Crystal bending, subgrain boundary development, and recrystallization in orthopyroxene during granulite-facies deformation

International audienceA prominent feature of a granulite-facies shear zone from the Hidaka Main Zone (Japan) is the folding of orthopyroxene (opx) porphyroclasts. Dislocation density estimated by transmission electron microscope (TEM) and chemical etching in homogeneously folded domains is too low to account for the amplitude of crystallographic bending, leading us to propose a model similar to "flexural slip" folding, where folded layers are micrometer-wide opx layers between thin planar clinopyroxene (cpx) exsolutions. Extension (compression) in the extrados (intrados) of the folded layer is accommodated by dislocations at the cpx-opx interfaces. Alternatively to distributed deformation, crystal bending also localizes in grain boundaries (GBs), mostly oriented close to the (001) plane and with various misorientation angles but misorientation axes consistently close to the b-axis. For misorientation up to a few degrees, GBs were imaged as tilt walls composed of regularly spaced (100)[001] dislocations. For misorientation angles of 7°, individual dislocations are no longer visible, but high-resolution TEM (HRTEM) observation showed the partial continuity of opx tetrahedral chains through the boundary. For 21° misorientation, the two adjacent crystals are completely separated by an incoherent boundary. In spite of these atomic-scale variations, all GBs share orientation and rotation axis, suggesting a continuous process of misorientation by symmetric incorporation of (100)[001] dislocations. In addition to the dominant GBs perpendicular to the (100) plane, boundaries at low angle with (100) planes are also present, incorporating dislocations with a component of Burgers vector along the a-axis. The two kinds of boundaries combine to delimit subgrains, which progressively rotate with respect to host grains around the b-axis, eventually leading to recrystallization of large porphyroclasts

Pan-African adakitic rocks from the Sør Rondane Mountains, East Antarctica

The Sør Rondane Mountains (22°E -28°E ) are located in eastern Dronning Maud Land, East Antarctica, and consist of amphibolite to granulite facies metamorphic rocks and various intrusive rocks. Peak metamorphism took place during the Pan-African event. Considering the mode of occurrence and previously reported age data, felsic magma activity started at the waning stage of the metamorphism. In this study, we report on the petrogenesis of discordantly intruding post-tectonic pyroxene tonalite (Px tonalite) with adakitic composition. In addition, the petrogenesis of posttectonic biotite granite (Bt granite), of which the geochemical features are similar to high-K adakite in the collision zone, are reevaluated with reference to the Px granite. The initial Sr isotopic ratios (SrI) corrected to 525Ma of Px tonalite and Bt granite represent limited values (0.7037-0.7052) that are included within the SrI of the Proterozoic metatonalite corrected to the same age (525Ma). These Sr isotopic signatures combined with geochemical modeling suggest that chemical variation of Px tonalite and Bt granite can be explained by partial melting of the Proterozoic metatonalite with various degrees of melting, leaving garnet as a residual phase, subsequent to fractional crystallization and/or accumulation of specific phases. Taking the tectonic situation and geochemical signature into account, Px tonalite and Bt granite from the Sør Rondane Mountains are correlated to the collision zone adakite and represent the post-collision magmatism in the suture zone during the Gondwana formation of the Pan-African event

Grain-sizereductionmechanisms and rheologicalconsequences in high-temperaturegabbromylonites of Hidaka, Japan

International audienceThe study of microstructures and crystallographic fabrics in a granulite-facies shear zone of the Hidaka Metamorphic Belt showed that the strong shearing localized within the mylonite resulted in the asymmetrical elongation of the inherited orthopyroxene porphyroclasts and the generation of fine-grained plagioclase and orthopyroxene layers as asymmetric tails of orthopyroxene porphyroclasts. The orthopyroxene porphyroclasts and the coarse plagioclase matrix surrounding them have a strong crystallographic preferred orientation acquired through deformation by dislocation creep. In contrast, the small orthopyroxene and plagioclase grains located in the tails have equant shapes and random fabric that are interpreted as the result of deformation by grain-boundary sliding. The small orthopyroxene grains are generated on the sheared rims of the orthopyroxene porphyroclasts by subgrain rotation, inheriting the orientation of the porphyroclasts before deforming by grain-boundary sliding (GBS) and losing this fabric. Additional mechanism of grain-sizereduction is the disruption of orthopyroxene porphyroclasts by synthetic shear zones localized on clinopyroxene exsolutions. The switch in deformation mechanism from dislocation creep to GBS, associated with the grain-sizereduction, yielded estimates of deviatoric stress one order smaller than lithostatic pressure. Besides, such rheological evolution attests of the mechanical softening during deformation, which contributed to the localization of the strain within the mylonite

