THERMOCHRONOLOGICAL AND PETROGRAPHIC CONSTRAINTS ON THE TIMING OF TERRANE ASSEMBLY IN EASTERN NEW ENGLAND

The processes of terrane assembly in mountain building are elusive, but can be approached by determining the metamorphic history in each tectonic block. ⁴⁰Ar/³⁹Ar techniques were used to date amphibole, white mica, biotite, and K-feldspar from rocks along two east-west traverses across four recognized west-dipping terranes in eastern New England to determine their metamorphic histories, discriminate between Acadian and Alleghanian orogenic events, and infer their tectonic histories.

Amphibole from the eastern-most, Neo-Proterozoic Avalon terrane (AT) yields a Neo-Proterozoic age; but near the western-bounding Bloody Bluff fault, Permian cooling ages show Alleghanian resetting. This is in sharp contrast to amphiboles from the adjacent migmatitic Putnam-Nashoba terrane (PNT), which has Mississippian cooling ages. Moreover, cooling ages of white micas from overprinting higher-grade fabrics have mostly Pennsylvanian cooling ages. Farther west, lower-grade Merrimack terrane (MT) rocks contain multiple generations of cleavage-forming white micas. Middle Devonian cooling ages are interpreted for white micas that grew at biotite-grade or higher metamorphic conditions, while Pennsylvanian and Permian growth ages are interpreted for white micas that define a regionally pervasive, overprinting chlorite-grade cleavage. Farthest west, fabric-forming amphiboles from the Central Maine terrane (CMT) yield Early Permian cooling ages. Near the eastern edge of the CMT, white micas have Early Permian cooling ages; but farther west, white mica ages are Late Permian to Early Triassic.

The distribution of ⁴⁰Ar/³⁹Ar ages demonstrates that all four terranes were affected by the Alleghanian orogeny. The PNT was cooling in the hanging wall of the Bloody Bluff fault as the AT was heating, showing a net thrust sense in the Alleghanian across this boundary. White micas from early fabrics in the MT show the middle Devonian end of the Acadian orogeny, but overprinting white mica crystallization ages show significant cleavage development during the latest Pennsylvanian. This is also the time of cooling of white micas in the adjacent CMT to the west, suggesting that shortening across the Central Maine-Merrimack boundary was responsible for both uplift of the hanging wall, and cleavage development in the footwall.