SOME LIKE IT HOT: REEVALUATING THE EXTENT OF ANATEXIS IN THE NASHOBA TERRANE, EAST-CENTRAL MASSACHUSETTS

The Nashoba terrane is a high-grade metamorphosed volcanic arc that records the collision of Avalonia with the trailing edge of Ganderia during the Acadian and Neo-Acadian orogenies. While the structural and geochronological history of these rocks have been well documented, the metamorphic history is largely poorly constrained. Constraining the geographic and volumetric extent of granulite-facies partial melting is key to discerning the applicability of ductile extrusion models like channel flow to the Nashoba terrane. Here we show how careful petrographic analysis and modern thermodynamic modeling techniques can be used to extract information from historically overlooked rocks, and reveal anatectic histories that are otherwise unmapped.

Sample 21-AC-02 outcrops just east of the Assabet river fault zone, and is mapped as a sulfidic rusty schist. It is historically thought to have reached upper-amphibolite facies conditions during Neoacadian metamorphism but to have escaped anatexis. Our petrographic analysis, however, indicates that this rock is an anatectic grt-sil-ky-bt-plg-qtz schist, with cm-thick layers of bt+sil restite in outcrop. There are also 3 generations of sil present: 1) fibrolitic sil after ms, 2) prismatic sil overgrowing matrix fabrics, and 3) ~1-2 cm large sil pseudomorphs after ky and st. Two generations of garnet are present as well, 1) up to ~6 mm in diameter grains with preserved Mn and Ca growth zoning, and 2) <1mm in diameter inclusion-free peritectic grains grown in bt-plg leucosomes. Both garnet types locally display melt back-reaction rims. Plg+bt+qtz leucosomes are also locally folded, and show evidence for high temperature deformation-induced recrystallization.

Synthesizing the petrographic data with thermodynamic modeling reveals a 3 stage clockwise P-T-d path from 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 anatexis. The dominant fabric developed syn- to post- anatexis. This study highlights both the utility of combining modern and tried and tested techniques to unravel metamorphic histories, and the need to revisit well-studied terranes in light of modern innovation.