Finite lattice distortion patterns in plastically deformed zircon grains

Abstract

Abstract. This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea-Verbano Zone (IVZ), southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are granitic orthogneisses, metalamprophyres and paragneisses, all of which are strongly deformed.The investigated zircon grains ranging from 10 to 50 µm were studied in situ using a combination of scanning electron microscope (SEM) techniques, backscattered electron (BSE) imaging, forward-scattered electron (FSE) imaging, cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction (EBSD), as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS) was applied to host phases.Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: type (I) is defined by gradual bending of the zircon lattice with orientation changes of about 0.6-1.8 • per micrometer without subgrain boundary formation. Cumulative grain-internal orientation variations range from 7 to 25 • within single grains. Type (II) represents local gradual bending of the crystal lattice accompanied by the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 15 to 40 • within single grains. Type (III) is characterized by formation of subgrains separated by a welldefined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour. The cumulative orientation variation within a single grain ranges from 3 to 10 • . Types (I) and (II) predominate in granulite facies rocks, whereas type (III) is restricted to the amphibolite facies rocks. The difference in distortion patterns is controlled by strain rate and by ratio between dislocation formation and dislocation motion rates, conditioned by the amount of differential stress.Investigated microstructures demonstrate that misorientation axes are usually parallel to the < 001 > and < 100 > crystallographic directions; dominant slips are < 010 > { 001 }, < 010 > { 100 } and < 001 > { 010 }, whereas in some grains cross-slip takes place. This study demonstrates that activation of energetically preferable slip systems is facilitated if zircon grain is decoupled from the host matrix and/or hosted by a soft phase.

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

“…Stronalite is defined as granulite-facies metapelite, consisting of garnet, sillimanite and biotite with leucocratic patches and veins, composed of quartz, plagioclase and K-feldspar (Bea and Montero, 1999) or as granoblastic graphite-sillimanite-garnet gneiss, one of the components of the IVZ septa (Barboza et al, 1999). Local foliation and/or compositional layering of stronalites is moderately folded (e.g., Kovaleva et al, 2014, their Fig. 4a.…”

Interpretation of zircon corona textures from metapelitic granulites of the Ivrea-Verbano Zone, Northern Italy: Two-stage decomposition of Fe-Ti oxides

“…4f). Rotation of the lattice reaches 7 • with respect to the undeformed lattice (Kovaleva et al, 2016), indicating that this vermicular zircon grain predated shearing and ductile deformation.…”

“…Stronalite is defined as granulite-facies metapelite, consisting of garnet, sillimanite and biotite with leucocratic patches and veins, composed of quartz, plagioclase and K-feldspar (Bea and Montero, 1999) or as granoblastic graphite-sillimanite-garnet gneiss, one of the components of the IVZ septa (Barboza et al, 1999). Local foliation and/or compositional layering of stronalites is moderately folded (e.g., Kovaleva et al, 2014, their Fig. 1C).…”

Textural Identification of Polycrystalline Magmatic, Tectonically-Deformed, and Shock-Related Zircon Aggregates

“…50 m-thick ductile fluid-aided shear zone is exposed on the NE slope of the small valley that joins the main Zemmbach valley from the southeast (47 • 01 39.308" N, 11 • 50 3.669" E). The shear zone represents a zone of alternating ultramylonitic and mylonitic portions of the quartz-biotite orthogneiss (samples BH12-01 and BH12-02 accordingly; BH12-02 contains sigma-clasts), which are hosted by weakly-to strongly-foliated gneiss of granitic composition (samples BH12-04 and BH12-06 accordingly), enriched in biotite and muscovite and crosscut by leucocratic veins (see also [14], "shear zone 1").…”

“…Rarely, in felsic igneous rocks such as anatectic granites, zircon demonstrates twinning and aggregates of 2-20 interconnected grains [7][8][9][10][11]. In metamorphic rocks, detrital zircon can be fragmented or corroded and then healed and overgrown by a new metamorphic rim, resulting in "cauliflower" or complicated polygranular or polycrystalline shapes [12][13][14].…”