MONAZITE GEOCHRONOLOGY OF MANHATTAN SCHIST FROM NEW YORK CITY

The Manhattan Schist is a Paleozoic high-grade pelite that occurs in New York City and adjacent counties in southern New York. These rocks show significant deformation fabrics (in some cases up to 4 generations of folding) and exhibit evidence for partial melting or migmatization. The Manhattan Schist has a distinctly Laurentian provenance and has been interpreted to have undergone metamorphism during the amalgamation of Pangea during the Taconic, Acadian, and Alleghanian Orogenies. Traditional interpretation from the Manhattan schist is the strongest deformation and metamorphism occurring during the Taconic Orogen ~460 Ma.

The data presented here is part of a larger petrochronologic study of deformation and regional metamorphism, and here we report U-Pb results obtained from monazite within one sample, taken from the “Rock of No Hope” near the Delacorte Theater in NYC’s Central Park. Results indicate complex metamorphic pathways, and multiple generations of monazite growth. Ages for monazite domains can be separated into three main groups: i) 420-440 Ma, ii) 350-370 Ma, and iii) 307 – 320 Ma. Of these, the most common age group is the 350-370 Ma group.

Ages presented in this study are consistent with other samples from NYC and suggest the major metamorphic event was most likely during the Acadian and NeoAcadian Orogeny, rather than during the Taconic Orogeny as previously suggested. Interestingly, the younger, ~307 Ma ages represent the first report of post-Acadian ages from these units. These could represent either collapse of the Acadian Altiplano (circa ~330 Ma) or deformation associated with the Alleghanian Orogeny. Such observations are important because they are yet to be documented in southern New York, despite being known in southern Connecticut. Ongoing work will focus on further refining the P-T-t pathways and correlating deformation here with other units in southern New England.