GEOCHEMISTRY AND ZIRCON U-PB AGE OF THE RAVENSWOOD GRANODIORITE, QUEENS, NY

The Ravenswood Granodiorite is one of the lesser known rock units in New York City. It is mapped along the western margin of Queens, in a broadly NNE-trending belt adjacent to the East River. Although known, this unit has not seen any dedicated geological study since Zeigler (1912). It has been tentatively correlated with either the Harrison Gneiss in Connecticut or the Yonkers Gneiss in Westchester County.

Here we present new whole-rock geochemistry and zircon LA-ICPMS U-Pb ages and trace element compositions for the Ravenswood Granodiorite and the Yonkers Gneiss. The weighted mean ages 583 +/- 9 Ma for the Ravenswood and 575 +/- 9 Ma for the Yonkers. Whole rock rare earth element patterns for both the Ravenswood and the Yonkers are similar; negatively sloping REE patterns with slightly depleted heavy REE compared to the light REE. However, the Yonkers is slightly enriched overall in REE and has a dramatic negative Eu anomaly not present in the Ravenswood. Moreover, there are zircon trace element differences between the units, including in Ti-in-zircon temperatures, total REE concentrations, U concentrations, and Nb/Th. These differences suggest that while these units are the same age within error, they were not derived from the same magma, suggesting a broader melting event ~580 Ma. Elevated Nb/Th in zircon from both units is consistent with the interpretation of them forming in association with extension and rifting of Rodinia, and may suggest that rift-related rocks are more common in the NYC area than previously recognized.