Metamorphic evolution of the Seward Peninsula blueschist terrane

Abstract

Blucschists and greenschists of the central Seward Peninsula (the Nome Group) crop out over an 8000 km2 area. The protolith package for the mid-Jurassic high P/T metamorphism consisted largely of Cambrian, possibly also Precambrian, to Devonian sedimentary rocks (limestones, marls, pelites, carbonaceous siltites, quartzites) of shallow water miogeoclinal origin, which maintained a coherent lithostratigraphy during metamorphism and concurrent high-strain, noncoaxial deformation.Several experimentally and empirically calibrated geothermobarometers have been integrated with textural and microstructural observations in order to derive a pressure-temperature (P-T) path for the metamorphism of the Nome Group. These data, coupled with previously obtained Rb-Sr phengite-whole-rock isochron ages, yield a P-T-t path showing a steep burial segment followed by a prolonged period (̃55 Ma) of isothermal decompression. Peak pressures, obtained from the jadeite content of clinopyroxene in rare, isofacial eclogites, and from the celadonite content in phengite yield ̃12 kb. Temperatures at this pressure, gained from garnet-clinopyroxene and microcline-plagioclase pairs, were 460±30°C. Decompression temperatures were essentially the same, as evidenced by calcite-dolomite and garnet-biotite thermometry. Uniformity of metamorphic conditions throughout the Seward Peninsula blueschist terrane coupled with the miogeoclinal nature of the protolith package indicate burial beneath a flat-lying (A-type) subduction zone. The derived P-T-t path is difficult to reconcile with one-dimensional thermal models of collisional blueschists, where peak P and T are not generally coincident. © 1989 Oxford University Press.