Petrology, Geochronology, Mineralogy, and Geochemistry of the Beemerville Alkaline Complex, Northern New Jersey

The Beemerville alkaline complex, New Jersey, occurs at the western end of the CortlandtBeemerville magmatic belt. The eastern end of the belt is defined by the Cortlandt complex, New York, which largely consists of ultramafic and mafic rocks. Between these two intrusive centers is a linear almost east-west trending zone of lamprophyre and felsic dikes. The Beemerville complex consists of two large nepheline syenite bodies, several diatremes, and associated lamprophyre and phonolite dikes, all of which intrude the shales and graywackes of the Ordovician Martinsburg Formation. The emplacement of these igneous bodies is post-tectonic and titanite fission-track ages indicate an emplacement age of 420 ± 6 Ma. The mean apatite fission-track age is 156 ± 4 Ma, which represents the time the rocks of the complex were last at temperatures of 60C to 100C. The major rock forming minerals are orthoclase, nepheline, and clinopryroxene. Minor minerals are biotite and titanite. Accessory minerals are sodalite, fluorite, calcite, and opaque oxides. Cancrinite alteration of nepheline is common. Based on electron microprobe data the pyroxenes range in composition from diopside through aegirine-augite to aegirine. The feldspars in the nepheline syenites and phonolites are orthoclase (>Or80), with albite noted in some specimens. Two biotite populations can be identified: one encompasses the nepheline syenites and phonolites and the other the lamprophyre dikes and matrix material from the diatremes, an observation which suggests a genetic linkage between the lamprophyres and the diatremes. Based on textural, phase equilibria, and chemical criteria, the phases fractionating from the syenitic magmas were aegirine, nepheline, and titanite. The nepheline syenites are essentially crystal cumulates with some trapped interstitial melt. Few, if any, of the phonolites represent liquid compositions, and this is partly due to post-magmatic alteration. Nb/Ta and Zr/Hf ratios are very variable, in part due to the fractionation of titanite and in part due to hydrothermal alteration. Three distinct REE patterns are observed for the phonolites: uniform slope downwards from LREE to HREE, flattening at the HREE end, and U-shaped. These patterns are partly due to the fractionation of titanite, but a more detailed explanation most likely involves the role of fluids in altering these patterns.