Subgrain Rotation Recrystallization Research Articles

Variscan deformation, microstructural zonation and extensional exhumation of the Moravian Cadomian basement

The Cadomian Dyje Batholith, in the foot-wall of the Variscan Moravian nappe pile, has been involved in Variscan ductile deformation. The Cadomian Brunovistulian rocks were obliquely underthrusted during Carboniferous dextral transpression. Strain intensity is inversely proportional to the distance from the contact of the Variscan thrust front. The microstructures of deformed granodiorites and quartz-diorites show a characteristic zonality marked by relatively high temperature flow in the west (550–580 °C) characterized by dynamic recrystallization of feldspars and grain boundary migration recrystallization of quartz. The size of quartz grains decreases with decreasing strain towards the east. At the easternmost part of the autochthonous Dyje massif, fracturing of feldspar and subgrain rotation recrystallization of quartz predominate. Flow stress estimates calculated from recrystallized quartz grain size show a regional increase of stress intensity from the highly strained margin towards the less deformed core of the Dyje massif. This microstructural zonation is oblique with respect to the major thrust boundary and corresponds roughly to metamorphic isogrades. The microstructural zonation reflects underthrusting of the Brunovistulian domain below the Moldanubian nappe. The main ductile tectonic event D 1 is followed by a retrogressive brittle-ductile and brittle deformation D 2. D 2 results in the development of shear zones and faults superimposed on the D 1 mylonite fabric. D 2 is related to extension oblique to the D 1 fabric, associated with detachment and the westward movement of the Moravian nappes.

Breakdown of feldspar, volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone

In a low metamorphic grade shear zone from Quadrilátero Ferrífero (southeastern Brazil), granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal-plastic processes in quartz and fluid-assisted reaction-softening in feldspars. Quartz deformed by basal 〈a〉 and prism 〈a〉 slip, with concurrent subgrain rotation recrystallization. Breakdown of matrix plagioclase (An22−35) via mica-producing softening reactions required cationic exchanges between Ca, Na and K, forming replacement perthites in the K-feldspar megacrysts, which occupy around 60% of the original rock volume. Subsequent breakdown of the perthitic plagioclase (An12–15) led to disruption of the megacrysts, enabling strain accommodation that localized the mylonite and phyllonite zones. Domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume (around 15%) in the whole system. However, accentuated differences (ranging from mass and volume losses of 25 and 35% at the margins, to gains of 85 and 95% in the centre, respectively) exist between the individual longitudinal subzones occupied by the different tectonite types. This scenario indicates that deformation proceeded as a nearly isochemical process, although with lateral mobility of major elements (principally Si, Al and K) within the shear zone. These results are different from those previously reported for other granitic mylonites, most of them indicating volume losses in compressional tectonic settings. It is suggested that the extensional tectonic framework attributed to the Moeda-Bonfim shear zone (where mass/volume losses are in principle not required) enhanced the access of fluid and consequent transport of components into the shear zone (principally Fe, Si and K), producing the small gains of mass and volume observed.

Syn-emplacement recrystallization and deformation microstructures in the Poe Mountain anorthosite, Wyoming

Plagioclase recrystallization microstructures and petrofabrics in the unmetamorphosed, 1.43 Ga Poe Mountain anorthosite, Wyoming, are indicative of very high-temperature deformation and recrystallization during the emplacement of the anorthosite body. The Poe Mountain anorthosite consists of a core of recrystallized, massive anorthosite transitional with a series of layered anorthositic cumulates at the margin of the intrusion. Irregular grain boundaries and dissected grain microstructures in the massive core and transitional anorthosites suggest that the anorthositic rocks recrystallized by “fast” grain boundary migration and possibly subgrain rotation recrystallization, at very high temperatures (≈1050°C) during emplacement of the intrusion in the mid-crust (3 kbar). The deformation and recrystallization of the Poe Mountain anorthosite was continuous from subliquidus to subsolidus temperature conditions during the emplacement of the intrusion. Anorthosites with the lowest modal percentages of ferromagnesian minerals and Fe-Ti oxides are always the most recrystallized. This suggests that melt interstitial to the plagioclase-crystal framework was removed during deformation and recrystallization of the intrusion. Bulging of plagioclase grain boundaries around Fe-Ti oxides together with deformed oikocrystic ferromagnesian minerals and plagioclase chadacrysts indicate that the deformation and recrystallization of the intrusion continued after the crystallization of the interstitial melt minerals.

