West Sudetes Research Articles

EPMA U-Th-total Pb dating of uraninite from the Stara Kamienica schist belt (West Sudetes, SW Poland): new data towards interpreting the genesis of cassiterite-sulphide mineralization

EPMA U-Th-total Pb dating of uraninite from the Stara Kamienica schist belt (West Sudetes, SW Poland): new data towards interpreting the genesis of cassiterite-sulphide mineralization

In situ white mica Rb/Sr geochronology of the Leszczyniec metaigneous complex, West Sudetes: evidence of upper plate deformation at the onset of Variscan collision

The Karkonosze-Izera Massif in the West Sudetes preserves evidence of subduction of the Saxothuringian Ocean beneath the Teplá-Barrandian Domain. Within the massif, the Leszczyniec metaigneous complex (LMC) is identified as the upper allochthon. It exhibits a unique structural history in comparison to the underlying allochthons, suggesting that the LMC records a distinctive tectonic history. To investigate the timing of this history, two orthogneisses were studied from a single outcrop of the LMC for in situ white mica Rb/Sr geochronology. The outcrop bears a southeast-dipping foliation (S1) and a north/northeast plunging stretching lineation (L1), defined by white mica and quartz. Quartz recrystallization textures, white mica chemistry (celadonite content of 0.23–0.47), preservation of igneous plagioclase phenocrysts, and the metamorphic mineral assemblage, all indicate maximum epidote–amphibolite facies conditions. Single-spot Rb/Sr dates were calculated from white mica using initial 87Sr/86Sr values obtained by titanite and epidote. Results are similar for both rocks, providing weighted averages of 352.4 ± 4.1 Ma (MSWD: 0.6; n: 24) and 349.3 ± 2.5 Ma (MSWD: 0.4; n: 31). The rocks are interpreted to have the same structural and metamorphic history; thus, a pooled weighted average of 350.1 ± 5.3 Ma (2σ) is reported as the timing of white mica (re)crystallization during S1 and L1 development in epidote–amphibolite facies conditions. This event is bracketed by the timing of blueschist-facies metamorphism for the subjacent middle (c. 364 Ma) and lower (c. 345–341 Ma) allochthons, associated with an east/southeast-plunging L1. Considering the different structural and metamorphic histories of the LMC compared to the subjacent allochthons, it is likely that the complex was extracted from the Teplá-Barrandian upper plate due to subduction erosion prior to collective exhumation and stacking of the Karkonosze-Izera Massif.Graphical abstract

Not simply volcanoes – The Geoheritage of the Cretaceous System in the Land of the Extinct Volcanoes Geopark, West Sudetes (SW Poland)

Volcanic geoheritage is emphasized as the main asset and distinctive characteristic of the Land of Extinct Volcanoes Geopark in the West Sudetes (SW Poland). However, the geoheritage values of the region are not limited to the legacy of ancient volcanism but include various other elements. This paper explores the contribution of geosites that expose sedimentary rocks of Cretaceous age and landforms developed upon these rocks. Six localities from the Geopark area, included in the comprehensive regional inventory of geosites, are presented. They represent natural and man-made sandstone outcrops and show, among others, lithological variations, small- and large-scale post-sedimentary deformation structures, landforms arising from differential weathering (rock shelters, honeycombs), boulder fields and a sandstone xenolith in volcanic rocks. Next, five localities from outside the Geopark, but still within the Pogórze Kaczawskie region, are described. Qualitative and quantitative evaluation of both groups is attempted, and the results show that, in general, geosites within the Geopark rank higher. However at least two from the other group also have significant geotourist potential. Finally, a brief comparative analysis with other parts of the Sudetes, where Cretaceous sedimentary rocks occur, is offered.

