2:30 PM - 2:45 PM
[SMP26-04] Crystallographic fabrics of the Gföhl granulite in the southern Bohemian Massif and replacement of kyanite by andalusite
Keywords:crystallographic fabrics, kyanite, andalusite, Gföhl granulite
We report the results of EBSD measurements of samples in and outside a ductile shear zone developed in the Gföhl granulite in the southern Bohemian Massif, Czech Republic.
Felsic granulite in the ductile shear zone 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 ~5% in the phyllonite, indicating that deformation increases in this order. 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 ~32%, ~6% and ~62%, respectively, in the phyllonite. Thus, kyanite fraction significantly decreases while andalusite fraction significantly increases with increasing deformation, indicating that deformation promoted the transformation of kyanite to andalusite.
EBSD measurements show that quartz c-axes are oriented subparallel to lineation in a felsic granulite outside the ductile shear zone, while they are strongly oriented subparallel to foliation and subperpendicular to lineation in the weakly and strongly deformed mylonites, and poorly oriented in the phyllonite. Such quartz c-axis fabrics suggest that the felsic granulite preserves crystal plastic deformation of quartz by {m}[c] slip at the granulite-facies condition, which has been modified by crystal plastic deformation by {m}< a > slip in the weakly and strongly deformed mylonites, and further by grain boundary sliding in the phyllonite. Applications of the two-feldspar and garnet–biotite thermometries yield amphibolite-facies temperatures of 400–550°C inside the ductile shear zone, which is consistent with the temperature range of dominant {m}< a > slip in quartz. We will also report crystallographic fabrics of plagioclase and K-feldspar.
In addition, EBSD measurements reveal an epitaxial relationship between kyanite and andalusite partly replacing it such that kyanite (100) coincides with andalusite (010), while kyanite [010] and [001] are close to andalusite [100] and [001], respectively.
Felsic granulite in the ductile shear zone 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 ~5% in the phyllonite, indicating that deformation increases in this order. 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 ~32%, ~6% and ~62%, respectively, in the phyllonite. Thus, kyanite fraction significantly decreases while andalusite fraction significantly increases with increasing deformation, indicating that deformation promoted the transformation of kyanite to andalusite.
EBSD measurements show that quartz c-axes are oriented subparallel to lineation in a felsic granulite outside the ductile shear zone, while they are strongly oriented subparallel to foliation and subperpendicular to lineation in the weakly and strongly deformed mylonites, and poorly oriented in the phyllonite. Such quartz c-axis fabrics suggest that the felsic granulite preserves crystal plastic deformation of quartz by {m}[c] slip at the granulite-facies condition, which has been modified by crystal plastic deformation by {m}< a > slip in the weakly and strongly deformed mylonites, and further by grain boundary sliding in the phyllonite. Applications of the two-feldspar and garnet–biotite thermometries yield amphibolite-facies temperatures of 400–550°C inside the ductile shear zone, which is consistent with the temperature range of dominant {m}< a > slip in quartz. We will also report crystallographic fabrics of plagioclase and K-feldspar.
In addition, EBSD measurements reveal an epitaxial relationship between kyanite and andalusite partly replacing it such that kyanite (100) coincides with andalusite (010), while kyanite [010] and [001] are close to andalusite [100] and [001], respectively.