Thermal overprint on blueschists of the Seward Peninsula: The Lepontine in Alaska

Abstract

On the Seward Peninsula, Alaska, blueschists and greenschists of the Nome Group and amphibolites and granulites of the Kigluaik Group all formed during a single episode of continent-arc collision. High-pressure/low-temperature mineral assemblages that initially developed in a structurally coherent package of miogeoclinal rocks were then variably overprinted during later uplift by lower P /higher T Barrovian facies series assemblages. Derived pressure-temperature-time ( P-T-t ) paths illustrate the early passage of both the Nome and Kigluaik Groups through high-pressure (> 12 kbar) conditions, subsequent nearly isothermal decompression of Nome Group blue- schists, and divergence during decompression of nested P - T paths to amphibolite and granulite conditions in the underlying Kigluaik Group. Similar field occurrences are described from the Lepontine Alps and the Greek island of Naxos. Standard geobarometric techniques applied to medium-grade Kigluaik Group rocks do not record the earliest burial phase and therefore underestimate peak pressures by ∼8 kbar.