Reconsideration of the age of blueschist facies metamorphism on the Seward Peninsula, Alaska, based on phengite 40Ar/39Ar results

Abstract ⁴⁰Ar/³⁹Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess ⁴⁰Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.     

Syn-tectonic pluton intrusion during contractional deformation: microstructural and metamorphic evidence from the aureole of the Acadian Victory Pluton, north-eastern Vermont, USA

Structures within the aureole of the Acadian (Devonian) Victory Pluton suggest that movement along the Monroe Fault occurred during pluton intrusion. Pre- or syn-tectonic garnet textures, including discordant internal and external foliations, sigmoidal inclusion trails and deflection of the matrix foliation around garnet, are found along the Monroe Fault. In sillimanite+K-feldspar grade rocks, the matrix foliation, defined by biotite and fibrolitic sillimanite, wraps around garnet and sillimanite porphyroblasts. Granite dykes and sills near the pluton contact are folded and boundinaged, and leucosome fills garnet pressure shadows and shear bands. Away from the fault and pluton contact, microstructures indicate much less deformation during metamorphism, suggesting that deformation was partitioned into the fault or the pluton. Deformation ceased before crystallization of the main body of the Victory Pluton was complete. It is possible that magmatism facilitated deformation along the Monroe Fault, and/or that the magma was transported to the mid- to upper crust along the Monroe Fault. This study suggests that Acadian deformation in north-eastern Vermont occurred as late as 390–370  Ma.