Ryszard Kryza

Kinematic data on major Variscan strike-slip faults and shear zones in the Polish Sudetes, northeast Bohemian Massif

The still highly disputable terrane boundaries in the Sudetic segment of the Variscan belt mostly seem to follow major strike-slip faults and shear zones. Their kinematics, expected to place important constraints on the regional structural models, is discussed in some detail. The most conspicuous is the WNW–ESE Intra-Sudetic Fault Zone, separating several different structural units of the West Sudetes. It showed ductile dextral activity and, probably, displacement magnitude of the order of tens to hundreds kilometres, during late Devonian(?) to early Carboniferous times. In the late Carboniferous (to early Permian?), the sense of motion on the Intra-Sudetic Fault was reversed in a semi-brittle to brittle regime, with the left-lateral offset on the fault amounting to single kilometres. The north–south trending Niemcza and north-east–southwest Skrzynka shear zones are left-lateral, ductile features in the eastern part of the West Sudetes. Similarly oriented (northeast–southwest to NNE–SSW) regional size shear zones of as yet undetermined kinematics were discovered in boreholes under Cenozoic cover in the eastern part of the Sudetic foreland (the Niedźwiedź and Nysa-Brzeg shear zones). One of these is expected to represent the northern continuation of the major Stare Mesto Shear Zone in the Czech Republic, separating the geologically different units of the West and East Sudetes. The Rudawy Janowickie Metamorphic Unit, assumed in some reconstructions to comprise a mostly strike-slip terrane boundary, is characterized by ductile fabric developed in a thrusting regime, modified by a superimposed normal-slip extensional deformation. Thrusting-related deformational fabric was locally reoriented prior to the extensional event and shows present-day strike-slip kinematics in one of the sub-units. The Sudetic Boundary Fault, although prominent in the recent structure and topography of the region, was not active as a Variscan strike-slip fault zone. The reported data emphasize the importance of syn-orogenic strike-slip tectonics in the Sudetes. The recognized shear sense is compatible with a strike-slip model of the northeast margin of the Bohemian Massif, in which the Kaczawa and Gory Sowie Units underwent late Devonian–early Carboniferous southeastward long-distance displacement along the Intra-Sudetic Fault Zone from their hypothetical original position within the Northern Phyllite Zone and the Mid-German Crystalline High of the German Variscides, respectively, and were juxtaposed with units of different provenance southwest of the fault. The Intra-Sudetic Fault Zone, together with the Elbe Fault Zone further south, were subsequently cut in the east and their eastern segments were displaced and removed by the younger, early to late Carboniferous, NNE–SSW trending, transpressional Moldanubian–Stare Mesto Shear Zone.

Geochemical evidence for progressive, rift-related early Palaeozoic volcanism in the western Sudetes

The volcanigenic rocks of the Kaczawa Mts (western Sudetes, Poland), in the eastern Variscides, show changing geological and geochemical evolution during early Palaeozoic time. The lower part of the succession (Cambrian (?)–Ordovician) has three components. 1. Shallow marine to subaerial metabasalts, associated with limestones and volcaniclastics. The lavas are dominantly of a transitional tholeiitic–alkaline type and their trace element patterns typically represent a rift-related environment. They pass laterally (and upwards ?) into more depleted basalts resembling enriched MORB, with Nd-isotopic characteristics indicating contamination by continental crust. 2. Interlayered rhyodacitic lavas and volcaniclastics which show negative ɛNd values, suggesting formation of the original magma by crustal melting. 3. An overlying alkaline bimodal suite of lavas and volcaniclastic rocks, as well as alkaline metabasites of shallow-intrusive character. The geochemistry of the latter resembles oceanic island volcanics, but they may well have been emplaced in the same evolving rift environment. The upper part of the Kaczawa succession, Ordovician-Silurian (?) in age, comprises a thick and monotonous sequence of deep-marine pillowed and massive metabasalts, associated with black shales and cherts. These lavas exhibit MORB trace element characteristics, with minor evidence of crustal contamination. During this stage of rifting, true oceanic crust probably formed. It is thus suggested that the studied part of the Kaczawa succession developed in a progressively evolving rift, initially within an ensialic environment, and finally reaching the stage of a basin underlain by oceanic-type crust. Together with similar Cambrian-Ordovician volcanic-sedimentary associations, widely distributed in western Europe, from Portugal, through France and Germany, they represent a record of the Early Palaeozoic rifting in the northern periphery of Gondwana.

