The Zero Fault Pegmatite is Associated with Marble at Franklin, New Jersey, USA

Abstract

AbstractThe Zero Fault Pegmatite is a large, unzoned pegmatite of graphic granite that is well exposed along quarry walls at Franklin, New Jersey. It is contained within the Franklin Marble and intersects the Zero Fault. New geochemical data has facilitated development of a three-step process responsible for its origin: (1) Anatectic partial fusion of a potassic metarhyolite (mapped as Microcline Gneiss) which occurred during peak-Ottawan (∼1045 Ma) granulite-facies metamorphism (∼770 °C and 600 MPa). Most of the resulting anatectic melt phase recrystallized in situ to form the leucosome of the Microcline Gneiss, but some persisted from late- to post-orogenesis and plots between the 720 and 730 °C isotherms of the 500 MPa phase diagram for the haplogranite, consistent with some relaxation and cooling from peak conditions. This residual and incompatible element enriched melt intruded along penetrating structures to form a local cluster of small, equally potassic granite plutons mapped as the Mt. Eve Granite. (2) Intrusion of the Mt. Eve Granite into marble layers, during which the anatectic melt underwent dense mineral fractionation, resulted in the formation of the Zero Fault Pegmatitic melt. (3) The melt encountered CO2 vapors, which reduced H2O activity, increased solidus temperatures, and resulted in significant undercooling. The latter led to rapid crystallization (at 720 to 740 °C), leading to development of a quenched, graphic texture. While the emplacement of graphic granite into marble is common among pegmatites, notably those in the Grenville Province, the proposed explanation provides insights into their general formation.