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

Abstract

ABSTRACTA 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, 700d̀ 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 600d̀ C and 6 kbar to 550d̀ 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.