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