Influence of transboundary transport of trace elements on mountain peat geochemistry (Sudetes, Central Europe)

Mountain ombrotrophic peatlands in Central Europe are an important stock of transboundary contamination both of natural and anthropogenic origin. The Śnieżka Mountain (West Sudetes) forms a significant orographic barrier and receives aerosols from broadly-recognized anthropogenic sources (production and use of stainless steel, processing of uranium, coal combustion, nuclear weapon tests, and Chernobyl accident). The main objective of the study was to assess the pattern of distribution and origin of trace elements and to distinguish the long-range transport vs. local signals in two 210Pb and 14C – dated peat cores from the highest summit of the Karkonosze (West Sudetes) spanning the last 280 years.Maximum values and accumulations of almost all investigated elements (Pb, Zn, Cu, Ni, Cr, Ti, Al, U, Sc, and REE) were identified around the 1970s. The analysis of peat using scanning electron microscopy (SEM) confirmed the occurrence of spheroidal aluminosilicate fly ash particles (SAP) in the topmost 40 cm (from AD 1938) together with a maximum of mullite (3Al2O3·2SiO2), an anthropogenic marker originating from coal-based power plants. The overall 206Pb/207Pb signature ranges from 1.160 to 1.173, indicating a predominant contribution of anthropogenic Pb. Human activities promote the release of mobile 234U, due to the weaker bonds to mineral structure, and cause the radiogenic disequilibrium between 238U and its daughter 234U.

Chronology of the Saxothuringian subduction in the West Sudetes (Bohemian Massif, Czech Republic and Poland)

Isotopic dating of monazite and garnet from high-pressure metamorphic rocks exposed in the northern part of the Saxothuringian palaeo-suture in the Bohemian Massif revealed a diachronous metamorphism of various rock types that are now closely associated within allochthonous units representing the Devonian–Carboniferous subduction–accretionary complex. Mafic blueschists of the middle unit yielded a Lu–Hf garnet age of 363.9 ± 1.3 Ma. The blueschists occur within high-pressure, garnet-free phyllites. Monazite extracted from this rock type yielded a U–Pb isotope dilution thermal ionization mass spectrometry age of 336.5 ± 0.5 Ma. Garnet-bearing micaschist of the lower unit contains monazite with a U–Pb secondary ion mass spectrometry age of 341 ± 3 Ma, consistent with Lu–Hf garnet–whole-rock ages of 344.5 ± 1.3 and 342 ± 7 Ma obtained from the same rock type. Existing tectonic models of the Bohemian Massif, and particularly of its northern part, assume that the period of oceanic subduction was terminated at c. 380–375 Ma by the arrival of an attenuated Saxothuringian continental crust, which was partly subducted and partly relaminated underneath the overriding Teplá–Barrandian Domain. However, our data, as well as data from mafic high-pressure rocks in the southern part of the Saxothuringian Domain, suggest that the initial collision was probably caused by the arrival of a smaller crustal block present within the Saxothuringian oceanic domain. After its subduction and relamination, the oceanic subduction was re-established and terminated by continental subduction and later collision of the Saxothuringian passive margin at c. 345–335 Ma.

UAV and SfM in Detailed Geomorphological Mapping of Granite Tors: An Example of Staro\u015bci\u0144skie Ska\u0142y (Sudetes, SW Poland)

The paper presents an example of using photographs taken by unmanned aerial vehicles (UAV) and processed using the structure from motion (SfM) procedure in a geomorphological study of rock relief. Subject to analysis is a small rock city in the West Sudetes (SW Poland), known as Starościńskie Skały and developed in coarse granite bedrock. The aims of this paper were, first, to compare UAV/SfM-derived data with the cartographical image based on the traditional geomorphological field-mapping methods and the digital elevation model derived from airborne laser scanning (ALS). Second, to test if the proposed combination of UAV and SfM methods may be helpful in recognizing the detailed structure of granite tors. As a result of conducted UAV flights and digital image post-processing in AgiSoft software, it was possible to obtain datasets (dense point cloud, texture model, orthophotomap, bare-ground-type digital terrain model—DTM) which allowed to visualize in detail the surface of the study area. In consequence, it was possible to distinguish even the very small forms of rock surface microrelief: joints, aplite veins, rills and karren, weathering pits, etc., otherwise difficult to map and measure. The study includes also valorization of particular datasets concerning microtopography and allows to discuss indisputable advantages of using the UAV/SfM-based DTM in geomorphic studies of tors and rock cities, even those located within forest as in the presented case study.

