PETROGRAPHY & GEOCHEMISTRY OF THE HONEY BROOK UPLAND, PENNSYLVANIA PIEDMONT

The Honey Brook Upland an approximately 700 km² unit of amphibolite grade gneisses, granulite grade gneisses, and anorthosite is nestled within the southeastern Pennsylvania Piedmont. Field and petrographic studies of the approximately 50km² anorthosite body indicate that the intrusive unit is texturally and compositionally variable. While plagioclase compositions of the anorthosite consistently fall within a range of An₅₀ to An₆₀, mafic minerals vary considerably. Hornblende abundance varies from 5-30% and grain size varies from 3-30mm. While outcrops are few, most anorthosite boulders within the field area exhibit a pervasive foliation defined by hornblende. Additional mafic minerals within the anorthosite consist of hypersthene and biotite all with abundances less than 5%. Samples of anorthosite collected along the margin of the unit additionally contain minor amounts of garnet and epidote. Surrounding the anorthosite to the north are both charnokitic and felsic gneisses; to the south the Brandywine Manor Fault cross-cuts these units abutting them to Paleozoic strata (Bascom and Stose, 1938). The anorthosite exhibits a petrographic affinity with the adjacent charnokitic gneisses. The charnokitic gneisses contain abundant plagioclase, hornblende, hypersthene, and variable amounts of augite. Felsic gneisses to the north of the Brandwine Manor fault are predominantly quartz and perthitic feldspar. Minor and accessory minerals within the felsic gneisses also include hypersthene, hornblende, and biotite. These results are consistent with previous petrographic studies of the Honey Brook Upland notably Crawford and others (1971) and Crawford and Hoersch (1984).