Pseudotachylytes from Langhovde and Skarvsnes in the Lützow-Holm Complex, East Antarctica, and their conditions of formation

The Tenth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 4 Dec. / Entrance Hall (1st floor), National Institute of Polar Researc

Geological field survey in the regions of L*tzow-Holm Bay, Prince Olav Coast and Enderby Land, 2018-2019 (JARE-60)

The Tenth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 4 Dec. / 3F Seminar room, National Institute of Polar Researc

リュッツォホルム岩体の大構造

第6回極域科学シンポジウム[OG] 地圏11月16日（月）　国立極地研究所１階交流アトリウ

Titanium behavior in quartz during retrograde hydration: Occurrence of rutile exsolution and implications for metamorphic processes in the Sør Rondane Mountains, East Antarctica

AbstractIn the central Sør Rondane Mountains, East Antarctica, orthopyroxene felsic gneiss (OPG) was converted to hornblende-biotite felsic gneiss (HBG) by hydration that accompanied the intrusion of pegmatite. The retrograde HBG contains exsolved rutile in quartz. The composition of orthopyroxene and clinopyroxene in OPG suggests a temperature of 840°C (interpreted as the near-peak temperature), and the composition of hornblende and plagioclase in HBG suggests a temperature of 670–700°C (interpreted as the temperature during hydration). Ti concentrations in quartz were measured using an electron probe micro-analyzer, and Ti-in-quartz thermometers were applied. Measured Ti concentrations were 110ppm (equivalent to 760–820°C) for homogeneous quartz from OPG and 35ppm (650–700°C) for an exsolution-free area of a quartz grain from HBG. The pre-exsolution Ti concentration in quartz from HBG was reconstructed with 100μm beam diameter and 25kV of accelerating voltage, giving 103ppm, similar to the value obtained for homogeneous quartz in OPG. The temperatures obtained using a Ti-in-quartz thermometer are consistent with those estimated using other thermometers. Although analysis of the main constitute minerals in HBG yields the conditions of hydration, the reconstructed pre-exsolution Ti content in quartz within HBG yields the pre-hydration conditions. Thus, the Ti-in-quartz thermometer is a potentially powerful tool with which to identify the peak or near-peak temperature conditions, even for retrogressed metamorphic rocks

Fluid inclusions in an osumilite-bearing granulite from Bunt Island in the Archean Napier Complex, East Antarctica: implications for a decompressional P-T path?

We report high-density CO_2-rich fluid inclusions in garnet,orthopyroxene,and quartz from an osumilite-bearing aluminous granulite from Bunt Island of the Archean Napier Complex,East Antarctica.The melting temperatures of fluids lie in the range of -56.8 to -57.8゜C being close to the triple point for pure CO_2 (-56.6゜C ). Homogenization of the CO_2,-rich fluids into the liquid phase occurs at temperatures in the range of -35.4 to 24.7゜C. This translates into CO_2 densities in the range of 0.788-1.084g/cm^3 .The estimated CO_2 ,isochore for high-density inclusions in garnet intersects the P-T trajectory of Bunt Island at around 10 kbar at 1050゜C, which corresponds to the peak metamorphic conditions of the region derived from mineral phase equilibria.We therefore infer that CO_2 ,was the dominant fluid species present during the ultrahigh-temperature metamorphism in Bunt Island.CO_2 inclusions with lower density occurring in quartz and garnet provide isochores that intersect the P-T path at <7 kbar and <950゜C indicating density reversal of originally high-density inclusions along a decompressional exhumation path of the ultrahigh-temperature rocks in the Bunt Island

Newly obtained data for geologic unit boundaries at central part of Sør Rondane Mountains

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月17日（木） 国立極地研究所　2階大会議室前フロ