Review of geochemical variation in Lower Palaeozoic metabasites from the NE Bohemian Massif: intracratonic rifting and plume-ridge interaction

Abstract During early Palaeozoic time the Cadomian basement of the northern margin of Gondwana underwent extensive rifting with the formation of various crustal blocks that eventually became separated by seaways. Attenuation of the continental lithosphere was accompanied by the emplacement of anatectic granites and extensive mafic-dominated bimodal magmatism, often featuring basalts with an ocean crust chemistry. Intrusive metabasites in deep crustal segments (associated with granitic orthogneisses) or extrusive submarine lavas at higher levels (associated with pelagic and carbonate basinal sediments) show a wide range of chemical characteristics dominated by variably enriched tholeiites. Most crustal blocks show the presence of three main chemical groups of metabasites: Low-Titholeiitic metabasalts, Main Series tholeiitic metabasalts and alkalic metabasalt series. They differ in the degree of incompatible element enrichment (depleted to highly enriched normalized patterns), in selected large ion lithophile (LIL) to high field strength element (HFSE) ratios, and abundances of HFSE and their ratios. Both the metatholeiite groups are characterized by a common enrichment of light REE-Th-Nb-Ta. High Th values (or Th/Ta ratios) and associated low εNd values (especially in the Low-Ti tholeiitic metabasalts) reflect sediment contamination in the mantle source rather than at crustal levels, although this latter feature cannot be ruled out entirely. The range of chemical variation exhibited is a consequence of the melting of (a) a lithospheric source contaminated by a sediment component (which generated the Low-Ti tholeiites), and (b) a high-level asthenospheric mid-ocean ridge basalt (MORB)-type source that mixed with a plume component (which generated the range of enriched Main Series tholeiites and the alkali basalts). It is considered that a plume played an important role in the generation of both early granites and the enriched MORB-type compositions in the metabasites. Its significance for the initial fragmentation of Gondwana is unknown, but its presence may have facilitated deep continental crust melting and the fracturing into small crustal blocks. The early-mid-Jurassic plume-instigated break-up of the southern Gondwana supercontinent is considered to be a possible tectonic and chemical analogue for Early Palaeozoic Sudetic rifting and its magmatic products.

New U–Pb monazite and zircon data from the Sudetes Mountains in SW Poland: evidence for a single‐cycle Variscan orogeny

New U–Pb monazite and zircon data combined with a study of their internal morphologies significantly clarifies the controversy surrounding the Palaeozoic geodynamic evolution of the Sudetes Mountains in the northeastern Bohemian Massif. Variably thin rims of secondary magmatic overgrowth on zircons from an anatectic granite in the Góry Sowie block are dated by a U–Pb zircon lower intercept age of 353±61 Ma, by magmatic monazite growth at 378±2 Ma, and by xenotime crystallization at 383–370 Ma in a late syntectonic pegmatite, reflecting the anatectic event associated with a single‐cycle Variscan orogeny in this area.

The Polish Sudetes: Caledonian or Variscan?

The Polish Sudetes on the NE margin of the Bohemian Massif comprise a complex mosaic of pre-Permian basement units, traditionally included in the Variscides. A hypothesis of significant Caledonian orogenesis in this area originated in the 1920s, was subsequently rejected, and then was recently revived in models which invoked Early Palaeozoic to Early-Mid Devonian subduction and continental collision along a proposed extension of the Tornquist suture zone. We reassess the evidence invoked in support of the Caledonian orogeny, such as supposed regional pre-Upper Devonian unconformity, Ordovician bimodal magmatism and radiometric, palaeontological, palaeomagnetic and structural data, and suggest these are either inconclusive or misinterpreted. On the other hand, the Sudetes record Mid?-Late Devonian blueschist metamorphism followed by an Early Carboniferous regional high temperature event, widespread Late Devonian/Early Carboniferous flysch/molasse sedimentation and abundant granite intrusion in the Carboniferous to Early Permian. We discuss the usage of the term ‘Caledonian’ in a plate tectonic context and suggest it should not be used simply to denote Early to Mid-Palaeozoic tectonic activity. The tectonic evolution of the Sudetes was temporally different from, and resulted from convergence of different crustal domains than that of the British-Scandinavian-Pomeranian Caledonides. The Sudetic Palaeozoic sequences most probably developed on Armorican Neoproterozoic crust and in adjacent oceanic(?) domains and, therefore, the Sudetes form part of the Variscan orogenic belt.

