AN UNUSUAL MAFIC DIKE IN EASTON, PENNSYLVANIA

We present a preliminary report on a newly (re-?)discovered and seemingly unusual mafic dike that cuts Precambrian rocks near Easton, PA (and new data for a previously identified dike/sill located nearby). At its one outcrop, the new dike is about 2m wide, and exhibits sharp contacts with enclosing Mesoproterozoic quartzo-feldspathic gneiss. Based on float located along its apparent strike, the dike also cuts both post-orogenic pegmatite and talc-bearing rocks derived from the Franklin Marble. The dike is porphyritic, and initial petrographic and geochemical analyses suggest that it is likely distinct from the Catoctin-related metadiabase dikes that are relatively common within the Precambrian rocks of the New Jersey Highlands and the Reading Prong. A total alkali vs. silica diagram suggests that the new dike has a basaltic to trachy-basaltic composition.