Fabric development in felsic granulites during multistage exhumation of the Saxonian Granulite Massive

Fabrics in the granulites of the Saxonian Granulite Massive (SGM) are interpreted to result from deformation related to early exhumation at deep crustal conditions, followed by subsequent shearing of the granulite roof against lower grade country rocks at retrograde conditions by top-SE shearing (e.g Reinhardt & Kleemann, 1994). For structural evidence and establishment of the rheological implications of such a multiphase deformation within the felsic granulites, detailed structural fieldwork and thin section scale microstructural analyses were carried out.

In the central SGM we observed a mylonitic foliation defined by interlayered quartz ribbons (20 vol.-%) and a homogeneous Qtz-Kfs-Pl-matrix that is transposed by a second mylonitic foliation towards the SE-rim, simultaneous with boudinage and proportion reduction of quartz ribbon (6 vol-%), and growth of biotite in the polyphase matrix. A NE-trending stretching lineation progressively rotates towards a SSE-trend. Quartz in the Qtz-Kfs-Pl-(Bt)-matrix generally has no CPO. Quartz ribbon’s (0001) CPOs change from two orthogonal, peripheral maxima to a central maximum.

Transposition of the foliation and lineation results from a change in the kinematics of flow at progressively lower temperature conditions, supported by a change from <0001> to <11-20> easy slip in the ribbons. Nevertheless, due to the generally high volume percentage of the Qtz-Kfs-Pl-(Bt)-matrix we assume diffusion creep s.l. involving grain boundary sliding to be the dominant deformation mechanism in all felsic granulites, suggesting a dominant linear-viscous rheology during the exhumation path.

References

Reinhardt, J., & Kleemann, U. (1994). Tectonophysics 238, 71–94, https://doi.org/10.1016/0040-1951(94)90050-7