Abstract
Zircon with polycrystalline or polygranular appearance is either produced in the magmatic environment through crystallization, or due to deformation in metamorphic settings (including regional metamorphism and ductile shear zones), or as a result of shock-induced recrystallization. All three types can be easily confused and potentially lead to incorrect interpretations, especially if the crystallographic orientation analyses of zircon are not conducted. It is particularly important to establish the difference between tectonically-deformed polygranular zircon and shock-induced polygranular zircon because the latter serves as an indicator of shock event and is often used for dating asteroid impacts. In this paper, a series of polycrystalline zircon grains from ductile shear zones and metamorphic rocks are analyzed using a combination of techniques (BSE, CL, orientation contrast, EBSD, and microprobe mapping), and their properties are compared to reported polycrystalline zircons from magmatic and impact settings. This work shows how appearance, crystallographic orientation, and CL signature of “granules” differ between the different types of deformed zircon.
Highlights
Zircon is an important accessory mineral, widely used for isotopic dating in geochronology [1,2,3,4].Usually, the zircon phase grows in magmatic and metamorphic rocks as regular single crystals with euhedral and subhedral shapes, elongation ratio from 1 to 5, and from 30 to 250 μm in size [5,6]
This study demonstrates the types and properties of deformed zircon from Alpine shear zones and metamorphic rocks that have a polycrystalline appearance and compares them to reported shock-recrystallized and magmatic grains
Zircon grains were examined for potential crystal–plastic deformation microstructures using orientation contrast images taken by a forescatter electron detector (FSD) mounted on the electron backscatter diffraction (EBSD)-tube of an 3D FEG instrument (FEI Quanta) equipped with a Schottky field emission electron source, at the Faculty of Earth Sciences, Geography and Astronomy, University of Vienna (Vienna, Austria)
Summary
Zircon is an important accessory mineral, widely used for isotopic dating in geochronology [1,2,3,4].Usually, the zircon phase grows in magmatic and metamorphic rocks as regular single crystals with euhedral and subhedral shapes, elongation ratio from 1 to 5, and from 30 to 250 μm in size [5,6]. Detrital zircon can be fragmented or corroded and healed and overgrown by a new metamorphic rim, resulting in “cauliflower” or complicated polygranular or polycrystalline shapes [12,13,14]. It was first discovered in the early 1990s that under shock conditions, zircon develops polygranular textures ( called “granular” or “polycrystalline” in the literature) with fine granules from 0.5 to 3 μm in diameter [15,16,17,18,19], but attempts to date an impact event with this polygranular zircon were not entirely successful [20,21]. Zircon was intensely studied for shock-induced deformation, and various polycrystalline types were distinguished using an electron backscatter diffraction (EBSD)
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