Contrasting microstructures and deformation mechanisms in metagabbro mylonites contemporaneously deformed under different temperatures ( c. 650 °C and c. 750 °C)

Abstract

Deformation mechanisms of amphibole and plagioclase were investigated in two metagabbroic sheets (the eastern and western metagabbros from the Stars M~sto belt, eastern Bohemian Massif), using petrology, quantitative microstructural and electron backscattered diffraction methods. After the gabbroic pyroxene was replaced by amphibole, both gabbroic bodies became progressively deformed. The eastern metagabbros were deformed under temperature of c. 650 ~ and the western metagabbros under c. 750 ~ Subgrain rotation and dislocation creep, characterized by strong crystallographic and shape preferred orientations, operated in plagioclase of the eastern belt during the early stages of deformation. Subsequent randomizing of plagioclase crystallographic preferred orientation is interpreted to be due to grain boundary sliding in the mylonitic stage. Large (50-150 ixm) grain sizes during the mylonitic stages are interpreted to be due to low strain rates. Amphibole is stronger and deforms cataclastically, leading to important grain size reduction when the bulk rock strength drops substantially. In the western belt, plagioclase deformed by dislocation creep accompanied by grain boundary migration (possibly chemically induced) while heterogeneous nucleation and syndeformational grain growth in conjunction with dislocation creep were typical for amphiboles.

Contrasting textural record of two distinct metamorphic events of similar P–T conditions and different durations

“…In the western mylonite metagabbro the temperature was estimated as 770 ± 50 °C using the thermometer by Holland & Blundy (1994) in metagabbro (GHT3), while in garnet‐amphibolite the temperature was estimated as 710–740 °C (GAHT). A similar temperature of 750 ±50 °C is reported by Baratoux et al. (2005) for 40 amphibole–plagioclase couples.…”

Application of SEM-EBSD to regional scale shear zone analysis: A case study of the Bhavani Shear Zone, South India

“…3A, E), are readily interpretable in terms of crystal slip in agreement with most published data on amphibole CPO obtained using a variety of measurement techniques (e.g. Siegesmund et al 1994;Berger and Stunitz, 1996;Egydio-Silva et al 2002;Imon et al 2004;Baratoux et al 2005;Diaz Aspiroz et al 2007). The coincidence of (100) with Z and [001] subparallel with X suggests activation of the (100)[001] slip system, consistent with those in middle-to-high grade metamorphic rocks reported previously from other areas (e.g.…”

“…Rigidbody rotation has been considered previously to play an important role in the CPO development of amphibole and biotite (e.g. Brodie and Rutter, 1985;Shelley, 1994;Berger and Stunitz, 1996;Baratoux et al 2005;Diaz Aspiroz et al 2007). During deformation, grains are rotated towards the XY plane of the finite strain ellipsoid (e.g.…”

Fabric Development in a Middle Devonian Intraoceanic Subduction Regime: The Careón Ophiolite (Northwest Spain)

“…[001] as a major slip system (Rooney et al 1975;Biermann and Van Roermund 1983;Cumbest et al 1989;Reynard et al 1989;Hacker and Christie 1990;Skrotzki 1990Skrotzki , 1992Kenkmann and Dresen 2002;Baratoux et al 2005). As the temperature decreases, other mechanisms such as twining, solution mass transfer, and dissolution-precipitation creep become more important (e.g., Dollinger and Blacic 1975;Biermann 1981;Imon et al 2002).…”

“…Documented crystal microcracks might be linked to rigid rotation, facilitating grain interactions (Ildefonse et al 1992;Díaz Azpiroz et al 2007). These mechanisms could help fabric development, thereby enhancing the strong correlation of SPO and texture (Shelley 1994;Baratoux et al 2005;Díaz Azpiroz et al 2007).…”