Development of garnet porphyroblasts by multiple nucleation, coalescence and boundary misorientation-driven rotations

International audienceGarnet (10 vol.%; pyrope contents 34-44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain-size reduction (from a few cm to 300 lm). Electron back-scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain-size evolution. The garnet grain-size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm-sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and ⁄ or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grainsize reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion-assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (1010 m)3 or lower). The rounded Type III garnet experiences rigid body rotation in fine-grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain-size- and shape-dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 lm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P-T conditions of garnet plasticity in the continental crust (‡950 C; 11 kbar) have been identified using two-feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain-size decrease in a fine-grained steady-state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone. Key words: deformation mechanisms; garnet plasticity; perthite; shear zone

Section: Deformation Mechanisms In Garnetmentioning

confidence: 99%

Garnet crystal plasticity in the continental crust, new example from south Madagascar

Martelat¹,

Malamoud²,

Randrianasolo³

et al. 2012

“…In nature, garnet represents an extremely versatile recorder of metamorphism (Baxter & Scherer, 2013) and in particular garnet coronas capture metamorphic processes 'in flagranti' (Carlson & Johnson, [Correction added on 25 September 2018 after first print and online publication: The name of the fourth author was previously incorrect and has been corrected in this version] 1991; Carlson, 2011;Ague & Carlson, 2013). Consequently, garnet coronas and their metamorphic significance have been intensely studied over the past decades (Mørk, 1985;Johnson & Carlson, 1990;Johnson, 1993;Spiess et al, 2001;Prior et al, 2002;Konrad-Schmolke et al, 2005;Massey et al, 2011;Goergen & Whitney, 2012).…”

Section: Introductionmentioning

confidence: 99%

The strain‐dependent spatial evolution of garnet in a high‐P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X‐ray microtomography study

Macente

Fußeis

Menegon

et al. 2017

Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono‐metamorphic processes, however, they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron‐based X‐ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite–garnet eclogite in four dimensions, where the 4th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain. The microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. The microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X‐ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low‐strain domain, garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow, and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet shows a near‐random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo‐nucleation and overgrowth. Microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies, becoming more almandine‐rich. We interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization, and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming.

“…The current data set does not allow analysis of the hypothesis of Spiess et al (2001) that coalescing grains rotate into coincidence. However, in a sample suite of 20 porphyroblasts from the Townshend mica schist, all but four are revealed by EBSD analysis to be polycrystals, indicating that polycrystals are abundant in this rock.…”

Section: Polycrystals and Garnet Growthmentioning

confidence: 99%

“…In the case of isometric minerals, grain boundary relationships may be more difficult to detect petrographically. For example, petrographic observation of polygonal aggregates shows that they are characterized by apparently smooth grain boundaries, but SEM and TEM imaging in some cases reveals the presence of steps and facets corresponding to boundaries with a high degree of lattice coincidence in adjacent crystals (Wolf & Merkle, 1992;Kruhl, 2001;Spiess et al, 2001;Kruhl & Peternell, 2002). Although the overall process of (re)crystallization affecting the textural evolution of these rocks may be reduction in interfacial area, at a local scale, interfaces between grains may grow or develop as relatively low-energy, crystallographically controlled grain boundaries (Vernon, 1968).…”

Section: Polycrystals and Garnet Growthmentioning

confidence: 99%

Formation of garnet polycrystals during metamorphic crystallization

Goergen

Ketcham

Kunze

2008

Garnet polycrystals may form throughout the metamorphic history of a rock, starting at the earliest stages of garnet growth when closely spaced nuclei coalesce. In mica schist from Townshend Dam, VT, electron back-scattered diffraction (EBSD) analysis shows that garnet polycrystals possess two or more distinct lattice orientations separated by high-angle boundaries (28-61°). The minimum rotational displacements required to bring these lattice orientations into concordance with each other are commonly normal to the same low-energy planes that occur as crystal faces of euhedral garnet. There is no evidence for intracrystalline deformation, and the polycrystals therefore probably represent individual garnet crystals that coalesced during growth. The boundaries cross-cut growth zoning and inclusion trails of the polycrystals, indicating that early-formed polycrystals, once coalesced, behave chemically and physically as single crystals. Statistical analysis of a 3D, high-resolution X-ray computed tomographic data set of a large sample (912 cm 3 ) of a Townshend Dam schist, combined with microprobe and EBSD analyses of garnet, are consistent with a high degree of clustering at all stages of garnet growth. The formation and prevalence of polycrystals implies that garnet nuclei impinged on each other and coalesced, and that coalescence was a common feature throughout garnet growth in the rock.