Multiphase formation of the Obří důl polymetallic skarn deposit, West Sudetes, Bohemian Massif: geochemistry and Re–Os dating of sulfide mineralization

The Obři důl Fe–Cu–As polymetallic sulfide skarn deposit is developed in a metamorphic series in the West Sudetes, Bohemian Massif. It consists of lenses of marble, calc–silicate rocks, and skarns. We studied the Gustav orebody, which is located few hundred meters away from the contact with a large, late-orogenic Variscan Krkonose–Jizera Plutonic Complex (KJPC) emplaced into shallow crust. Mineralogical and fluid inclusion study evidence indicates that the main sulfide stage, dominated by pyrrhotite, arsenopyrite, and chalcopyrite originated from aqueous hydrothermal fluids with salinity up to 8 wt% NaCl eq. with minimum homogenization temperatures ranging from 324 to 358 °C. These fluids mainly replaced carbonate-rich lithologies. Carbon, oxygen, and strontium isotope data in Ca-rich rocks imply total overprinting by channelized metasomatic fluid flow, which is most probably related to the intrusion of the KJPC, whereas δ34S values of sulfides argue for a magmatic source of sulfur. The Re–Os age of arsenopyrite overlaps published age data for the KJPC and suggests synchronous formation of the main sulfide mineralization and pluton emplacement.

Customer-Oriented Evaluation of Geoheritage—on the Example of Volcanic Geosites in the West Sudetes, SW Poland

Thirty geosites representing ancient volcanism in the West Sudetes are subject to quantitative, multi-variant evaluation that takes into account the geoscientific background and likely preferences of visitors. To account for the diversity of visitors, different weightings of criteria included in the evaluation procedures are applied. Four rankings were prepared: a ‘non-weighted’ one and three ‘customer-oriented’. Differences between rankings are relatively small, although notable individual exceptions are noted. Top positions in different rankings are invariably occupied by the same group of geosites which are highlights of the region, although apparently for different purposes. However, scientific, educational, additional and scenic values may be causally related. Some geosites are of outstanding significance for the geoscientific community but appear less appealing to others, particularly unaware visitors. The results of the exercise may assist in resource allocation for developing interpretative facilities and diversifying the level of interpretation between geosites.

Magmatic and post-magmatic phenomena in the Karkonosze granite and its metamorphic envelope (West Sudetes, SW Poland)

AbstractMineralogical studies of the Karkonosze granite (ca. 322-312 Ma) and its surroundings in West Sudetes (SW Poland) have provided data on Nb-Ta-REE minerals from pegmatites in the NE part of the pluton and several new finds of Ag minerals and 15 oxygenic Bi phases, hitherto not reported from the massif. The Karkonosze pegmatites are enriched in HREE as fergusonite-(Y) or xenotime-(Y) appear in almost every studied pegmatite, together with a subordinate assemblage of the aeschynite, euxenite or columbite group. The abundance of LREE minerals such as allanite-( Ce) and the monazite group, correlates inversely with the Nb-Ta-Ti minerals, whilst an early generation of monazite-(Ce) revealed an exceptionally high amount of Nd (up to 22 wt.% of Nd2O3). The physical and chemical conditions during the magmatic and post-magmatic processes were reconstructed and the effects of contact metamorphism in amphibolites from hornfelsed zones examined. Changes in solution composition and concentration at the early magmatic stage (825-920°C), pegmatitic stage overlapping with hydrothermal (560°C which ended at 160-90°C) and clearly hydrothermal stage (400 to 110°C) were studied in detail by means of melt and fluid inclusions in quartz. Furthermore, post-magmatic fluids, including some enriched in Li and B, were identified in rock-forming quartz from the whole pluton. In turn, study of the amphibolites indicates that the pair cummingtonite + anorthite or the presence of Ca-rich plagioclase with actinolite seem to be reliable mineral proxies of the thermal impact of the granitoid body on amphibolites in its envelope. The inferred conditions of the contact processes (450-550°C, 2.5- 4.8 kbar) point to an elevated geothermal gradient (ca. 32-45°C/km) probably reflecting the heat flow induced by the Karkonosze intrusion. Moreover, despite the textural and mineral changes imposed by regional and contact metamorphism, the amphibolites have their pre-metamorphic (magmatic) geochemical features undisturbed.

