Metamorphic Evolution of the Seward Peninsula Blueschist Terrane

• Published in: Journal of Petrology
• Main Authors: PATRICK, BRIAN E., EVANS, BERNARD W.
• Format: Text
• Language: English
• Published: Oxford University Press 1989
• Subjects: Articles; Seward Peninsula
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/30/3/531; https://doi.org/10.1093/petrology/30.3.531

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 ({small tilde}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 {small tilde}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.