(REE)XAMINING NYC: MONAZITE PETROCHRONOLOGY OF THE MANHATTAN PRONG

The Appalachian bedrock is complexly metamorphosed and deformed, reflecting a history of Paleozoic collisions between the Laurentian passive margin and a series of exotic terranes. The Manhattan Prong is a physiographic province between the northern and southern Appalachians and is often considered the link between these domains. Despite this, the tectonic history of the prong is largely unconstrained and has been considered “Southern” and “Northern” in the past. Here, we employ petrographic analysis, thermodynamic modeling, and U-Th-Pb monazite chemical dating on rocks from the Manhattan Schist and Hartland Formations to constrain the tectonic history of the Manhattan Prong and explore its relationship to the broader Appalachian system.

GWB-03 is a gt-ky-bt-ms migmatitic schist within the Manhattan Schist. Sil needles crosscut ky-bearing leucosomes, and gt grains are ~0.5-2 mm in diameter. CRT-06 and EIN-02 are migmatitic schists within the Hartland Formation and preserve gt-ky-bt-ms and gt-bt-ms assemblages, respectively. Sil in CRT-06 occurs as fibrolite psuedomorphs after ky. Gt grains are ~0.5-1 mm in diameter, and are partially replaced by chlorite. Sil occurs as small (<0.2 mm) tabular inclusions in Gt.

Monazite in all three samples were chemically dated via EPMA. There are four age groups: 654±20 Ma (G1), 542±25 Ma (G2), 468±22 Ma (G3), and 385±21 Ma (G4). G3 and G4 are interpreted as metamorphic ages associated with the Taconic and Acadian orogenies, respectively. The youngest monazite inclusions in gt are ~420 Ma in GWB-03, and ~375 Ma in EIN-02.

The synthesis of petrography and thermodynamic modeling predicts nearly identical Barrovian-style P–T paths for GWB-03 and CRT-06. Peak conditions are constrained by the stability of ky+melt at ~8-11 kbar and 725 °C. G3 inclusions in gt from GWB-03 suggest peak conditions are post-Taconic, likely associated with the Acadian orogeny. EIN-02 records an anticlockwise P–T path with peak conditions at ~8 kbar and 725°C. G4 monazite inclusions in gt suggest peak conditions are Acadian at the youngest. These results support a history of polymetamorphism in the Manhattan prong, with Taconic greenschist facies conditions overprinted by upper amphibolite facies conditions during the Acadian orogeny- a history most similar to the Northern Appalachians.