PROVENANCE, TECTONIC HISTORY, AND GEOCHEMISTRY OF THE MANHATTAN AND HARTLAND SCHISTS IN NEW YORK CITY

The rocks of New York City lie within the broader physiographic province referred to as the Manhattan Prong, which includes the boroughs of NYC and much of southern Westchester County. Commonly referred to as the New York City group, the bedrock geology consists of the Inwood Marble, and the Walloomsac, Hartland, and Manhattan Schists. Structurally, the Manhattan Prong sits at an important junction of the Appalachian Mountain belt, marking the boundary between the northern and southern Appalachians. It is generally considered to be at (or near) the major boundary between rocks of Laurentian and Gondwanan affinities. Serpentinites and metabasites occur in the region and support interpretations of a major suture zone separating these Laurentian and Gondwanan rocks occurring within the Manhattan Prong. Geochemically, however, the schist units do not show the differences expected across a major terrane boundary. Major element geochemistry of garnet remains consistent across all units. Nd isotope values for samples of the Manhattan Schist across Manhattan and the Bronx (including those at Pelham Bay Park) range from -7 to -13 eNd and are consistent with a Laurentian source. Detrital zircon populations are also suggestive of a Laurentian-only source similar to the Rowe belt in Connecticut and Massachusetts. Our systematic detrital zircon survey of the Manhattan Prong has yet to convincingly identify any evidence for exotic terranes (e.g., Mooretown or arc terrane). Based on this geochemistry, we suggest that all of the schists within NYC are derived from the same Laurentian sourced sediments and lie to the west of the major Appalachian suture, which has yet to be identified but likely lies east of New York City.