Macrofabric fingerprints of Late Devonian–Early Carboniferous subduction in the Polish Variscides, the Kaczawa complex, Sudetes

We describe a remarkably preserved assemblage of sedimentary and tectonic fabrics in cores from the Kaczawa complex, Sudetes, SW Poland. These fabrics indicate a continuum of process from repeated remobilization of Upper Devonian–Lower Carboniferous muddy flysch and volcaniclastic sediments as debris flows and olistostromes, to fracturing, fluid‐streaming and soft‐sediment injection triggered by high pore‐water pressures during the initial stages of tectonic deformation, to contractional cleavage formation and local cataclasis while the sediment was still only partially consolidated. These structures are similar to those described from ODP cores through the toes of active accretionary prisms. They indicate active subduction of oceanic crust during the Late Devonian, suggesting that ophiolite obduction and significant overthrusting in the Sudetes occurred as an integral part of the Variscan orogeny.

Devonian deep-crustal metamorphism and exhumation in the Variscan Orogen: evidence from SHRIMP zircon ages from the HT-HP granulites and migmatites of the Góry Sowie (Polish Sudetes)

The high-temperature and high-pressure granulites in the internal zone of the Variscan belt are witnesses of deep crustal processes and subsequent exhumation of deeper lithospheric fragments. The Góry Sowie Block granulites in SW Poland and the surrounding gneisses and migmatites contain different inherited zircon age spectra, testifying different sources of their protoliths: mainly Cadomian (ca. 580 Ma) igneous rocks, and early Ordovician (ca. 500 Ma) granitoids (or their reworked products), respectively. The metamorphic spheric and oval zircons in the granulites give two statistically distinct ages of c. 395 and 380 Ma, in excess of experimental uncertainty. These ages may correspond to the high-temperature (HT) and high-pressure (HP) granulite facies metamorphism and subsequent amphibolite facies retrogression. However, essentially isochemical reconstitution of zircons and inheritance of radiogenic Pb cannot yet be excluded. The retrogression in the granulites coincided with the amphibolite facies metamorphism in the surrounding gneisses evidenced by a range of various isotopic ages between 384 and 370 Ma. The new SHRIMP zircon data provide new evidence of the presence of “old” (c. 400–395 Ma) HT-HP granulites in the central European part of the Variscides. The granulites were subsequently exhumed to mid-crustal levels, and tectonically interleaved with gneisses. Continuing uplift and decompression caused migmatization in the surrounding gneisses and partial re-equilibration of the granulites, at around 380 Ma. This high-temperature and medium-pressure (HT-MP) event was followed by rapid uplift and exhumation at c. 360 Ma due to Eo-Variscan orogenic movements.

Ślęża Ophiolite: geochemical features and relationship to Lower Palaeozoic rift magmatism in the Bohemian Massif

