The Manhattan project: Isotope geochemistry and detrital zircon geochronology of schists in New York City, USA

The geology of New York City (USA) consists primarily of metasedimentary rocks that were deformed during the series of orogenies between ca. 470 Ma and ca. 300 Ma that culminated in the amalgamation of Pangea. The rocks in New York City play a key role in understanding the tectonic history of these orogenies because they lie at a critical location at the boundary between the Northern and Southern Appalachian Mountains. The primary question addressed here is where these metasedimentary rocks originated prior to the assembly of Pangea. Through detrital zircon and whole-rock Nd isotope analyses, we show that all the metasedimentary rocks of New York City, mapped as the Manhattan Schist and the Hartland Group, are primarily derived from Laurentia as indicated by detrital zircon populations dominated by 1200−900 Ma grains and εNd values between −7 and −13. The results presented here do not necessitate an exclusively Laurentian source for the detrital material found in New York City, but the data strongly suggests protoliths represent sedimentary units that are primarily derived from the Laurentian margin. Another important result from this study is the limited contributions from any rift volcanics and/or Gondwanan material(s). There is some subtle variability across our zircon sample suite, but there is no convincing evidence for major changes in bulk provenance signal that would be consistent with derivation from vastly different continental sources for these rocks. The shared provenance signal observed here is counter to the previous suggestions that a major terrane boundary, often called Cameron’s Line, exists in New York City, separating Laurentian rocks from those of a Gondwanan affinity.