Albite porphyroblasts with sigmoidal inclusion trails and their kinematic implications: an example from the Taconic Allochthon, west-central Vermont

Microstructural analyses of albite porphyroblasts in phyllite of the Taconic Allochthon were conducted in order to determine the origin of the sigmoidal inclusion trails within the porphyroblasts and to evaluate if and when the porphyroblasts rotated during the deformation history. Three cleavages (S2, S3, and S4) with similar strike are present in the phyllite. The albite porphyroblasts are characterized by inclusion-rich cores and inclusion-free rims, and the boundary between the cores and overgrowths is characterized by relatively straight and parallel zones of concentrated rutile. The sigmoidal inclusion trails within the cores are inferred to be trapped microfolds. The zones of concentrated rutile are interpreted to be truncation planes that formed during the development of S3. In contrast to the tight orientation distribution of S3, the truncation planes show a wide distribution pattern. The mean orientations of S3 and the truncation planes also differ. These orientation data indicate that the porphyroblasts have not only rotated with respect to S3 but also to the geographic reference frame. In addition, the growth of the porphyroblasts was influenced by differentiation associated with S3 crenulation cleavage formation; the porphyroblasts grew preferentially within microlithon domains where chlorite, a reactant mineral for the porphyroblasts, is abundant.