SEWARD PENINSULA. ALASKA

• Main Authors: Conodian Minerologist, Phengite-t In, Granitic Orthogneisses, Bernard W. Evans, Brian, E. Patrick
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: phengite geobarometer; two.feldspar geothermometer; blueschist biotite; Seward Peninsula; Alaska; Ferro
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.516.9758; http://rruff.info/doclib/cm/vol25/CM25_141.pdf

Orthogneisses of granite, granodiorite and tonalite com-position, 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 con-ditions. The mineral paragenesis of ltre blueschist facies in granitic orthogneiss is quartz + albite + microcline + phengite + biotite; almandine-rich garnet occurs in tona-Iitic 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 andmetaba-sites, 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 bimo-dally. The low.Si frequency maximum at 6.4 atoms is inter-preted as being due to the greenschist-:facies decompression and is accompanied by growth of biotite. Positive corela-tions between tschermak, paragonite, and (possibly) trioc-tahedral mica components in the phengite are ascribed to crystakhemical interactions. The granitic orthogneiss para-genesis is ideal for phengite geobarometry, but existing calibrations differ considerably from one another. Mcrocline- low albite pairs give equilibration tempera-tures 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.