SYNTHESIZING PETROGRAPHIC ANALYSIS AND MODERN THERMODYNAMIC MODELING TO DETERMINE THE P–T CONDITIONS OF METAMORPHISM IN THE NASHOBA TERRANE, EAST-CENTRAL MA

The Nashoba terrane in east-central Massachusetts contains high-grade metamorphosed volcanic, volcanic-clastic, and pelitic rocks. The Nashoba terrane records a polymetamorphic history, including amphibolite facies arc-related regional metamorphism as a result of late Silurian subduction of Avalonia under the trailing edge of Ganderia and up to granulite facies metamorphism with local anatexis due to the collision of Avalonia with Laurentia through the late Devonian. While the origin and parts of the evolution of the Nashoba terrane have been previously documented, more detailed pressure–temperature–time–deformation paths are needed to truly unravel the metamorphic history of the terrane from convergence to exhumation. Here, we present petrographic analysis, phase equilibria modeling, and melt reintegration modeling of three metapelitic rocks and interpreted P–T paths along a N-S transect in the Shrewsbury Quadrangle in the southern part of the terrane.

Sample 21-AC-02 is the southernmost sample, and lies immediately west of the N-E trending Assabet River Fault zone, which bisects most of the terrane. In outcrop, it appears as a sulphidic rusty schist, but careful petrographic analysis reveals a bimodal bulk composition with thin layers of bt-sil restite and anatectic grt-sil-ky-bt-plg-qtz schist. Aluminosilicate and anatectic reaction textures coupled with thermodynamic modeling suggest that this sample experienced three stages of prograde metamorphism: M1 at ~650 °C and 8.5 kbar, M2 at ~700 °C and 6.5-8 kbar, and M3 at ~750 °C and 7-10 kbar with associated anatexis. These peak conditions are ~100 °C hotter and 2-4 kbar deeper than previously reported. North of this, sample 21-AC-04 is a grt-sil schist. Preliminary results suggest two stages of metamorphism: M1 at 575-600 °C and 4-5.5 kbar, and M2 at ~700 °C and 5-6 kbar. Sample 21-AC-06, the northernmost sample, is a grt-bt-kfs-sil schist. Grt occurs as 4-6 mm diameter poikiloblasts rimmed by qtz. Kfs and qtz occur in 1-2 mm diameter leucosomes. P–T estimates will be reported.

These results represent an evolution in our understanding of the metamorphic history of the Nashoba terrane, and a key step in constraining the history of orogenesis in east-central Massachusetts.