The Skalice Crag: The Variscan migmatization of gneisses in the southern part of the Strzelin Massif

The paper presents the relationships between the rocks of the Strzelin Massif: metamorphic sillimanite gneiss, anatectic pegmatite/leucogranite and igneous biotite-muscovite granite, exposed in the Skalice Crag, one of the largest natural exposures in the massif. The interpretation of these relationships enables reconstruction of the tectono-metamorphic and magmatic history of these rocks. The age data provide the time frame for their origin and evolution.Praca przedstawia relacje między skałami masywu strzelińskiego: metamorficznym gnejsem silimanitowym, anatektycznym pegmatytem/leukogranitem i magmowym granitem biotytowo-muskowitowym w Skalickich Skałach, które są jednym z największych naturalnych odsłonięć w masywie. Interpretacja tych relacji umożliwia odtworzenie tektono-metamorficznej i magmowej historii badanych skał. Przytoczone dane o wieku skał określają czasowe ramy ich dla powstania i ewolucji.The paper presents the relationships between the rocks of the Strzelin Massif: metamorphic sillimanite gneiss, anatectic pegmatite/leucogranite and igneous biotite-muscovite granite, exposed in the Skalice Crag, one of the largest natural exposures in the massif. The interpretation of these relationships enables reconstruction of the tectono-metamorphic and magmatic history of these rocks. The age data provide the time frame for their origin and evolution.Praca przedstawia relacje między skałami masywu strzelińskiego: metamorficznym gnejsem silimanitowym, anatektycznym pegmatytem/leukogranitem i magmowym granitem biotytowo-muskowitowym w Skalickich Skałach, które są jednym z największych naturalnych odsłonięć w masywie. Interpretacja tych relacji umożliwia odtworzenie tektono-metamorficznej i magmowej historii badanych skał. Przytoczone dane o wieku skał określają czasowe ramy ich dla powstania i ewolucji

Cambrian Zawidów granodiorites in the Cadomian Lusatian Massif (Central European Variscides): what do the SHRIMP zircon ages mean ?

International audienceThe Lusatian Massif in the Central European Variscides, composed of upper Neoproterozoic (c. 570 Ma) greywacke intruded by c. 540 Ma old Cambrian granodiorites (and the somewhat younger Zawidów granodiorites), constitutes a fragment of the Cadomian basement of the Saxo-Thuringian Zone. The Lusatian Massif adheres on the east to the Karkonosze-Izera Massif composed of the c. 500 Ma Izera/Rumburk granites related to the Cambro-Ordovician rifting of the Cadomian basement, and narrow belts of micaschists. Trace-element and Sm-Nd isotope data suggest that the source rocks for the Lusatian greywacke, the Zawidów granodiorite and the Izera/Rumburk granite could have been similar, though not the same. The new SHRIMP U-Pb zircon data for the Zawidów granodiorite reveal, apart from the expected c. 540 Ma ages, three other zircon age populations of around 630 Ma, 600 Ma and 510 Ma, the latest being evidently younger than 540 Ma that was considered as the age of the granodiorite magma emplacement. Zircons with 206Pb/238U ages around 510 Ma, and somewhat older (up to c. 538 Ma), have also been reported from the Izera granites. This could mean that the granitic plutonism related to the Cadomian orogenic cycle and the Cambro-Ordovician rifting triggered two or more magmatic pulses during at least c. 30 My. During the prolonged period of igneous activity, the plate-tectonic environment at the Gondwana margin changed from collisional (Cadomian Orogeny), to initial rifting (Cambro-Ordovician)

Ore mineralization in the Miedzianka area (Karkonosze-Izera Massif, the Sudetes, Poland): new information

