242-20 New Constraints on the Paleozoic Tectonic Evolution of Central New England from Metamorphic Petrochronology

Abstract:

The crystalline bedrock underlying Central New England has been thought to preserve a signature of deformation and metamorphism dominated by the accretion of Avalonia onto the Laurentian margin during the (Neo)Acadian orogeny (c.420–360 Ma). Understanding the nature of the tectonics in this region has strong implications for orogenic evolution, exhumation rates, paleoclimate reconstruction, topography, and petrogenesis of economic minerals.

Competing models for Acadian tectonism in central Massachusetts include: 1) a high elevation, low relief altiplano, characterized by periods of crustal thickening (50–60 km; 410–380 Ma), cooling (380–330 Ma), and rapid exhumation (330–300 Ma), and 2) a thin-skinned thrust nappe emplacement model. The plateau model suggests that the rocks underlying central Massachusetts, from the Iapetan suture to the Bloody Bluff fault, record essentially the same pressure-temperature-time (P–T–t) evolution, whereas the thrust nappe model implies the existence of relatively thin and distinct metamorphic zones recording distinct P–T histories in different parts of the orogen. Here, we employ equilibrium thermodynamic modeling in the MnNCKFMASHTi chemical system, petrography, and quartz-in-garnet Raman inclusion barometry to constrain robust P–T histories for three samples from Central Massachusetts.  

Sample 21-EM-02 is a garnet-staurolite schist from the Littleton Formation near Erving, MA, with peak conditions at 550–650 °C and 5–8 kbar. West of this sample, 25-AP-01A is a garnet-kyanite schist from the Mt Mineral Formation near Pelham, MA, which records a counterclockwise P–T path with peak conditions at ~700 °C and 8 kbar constrained by quartz-in-garnet Raman inclusion barometry. Garnet rim isopleths indicate cooling during burial to ~625 °C and 9 kbar. 25-AP-02A is a garnet-kyanite migmatite gneiss also from the Mt Mineral Formation that records peak conditions of at least 800 °C and 11 kbar, determined by garnet rim isopleths. 

These results suggest a significant decrease in metamorphic grade from W-E of ~1–4 kbar and 400 °C, over <2 km perpendicular to strike. The peak P–T conditions constrained here are generally higher P–T than previous work. We therefore favor tectonic models involving diachronous emplacement of ductile thrust nappes through the Late Pennsylvanian. These results will be bolstered with additional in situ monazite U-Pb dating and bulk garnet Sm-Nd geochronology.

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

doi: 10.1130/abs/2026AM-18450

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