PHASE EQUILIBRIA CONSTRAINTS ON TACONIAN METAMORPHISM IN THE MANHATTAN AND HARTLAND SCHISTS, NEW YORK CITY: CONSTRAINING THE NATURE OF CAMERON’S LINE

The public parks in New York City preserve outcrops of metasedimentary bedrock metamorphosed and deformed during the ~520-470 Ma Taconic Orogeny. The metamorphic histories of the Manhattan and Hartland Formation are of particular interest, as they are thought to be Laurentian-, and Taconic arc-derived sediments, respectively. The contact between the two units is the map-scale thrust fault Cameron’s Line, which is historically interpreted as the Taconic suture. While these units have been extensively mapped, much of their history is based on correlations with potentially equivalent lithologies elsewhere in New England rather than detailed petrographic study of these specific formations. As a result, quantitative pressure–temperature–time (P–T–t) paths and the nature of Cameron’s Line are largely unconstrained.

Here we present the results of thermobarometry, and thermodynamic modeling to quantitatively determine the P–T evolution for several samples across the City. Manhattan Formation samples in Central Park, Inwood Hill Park, and Crotona Park are grt-ky-sil-bt-ms migmatite schists. Our results indicate consistent P–T conditions in all three samples, and suggest the following clockwise P–T path: (1) garnet growth at 550 °C and 6 kbar, (2) muscovite-out melting and kyanite growth 750 °C and 9 kbar, and (3) exhumation to 700 °C and 6.5 kbar and associated sillimanite growth.

Hartland formation ms-bt-pl-qtz-gt metagreywacke schists are dominated by ~80 mode% qtz+pl, with <1 mode% gt. Constraining a full P–T path is difficult in these rocks, as the bulk composition does not allow for traditional index mineral growth (e.g. kyanite, staurolite, etc). Nevertheless, traditional thermobarometry can still be used to constrain the P–T conditions of equilibration at garnet rims. Garnet rim thermobarometry constrains rim-equilibration conditions of 600-700 °C and 6-7 kbar.

Taken together, the Manhattan and Hartland schist appear to share a similar metamorphic history, with differences in assemblage largely attributable to differences in bulk composition. The lack of metamorphic break across the fault expected of a suture zone in combination with Laurentian detrital zircon signatures suggests that Cameron’s Line is not a suture but is instead a pre- to syn-metamorphic structural feature.