Textural relationships between different mineral phases in rock such as grain size, shape, associated minerals, inclusions, overgrowths may provide information on crystallization conditions. Such relations between zircon and associated mineral phases can affect interpretation of zircon chemical composition and age. The information on the rock texture is best observed in thin sections. However, observations on thin sections are often neglected due to low number of zircon grains in a thin section, and usually only separates from larger rock volume are being analysed. By doing so, the information on occurrence of zircon within the rock is lost and different zircon generations cannot be traced back to a particular textural position in the rock. In this study we analysed thin sections from silicic volcanic rocks in order to characterize the textural position of zircon in the rock and its diversity. Zircon grains in lava flows and domes samples analysed in this study are rare and usually small. They often occur within the rim of phenocrysts e.g quartz, plagioclase, feldspar, biotite, which indicates that zircon crystallized during the last stage of magma evolution, probably not long before eruption. Therefore, dating of this zircon provides the age of this last stage, but its chemistry may be affected by crystallization in melt pocket concentrated along mineral boundaries. On the other hand, such zircon is absent from ignimbrite samples, which contain abundant and large grains in the glassy matrix. The difference between the zircon appearance in lavas and ignimbrites might be explained by many factors e.g. the difference in analysed localities, crystallization in volatile-rich magma chamber, zonation within magma chamber prior to eruption, sorting and concentration during pyroclastic deposition. Relacje teksturalne i strukturalne wielkośc, wyksztalcenie ziarna, wspolwystepowanie z innymi mineralami, inkluzje i przerosty miedzy roznymi fazami mineralnymi w skale mogą dostarczyc informacji na temat zroznicowanych warunkow krystalizacji. Takie relacje miedzy cyrkonem a fazami towarzyszącymi mogą miec kluczowe znaczenie, jeśli chodzi o interpretacje wieku cyrkonow datowanymi metodą U-Pb. Aby uzyskac informacje na temat teksturalnych relacji miedzy cyrkonem a pozostalymi mineralami glownymi skaly Qtz, Bt, Pl, zalecane są dokladne analizy plytek cienkich probek skalnych. Bardzo czesto ten rodzaj badan jest pomijany i bezpośrednio przechodzi sie do procesu separacji ziaren. W ten sposob utracone są informacje na temat wystepowania cyrkonu w skale, co jednoznacznie pozwala wydzielic i rozroznic poszczegolne etapy jego krystalizacji. W artykule przedstawiono wyniki analiz ponad 500 plytek cienkich skal wulkanicznych. Analizy te zostaly wykonane w celu opisania wystepowania cyrkonu oraz potencjalnego zroznicowania cyrkonu ze wzgledu na sposob jego krystalizacji. Ziarna cyrkonu zidentyfikowane w lawie i kopule lawowej są zazwyczaj male i wbudowane w brzegi mineralow. Świadczy to o ich krystalizacji podczas ostatniego etapu ewolucji magmy, najprawdopodobniej zaraz przed jej erupcją. Dzieki takim ziarnom uzyskujemy wiek tego etapu, niemniej jednak ich zapis chemiczny najprawdopodobniej odzwierciedla warunki krystalizacji w brzegowej strefie roznych mineralow. W ignimbrytach nie spotyka sie malych ziaren zamknietych w innych mineralach, natomiast wystepują liczne duze ziarna w szklistym tle skalnym. Taka roznica miedzy wystepowaniem cyrkonu w lawach i iginmbrytach moze byc wynikiem analiz konkretnych lokalizacji. Jednak mogą miec na to wplyw zarowno charakterystyczne warunki krystalizacji magmy np. w komorach silnie nasyconych gazami, jak i jej stratyfikacja w obrebie komory. Stąd tez ziarna mogą byc selektywnie gromadzone w osadach wulkanicznych odzwierciedlających cześci komory magmowej bogate bądź ubogie w krysztaly.
Abstract 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. Graphical abstract
Voluminous rhyolitic lavas and ignimbrites (c. 34 000 km3) were formed in the NE German Basin in a post-collisional setting at c. 295 Ma. Trace elements, εHf and δ18O 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.
The 500 m long section through the upper part of the Permo-Carboniferous Landsberg laccolith (Halle Volcanic Complex) was sampled every 25 meters. The modal proportions between plagioclase and K-feldspar phenocrysts vary in the section and the laccolith may be divided into four parts with different proportions of Pl/Kfs, which, in subvolcanic rocks, should reflect different proportions of these minerals in the magma plumbing system. Chemical composition of whole rock samples is uniform, but the correlations of Si and other elements with depth within all of the four sections suggest that the sections based on modal composition are also reflected in chemical composition of the rock. Also, the uppermost 100 meters of the laccolith has slightly higher contents of Fe, Ti, Zr and Nb compared to those in the rest of the laccolith and this is consistent with it being a separate magma pulse derived from a distinct source. Detailed analyses of chemical variations within each section are consistent with the model that the upper 500 meters of the Landsberg laccolith was formed by three successive pulses with slightly different chemical compositions. The best documented is the uppermost pulse, which was over-accreted on the first pulse. Another pulse was probably emplaced in the middle of the first pulse. The thickness of the pulses was 100–300 m, which is consistent with previous 2D and 3D emplacement models of the Halle laccoliths. However, the contacts between the pulses based on modal and chemical compositional variations are not always concurrent with the presence of shearing zones, the discrepancy that is not yet well understood. In general, because silica-rich laccoliths are relatively small bodies that cool quickly due to high level of emplacement, they may preserve better evidence for separate magma pulses compared to plutonic batholiths.
