Origin and tectonic evolution ofthe metamorphic sole beneath the Brooks Range ophiolite, Alaska

Structural field relations, petrologic and geochemical studies, and radiometric age aualysis of metamorphic rocks at the base of the Brooks Range ophiolite provide a record of the time, tectonic setting, and initial conditions of ophiolite emplacement during the Middle Jurassic western Brookian orogeny. The metamorphic sole of the Misheguk, Siniktanneyak, and Avan Hills ophiolite bodies is transitional with and geochemically similar to basalt, chert, limestone, and psammitic material mostly of the Copter Peak allochthon. Dynamothermal metamorphism of these rocks produced garnet-bearing amphibolite and quartz-mica schists that decrease in metamorphic grade and ductile strain inteusity downward. These high-grade rocks are separated by thin, fault-bounded intervals from underlying greenschist facies and unmetamorphosed basalt and sedimentary rocks. The maximum total thickness of both parts of the metamorphic sole is <500 m. Minimum temperature estimates, from garnet-biotite and garnet-amphibole geothermometric studies of rocks currently 5-50m below the ophiolite, range from 500 to 560°C at around 5 Kb. These conditions are similar to closing temperatures estimated for Ar retention in hornblende near the base of the ophiolite, which yields 4oAr/39Ar ages of 164-169 Ma. This age is concordant with, but slightly youuger than the mean age of ophiolite crystallization. The limited time between crystallization and tectonic emplacement of the Brooks Range ophiolite, and compositional similarities between the metamorphic sole and continental margin material imply spacial and temporal proximity of the ophiolite with the Brookian orogen prior to emplacement. Localization of strain at the base of the ophiolite during emplacement produced a ductile shear zone in the metamorphic sole that is characterized by mylonitic textures overprinted by postmetamorphic deformation. Microstructures document synkinematic crystal growth at high flow stress and strain rates. Kinematic indicators show mostly top-to-northwest sense of shear at the base of the ophiolite. Postmetamorphic deformation associated with the Brookian orogen shuffled various parts of the metamorphic sole and thrust it over shelf facies sedimentary material.