Abstract:

The Taconic and Salinic orogenies in the northern Appalachian
Mountains record the closure of the Iapetus Ocean, which separated
peri-Laurentian and peri-Gondwanan terranes in the early Paleozoic.
The Taconic orogeny in New England is commonly depicted as
an Ordovician collision between the peri-Laurentian Shelburne Falls
arc and the Laurentian margin, followed by Silurian accretion of peri-
Gondwanan terranes during the Salinic orogeny. New U-Pb zircon
geochronology demonstrates that the Shelburne Falls arc was instead
constructed on a Gondwanan-derived terrane preserved in the Moretown
Formation, which we refer to here as the Moretown terrane.
Metasedimentary rocks of the Moretown Formation were deposited
after 514 Ma and contain abundant ca. 535–650 Ma detrital zircon that
suggest a Gondwanan source. The Moretown Formation is bound to the
west by the peri-Laurentian Rowe belt, which contains detrital zircon
in early Paleozoic metasedimentary rocks that is indistinguishable in
age from zircon in Laurentian margin rift-drift successions. These data
reveal that the principal Iapetan suture in New England is between the
Rowe belt and Moretown terrane, more than 50 km farther west than
previously suspected. The Moretown terrane is structurally below and
west of volcanic and metasedimentary rocks of the Hawley Formation,
which contains Laurentian-derived detrital zircon, providing a link
between peri-Laurentian and peri-Gondwanan terranes. The Moretown
terrane and Hawley Formation were intruded by 475 Ma plutons
during peak activity in the Shelburne Falls arc. We propose that the
peri-Laurentian Rowe belt was subducted under the Moretown terrane
just prior to 475 Ma, when the trench gap was narrow enough
to deliver Laurentian detritus to the Hawley Formation. Interaction
between peri-Laurentian and peri-Gondwanan terranes by 475 Ma is
20 m.y. earlier than documented elsewhere and accounts for structural
relationships, Early Ordovician metamorphism and deformation, and
the subsequent closure of the peri-Laurentian Taconic seaway. In this
scenario, a rifted-arc system on the Gondwanan margin resulted in the
formation of multiple terranes, including the Moretown, that independently
crossed and closed the Iapetus Ocean in piecemeal fashion.