3D ANALYSIS OF GARNET PORPHYROBLAST ORIENTATION IN THE HONEY BROOK ANORTHOSITE OF PENNSYLVANIA

The Honey Brook Anorthosite, map area of ~40 sq km, intrudes Mesoproterozoic gneiss of the Honey Brook Upland in the Piedmont of southeastern Pennsylvania. The anorthosite contains garnet porphyroblasts ranging from 1-3cm in diameter, which are always surrounded by leucocratic halos, characterized by an absence of mafic minerals and lower-An plagioclase than in the surrounding rock. This research focuses on a 150m long railroad cut/outcrop in the anorthosite in which garnet porphyroblasts typically occur in linear arrays on outcrop faces. This geometry which suggests that the garnet may have preferentially nucleated along particular planes.

This study employs field based measurements and 3D modeling to assess whether or not these preferred nucleation planes are present. A structure-from-motion model of the outcrop was created with VisualSFM, and analyzed in Meshlab. 102 garnet porphroblasts were identified on the 3D model and the orientations of all possible planes that could be fit through the porphyroblasts were calculated. A Montecarlo simulation of random points on the model was used to create a null set of garnet porphyroblasts, and all possible planes that null garnet points could lie on were identified. Garnet planes will be compared with null planes to identify potential significant garnet bearing planes.