Pressure–temperature estimates of the blueschists from the Kopina Mt., northern Bohemian Massif, Poland – constraints on subduction of the Saxothuringian continental margin

The blueschist occurrence at the Kopina Mt. is situated at the eastern margin of the Karkonosze–Izera Massif in the West Sudetes of the northern Bohemian Massif. Metabasic rocks with continental crust affinity occur as lenses within the prevailing metasediments. The metabasites consist mainly of garnet, blue amphibole, epidote, chlorite-I, titanite, hematite and quartz forming the high-pressure assemblage. Synkinematic garnet exhibits prograde zoning, a feature allowing for assessment of the prograde course of the pressure–temperature ( P–T ) path. Phase equilibrium modelling has been used to estimate prograde and peak metamorphic conditions. Based on the P–T pseudosections, calculated in the system Na 2 O–CaO–K 2 O–FeO–MgO–MnO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O 2 (NCKFMMnASHTO), the garnet + glaucophane + epidote + chlorite + white mica(?) + Fe oxide + quartz ± titanite assemblage was formed between 12–15 kbar and 480–520 °C. These results are based on garnet and Na-amphibole compositional isopleths. Garnet shows a prograde zoning expressed by the rapid rim-ward decrease of spessartine, moderate increase of almandine and as light increase of other components. It is inferred here that white mica must have decomposed to secondary albite–chlorite-II–K-feldspar. The obtained results point to the formation of the Kopina blueschists along a low-temperature P–T gradient of 8–10 °C/km, typical of rocks from a subduction–exhumation channel. We postulate that the Kopina blueschists were formed in a subduction system developed along the eastern termination of the Saxothuringian Ocean and operating beneath the Tepla–Barrandian upper plate. When combined with previous data on continental crust affinity for the protolith, our results demonstrate a derivation of the Kopina blueschists from subducted part of the Saxothuringian margin.

Geoturystyka – nowe możliwości wykorzystania dziedzictwa ziemi w turystyce. Studium przypadku Krainy Wygasłych Wulkanów W Sudetach Zachodnich

Wzrost zainteresowania mozliwościami wykorzystania dziedzictwa Ziemi na potrzeby rozwoju turystyki sklania do rozwazan, czy geoturystyka jest nową formą turystyki, czy tylko nowym terminem określającym turystyke uprawianą juz dawniej, związaną z odwiedzaniem miejsc interesujących pod wzgledem geologicznym. Studium przypadku Krainy Wygaslych Wulkanow w Sudetach Zachodnich, obszaru kreującego swoją marke i produkt turystyczny oparte na wulkanicznej przeszlości geologicznej, uzasadnia traktowanie geoturystyki jako nowej formy turystyki. Od niedawna akcentowany jest bowiem poznawczy charakter odwiedzania obiektow związanych z dziejami Ziemi i ścisle powiązania z geoedukacją. Wyroznikiem geoturystyki jest tworzenie odpowiedniej infrastruktury edukacyjnej. Początki geoturystyki na badanym obszarze siegają lat 90. XX w., ale jej rozwoj przypada na ostatnią dekade i jest związany glownie z dzialaniami oddolnymi, inicjowanymi przez liderow spoleczności lokalnych.

Overlooked Geomorphological Component of Volcanic Geoheritage—Diversity and Perspectives for Tourism Industry, Pogórze Kaczawskie Region, SW Poland

In the West Sudetes (SW Poland), volcanic activity of Oligocene and Miocene age has left a suite of landforms of considerable geomorphological interest. Besides being relevant to the subject of volcanology, they illustrate how volcanism influences interplays with landscape development in the long-term. Three main geomorphological themes are explored: structural geomorphology of ancient volcanic terrain, volcanic remnants as markers of long-term denudation and hillslope evolution under periglacial conditions of the Pleistocene. Volcanic geosites are used as a resource for geotourism and promotion of geoheritage has intensified recently, with the region branding itself as the ‘Land of Extinct Volcanoes’. Ten localities are evaluated as geomorphosites of significant educational potential, taking into account geomorphological values, added values as well as physical accessibility and information supply. Successful interpretation remains a key challenge in further geotourism development, and several associated practical issues are pointed out such as choice of terminology, uncertainties in scientific understanding and stand-alone attractiveness of geoheritage.

