CONSTRAINTS ON DEVONIAN-CARBONIFEROUS DEFORMATION IN THE NASHOBA TERRANE, EASTERN MASSACHUSETTS

The Nashoba terrane is a NW-dipping, fault-bounded, Cambrian-Ordovician, arc-backarc complex intruded by Silurian to earliest Carboniferous plutons. The Tadmuck Brook Schist (TBS) occurs along the NW margin of the terrane and metamorphic grade decreases from the sill-ksp zone in the SE to the chl zone in the NW. The Nashoba Formation (NF), immediately SE of the TBS, consists mostly of bt and sill gneiss and migmatite. Electron microprobe monazite dating was used to constrain the timing of deformation within the TBS and NF.

The high-grade TBS preserves small, folded domains within a broadly sinistral strike-slip shear zone. Monazite grains within the folded fabrics are high in Y, and are unzoned. Their ~405-385 Ma ages broadly indicate a maximum age for the time of folding, coincident with melting in the NF. Monazite cores from the sheared TBS also have broadly coeval ~405-380 Ma crystallization ages. Monazite rims aligned with the sinistral shear fabric of the TBS are ~345-340 Ma and have higher in Y than the cores. Sinistral shearing within the TBS is interpreted to have continued until this time.

The isoclinally folded gneissic and migmatitic fabrics of the NF are cut by NE-trending sub-vertical NW-side-down shear zones. A sample from one of these shear zones, where it was cut by a NW-side-down ultracataclasite, has high-Y monazite cores of ~410-360 Ma within both the shear zone and ultracataclasite and low-Y overgrowths (~360-330 Ma) in the shear zone, but not in cataclastite. The youngest ages within the NF (~310-305 Ma) come from high-Y rims on monazite grains present only within steeply south-dipping joints.

In summary, sinistral strike-slip and folding within the TBS were coeval, indicative of strain partitioning. The youngest ~345-340 Ma sinistral shear within the TBS is coeval with NW-side-down shear within the NF, suggesting that previously interpreted extrusion of the Nashoba terrane during sinistral convergence continued until later than previously thought. The ~310-305 Ma ages within the NF are interpreted to represent the distal effects of the Alleghanian orogeny, possibly fluids in late-stage fractures. The sparsity of Alleghanian-age monazite is consistent with previous work and an indication that an Alleghanian overprint within the Nashoba terrane is minimal.