The Miedzianka mining district has been known for ages as a site of polymetallic ore deposits with copper and, later, uranium as the main commodities. Although recently uneconomic and hardly accessible, the Miedzianka ores attract Earth scientists due to the interesting and still controversial details of their ore structure, mineralogy and origin. Our examination of the ore mineralization from the Miedzianka district was based exclusively on samples collected from old mining dumps located in the vicinity of Miedzianka and Ciechanowice, and on samples from the only available outcrop in Przybkowice. In samples from the Miedzianka field, chalcopyrite, pyrite, galena, bornite, chalcocite, digenite, arsenopyrite, magnetite, sphalerite, tetrahedrite-tennantite, bornite, hematite, martite, pyrrhotite, ilmenite, cassiterite and covellite are hosted in quartz-mica schists and in coarse-grained quartz with chlorite. In the Ciechanowice field, the ore mineralization occurs mainly in strongly chloritized amphibolites occasionally intergrown with quartz and, rarely, with carbonates. Other host-rocks are quartz-chlorite schist and quartzites. Microscopic examination revealed the presence of chalcopyrite, pyrite, sphalerite, galena, tetrahedrite-tennantite, bismuthinite, native Bi, arsenopyrite, löllingite, cassiterite, cobaltite, gersdorffite, chalcocite, cassiterite, bornite, covellite, marcasite and pyrrhotite. Moreover, mawsonite and wittichenite were identified for the first time in the district. In barite veins cross-cutting the greenstones and greenschists in Przybkowice, we found previously-known chalcopyrite, chalcocite and galena. The composition of the hydrothermal fluids is suggested to evolved through a series of consecutive systems characterized, in turn, by Ti-Fe-Sn, Fe- As-S, Fe-Co-As-S, Cu-Zn-S and, finally, Cu-Pb-Sb-As-Bi compositions

The Saxothuringian Terrane affinity of the metamorphic Stachów Complex (Strzelin Massif, Fore-Sudetic Block, Poland) inferred from zircon ages

International audienceThe Saxothuringian Terrane de fined in the west ern part of the Bo he mian Mas sif is re garded to have east erly con tin u a tions inthe Karkonosze–Izera Mas sif, the Kamieniec Z¹bkowicki Belt and the Orlica–OEnie¿nik Dome. All these units com prise EarlyOr do vi cian (~500 Ma) metagranites as so ci ated with mica schists. Even more to the east, ~500 Ma metagranites andmetasedimentary rocks oc cur also in the Strzelin Mas sif of the East Sudetes, where they are known as the pale and darkStachów gneiss es, re spec tively. Al to gether, these rocks form the Stachów Com plex which was thrust on the Strzelin Com -plex of the Brunovistulicum Terrane dur ing the Variscan Orog eny. The con tri bu tion pres ents lines of ev i dence for aSaxothuringian af fin ity of the Stachów Com plex rocks: (1) the new SHRIMP U-Pb age data of zir cons from both the pale anddark Stachów gneiss es; (2) the in di ca tion that the zir con age spec tra from the ~500 Ma granitoids and their ac com pa ny ingmetasedimentary rocks are sim i lar to those found in other parts of the Sudetes; (3) the “Armori can” age pat tern of in her itedzir cons of the pale Stachów gneiss es, as also ob served in the Saxothuringian Terrane; (4) the sim i lar ity of trace el e mentsand Sm-Nd iso tope data of the Stachów gneiss es and cor re la tive rocks from the Karkonosze–Izera Mas sif and theOrlica–OEnie¿nik Dome

SHRIMP U-Pb zircon geochronology of the Strzelin gneiss, SW Poland: evidence for a Neoproterozoic thermal event in the Fore-Sudetic Block, Central European Variscides

Zircon ages recorded in gneissic rocks have recently been used as criteria to define and correlate various tectonic units and crustal blocks in the central European Variscides. A SHRIMP U-Pb zircon geochronological study of the Strzelin gneiss in the Fore-Sudetic Block (SW Poland) indicates the presence of: (1) inherited zircon cores of Palaeo- to Mesoproterozoic 206Pb-238U ages (between ca. 2,000 and 1,240 Ma), and (2) zoned rims of Neoproterozoic age with two distinct means of 600±7 and 568±7 Ma. The Proterozoic age range of the cores suggests that different Precambrian crustal elements were the source for the protolith of the gneiss. A likely scenario is the erosion of various Proterozoic granites and gneisses, sedimentation (after 1,240 Ma), and partial resistance of the original components to subsequent metamorphic dissolution and/or anatectic resorption (in Neoproterozoic times). The zoned zircon rims of both of the younger Neoproterozoic ages are indistinguishable in the cathodoluminescence images. The data are interpreted in terms of two different thermal events inducing zoned zircon overgrowth at ca. 600 and 568 Ma. In general, the new results confirm earlier assumptions of the Proterozoic age of the gneiss protoliths, and indicate their similarity to orthogneisses in the East Sudetes tectonic domain (e.g. the Velké Vrbno and Desná gneisses). The Neoproterozoic dates are different from the age of 504±3 reported earlier for the Gościȩice gneiss from a neighbouring locality in the Strzelin Massif. The new data strongly indicate a Moravo-Silesian (Bruno-Vistulian) affinity for the Strzelin gneiss and support the hypothesis that the Strzelin Massif lies within the tectonic boundary zone between the West- and East Sudetes domains, which represents the northern continuation of the Moldanubian Thrust

