Phengite-3T in high-pressure metamorphosed granitic orthogneisses, Seward Peninsula, Alaska

ABsrRAcr Orthogneisses of granite, granodiorite and tonalite composition, in the lorr-grade schist terrane of the Seward Peninsula, Alaska, underwent structurally coherent, progressive regional blueschist-facies metamorphism in tle mid-to-late Jurassic, followed by partial re<quilibration during decompression under higb-greenschist-facies conditions. The mineral paragenesis of ltre blueschist facies in granitic orthogneiss is quartz + albite + microcline + phengite + biotite; almandine-rich garnet occurs in tonaIitic compositions in tle absence of microcline. No blueschist-facies minerals, such as jadeite, ferro-omphacite or ferro-glaucophane, or obvious pseudomorphs tlereof, occur in tie orthogneisses. Diagnostic blueschist-facies paragene€s are found, in adjacentmetapelites andmetabasites, in parts of the peninsula. The phengite in orthogneiss is uniaxial (3T) and Si-rich; maximum formula content of Si is 7.2 atoms for phengite in central and southern Seward Peninsula. TLe Si content of phengite varies considerably within mmples, in gome cases continuously, in otlers bimodally. The low.Si frequency maximum at 6.4 atoms is interpreted as being due to the greenschist-:facies decompression and is accompanied by growth of biotite. Positive corelations between tschermak, paragonite, and (possibly) trioctahedral mica components in the phengite are ascribed to crystakhemical interactions. The granitic orthogneiss paragenesis is ideal for phengite geobarometry, but existing calibrations differ considerably from one another. Mcrocline - low albite pairs give equilibration temperatures in the range 420 - sNC. One garnet-biotite pair gives 390oC. Accessory nrinerals are: epidote, allanite, titaaite, zircon, apatite, and calcite. The titanite contains between 7 and 27Vo of CaAlSiO4F.