High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea–Verbano Zone, Northern Italy. Part I: Petrography and Silicate Mineral Chemistry

K-feldspar–plagioclase–quartz mineral textures as well as biotite and hornblende compositions are compared for suites of metamorphosed mafic rocks from two widely separated traverses. A portion of either traverse has experienced a high-grade dehydration event transforming it from an H2O-rich, hornblende-bearing zone to an H2O-poor, hornblende-free, orthopyroxene-bearing, ‘granulite facies’ zone at 700–800°C and 7–8 kbar. In the Kigluaik Mountains, Seward Peninsula, Alaska, dehydration took place over an 85 cm thick layer of metatonalite in contact with a marble during regional metamorphism and involved a CO2-rich fluid, whereas for the Val Strona di Omegna traverse, Ivrea–Verbano Zone, northern Italy, dehydration took place over a 3–4 km thick sequence of metabasites interlayered with metapelites in a contact metamorphic event involving basaltic magmas intruded at the base of the sequence. Orthopyroxene-bearing samples from both dehydration zones show micro-veins of K-feldspar along quartz and plagioclase grain boundaries as well as replacement antiperthite in plagioclase. K came primarily from the breakdown of hornblende + quartz to orthopyroxene ± clinopyroxene, feldspar and fluid. Biotite either was stabilized or formed in the dehydration zones and is enriched in Ti, Mg, F and Cl relative to biotite in the amphibolite facies zone.