Deformation-induced reconstitution and local resetting of mineral equilibria in polymetamorphic gneisses: tectonic and metamorphic implications

A sequence of at least three Al₂SiO₅-bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the Willimantic window, Connecticut. The ductile deformation, localized at and near the boundary between the Putnam-Nashoba terrane and underlying Avalon terrane is characterized by a network of anastomozing shear zones that outline metre-scale tectonic blocks of migmatitic Kfs + Sil + Gt + Bi + Pg + Qtz + Ilm + Ru gneiss. These assemblages record Acadian or older metamorphic conditions of 6 kbar, 700d? C. Mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss. The reconstituted rocks exhibit relict Ky + St + Grt (+Pl + Bt + Qtz + Rt + Ilm) assemblages and require a minimum pressure for the Ky-Str grade metamorphism of 8.5 kbar. Kyanite in block margins is widely replaced by sillimanite, and locally by andalusite, during a period of post-Alleghanian ductile deformation. The interiors of blocks do not record this sequence of polymorphs. The pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane-bounding fault zone. Strain provided the activation energy for recrystallization and retrograde mineral reactions. The P-T conditions of post-Alleghanian ductile deformation evolved from 600d? C and 6 kbar to 550d? C and 3 kbar. The occurrence of Ky + Str-bearing assemblages, overprinting Acadian Kfs + Sil-bearing assemblages and subsequently overprinted by Alleghanian sillimanite- and andalusite-bearing assemblages, along with reset hornblende ⁴⁰Ar/³⁹ Ar mineral ages from Mississippian to Permian, requires a prograde Alleghanian metamorphism of rocks previously metamorphosed during the Acadian. Thus, mineral assemblages from gneisses in the Willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high-grade Acadian orogenesis, tectonic loading resulting from Alleghanian collision of Avalon with North America, and tectonic exhumation in Permo-Triassic time. The c.3-kbar pressure decrease between prograde and retrograde Alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the Putnam-Nashoba terrane cover sequence following Alleghanian orogenesis in south-eastern New England.