Rotational behaviour of complex shaped porphyroblasts during general flow: microstructural investigation of staurolite-mica schist of the Bossòst structural-metamorphic dome, central Pyrenees

Mezger, J. E. (2009)
International Conference and 99th Annual Meeting of the Geologische Vereinigung (GV)

Non-equidimensional objects tend to attain stable positions during general flow, their long axes become aligned to the shear plane. The amount of finite rotation therefore depends on the initial orientation of long axes prior to deformation, and can be inferred from inclusion trails preserved in metamorphic porphyroblasts. Staurolite is an important phase of metapelitic rocks of middle pressure amphibolite facies forming euhedral poikiloblastic porphyroblasts. In contrast to spherically growing garnets, staurolites have elongated prismatic shapes often occurring as interpenetrating growth twins with a variety of forms, such as crosses. Flow behaviour of staurolite porphyroblasts from the Bossòst structural and metamorphic dome was investigated by statistical analyses. Angular relations of porphyroblast long axes and inclusion trails (Si) with the schistosity were measured in sections parallel to mineral lineation and perpendicular to schistosity (xz), orthogonal to mineral lineation and schistosity (xy) and parallel to the schistosity plane (xy). Aspect ratios and long axes orientation were obtained from best-fit ellipses calculated with the free-ware ImageJ. Initial pre-deformational growth of staurolite is random. In xz-sections Si lies at higher angles to Se as in yz-sections, as to be expected from rotation of staurolite grains in the direction indicated by the stretching lineation. Staurolites with a rather spherical shape display continues angles (0–180°) between length axes and Se, while long axes of staurolites with an elongated shape (aspect ratios >2) cluster at low angles with Se, implying the attainment of stable positions of long axes parallel to flow plane. An important parameter is the ratio between long axis length and inclusion trail length. High ratios imply orthogonal long axes-inclusion trails angles and high crystal aspect ratios. Positive correlation with high Si-Se angle shows that these porphyroblasts experienced maximum rotation up to 110°.