Abstract The Ślęża Ophiolite is one of several thrust-bounded crustal slices dominated by metabasites in the western Sudetes. The apparent field association of serpentinites, gabbros and amphibolitic components led previous workers to consider that this lithological assemblage represented an ophiolite sequence. Fieldwork suggests that the ophiolite is now highly inclined, partly overturned, so that an ophiolitic pseudostratigraphy can be deduced, grading from serpentinites and gabbros in the south to metabasite lavas in the north. The recent discovery of pillow lava structures (at Gozdnica Hill, to the west of Sobótka town) confirms that the volcanic top of the ophiolite lies in the northern section, as might be expected from the ophiolite model. The gabbros have undergone greenschist facies metamorphism with the random development of low-grade amphibole. The volcanic portion of the sequence comprise metamorphosed dolerites and basalts partly within the contact aureole of the Variscan Strzegom-Sobótka granite. Previous work dated plagiogranites associated with the gabbros at about 400–420 Ma (U-Pb zircon ages). Geochemical data suggest that the gabbros are distinct and apparently not comagmatic with the volcanic section of sheeted dykes and lavas. The gabbros, in particular, although very depleted in incompatible elements are dissimilar to supra-subduction zone ophiolites, exhibiting instead N-MORB-like light REE depleted patterns. Depletion is both a feature of the cumulate character of many of the gabbros, as well as a source effect (especially the uniformly low Nb content). The metabasalts and metadolerites, on the other hand, are a well-evolved single comagmatic suite with high incompatible element contents, Zr/Y approximately 3–4, and generally flat to light REE-depleted patterns. The geochemical dichotomy of the plutonic and volcanic segments calls into question a simple interpretation of the body as a single-stage coherent stratiform ophiolite. Chemical comparison with Sudetic metabasites from within the nearby Rudawy-Janowickie and Kacazawa Complexes shows that the Ślęża metabasites have a number of features in common, including the presence of both low-Ti (gabbros) and high-Ti (dykes and lavas) chemical groups. The correlation of the gabbros, dykes and lavas with the low-Ti and high-Ti (Main Series) metatholeiites respectively, seen throughout the Bohemian Massif, as well as the Sudetes, places them within the regional collage of Palaeozoic crustal blocks separated by the Saxothuringian Seaway. Comparison with Bohemian Massif metabasites also indicates that sediment contamination of the Ślęża Ophiolite sources was not an important process and that an enriched plume source played no part in the generation of the ophiolitic melts. The two Ślęża chemical groups were derived from variably depleted asthenospheric mantle sources. Simple modelling suggests that the volcanic segment could have been derived by 10–15% partial melting of a depleted N-MORB source, whereas the plutonic segment represents around 30% partial melting of a more depleted source. To develop varying degrees of depletion in an oceanic environment, the two sources could be related via incremental partial melting of a shallow MORB-type source.

SHRIMP zircon study of a micromonzodiorite dyke in the Karkonosze Granite, Sudetes (SW Poland): age constraints for late Variscan magmatism in Central Europe

The large Variscan Karkonosze Granite in the West Sudetes, representative of the vast Variscan granite plutonism in Central Europe and located adjacent to regional tectonic suture and strike-slip-zones, has been difficult to date precisely; a range of published data varies between c. 304 and 328 Ma. However, the granite is cut by locally numerous lamprophyre and other dykes. Dating of the dyke rocks, emplaced shortly after the granite intrusion and cooled more rapidly, provides a promising tool for the verification of published SHRIMP results on the granite itself. SHRIMP zircon geochronology of a studied micromonzodiorite dyke indicates substantial admixture of inherited zircons of c. 2.0, 1.4 Ga ( 207 Pb- 206 Pb minimum ages), and c. 570 (and 500?) Ma. The average concordia age of the main magmatic population of the zircons in the dyke is 313 ± 3M a (2σ); however, the true magmatic age might be older, around 318 Ma. This would constrain the age of the hypabyssal magmatism in the Karkonosze Massif and the minimum age of the host Karkonosze Granite. Thus, the Karkonosze Granite is confirmed as representative of an early phase of Variscan granite plutonic activity in the central-European Variscides.

Early Palaeozoic initial-rift volcanism in the Central European Variscides (the Kaczawa Mountains, Sudetes, SW Poland): evidence from SIMS dating of zircons

Early Palaeozoic volcanic suites are widespread throughout the Variscan Belt, and have commonly been ascribed to incipient rifting along the northern periphery of Gondwana in Cambrian to Ordovician times. Their distribution across Europe defines the present-day extent of Gondwana-derived terranes and constrains the timing of their separation from the Gondwanan margin. The Kaczawa Mountains in the West Sudetes, at the eastern termination of the Variscan Belt, include bimodal rift-related rocks, but their protolith age and, hence, their significance have been highly uncertain. We have applied secondary ionization mass spectrometry zircon geochronology to a metarhyodacite and a metatrachyte from this suite, yielding ages of 502.4 ± 2.6 Ma and 485.7 ± 1.6 Ma, respectively. This constrains the initial rift magmatism to c. 500–485 Ma in this part of the European Variscides. The rift-related Early Palaeozoic volcanism thus seems to have been broadly synchronous throughout the peri-Gondwanan terranes of Europe.