UNRAVELING THE COMPLEX TEXTURAL HEREDITY OF MELANOGRANOPHYRES IN THE PALISADES SILL, NY AND NJ

Melanogranophyres in the Palisades Sill exhibit considerable variation in texture despite occupying a relatively narrow compositional spectrum. The silicic products of late stage crystallization possess high iron content (>13% FeO_T) and have been described by various authors to include fayalite granophyre, ferrodolerite, and granophyric dolerite. We present textural, mineralogic and chemical data on granophyres from two distinct sections in the Palisades and discuss metasomatic textures in the context of the greater Palisades-Watchung system. In one section, granophyres are fine-grained, containing crystals on the order of several hundred microns and including assemblages of remnant ferroaugite often corroded or altering to annite with associated earlier Fe-mica, mesostasis or residual pockets of oligoclase and myrmekite with accessory titanomagnetite and apatite. In contrast, melanogranophyres of nearly identical bulk composition from a section several miles north contain grain sizes closer to the millimeter-scale and deuteric assemblages characterized by gedrite-actinolite, and microperthitic anorthoclase with remnant plagioclase, ferropigeonites and ferroaugites with early alteration hornblende. Both sections have crystallites of quartz, albite and potassium feldspar and titanomagnetite displaying exsolution to a ferrianilmenite-magnetite association. This diversity in mineralogy and texture corresponds to a compositional continuum that provides snapshots of the various end conditions resulting from punctuated magma input, migration and storage.