⁴⁰AR/³⁹AR AND PETROLOGIC EVIDENCE FOR AN ALLEGHANIAN TECTONOTHERMAL EVENT IN THE NEW JERSEY COASTAL PLAIN BASEMENT FROM AMPHIBOLITE-FACIES GNEISS IN THE ISLAND BEACH STATE PARK WELL

New Jersey occupies a critical position in terms of correlating terranes in the central and northern Appalachians related to the Appalachian orogen. The Island Beach State Park (IBSP) well in the outer New Jersey Coastal Plain, drilled by the US Geological Survey in the 1960's, is especially noteworthy because it represents the easternmost sample of crystalline basement available for examination. Drill core from a depth of -3,890 ft. returned dark-gray, medium-grained, penetratively foliated, locally migmatitic gneiss composed of plagioclase, quartz, microcline, biotite, and garnet. The mineral assemblage and textural relationships indicate a single metamorphism under conditions of amphibolite facies. The geochemical composition constrains the protolith to either paragneiss (metagraywacke) formed from a calc-alkaline, arc-related sediment source, or orthogneiss that is dacitic. The geochemical data are more consistent with metagraywacke because the gneiss is strongly peraluminous (aluminum saturation index = 1.16) and corundum normative (C = 2.18 %). ⁴⁰Ar/³⁹Ar dating of biotite from a drill core sample yields a well-developed plateau age of 243.98 ± 0.1 Ma. Evidence for disturbance by reheating of older biotite is lacking, implying that the biotites crystallized following a single thermal event. We interpret the ⁴⁰Ar/³⁹Ar data to represent a cooling age following high-grade metamorphism during the Alleghanian orogeny. Slow cooling following Acadian metamorphism is precluded by the absence of evidence for a thermal event of this age in New Jersey. The IBSP well gneiss occurs along the same structural and geophysical anomaly trend as rocks in the Connecticut Valley Synclinorium (CVS). The recovery of graphitic schist from wells west of IBSP, and the results of a seismic survey to the north strengthen this correlation. Our new ⁴⁰Ar/³⁹Ar data thus support the interpretation of an Alleghanian high-grade metamorphic event in the New Jersey Coastal Plain basement, but in rocks that appear to correlate to those in the CVS.