Decoupled monazite and garnet petrochronology reveals short-duration, high-temperature Acadian metamorphism in the Manhattan Schist, New York City, USA

Abstract The Manhattan Prong (a belt of ancient rock in southern New York, USA) underlies one of the largest population bases in North America yet its tectonometamorphic history remains poorly understood. We performed monazite U-Pb and garnet Sm-Nd petrochronology in association with garnet diffusion chronometry on a sample of the Manhattan Schist from Central Park, New York City. Monazite included in both garnet and matrix yielded a 206Pb/238U date of ca. 438 Ma, though one matrix grain yielded a distinctly younger date of ca. 391 Ma; all monazite analyzed showed identical rare earth element compositions. Garnet yielded a robust isochron date of ca. 386 Ma, interpreted to constrain the timing of garnet growth at near-peak metamorphic conditions. Retention of manganese zoning in 1-mm-scale garnet requires that near-peak metamorphic conditions at ca. 386 Ma persisted for <5 m.y. The Manhattan Schist records a polymetamorphic history involving Taconic greenschist-facies metamorphism at ca. 438 Ma, recorded by relict monazite, then short-lived Acadian kyanite-grade metamorphism at ca. 386 Ma, recorded by garnet and the major rock-forming assemblage. These results are consistent with tectonic models that involve localized, episodic heating of the middle crust, rather than gradual and more pervasive processes of heating and cooling. Our study is an example of the nuanced tectonometamorphic understanding that can be gained by combining accessory and major phase petrochronology.