DATING NYC: CONSTRAINTS ON THE TIMING OF METAMORPHISM IN THE MANHATTAN SCHIST FROM GARNET PETROCHRONOLOGY

The metamorphic bedrock of the Manhattan Prong, New York City records the history of collisional events associated with Paleozoic Appalachian mountain-building. Although considered the physiographic link between the northern and southern Appalachians, the tectonic history of the Manhattan Prong remains geochemically and geochronologically underexplored, with much tectonic interpretation based largely on stratigraphic correlations with units in Western Connecticut. Here, we synthesize the results of garnet Sm-Nd dating, petrography, and thermodynamic modeling to constrain the timing of metamorphism recorded in the Manhattan Prong.

The Manhattan Schist is a Laurentian-derived metasedimentary unit within the Paleozoic bedrock of the Manhattan Prong. Sample MAT-2017-01a is a grt-ky-sil-bt-ms schist and exhibits ~1-3 mm subidioblastic garnet, that are pre- to syn-deformation. Garnets preserve typical growth zoning in Mn, Fe, Mg, and Ca. Garnet cores are inclusion-rich and contain quartz, plagioclase, and zircon. Kyanite nodules occur in leucosomes that are rarely cross-cut by sillimanite, and fibrolite is preserved in late muscovite. Coupled with phase equilibria modeling, these textures indicate a clockwise pressure-temperature path: 1) garnet growth starting at ~550°C and 4-6 kbar, 2) burial and heating to kyanite-grade anatexis at 700-750°C and 7-11 kbar, 3) isothermal exhumation to fibrolite stability, and 4) continued exhumation to ~700°C and ~6 kbar.

Garnet is a valuable petrochronometer, as its fractionation of Sm over Nd and high closure temperature produces reliable ages that can be tied to specific P–T conditions. Here, we analyzed bulk garnet separates via TIMS. An isochron constructed with the whole rock and four garnet aliquots yields a late Acadian age of 385.96 ± 3.68 Ma (n=5, MSWD=0.94). The MSWD close to one suggests the bulk sample represents a single age population. Additionally, the preservation of major element growth zoning in garnet suggests limited time at peak conditions, and minimal retrograde modification. This age likely constrains the timing of prograde to peak metamorphic conditions. Taken as a whole, our results suggest that the high-grade metamorphism preserved within the Manhattan Schist is primarily Acadian rather than Taconic, as previously interpreted.