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| Geological Magazine | ![]() |
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TYSZKA*


* University of Wroc
aw, Institute of Geological Sciences, Department of Mineralogy and Petrology, ul. Cybulskiego 30, 50-205 Wroc
aw, Poland
University of Leicester, Department of Geology, University Road, Leicester LE1 7RH, UK
Centre of Isotopic Research, A.P. Karpinsky All Russian Geological Research Institute (VSEGEI), 74 Sredny Pr, St Petersburg, 199 106, Russia
Author for correspondence: rkryza{at}ing.uni.wroc.pl
(Received 18 November 2006; accepted 16 March 2007)
| Abstract |
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Key Words: SIMS zircon dating slates Kaczawa Complex Sudetes Variscides Gondwana
| 1. Introduction |
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ski, 1992; Kryza, Mazur & Oberc-Dziedzic, 2004; Mazur et al. 2006; and references therein). The component rock masses are mostly poorly exposed, tectonically dismembered and variably metamorphosed up to granulite grade. Even the lower-grade, (anchi-/epizone) sedimentary successions are pervasively sheared, rendering most searches for macro- and microfossils futile and seriously hindering attempts to determine depositional age and to construct stratigraphic successions.
These difficulties have created sufficient uncertainty in geological interpretation to lead to serious debate, even in recent years, over whether this region is indeed Variscan in construction, or is Caledonian with a minor Variscan overprint (Oliver, Corfu & Krogh, 1993, Aleksandrowski et al. 2000). Its Variscan nature has recently been confirmed by combining detailed field studies with the application of modern geochronological techniques, including SIMS dating. This has allowed elucidation of a generalized history for the region of Cambro-Ordovician rifting and continental break-up, Silurian–early Devonian ocean opening, and ocean closure in late Devonian to early Carboniferous times culminating in continent–continent collision (e.g. Baranowski et al. 1990; Furnes et al. 1994; Collins, Kryza & Zalasiewicz, 2000; Seston et al. 2000; Kryza & Muszy
ski, 2003; Kryza, Mazur & Oberc-Dziedzic, 2004; Mazur et al. 2006).
While this broad regional framework now seems to be commonly accepted, there remain many substantial questions related to extensively outcropping rock units whose position within this framework, and hence whose contribution to the geological picture, is unknown.
One notable example has been the Radzimowice Slates of the Kaczawa Complex in the West Sudetes (Fig. 1
), a northeastern part of the Bohemian Massif. The Radzimowice Slates have been attributed variously to the Neoproterozoic (Teisseyre, 1963) and early Palaeozoic, the latter assignation being based on sparse and low-resolution conodont evidence (Urbanek & Baranowski, 1986), while a sedimentological analysis (Baranowski, 1988) inferred a trench-fill setting, suggesting a further alternative of possible late Palaeozoic deposition during ocean closure.
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| 2. Geological setting |
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wierzawa Unit (Teisseyre, 1963; Fig. 1
ski, 1992).
The Radzimowice Slates comprise a set of variably deformed rocks, locally mylonitic, but with low-strain domains preserving primary sedimentary features. Strongly foliated, white-mica-rich varieties could be termed phyllites or even mylonites, but here we prefer to use the local traditional term slates for this rock assemblage. The metamorphic grade corresponds to the epizone (greenschist facies) as shown by their fabric and mineral composition, as well as by the white mica characteristics (Baranowski, 1988, Kryza & Muszy
ski, 2003).
Baranowski (1988) recognized relict sedimentary structures and distinguished a range of lithofacies, including: mudstones that are variably siliceous, graphitic or silt-laminated; siltstones; fine-grained sandstones (quartz wackes); medium- and coarse-grained sandstones (lithic wackes with volcanic component); chaotic deposits (sedimentary breccias and olistoliths of mafic volcanics and limestones); and mafic tuffites. He interpreted this suite as representing turbidites and hemipelagites/pelagites with intercalated slide to debris flow deposits. The lithic wackes were interpreted as sourced from a magmatic arc, and the quartz wackes from a continental block. The facies association and the petrographic composition of the lithic wackes were ascribed to deposition in an oceanic trench or an immature slope basin.
Seston et al.(2000) suggested that the Radzimowice Slates represent a high-strain zone sandwiched between the low-strain
wierzawa Unit and the moderate-strain Bolków Unit (Fig. 1
). In such a structural position, the slates could incorporate a range of rocks of various ages, including those seen in the neighbouring units. However, the lithological association of the Radzimowice Slates is distinct and represents an internally consistent sedimentary succession markedly different from those exposed in the neighbouring units. For instance, they include negligible volcanic rocks other than a few probable olistoliths.
