ALBERT

Description: Voluminous rhyolitic lavas and ignimbrites ( c . 34 000 km 3 ) were formed in the NE German Basin in a post-collisional setting at c . 295 Ma. Trace elements, Hf and 18 O have been measured in dated magmatic and inherited rhyolitic zircons from three drill cores across the basin. Magmatic zircons crystallized in two stages: the first stage in less differentiated and isotopically heterogeneous magmas and the second stage in more differentiated and isotopically homogeneous magmas. The first and second stages can be related to the crystallization of zircons within a heterogeneous pluton and in an evolved, silicic magma that was later erupted. Some zircons crystallized entirely during the evolved magma stage and provide good resolution for identifying and characterizing processes happening shortly before eruption. The isotopic compositions of the magmatic zircons constrain the proportions of juvenile and reworked materials involved in the formation of voluminous silicic magmas in a post-collisional tectonic setting. The advantage of studying the volcanic rocks from the NE German Basin is that their petrogenesis involved a relatively simple, two-source interaction during magma production, which permits quantitative estimation of the amount of each source component. AFC modelling shows that the first stage zircons crystallized from magmas containing 5–80% of the juvenile component, whereas the final rhyolitic magmas contained 30–40% of this material. The inherited zircons have Hf model ages of 1·9–2·2 Ga, suggesting that much of the local basement was initially derived from the mantle at that time and that it was subsequently reworked at around 1·5 Ga. Similar model ages are a feature of Baltica-derived sediments and the implication is that such sediments underlie large areas of the NE German Basin. The lack of any record of Avalonian basement in the NE German Basin may indicate that both the sedimentary cover and the underlying basement are part of Baltica.

Description: The Polish Lowlands, located southwest of the Teisseyre–Tornquist Zone, within Trans-European Suture Zone, were affected by bimodal, but dominantly rhyolitic, magmatism during the Late Paleozoic. Thanks to the inherited zircon they contain, these rhyolitic rocks provide a direct source of information about the pre-Permian rocks underlying the Polish Lowland. This paper presents zircon U–Pb geochronology and Hf and O isotopic results from five drill core samples representing four rhyolites and one granite. Based on the ratio of inherited vs. autocrystic zircon, the rhyolites can be divided into two groups: northern rhyolites, where autocrystic zircon is more abundant and southern rhyolites, where inherited zircon dominates. We suggest that the magma sources and the processes responsible for generating high silica magmas differ between the northern and southern rhyolites. Isotopically distinct sources were available during formation of northern rhyolites, as the Hf and O iso-topes in magmatic zircon differ between the two analysed localities of northern rhyolites. A mixing between magmas formed from Baltica-derived mudstone–siltstone sediments and Avalonian basement or mantle can explain the diversity between the zircon compositions from the northern localities Daszewo and Wysoka Kamieńska. Conversely, the southern rhyolites from our two localities contain zircon with similar compositions, and these units can be further correlated with results from the North East German Basin, suggesting uniform source rocks over this larger region. Based on the ages of inherited zircon and the isotopic composition of magmatic ones, we suggest that the dominant source of the southern rhyolites is Variscan foreland sediments mixed with Baltica/Avalonia-derived sediments.

Description: Isotope and chemical analyses of zircon from the Po lish Lowland provide information on magma sources and processes of magma evolution during the period of intense magmatism that affected Central Europe at the Carboniferous/Permian boundary. Since zircon reflects 18 O/16 O of the magma from which it crystallized, we have measured the 18 O/ 16 O in zircon to establish the fraction of the rhyolite precursors that was derived from juvenile magmas and also to reconstruct magma evolution during the zircon crystallization. Zircon isotope ratio determinations were conducted at the GFZ-Potsdam using the Cameca 1280-HR SIMS instrument. The reference zircon 91500 (Wiedenbeck et al. 2004) was analyzed 39 times as part of this project, yielding a repeatability of ± 0.2 ‰ (1SD). We studied zircon grains from four localities Pniewy, Wysoka, Daszewo and Chrzypsko. Altogether more than 100 magmatic grains were measured. The highest δ 18 O values were measured in Daszewo and the lowest in Wysoka suggesting that magmas from different localities represent different sources. Each locality have zircon with the range of δ 18 O values exceeding the error, which is consistent with zircon crystallizing in an evolving magma or in separate magma batches. The latter seems to be more probable as zircon is not zoned in δ 18 O from the core to the rim. As the whole, zircons from the Polish Lowland have the greatest variability in δ 18 O observed so far amongst all of the localities studied from the Central European basin large igneous province (Piet ranik et al. 2013). The results extend from low values typical for derivation of the magmas from the sources altered by hydrothermal activity up to results consistent with a important sedimentary input into the system. So far the localities from the Polish Lowland are the first ones studied within the CEB-LIP, which show δ 18 O lower than the mantle values (δ 18 O 〈5.3 ‰).

Description: The Central European Basin LIP extends from the North Sea across northern Germany into Poland [1]. Strong magmatic activity occurred in this area at the Carboniferous–Permian transition and produced about 70000 – 80000 km3 of volcanic material. The majority of the material consists of felsic rocks (48000 km3) [2] which are covered by thick sediments. Hundreds of deep hydrocarbon exploration wells exposed the Late Paleozoic volcanic province with different regions being now under detailed examination. In this study we focused on the comparison of zircons from the NE German Basin [3] and the Halle Volcanic Complex [in prep] with new samples from the Polish Lowland. We investigated three samples from ignimbrite (Daszewo Wysoka,. Pniewy), which were dated respectively: 293 Ma, 294 Ma and 298 Ma [1] and two undated samples from Chrzypsko (from rhyolite and underlying granite). The analyses of zircon include dating, O and Hf isotopes as well as detailed measurements of Hf concentration. The basement in the Polish Lowland is hidden by a thick sedimentary cover and zircons are the only grains that provide information on the structure and composition of Permain magmatism in this area as well as on the pre-Permian evolution of the continental crust on the boundary between several important crustal fragments.

Description: The late Carboniferous/early Permian post-collisional rhyolites (305–285 Ma) that formed in Central Europe have generally similar whole rock compositions to that of older Late-Variscan rhyolites (330–310 Ma). However, data compilation combining zircon age with the chemical composition of rhyolites from 20 units shows a trend of increasing zircon saturation temperature with decreasing age. This trend is particularly well identified in rhyolites from the Central European Lowlands (CEL)—consisting of the NE German and NW Polish Basin—and also correlates their location with the zircon saturation temperature increasing from SE to NW from 750°C to 850°C. We infer that these higher temperatures of zircon saturation reflect a contemporaneous change in the tectonic setting from collisional to divergent, reflecting the onset of the Central European continental rifting. This interpretation is further corroborated by the trace element compositions of the CEL zircons, which resembles zircon crystallized in a divergent setting. Interestingly, the zircon formed globally in this type of setting is chemically diverse, especially considering uranium concentration. For example, zircon from locations dominated by mafic magma fractionation, such as rhyolites from Iceland, have low U concentrations and low U/Yb ratios. On the other hand, zircon formed in rhyolites in rifted margins, like western North America, tends to have much higher U and U/Yb ratios. Such high concentrations are not observed in zircon from the CEL, suggesting that the mantle input could be higher and residence times within continental crust shorter than those for rhyolites from the Cenozoic western USA. This may, in turn, suggest that the region might have been affected by a hot spot, similar to that responsible for rhyolite formation of the Snake River Plain.