Historical and recent floods in the West Sudetes, Central Europe – the geomorphological dimension

Historical and recent floods in the West Sudetes, Central Europe – the geomorphological dimension

Post-Variscan cooling history of the central Western Sudetes (NE Bohemian Massif, Poland) constrained by apatite fission-track and zircon (U-Th)/He thermochronology

Apatite fission track (AFT) and zircon (U-Th)/He (ZHe) dating techniques were combined to reveal post-orogenic history of the Rudawy Janowickie Mts. in the West Sudetes (Bohemian Massif). Permian to Jurassic ZHe ages of 271.2±24.8–171.3±11.5Ma obtained for crystalline basement rocks indicate post-orogenic cooling and progressive unroofing of the NE Bohemian Massif. On the other hand, Cretaceous ZHe ages of 130.5±8.8–99.7±6.7Ma prove that the Rudawy Janowickie Mts. region remained under ~190°C during the mid-Cretaceous, suggesting a deep burial. ZHe ages from the Cretaceous sediments located to the east of the Rudawy Janowickie Mts. range between 298.8±16.8 and 181.8±11.9Ma, documenting that the burial did not exceed 6km in the axial part of the Intra-Sudetic Through. AFT ages range from 105.9±6.1 to 51.1±4.3Ma. The Eocene AFT ages revealed by the granite basement and the Cretaceous AFT age revealed by the metamorphic cover suggest the existence of at least two different thermo-tectonic blocks experiencing different post-Variscan cooling and exhumation. AFT results strongly support 3.3–3.5km exhumation for the Rudawy Janowickie Mts. since the Cenomanian, which in combination with ZHe results, clearly contradicts some of the previously proposed evolutionary scenarios for the Sudetes.

Controlling factors limiting timberline position and shifts in the Sudetes: A review

Three isolated massifs in the Sudetes, Central Europe, are elevated sufficiently high to allow for the development of the treeline ecotone. These are the Karkonosze/Krkonose in the West Sudetes and Hruby Jesenik and Masyw Śnieznika/Kralický Sněžnik in the East Sudetes. The upper limit of closed tree stands (i.e. timberline) is located at c. 1250 m a.s.l. on average in the Krkonose, but with significant variability spanning more than 500 m. In the East Sudetes the respective elevation is higher, above 1300 m a.s.l., and the variability is smaller. While temperature is the primary factor governing the uppermost tree stands, second-order climatic factors play an important role in shaping treeline ecotone position, particularly wind and snow accumulation patterns. Active surface processes such as debris flows and snow avalanches force the timberline to descend and account for its locally very irregular course. There is a history of long-term human impact on the position of the timberline, with its peak in the 17-19th centuries when high-mountain meadows were extensively used for grazing and haymaking. In the last century the overall trend of timberline ascent associated with abandonment of agricultural land and temperature rise has been interrupted by the episode of catastrophic forest decline due to air pollution.

Monazite to the rescue: U–Th–Pb dating of the intrusive history of the composite Karkonosze pluton, Bohemian Massif

The large (~700km2) composite Karkonosze (Krkonoše) pluton in the West Sudetes, on the border between Poland and the Czech Republic, consists mainly of porphyritic and equigranular granitoids, but contains a range of lithologies from lamprophyre to leucogranite. The absolute age and duration of the plutonism have proved difficult to determine. Previous age measurements by Rb–Sr, Ar–Ar and U–Pb range from ~330 to 290Ma, with more recent results converging to ~320–300Ma. Dating of zircon and monazite from samples of a variety of major and minor lithologies by SIMS U–Th–Pb, several from the geochemical study of Słaby and Martin (2008), has narrowed the possible age range further. U–Pb ages measured on eight of ten zircon and monazite samples are in the range ~314–311Ma. Zircon ages measured on the two major types of porphyritic granitoid are 313±3 and 311±4Ma, and monazite ages are 312±2, 313±3 and 311±3Ma. Monazite from one hybrid granitoid has an age of 314±3Ma, and zircon from another an age of 314±4Ma. Zircon from a composite dyke has an age of 311±6Ma. The monazite U–Pb age of an equigranular granite, at 318±6Ma, is consistent with geological evidence that it is older than the porphyritic granitoids but, because of the relatively large uncertainties, is not conclusive. Zircon from one microgranular enclave is anomalously young, 302±4Ma. Evidence is mounting that the main porphyritic granitoids, hybrid granitoids and composite dykes were emplaced within a short time interval between 314±4 and 311±3Ma. Given the uncertainties, emplacement of these units could have been effectively simultaneous. The larger difference between the ages from the equigranular granite and microgranular enclave, however, indicates that the whole Karkonosze thermal episode possibly lasted as long as 15Ma.

