Micro‐zircon: origin and evolution during metamorphism

Abstract

The size and distribution of zircon within a garnet-mica-schist from the Scottish Highlands were assessed using scanning electron microscopy. The study reveals that abundant 0.2-3.0 lm sized zircon is preferentially concentrated within garnet and biotite porphyroblasts. The micro-zircon has grown during regional metamorphism and represents >90% of the total number of zircon in the schist. It is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution, and more rarely, small metamorphic outgrowths. The sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified. Zircon is reactive and soluble, and responds to medium-grade metamorphism in a series of dissolution and crystallization events, linked to possible changes in fluid composition. Deformation also has a significant influence on the distribution of zircon, allowing inclusions previously trapped within biotite to react. About 8 · 10 6 micro-zircon occur as inclusions within a typical individual 5-mm garnet porphyroblast and their presence must be considered prior to trace-element or isotopic analysis of such metamorphic phases.

Zircon dissolution in a ductile shear zone, Monte Rosa granite gneiss, northern Italy

“…2b,c). Similar behaviour has been reported by Dempster et al (2008) in amphibolite-facies mica schists and attributed to zircon solubility being buffered by the presence of F-rich fluids. However, the spatial controls on zircon dissolution within the orthogneiss suggests that either fluid composition is subject to extremely local buffering or that infiltration is restricted to certain specific mineral boundaries.…”

“…Similar sub-solidus changes are now recognized in a variety of rocks and point to zircon acting as a metamorphic mineral in a range of crustal environments (Rubatto et al, 2001;Williams, 2001;Tomaschek et al, 2003;Dempster et al, 2004;Rasmussen, 2005;Dempster et al, 2008). Within the sheared gneiss, dissolution is the most commonly observed response of zircon and the difference in behaviour between inclusions and those crystals on grain boundaries point to the importance of a fluid phase (Tomaschek et al, 2003;Geisler et al, 2003).…”

“…The Zr is presumably removed in the fluid phase (Schmidt et al, 2006) and potentially low-temperature hydrothermal zircon (Rubin et al, 1989) or metamorphic zircon may crystallize elsewhere. Sub-solidus metamorphic zircon has been reported in other studies but typically largely involves recrystallization of earlier zircon (Gebauer et al, 1997;Rubatto et al, 1999;Tomaschek et al, 2003) or new crystallization either as outgrowths on greenschist-facies zircons (Dempster et al, 2004) or as discrete micro-zircons in amphibolite-facies schists (Dempster et al, 2008). In the latter study, alternating periods of zircon dissolution and growth are inferred from the distribution of zircon with respect to porphyroblast phases.…”

Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides

“…Rarely, zircon has unusual saccharoidal or needleshaped morphology or forms coronae (Corfu et al, 2003 and references therein). Mineral-fluid interactions, decomposition of Zr-bearing minerals, and exsolution from Zr-bearing accessory and rock-forming minerals can result in such unusual zircon textures (e.g., Corfu et al, 2003 and references therein;Dempster et al, 2004Dempster et al, , 2008Rasmussen, 2005), even at low metamorphic grades (e.g., Dempster et al, 2008). In natural samples, there are several documented examples of zircon coronae textures from igneous and metaigneous rocks (e.g., Bingen et al, 2001;Söderlund et al, 2004;Austrheim et al, 2008), as well as from metapelites of the IVZ (Pape et al, 2016).…”

SHRIMP allanite U-Th-Pb dating of bimodal Triassic metamorphism of Neoarchean tonalitic gneisses, Daeijak Island, central Korea

“…However, zircon rarely crystallises below upper amphibolite-facies conditions in the absence of melt (Williams, 2001;Rubatto et al, 2001). Although micro-zircon grains (up to 3 μm diameter) possibly can form in this temperature range (e.g., Dempster et al, 2008), it is presently technically difficult to measure their U−Th−Pb isotopic compositions with high precision. Instead, monazite has been extensively investigated for a decade because of its ability to record amphibolite-facies metamorphism (e.g., Williams, 2001;Rubatto et al, 2001).…”