early Permian.

Previous views on the timing of volcanism in the ISB and its regional context must be reappraised. The early Carboniferous stage of volcanism is discredited. The onset on volcanic activity post-dated basin opening by some 20 Ma. Two main stages of igneous activity are identified in the ISB: the initial late Carboniferous, late orogenic stage, and the climactic early Permian, post-orogenic stage.