Insights from the Application of Accessory and Major Phase Petrochronology from the Manhattan Schist, NYC

Accurately constraining the timing and tempo of specific metamorphic events is necessary to constrain the rates of important geodynamic processes, such as burial and exhumation in orogens and devolatilization in subduction zones. Accessory phase petrochronology is a popular tool employed to constrain the age and duration of metamorphic processes, including mineral nucleation and deformation fabric development. While accessory phases can produce robust dates, their thermodynamic data is often poorly constrained, requiring careful petrographic analysis to link dates to the pressure-temperature-deformation histories of metamorphic rocks. The dating of rock-forming minerals, like garnet and plagioclase, is typically more resource intensive than accessory phase petrochronology, but allows for directly linking dates to specific metamorphic events. Here, we consider the results of monazite accessory phase U-Th-Pb chemical dating with that of Sm-Nd dating of garnet from the Manhattan Schist, New York City, to explore how the synthesis of these two methods can be used to constrain robust tectonic histories.

The Manhattan Schist is a Laurentia-derived aluminous pelite historically thought to record polymetamorphism characterized by: 1) upper amphibolite facies metamorphism and anatexis associated with the Taconic Orogeny (~520-470 Ma) and 2) lower grade overprinting during the Acadian orogeny (~416-360 Ma). This interpretation, however, is based on correlation with seemingly similar units elsewhere in New England rather than direct study of the Manhattan Schist, and is called to question by the data presented here.

This work explores three samples, MAT-2017-01a, GWB-03, and CRT-06. All samples are grt-ky-bt-ms migmatites, that record a four-stage metamorphic history: 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) ~1-2 kbar of isothermal exhumation to sillimanite (fibrolite) stability, and 4) continued exhumation to ~700°C and ~6 kbar. Monazite grains in all samples occur both in the matrix and as inclusions in garnet. Metamorphic monazite dates are dominated by a ~465 ±30 Ma (n=15) age consistent with growth during the Taconic Orogeny, with only a single date at ~383 ±25 Ma found in CRT-06. Bulk garnet Sm-Nd dating via TIMS in MAT-2017-01a, however, yields a late Acadian age of ~386 ±3.68 Ma (n=5, MSWD=0.94). The low MSWD coupled with the preservation of major element growth zoning in garnet, suggests that this value represents a single garnet age population reflective of growth at peak or near-peak conditions, entirely during the Acadian orogeny.

Taken as a whole, our results suggest that the Manhattan schist experienced: 1) early greenschist facies metamorphism during the Taconic orogeny recorded in metamorphic monazite, and 2) peak metamorphism associated with collision and burial in the Acadian orogeny recorded in garnet. Additionally, the sample-to-sample heterogeneity in monazite ages within the same lithology suggests that monazite growth is controlled by (sub)cm-scale processes. Therefore, future studies should investigate multiple samples of the same formation, if not the same outcrop, before making tectonic interpretations based on accessory phase petrochronology.