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 isotopes 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.
The Halle Volcanic Complex is composed of rhyolites interpreted as intrusive-extrusive complexes that pierced host sedimentary cover during their vertical growth. Zircon ages from several units vary from 291.7 ± 1.8 Ma to 301 ± 3 Ma suggesting the prolonged evolution of this subvolcanic-volcanic system. In this study, we sampled the Landsberg (301 ± 3 Ma) and the Petersberg (292 ± 3 Ma) laccoliths to better identify the magmatic processes involved in silicic magma formation and their duration.  Altogether seven depths have been analyzed from these two laccoliths including electron microprobe analyses of zircon and apatite and U-Pb SHRIMP dating of zircon. At the first sight, zircon is chemically similar within and between laccoliths. Additionally, SHRIMP ages are scattered over 30 Ma for each sample in Landsberg. These ages overlap with two Concordia ages obtained for the uppermost horizon (289.7±2.8 Ma) and the lowermost horizon (297.1±1.7 Ma) in the Petersberg laccolith. The ages suggest that the volcanic system was active for at least 10 Ma and similar age range is recorded in both laccoliths. The scatter of ages seems to indicate the formation of the laccoliths over a prolonged period of time with periodic reactivation of the magma chamber, but the lead loss cannot be excluded. Also, prolonged formation may indicate either younger pulses reactivating previously formed parts of the magma chamber or multiple unrelated  magma injections amalgamated separately within the system.The processes involved in the prolonged evolution of the magmatic system in Halle are evident from petrographic analyses of thin sections, where zircon can be imagined in association with other phases. Both zircon and apatite occur almost exclusively within complex glomerocrysts, an assemblage of major phases (variably altered biotite, feldspar, pyroxene). Such glomerocrysts were described in the literature and interpreted as remnants of crystal mush, probably re-mobilized at the final stage (heating episode) before laccoliths emplacement. The glomerocrysts in Petersberg and Landsberg laccoliths are similar leftovers of previous magmatic episodes, but they are special in that they contain abundant zircon and apatite. Such a picture is consistent with the evolution of magma in a long-lived magmatic system that underwent at least one reactivation. The major implication is that in some systems large proportion of zircon may represent the early stages of magma evolution, this context may be missed without detailed textural observations of zircon occurrence and associations.Acknowledgements: Christoph Breitkreuz is thanked for his constant help with our rhyolitic research. The research has been funded by the NCN research project to AP no. UMO-2017/25/B/ST10/0018
Abstract Permo-Carboniferous rhyolitic rocks are widespread in the NE German Basin and NW Polish Basin. Hafnium (Hf) and oxygen (O) isotopes analysed in zircon from these rocks suggest diverse sources and processes involved in the formation of rhyolitic magmas. In this study, detailed core-to-rim trace element compositions were analyzed in zircon from four localities that were previously analyzed for Hf and O isotopes. The trace element analyses, in particular Hf concentrations as well as Eu/Eu*, Ce/U, Yb/Gd, and Th/U ratios, are consistent with prolonged magma evolution in three localities from the NE German Basin (Fehmarn, Slazwedel and Penkun). The fourth locality within the NW Polish Basin (Wysoka Kamieńska) is consistent with a shorter period of magma evolution. Similar stages were distinguished in zircon from the three NE German Basin localities that include: early crystallization followed by rejuvenation with more primitive magma (stage A), subsequent fractional crystallization (stage B) and finally late crystallization in a saturated system or alternatively late rejuvenation with a more primitive magma (stage C). Interestingly magmatic rims on inherited zircon grains have compositions typical for late stage B and stage C, which is consistent with their late addition to evolving rhyolitic magma, most probably during assimilation and not during source melting. The zircon from the fourth, NW Polish Basin locality shows limited compositional variability consistent with the eruption of hot magma not long after the zircon started crystallizing. Thus trace element analyses in zircon provide a record of magmatic processes complementary to that of Hf and O isotope analysis, in that, a detailed analyses of core-to-rim compositional variations are particularly useful in distinguishing respective stages of magma evolution and can pinpoint the relative timing of inherited grains being incorporated into magma.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