Contrasting tectono-metamorphic evolution of orogenic lower crust in the Bohemian Massif: A numerical model

The Bohemian Massif, located at the eastern margin of the European Variscan belt, is characterised by an exceptional accumulation of felsic high-pressure granulites. The petrological, structural and geochronological studies of this region revealed systematic differences between the tectonometamorphic evolution of the southern (Moldanubian) and northern (West Sudetes) parts of the orogen. Two contrasting tectonic scenarios have been proposed: gravity-driven vertical mass exchanges followed by continental indentation in the Moldanubian domain, and crustal-scale folding leading to gneiss dome formation in the West Sudetes. We present a numerical model in order to correlate the apparent differences between these two regions with the variations in the dynamics of the modelled system. We model two colliding blocks: an orogenic root, where a felsic lower crust is overlain by a mafic layer and a middle crust, and a continental indentor. We examine the role of the rate of convergence of the two blocks, radiogenic heat production within the felsic lower crust and efficiency of erosion. The prograde part of the metamorphic evolution is controlled by the rate of convergence and the peak temperature depends on the heat production. The retrograde evolution is controlled mostly by erosional processes. In the models, where the material is weakened due to the heating in the felsic lower crust, the gravitational instability of the mafic and felsic layers causes their complete vertical exchange followed by a flow above the indentor. In colder and/or faster models, the thickening is dominated by the buckling of the mafic layer. These two styles of deformation, i.e. gravity-dominated and fold-dominated models, correspond to the structures observed in the Moldanubian and the West Sudetes. Moreover, the calculated pressure–temperature paths of the felsic lower crust are in agreement with available data.

Amphibolites from the Szklarska Poręba hornfels belt, West Sudetes, SW Poland: magma genesis and implications for the break-up of Gondwana

Amphibolites from the Szklarska Poreba hornfels belt (northern part of the Karkonosze-Izera Massif) represent rocks of alkali-basalt composition metamorphosed during Variscan times. Despite the intense thermal influence of the Karkonosze granite superimposed on the effects of regional amphibolite-facies metamorphism, the geochemical affinities of the Szklarska Poreba amphibolites are well preserved. They are similar to alkaline OIB basalts derived from an enriched (undepleted) sub-lithospheric source in the garnet stability field at depths ca 80–120 km. Trace-element characteristics and geochemical modelling indicate that the source was not modified by metasomatism in a supra-subduction zone or by alkali (silicate, carbonatitic) infiltration. Subsequent intra-crustal fractional crystallization involved olivine and clinopyroxene, and subordinate spinel and, presumably, plagioclase. The chemical composition of the rocks is most similar to that of modern magmas generated in an extensional setting (intra-continental rift). Neither geochemical characteristics nor estimated mantle temperatures only slightly higher than those of ambient mantle convincingly attest to the involvement of deep-mantle plume activity. Instead, decompression melting of passively upwelling asthenosphere beneath opening fractures in fragmented lithosphere is invoked. The origin of the amphibolite protolith was presumably associated with the Early Palaeozoic rifting of northern Gondwana, well documented throughout the Karkonosze-Izera massif. Locally rifting must have ceased earlier (immature rift) as reflected by mafic dykes exposed in the northern part of the massif, i.e., in the Szklarska Poreba hornfelses, and by the Izera gneisses and the Stara Kamienica metapelites. A passive rift system controlled by lithosphere extension provides a plausible explanation for the origin of mafic rocks in the Karkonosze-Izera Massif and sheds light on possible mechanisms involved in the break-up of Gondwana.

