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

The Honey Brook anorthosite is poorly studied, and an ideal target for advances in 3-D photogrammetric analysis. This anorthosite unit lies within the Piedmont HIghlands of Southeastern Pennsylvania and is of precambrian origin. The anorthosite originated from a 70 km deep mantle plume, and initially cooled at conditions of approximately 900 °C and 0.5 GPa. During cooling, mafic crystals settled, leaving a relatively pure plagioclase which crystallized to a coarse-grained anorthosite. The anorthosite massif later underwent granulite-facies metamorphism, forming garnet porphyroblast under an increase in pressure. Garnet porphyroblasts in the Honey Brook Anorthosite average 2.4 cm in width, and are ringed by a bleached An-depleted zone. In present day outcrops these garnet porphyroblasts appear in linear zones parallel to near horizontal fractures. The linear orientation of these garnet porphyroblasts and their spatial relationship to other features in anorthosite outcrops can be assessed using recent advances in structure from motion (SFM) based photogrammetry. Photo based 3-D rock outcrops, relying on SFM technology have recently become increasingly accessible to the geological sciences, yet applications on crystal orientation and structural geology have been limited. Using this SFM technology a semi-automatic method for porphyroblast identification and analysis has been developed. This method uses high resolution imagery of anorthosite outcrops to create a 3-D model of the unit. By combining 3-D modeling with imagery analysis; garnet porphyroblasts can be identified and analyzed in three dimensions. Using this novel method, garnet porphyroblasts can be identified and porphyroblast orientation can be properly assessed. Understanding the prefered orientation of these porphyroblasts can shine new light on the petrological history of the Honey Brook formation, and how it relates to other Appalachian anorthosites. This method can be applied to the analysis of other anorthositic crystals, and features in the surrounding parent rock and intrusions to develop a better idea of Piedmont Highland petrology.