DUCTILE EXTRUSION OF THE NASHOBA FORMATION DURING CONVERGENCE OF THE AVALON TERRANE WITH THE COMPOSITE LAURENTIAN MARGIN, EASTERN MASSACHUSETTS

The Nashoba terrane is a NW-dipping, fault-bounded, Cambrian-Ordovician, arc-backarc complex intruded by Silurian to earliest Carboniferous plutons. The Nashoba Formation (NF) is mostly composed of highly deformed migmatitic bt±sill gneiss, interpreted to be part of the back-arc. Zones of leucosome-rich NF are interlayered with garnet-bearing leucosome-poor NF that outcrop in curvilinear NE-trending belts. Variation in leucosome content indicates that the level of deformation-driven melt extraction was variable across the formation.

Sinistral shear fabric in the NF gneiss is associated with shallowly plunging (NE or SW) to sub-horizontal sillimanite lineations. The timing of this fabric was investigated using electron microprobe analysis (EPMA) monazite dating on two leucosome-poor samples. Monazite grains have distinct low Y cores and high Y rims. Monazite cores crystallized during regional metamorphic prograde garnet growth from ~415 to ~400 Ma with a second growth pulse at ~390 Ma. These ages are consistent with ages of migmatization within the NF, based on U-Pb zircon sensitive high resolution ion microprobe (SHRIMP) data. Garnet has sigmoidal shapes indicative of sinistral deformation during growth. High Y, ~371-360 Ma rims are oriented within the sinistral shear fabric, and are coeval with garnet breakdown, indicating sinistral shear was ongoing before and during that time.

Leucosome in the leucosome-rich gneiss is isoclinally folded and then refolded by NW-side-down asymmetric folds. NW-side-down folding occurred between 367 and 360 Ma, based on zircon U/Pb SHRIMP and chemical abrasion thermal ionization mass spectrometry age constraints on folded and crosscutting felsic dikes. This folding is interpreted to have occurred during ductile extrusion of the NF into the upper crust.

In summary, in the latest Devonian, NW-side-down asymmetric folds developed in the migmatitic leucosome-rich gneiss while sinistral shear was localized along zones in the leucosome-poor gneiss. These differing forms of coeval deformation indicate strain partitioning on a formation-wide scale. Based on the age constraints both sinistral shear and ductile extrusion is attributed to the late-Acadian continued convergence of the Avalon terrane with the composite Laurentian margin.