Phase Equilibria Evidence for Intermediate P/T Metamorphism in the Nashoba Terrane and Implications for Acadian Tectonics in Ganderia

Abstract

Abstract The bedrock exposed in the Nashoba terrane of east-central Massachusetts records a complex history of deformation and metamorphism associated with the collision of Avalonia with the Laurentian margin during the Devonian Acadian orogeny. Although the structural history of the terrane has been well studied, its pressure-temperature (P–T) history is less well constrained, and the mechanisms by which the upper amphibolite facies Nashoba terrane was juxtaposed between greenschist facies rocks to the southeast and northwest have not been established. Here, we apply phase equilibria modeling, geothermobarometry, and petrographic analysis to three garnet-bearing migmatitic rocks from the Nashoba terrane to determine their P–T histories and provide key metamorphic constraints relevant to Acadian orogenic processes. All three samples are from the Nashoba Formation, a unit within the Nashoba terrane consisting of volcaniclastic rocks likely deposited in an arc/back-arc setting near the trailing edge of the Ganderia terrane. Peak subsolidus conditions are determined via the integration of petrographic analysis and thermodynamic modeling. Peak anatectic conditions are constrained with garnet-biotite thermometry + GASP barometry, garnet intersecting rim isopleths, and thermodynamic modeling. Phase equilibria constraints suggest peak subsolidus conditions ranging from ~550°C to 700°C and ~6–12 kbar. Geothermobarometry, phase equilibria modeling, and garnet rim isopleths constrain biotite-out anatexis to ~700°C–715°C and ~5–8.5 kbar, up to ~2–4 kbar deeper than previously suggested. This synthesis of phase equilibria modeling and petrographic analysis suggests that all three samples record a clockwise P–T path with peak pressures achieved before anatexis associated with the Acadian orogeny at ~394 Ma. These results are inconsistent with a previously interpreted pre-Acadian period of low P/T metamorphism (<5 kbar peak pressures). Instead, this implies a previously unrecognized phase of intermediate P/T crustal thickening preceding anatexis, which we interpret as a result of Acadian orogenesis. We suggest that the Nashoba terrane exposes mid-crustal levels of the Acadian hinterland.