A LARGE PURE GRAPHIC-GRANITE PEGMATITE, THE FUSION PRODUCT OF META-RHYOLITE AT THE ZERO FAULT, NEW JERSEY

A pegmatite composed of large grains of microcline perthite intergrown with quartz (graphic-granite) is exposed along the Zero Fault, Franklin, New Jersey. Traces of graphite (~0.5 cm blebs) and pyroxene (~0.2 cm grains) make up less than 0.1 percent of the pegmatite with minor traces of hydromuscovite found along slickensides through the pegmatite. The pegmatite is unzoned and is emplaced between layers of Proterozoic marble and microcline gneiss. The microcline gneiss is composed of microcline and quartz with variable concentrations of pyrite, mica, chlorite, and graphite. The chemical composition of the pegmatite closely resembles the composition of the adjacent microcline gneiss. In particular, the CaO, Na₂O, and K₂O contents of the pegmatite and adjacent gneiss average 1.3, 3.2, and 6.9 weight percent and 1.0, 2.9, and 6.4 weight percent respectively. These values are typical of rhyolite but fall well outside the range of common arkose. In addition, the protolith of the microcline gneiss was deposited on calc-alkaline lithologies devoid of any potassic source rock. We, therefore, propose that the protolith of the microcline gneiss was rhyolitic rock that may be genetically related to Byram Granite exposed near Franklin, New Jersey. The rhyolite was extruded within a few million years before the Byram Granite intruded but may represent an early stage of magmatism. Subsequent granulite facies metamorphism resulted in local fusion and the generation of pegmatites. The Zero Fault pegmatite is unlike the amphibole bearing pegmatites typical of the Byram Granite and unlike the magnetite and biotite bearing pegmatites commonly contained within thicker layers of microcline gneiss but closely resembles other graphic-granite pegmatites found within or adjacent to marble. The unusual characteristics of the Zero Fault pegmatite may, therefore be related to the effects of carbonates and perhaps sulfides on the pegmatite melt.