Thus, the primary nature, age and tectonic position of the Radzimowice Slates have remained controversial, and their resolution is critical to constructing wider regional geological interpretations.
| 3. Methods |
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nica, in the western part of the Radzimowice Slates outcrop. Both the samples represent a facies of thinly laminated mudstones with silt laminae, as defined by Baranowski (1988); in the former exposure they are associated with siliceous and graphitic slates. The relative stratigraphic position of both the samples is unknown.
The samples, each about 3 kg in weight, were crushed, and heavy minerals separated by a conventional heavy liquid (sodium polytungstate, d 3.0 g cm–1) method. Hand-picked zircon grains representing various morphological and structural types were studied by optical microscope and afterwards mounted in Buehler Epoquick® resin, ground and polished for CL imaging and in situ U–Pb dating. The analyses were performed on the SHRIMP II at VSEGEI, St Petersburg. The analytical conditions and data treatment procedures were as described in Larionov, Andreichev & Gee (2004). The results were processed using SQUID v1.12 (Ludwig, 2005a) and ISOPLOT/Ex 3.22 (Ludwig, 2005b).
| 4. Results |
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ski et al. 2002). On these grounds, we reject these three youngest ages as geologically unreliable (although they are shown in Table 1
4.b. Sample RDZ214
The zircons in this sample also vary widely, from idiomorphic to rounded, mostly colourless and transparent grains; some contain more or less distinct cores, and most crystals display zoning (Fig. 4
). A minor portion (about 5 %) is represented by brownish semi-transparent grains. In CL the zircons of this sample range from mostly dark to less common bright grains.
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The youngest calculated 206Pb–238U date of 358 ± 7 Ma (Table 1
, Fig. 5
) from a single grain is high in common Pb. This date (and also those of two other grains with high Pbc, 3.1 and 20.1) is unreliable and should be rejected (see Section 4.a).
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| 5. Discussion |
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Thus, the Radzimowice Slates cannot form the base of the Kaczawa succession (Teisseyre, 1963) or of a major component of this, the Bolków Unit (Fig. 1
), but must be at least synchronous with, and most likely postdate, acid igneous rocks in the middle of the Bolków Unit succession recently dated at c. 500 Ma (Kryza et al. 2007a,b).
The minimum age of sedimentation is uncertain. Baranowski (1988) suggested deposition in an ocean trench environment, which would imply association with ocean closure. The combination of our new dates and current understanding of the geological evolution of the Sudetes (Franke &
ela
niewicz, 2000; Seston et al. 2000; Crowley et al. 2001; Aleksandrowski & Mazur, 2002; Kryza, Mazur & Oberc-Dziedzic, 2004; Mazur et al. 2006) imply that, if so, this contractional phase, following the protracted extensional regime (Cambrian to late Devonian), would be the late Devonian to early Carboniferous subduction that preceded final Variscan orogenesis in this region. In such an interpretation, the Radzimowice Slates would effectively form part of the mudrock-dominated mélange deposits of this age that are widespread in the region (Baranowski et al. 1990; Collins, Kryza & Zalasiewicz, 2000; Kryza & Muszy
ski, 2003; J. Kostylew, unpub. Ph.D. thesis, Wroc
aw Univ. 2006; Fig. 7
, scenario B).
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Given the maximum age we establish for the Radzimowice Slates, its extensive outcrop, isolated from the main Kaczawa successions (e.g. in the
wierzawa and Bolków units; Fig. 1
), needs explanation. We consider as most likely an origin in a restricted basin, adjacent to and sourced from a combination of older Precambrian crust and early Palaeozoic volcanic–sedimentary successions (Fig. 8
).
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Both the samples studied have major populations of zircons with ages dispersed within the Cadomian range, roughly between 550 and 650 Ma. The ages are evenly distributed within that 100 Ma interval, suggesting that the source rocks comprised magmatic protoliths of various ages. The dates reflect a shared history between the Bohemian Massif, of which the Radzimowice Slates forms a part, and that part of Gondwana affected by intense Cadomian magmatism between 700 and 540 Ma (e.g. Pereira et al. 2006).
The Radzimowice Slates also contain a variety of older components of c. 750 Ma, 1050 Ma, 1900 Ma, 2150 Ma, 2450 Ma, 2650 Ma, the oldest date recorded being 3271 ± 41 Ma (Fig. 6
). Similar zircon ages have been reported from North Africa, which formed a part of Gondwana (Linnemann et al. 2000, 2004; Nance, Murphy & Keppie, 2002; Von Raumer, Stampfli & Bussy, 2003; Friedl et al. 2004; Inglis et al. 2005; Samson et al. 2005). Thus, our new data support a close similarity in zircon ages and, consequently, genetic links between this part of the Bohemian Massif and the North African part of Gondwana (Kryza et al. 2007b).
| 6. Conclusions |
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| Acknowledgements |
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aw University grants 1017/S/ING, and 2022/W/ING. Quentin Crowley and John Winchester are thanked for their constructive reviews. | References |
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