"BARROVIAN" METAMORPHISM IN THE NASHOBA TERRANE: EVIDENCE FROM COMBINED PHASE EQUILIBRIA MODELING, GEOTHERMOBAROMETRY, AND PETROGRAPHIC ANALYSIS

The Nashoba terrane in central Massachusetts is thought to record a complex history of high-grade metamorphism and deformation, with recent tectonic models proposing a history of arc metamorphism in the late Silurian followed by the collision of the Avalon terrane to the southeast, resulting in anatexis in the Nashoba terrane in the Devonian. While the structural history of the terrane is well studied, here we explore the pressure–temperature (P–T) history of the terrane, in order to help inform our understanding of Laurentian evolution during the Paleozoic Wilson cycle. We present metamorphic conditions estimated from integrated petrographic analysis, geothermobarometry, and phase equilibria modeling for three samples along a N-S transect in the Nashoba terrane.

Samples were collected from the south-central portion of the Nashoba formation, a unit of volcanic and marine metasediments. Sample JWB13-8 is a grt-plg-qtz-ky-sil-bt gneiss. The dominant foliation is defined by alternating layers of intergrown bt+ky+sil, and qtz+plg and quartz leucosomes. AlSi and anatetic reaction textures paired with thermodynamic modeling suggest two stages of metamorphism: M1 at 650-700 °C and 7-11 kbar and M2 at 700-750 °C and 3.5-7 kbar. Sample 21-AC-06 is a grt-plg-qtz-sil-bt gneiss. Gneissic banding is defined by alternating layers of bt+sil and qtz+plg ribbons. Sil occurs exclusively as prismatic intergrowths within matrix bt. Grt appears as subidioblastic or xenoblastic grains. Sample 21-AC-06 experienced M1 at 575-675 °C and 6-9 kbar and M2 at 675-750 °C and 3.5-7.5 kbar. Sample 21-AC-04 is a grt-plg-qtz-ky-sil-bt-ms gneiss. The matrix foliation is defined by intergrown bt+prismatic sil+fibrolite with rare ms. Grt is rare and mostly inclusion-free. Ky is commonly replaced by sil. 21-AC-04 experienced M1 at 625-700 °C and 7.5-12 kbar and M2 at 675-725 °C and 5-8 kbar.

Our results suggest a clockwise P–T path for all samples, with no apparent geographic variation in peak P–T. This is consistent with classic orogenic Barrovian-type metamorphism, rather than the low pressure, nearly isobaric paths presented in previous studies. These results represent the first documentation of orogenic P–T paths in the Nashoba, and are an important evolution in our understanding of the metamorphic and orogenic history of the Nashoba.