Abstract
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.
Highlights
Magmatic accessory minerals may crystallize during different episodes of magma formation and differentiation
This is true for zircon, which can represent crystals formed in contemporary magma, crystals formed in earlier magmatic episodes, unresorbed remnants
This diversity for Permo-Carboniferous rhyolites of central Europe was previously characterized on zircon separates [1,23,39]; we showed that more information on the presence and source of antecrystic/inherited components can be obtained from mineral liberation analysis (MLA)-Scanning Electron Microscope (SEM) analyses of the structural positions of zircon in thin sections of rock
Summary
Magmatic accessory minerals may crystallize during different episodes of magma formation and differentiation. The abundance of inherited zircon can be fairly identified using Scanning Electron Microscope (SEM) analysis, and confirmed by in situ zircon dating. Rhyolites often contain mixed autocrystic and antecrystic zircon cargos [1,3,4,5,12,13] and any technique that facilitates the recognition of these populations provides additional information on the origin of rhyolite magma. This in turn may be important to improving the understanding of zircon ages in rhyolitic rocks [14]
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