Age constraints for the thermal evolution and erosional history of the central European Variscan belt: new data from the sediments and basement of the Carboniferous foreland basin in western Poland

Three Carboniferous-age detrital muscovites from the Variscan foreland basin of SW Poland and two muscovites from phyllites underlying the basement have been dated by the40Ar/39Ar step-heating and single-crystal laser fusion method.40Ar/39Ar analysis of the detrital micas defines step-heating preferred ages of 370.7 ± 1.4, 363.0 ± 1.9 and 355.0 ± 1.3 Ma. Single-crystal laser fusion data indicate little dispersion for the first of three samples, with an integrated age that closely matches the step-heating data, but the latter two describe inhomogeneous populations. The white mica concentrate from one phyllite yields a step-heating preferred age of 358.6 ± 1.8 Ma. The second phyllite sample displays an incremental discordant apparent age spectrum representing a mixture of white mica grains of varying ages. Our most important finding is that the Variscan foreland basin was supplied by source rocks that were exhumed and cooled in the Late Devonian, probably as a result of an early Variscan collisional event, previously largely undocumented. Although accessible exposures of the Variscan basement in SW Poland exhibit only a minor component of rocks exhumed before the Carboniferous, our work suggests that large tracts of rocks with a Devonian cooling signature are preserved at depth beneath the foreland basin

Gneiss protolith ages and tectonic boundaries in the NE part of the Bohemian Massif (Fore-Sudetic Block, SW Poland)

Published geochronological data, petrology, geochemistry and geological context of orthogneisses; in the Strzelin and the Stachów complexes (NE-part of the Fore-Sudetic Block), together with structural observations help to locate the northern extension

Chemical abrasion applied to SHRIMP zircon geochronology: An example from the Variscan Karkonosze Granite (Sudetes, SW Poland)

International audienceThermal annealing followed by acid etching of zircon (chemical abrasion or CA) can be successfully utilised to minimize or eliminate the effects of major and cryptic Pb-loss for SIMS U-Pb zircon dating. The procedure is demonstrated by applying the U-Pb SIMS technique to both untreated and chemically abraded zircons from the Karkonosze Granite, Sudetes, SW Poland. Conventional U-Pb SIMS dating of untreated zircons yields an apparently coherent age population (n = 9) with a weighted mean 206Pb/238U age of 306 ± 4 Ma. Some untreated zircons display anomalously young 206Pb/238U ages (c. 225 and 238 Ma) and are likely to have suffered substantial Pb-loss. A sub-set of zircons from the same sample was chemically abraded. Physically, zircons treated in this manner display a range in the degree of etching and partial dissolution. Extreme examples developed a 3D network of sub-μm channels which follow high-U (dark CL) zones or linear defects, such as micro fractures or indistinct cleavage planes. U-Pb SIMS dating of treated zircons (n = 11) yields a mean 206Pb/238U age of 322 ± 3 Ma. Two analyses of treated zircons still display younger 206Pb/238U ages (c. 297 and 301 Ma) ascribed to the effects of Pb-loss. For the analysed sample, U-Pb ages determined from chemically abraded zircons are c. 5% older than those from untreated zircons. This is attributed to effective removal of metamict domains susceptible to Pb-loss. The CA technique also removes micro-inclusions thus lowering common Pb and reducing matrix effects. A cryptic Pb-loss in untreated zircons is only recognised when compared with chemically abraded counterparts or ages determined using other isotope techniques. This clearly demonstrates the utility of CA to high-spatial resolution methods and stresses that Pb-loss is detectable at a range of scales, regardless of the analytical technique used