242-21 Preliminary Constraints on the Timing and Tempo of Anatexis of the Hartland Schist at Orchard Beach, New York City

Abstract:

Crustal anatexis during regional metamorphism acts as a control of lower crustal rheology, strain partitioning, the composition and generation of continental crust, and exhumation of deep orogenic crust. The relationship between anatexis, deformation, and exhumation is underexplored in the Central Appalachians. The Hartland Formation in Pelham Bay, The Bronx, New York is a natural lab for exploring the relationship between anatexis and deformation, and how melt segregation accommodates strain. Here, we apply structural analysis in the field, petrological observations, and thermodynamic modeling to constrain the pressure-temperature (P–T) evolution of the Hartland Formation. The Hartland Formation is a unit of metasedimentary rocks underlying the Manhattan Prong thought to record metamorphism and deformation (d) associated with the Devonian Acadian orogeny. Constraining the P–T–d history of these rocks will thus provide key insights into the dynamics of Acadian orogeny and post-Acadian exhumation.

The Hartland Formation at Orchard Beach is exposed as a ~320 m-long discontinuous outcrop consisting of complexly deformed metasedimentary migmatite and minor amphibolite. Diatexite preserves m-scale similar folds with cm-scale parasitic folds (F1). Metatexite preserves extensional shear bands with ponded leucosome at a low angle to foliation. Both structures suggest suprasolidus deformation. Discontinuous high strain zones through diatexite and ptygmatic folding (F2) suggest subsequent subsolidus deformation. Diatextite preserves the assemblage kyanite-plagioclase-muscovite-biotite-garnet-rutile-ilmenite-quartz. Preliminary phase equilibria modeling in the MnNCKFMASHTi system constrains peak P–T conditions to ~10 kbar and 800 °C, consistent with the petrographically determined peak assemblage kyanite-plagioclase-muscovite-biotite- garnet-rutile-melt. Sillimanite pseudomorphs after kyanite require subsequent (near)isothermal decompression to 8 kbar. These results are consistent with an overall clockwise P–T path, rapid (melt facilitated?) exhumation, and syn- to post-peak P-T strain accumulation in a transpressional regime. These results will be supplemented by in situ U-Th-Pb monazite dating and bulk garnet Sm-Nd geochronology to better constrain the P-T-t-d evolution of the Hartland Formation. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-18241

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