Polyphase late Paleozoic tectonothermal evolution of the southwestern Meguma Terrane, Nova Scotia: evidence from 40Ar/39Ar mineral ages

Abstract

40Ar/39Ar incremental-release ages of hornblende, muscovite, and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late Paleozoic tectonothermal evolution for the southwestern Meguma Terrane. Regional D1 folding with cleavage formation under greenschist – lower amphibolite facies, M1 metamorphic conditions, occurred at ca. 400–410 Ma and was followed by emplacement of a series of granitic stocks ranging in age between ca. 375 and 315 Ma. These were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade. These are locally superposed on M1 regional metamorphic assemblages and result in a complex isograd pattern. 40Ar/39Ar mineral ages suggest episodes of contact metamorphism occurred at (1) 360–375 Ma (possibly related to emplacement of the South Mountain Batholith or temporal equivalents), (2) 350–356 Ma around the Port Mouton Pluton and northeastern Shelburne Pluton, (3) ca. 315–325 Ma near the Wedgeport Pluton and in several other isolated localities, and (4) ca. 287 Ma along the northern margin of a large, low gravity anomaly located off the southwestern coast of Nova Scotia (inferred to reflect a subsurface pluton). Dextral shear deformation was locally associated with all of these thermal events. It is suggested that the Meguma Terrane experienced a similar stress system throughout the Late Devonian – Permian, with shear deformation localized in areas where increased temperatures resulted in decreased viscosity.