Zircon is a main mineral used for dating rhyolitic magmas as well as reconstructing their differentiation. It is common that different populations of zircon grains occur in a single rhyolitic sample. The presence of both autocrystic and antecrystic zircon grains is reflected in their strongly varied chemical compositions and slight spread of ages. However, postmagmatic processes may induce lead loss, which is also recorded as a spread of zircon ages. Therefore, new approaches to identify different zircon populations in rhyolitic rocks are needed. In this study, we suggest that detailed examination of zircon positions in the thin sections of rhyolitic rocks provides valuable information on zircon sources that can be used to identify autocrystic and antecrystic zircon populations. Automated Scanning Electron Microscope (SEM) analyses are of great applicability in determining this, as they return both qualitative and quantitative information and allow for quick comparisons between different rhyolite localities. Five localities of Permo-Carboniferous rhyolites related to post-Variscan extension in Central Europe (Organy, Bieberstein, Halle, Chemnitz, Krucze) were analyzed by automated SEM (MLA-SEM). The samples covered a range of Zr whole rock contents and displayed both crystalline and glassy groundmass. Surprisingly, each locality seemed to have its own special zircon fingerprint. Based on comparisons of whole rocks, modal composition and SEM images Chemnitz ignimbrite was interpreted as containing mostly (or fully) antecrystic zircon, whereas the Bieberstein dyke was shown to possibly contain both types, with the antecrystic zircon being associated with disturbed cumulates. On the other hand, Organy was probably dominated by autocrystic zircon, and Krucze contained dismembered, subhedral zircon in its matrix, whereas Halle zircon was located partly in late veins, filling cracks in laccolith. Both localities may, therefore, contain zircon populations that represent later stages than the crystallization of the main rhyolitic body.
<p>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 &#177; 1.8 Ma to 301 &#177; 3 Ma suggesting the prolonged evolution of this subvolcanic-volcanic system. In this study, we sampled the Landsberg (301 &#177; 3 Ma) and the Petersberg (292 &#177; 3 Ma) laccoliths to better identify the magmatic processes involved in silicic magma formation and their duration.&#160; 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&#177;2.8 Ma) and the lowermost horizon (297.1&#177;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&#160; magma injections amalgamated separately within the system.</p> <p>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.</p> <p>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/ST10/00180</p>
Amphibole- and clinopyroxene-bearing monzodiorites were emplaced at 340 Ma (CA-ID-TIMS zircon age), suggesting the formation of hydrous and dry magmas closely related in space and time in the NE Bohemian Massif. Hafnium and oxygen isotopes of zircon in less evolved rocks (<55 wt% SiO2) are similar between Amp and Cpx monzodiorites (εHf = −3.3 ± 0.5 and − 3.5 ± 0.8; δ18O = 6.4 ± 1.0 and 6.8 ± 0.7, respectively), consistent with a common source—a contaminated mafic magma derived from an enriched mantle. At the same time, the conditions of crystallization are distinct and zircon appears to be an excellent tool for distinguishing between hydrous and anhydrous crystallization conditions, a process that may be more ambiguously recorded by whole rock and major mineral chemistry. In particular, elements fractionated by either amphibole or plagioclase crystallization, such as Hf, Dy, and Eu, differ in zircon from amphibole- and clinopyroxene-bearing rocks, and Zr/Hf, Yb/Dy, and Eu/Dy are therefore useful indices of crystallization conditions. We show that the composition of zircon from hydrous dioritic magmas is not comparable with that of typical zircon from dioritic-granitic suites worldwide, suggesting a specific process involved in their formation. Here, we propose that fluid-present remelting of a mafic underplate is necessary to explain the rock textures as well as the composition of the whole rock, zircon, and other minerals of amphibole-bearing monzodiorites and that a similar process may control the formation of amphibole-rich dioritic rocks worldwide, including appinitic suites. Overall, we show that dioritic rocks represent snapshots of differentiation processes that occur in the early stages of magma evolution before the magma is homogenized into large-scale batholiths.
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