The metamorphic bedrock of New York City (NYC) records the closure of the Neo-Iapetus ocean and accretion of the Taconic arc onto Laurentia during the Taconic orogeny. The rocks preserved in Central Park, NYC offer direct insight into the nature of this orogenic history. The Manhattan and Hartland formations outcrop throughout the park, and are thought to be Laurentian and Taconic arc-derived metasediments, respectively. The suture between the two is known as Cameron’s line. While these rocks were mapped as part of infrastructure development and structural studies, much of their proposed history is extrapolated from correlations to rocks elsewhere in New England. The specific petrogenesis of these units has gone largely understudied, so an accurate pressure-temperature history is necessary to understand the dynamics of Taconian metamorphism in NYC.
Here, we applied a combination of petrographic analysis, geothermobarometry, and phase equilibria modeling to 4 samples from Central Park and northern Manhattan. MAT-2017-01 is from the Manhattan formation and is a gt-sil-bt-ms schist. There are three generations of sil: 1) early fibrolite after ms, 2) peritectic sil formed via ms melting at peak conditions, and 3) post-anatectic sil. Combined with phase equilibria modeling, these textures suggest a clockwise P–T path with peak conditions around 5–6.5 kbar and 700–750 ˚C. Sample East-79 is a ms-bt-plg-qtz-grt schist from the Hartland formation. Preliminary phase equilibria modeling suggests peak conditions of ~600-700 °C and ~7.5-10 kbar. Traditional geothermobarometry for both samples retrieved inconsistent results, and potential reasons for this discrepancy will be explored. IWP-01 is a leucocratic grt-ms-bt schist from the Manhattan formation while MOR-NYC-04 is a grt-bt-ms schist from the Hartland formation. Results for thermodynamic modelling of both samples will be reported.
The preliminary results indicate a metamorphic break of at least ~2 kbar between the units. Coupled with the structural data, this suggests that Cameron’s line is a syn- to post-metamorphic thrust fault. Additionally, our study provides a concrete example of the utility of urban geology as a tool for tectonic study to the >140,000 students who study Earth Science in NYS, and >42 million people who visit Central Park each year.