The Izera metabasites, West Sudetes, Poland: Geologic and isotopic U–Pb zircon evidence of Devonian extension in the Saxothuringian Terrane

In the Lausitz–Izera Massif of the Saxothuringian terrane, part of the central European Variscides, 515–480 Ma metagranites have been intruded by a swarm of basic veins that have within-plate alkali basalt geochemistry, or, rarely, normal mid-ocean ridge basalt (N-MORB) geochemistry. Based on their spatial orientation, distribution in the region and structural relationships to the host metagranites, these basic veins could be grouped into three types that displayed six varieties of host–vein contacts. Zircons extracted from eight of the basic veins revealed that there were two populations: clear zircons and brown zircons. Each type were subjected to both single grain conventional U–Pb analysis and to sensitive high-resolution ion microprobe (SHRIMP) U–Pb analysis. The clear grains yielded ages > 480 Ma, clustering at ~ 500 Ma; the brown grains yielded ages between ~ 390 and 365 Ma, clustering at ~ 370 Ma. The colourless zircons possess an oscillatory zoned structure and rare earth element (REE) distribution pattern that suggests they come from the host granites and from deeper Neoproterozoic granitoid basement. However, the basic host rocks show almost no obvious contamination by felsic material. The turbid brown grains, which are dark under cathodoluminescence, also seem to be mainly magmatic because of their internal structures, high Th/U ratios and REE pattern. When the zircon results are integrated into the regional geology, an ~ 30 m.y. period of episodic, intra-plate, predominantly alkaline basic magmatism at the passive margin of the Saxothuringian Terrane is suggested, with maximum activity at ~ 370 Ma during an extensional regime. Combining the factors of the relatively high thermal gradient during early metamorphism of the metabasites (~ 450 °C, 2 kbar), the geology of Saxothuringia and the regional structural data, we propose a plume model of “hot fingers” type whereby the basic magmatism of the Izera region was due to repeated Devonian intrusions derived from garnet-bearing enriched (OIB type) and less commonly depleted asthenosphere sources. The waning stages of this activity were coeval with the early deformation of the Izera granite host–basic vein system and matched by rifting and extensional widening of the Saxothuringian Basin in its Izera–Kaczawa segment.

Variscan progradeP-Tevolution and contact metamorphism in metabasites from the Sowia Dolina, Karkonosze-Izera massif, SW Poland

AbstractSeveral bodies of moderately foliated and porphyroblastic metabasites crop out on the SE side of the metamorphic cover of the Karkonosze granite within metapelites of the Sowia Dolina area (West Sudetes, Saxothuringian zone). Depending on the microstructural setting of the Ca-amphiboles in the rocks, different mineral-chemical trends have been determined for Si,XMg, AlVI,A[Na+K] which serve as semi-quantitative indicators of temperature and pressure changes. Porphyroblasts and prisms oblique to the main foliation in schistose metabasites show zoning from Mg-hornblende and actinolite to tschermakite, and then to Mg-hornblende (or actinolite). Matrix amphiboles and those in pressure shadows around some porphyroblasts have tschermakitic cores and actinolitic rims. Rarely, Ca-amphibole is accompanied in schists by late- to post-tectonic cummingtonite. Thermobarometric calculations involving empirically calibrated amphibole equilibria enable a reconstruction ofP-Tpaths for individual rocks and the unravelling of the metamorphic evolution of the metabasites. Peak metamorphic temperatures of 615–640°C and pressures of 7.3–8.2 kbar were preceded by a variably preserved earlier stage (T = 370–550°C, P = 2.8–6.2 kbar). The final metamorphic episode took place at 450–550°C and 2.5–4.8 kbar and is recorded particularly in rocks close to the Karkonosze pluton. The metabasites shed new light on the history of metamorphism in the Sowia Dolina area. The first two stages ofMP-MTmetamorphism, coeval with Variscan deformation events (continental collision, burial and subsequent exhumation), took place under epidote-amphibolite then amphibolite facies conditions. The last stage partly concurred with the final stages of Variscan deformation and overlapped the onset of thermal activity associated with the Karkonosze granite. This metamorphic event is documented by metabasites (occasionally cummingtonite-bearing) outcropping close to the granite. Finally, a prehnitebearing assemblage reflects retrograde re-equilibration under greenschist/sub-greenschist facies conditions (T<300–350°C,P<2.5–3 kbar), which might also be partly due to hydrothermal activity around the pluton.