GEOCHEMICAL VARIATION IN METASEDIMENTARY ROCKS ADJACENT TO THE PALISADES SILL – METAMORPHIC INFLUENCE OR ORIGINAL SEDIMENT COMPOSITION?

An outcrop of the contact between the base of the Palisades Sill and the Newark Basin sedimentary sequence has been exposed by construction at Fort Lee, NJ. An earlier study established that all of the sedimentary units in this outcrop had been recrystallized by contact metamorphism resulting from the intrusion of the Palisades Sill. Over most of the outcrop, these metasedimentary rocks maintain their clear sedimentary depositional structures. However, in one location, the sediments had been remobilized as the sill was intruded, resulting in the formation of a clastic dike. This work seeks to establish whether the geochemistry of these metasedimentary rocks has been influenced by metamorphic processes, or represents the original sediment composition.

Geochemical analysis of samples from a transect through undisturbed metasedimentary rocks (from 2 m – 7 m below the contact with the sill) indicates that there has been no chemical influence from the sill itself. Variations in major, trace and rare earth elements (REE) are related to the composition of the original sediments, and bear no relation to distance from the contact. However, the trace elements and REE in the clastic dyke sample strongly suggest an influence from the sill.

Finally, there is a sample from a metapsammite, taken around 8 m below the contact, which displays an unusual mineralogical and geochemical profile. Much of the quartz in this sample is smoky quartz, and this first indicated that there should be a source of gamma radiation. Geochemical analysis revealed a strong enrichment of thorium (148 ppm), along with smaller enrichments in uranium and the heavy rare earth elements. There is no evidence of any post-depositional disturbance of the sedimentary structures in this area, so the assumption is that the anomalies stem from the original sediment, possibly representing a volcanic ash deposit.