11:45 AM - 12:00 PM
[SMP27-05] Deformation-induced transformation of kyanite to andalusite: An example of Gföhl granulite in the southern Bohemian Massif
Keywords:kyanite, andalusite, phase transformation, ductile shear zone, Gföhl granulite
The Al2SiO5 minerals of andalusite, kyanite and sillimanite are a typical example of polymorphs. Transformation of andalusite to sillimanite or kyanite commonly occurs with increasing temperature or pressure, while sillimanite or kyanite is rarely transformed to andalusite even if the temperature or pressure is decreased. We report here an example of deformation-induced transformation of kyanite to andalusite in a ductile shear zone developed in the Gföhl granulite in the southern Bohemian Massif, Czech Republic.
The ductile shear zone has been repeatedly subjected to plastic shear deformation with mylonite formation and brittle shear deformation with pseudotachylite formation, suggesting its development in the brittle–plastic transition zone. Plastically sheared felsic granulite is divided into weakly deformed mylonite, strongly deformed mylonite and phyllonite. The modal content of porphyroclasts decreases from ≈22% in the weakly deformed mylonite, through ≈13% in the strongly deformed mylonite, to ≈2% in the phyllonite, indicating that plastic shear deformation increases in this order. Decomposition of K-feldspar into myrmekite proceeds with increasing deformation. Andalusite commonly occurs in the domain where shear strain γ exceeds 20, and is observed to partly replace kyanite. Fractions of kyanite, sillimanite and andalusite among Al2SiO5 minerals are ≈95%, ≈5% and 0%, respectively, outside the ductile shear zone, while they are ≈87%, ≈7% and ≈7%, respectively, in the weakly deformed mylonite, ≈58%, ≈16% and ≈26%, respectively, in the strongly deformed mylonite, and ≈33%, ≈9% and ≈58%, respectively, in the phyllonite. Thus, although sillimanite fraction does not change systematically, kyanite fraction decreases significantly while andalusite fraction increases significantly with increasing plastic shear deformation, indicating that deformation promoted the transformation of kyanite to andalusite.
The pressure and temperature estimated from the chemical compositions of the garnet core, biotite, and plagioclase in a sample outside the ductile shear zone are 1.9–2.1 GPa and 910–920°C, which are likely to represent the pressure and temperature at the metamorphic peak of eclogite facies. The temperatures inside the ductile shear zone inferred from the chemical compositions of myrmekite plagioclase and its adjacent K-feldspar as well as those of garnet and its adjacent biotite are 420–475°C, assuming a pressure of 300 MPa within the stability field of andalusite. These temperatures were also consistent with the temperature range of the observed c-axis fabrics of crystal-plastically deformed quartz inside the ductile shear zone.
The ductile shear zone has been repeatedly subjected to plastic shear deformation with mylonite formation and brittle shear deformation with pseudotachylite formation, suggesting its development in the brittle–plastic transition zone. Plastically sheared felsic granulite is divided into weakly deformed mylonite, strongly deformed mylonite and phyllonite. The modal content of porphyroclasts decreases from ≈22% in the weakly deformed mylonite, through ≈13% in the strongly deformed mylonite, to ≈2% in the phyllonite, indicating that plastic shear deformation increases in this order. Decomposition of K-feldspar into myrmekite proceeds with increasing deformation. Andalusite commonly occurs in the domain where shear strain γ exceeds 20, and is observed to partly replace kyanite. Fractions of kyanite, sillimanite and andalusite among Al2SiO5 minerals are ≈95%, ≈5% and 0%, respectively, outside the ductile shear zone, while they are ≈87%, ≈7% and ≈7%, respectively, in the weakly deformed mylonite, ≈58%, ≈16% and ≈26%, respectively, in the strongly deformed mylonite, and ≈33%, ≈9% and ≈58%, respectively, in the phyllonite. Thus, although sillimanite fraction does not change systematically, kyanite fraction decreases significantly while andalusite fraction increases significantly with increasing plastic shear deformation, indicating that deformation promoted the transformation of kyanite to andalusite.
The pressure and temperature estimated from the chemical compositions of the garnet core, biotite, and plagioclase in a sample outside the ductile shear zone are 1.9–2.1 GPa and 910–920°C, which are likely to represent the pressure and temperature at the metamorphic peak of eclogite facies. The temperatures inside the ductile shear zone inferred from the chemical compositions of myrmekite plagioclase and its adjacent K-feldspar as well as those of garnet and its adjacent biotite are 420–475°C, assuming a pressure of 300 MPa within the stability field of andalusite. These temperatures were also consistent with the temperature range of the observed c-axis fabrics of crystal-plastically deformed quartz inside the ductile shear zone.