Experimental study of the influence of stress, temperature, and strain on the dynamic recrystallization of Carrara marble

Carrara marble has been deformed experimentally at temperatures ranging between 500° and 1000°C, at confining pressures of 200 and 300 MPa, and up to very large strains in extension and compression, in order to study the microstructural and mechanical property changes associated with dynamic recrystallization. Microstructural studies were made by optical microscopy on ultrathin sections, and automatic grain size measurement techniques were used. When the temperature is sufficiently high (≈ 600°C) and the stresses are high enough for deformation twinning to occur, twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size. At stress levels too low to activate twinning, recrystallization occurs in two stages: the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions, followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume. The recrystallization process requires larger prestrains at the lower temperatures. The size to which migration‐recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms. Within the range of experimental observations, dynamically recrystallized grain size does not depend on strain rate or temperature. Separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry. No unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found. However, extrapolation of the experimental data to low, natural strain rates suggests that weakening following recrystallization may occur in nature.

The role of recrystallization in the preferred orientation of olivine

The effects of recrystallization on the preferred orientation in olivine have been studied. The recrystallization mechanisms studied include grain growth, the deformation-induced grain boundary migration (DIGBM) and subgrain rotation (SGR). Grain boundary migration is involved in the first two, but not in SGR. Significant preferred orientation was found only in the latter two where dislocations are involved. The combined microstructural and fabric (preferred orientation) observations indicate that the grain boundary migration produces strong preferred orientation when the driving force is the dislocation energy but not when the grain boundary energy is the driving force. The nature of preferred orientation due to dislocation-related recrystallization (DIGBM or SGR) was found to depend on the mechanism of recrystallization. The DIGBM produces preferred orientation in which the low dislocation density grains with unfavourable orientation for easy slip dominate, thereby significantly altering the preferred orientation formed by dislocation glide. In contrast, SGR does not essentially alter the deformation fabric, and therefore the fabric associated with SGR recrystallization is related to the geometry of flow. The present results demonstrate that the existence of seismic anisotropy in the upper mantle is strong evidence for the dislocation mechanism(s) of deformation, and suggest that care must be exercised in applying the results of laboratory fabric studies made for a particular mechanism of recrystallization to the Earth's interior where the dominant mechanism(s) of dynamic recrystallization can be different.

Subgrain rotation and dynamic recrystallization of olivine, upper mantle diapirism, and extension of the basin-and-range province

On the basis of an abrupt change of olivine grain size at certain depths in the upper mantle beneath the Basin-and-Range province in the western U.S.A., Mercier (1980b) proposed that subgrain rotation (SGR) recrystallization had taken place above the grain size discontinuity, and grain boundary migration (GBM) recrystallization at depths greater than the discontinuity. The rotation of subgrains and the characteristics of dynamically recrystallized neoblasts of olivine were analyzed in a series of dunite samples deformed experimentally to compressive strains of about 15–60%. Whereas the rotation angle between adjacent subgrains increases with strain, the mean rotation angle did not exceed 2° even in the most heavily deformed samples. Rotation angles between kink bands, which formed at all experimental conditions, also increase with strain; at the highest strains rotations of up to 110° are not uncommon. At low strains the dominant rotation axis between kinks is [001] and less frequently [ovw]; at high strains [uvw] rotation axes are common. Neoblasts did form mainly on old grain boundaries and less frequently within old grains. Rotation axes between intragranular neoblasts and their hosts are of the type [uvw] and the rotation angles are always large, features which seem to be inconsistent with SGR recrystallization and suggest GBM recrystallization. Neoblasts may have formed in highly strained regions of old grains where slip occurs with [uvw] rotation axes. The olivine textures and fabrics of lherzolite nodules originating in the uppermost mantle above the grain size discontinuity are similar to those of nodules from the Dreiser Weiher, Germany (Mercier, 1980b). A study of Dreiser Weiher samples shows, however, that they have features difficult to reconcile with the SGR mechanism. It is proposed here that GBM recrystallization occurred throughout a rising upper mantle diapir beneath the Basin-and-Range extension zone. As the upward flow crossed the regional lithosphere-asthenosphere boundary at about 65 km depth, it diverged causing flow velocities to decrease abruptly and the strain rate dropped approximately by an order of magnitude. As a consequence the differential stress decreased by about a factor of two and the olivine grain size increased by a factor of two.