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 |
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Authors: | HARLOV DANIEL E; FORSTER HANS-JURGEN |
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Institution: | 1GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, D-14473 POTSDAM, GERMANY
2INSTITUTE OF EARTH SCIENCES, UNIVERSITY OF POTSDAM, PO BOX 601553, D-14415 POTSDAM, GERMANY |
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Abstract: | K-feldsparplagioclasequartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H2O-rich, hornblende-bearingzone to an H2O-poor, hornblende-free, orthopyroxene-bearing,granulite facies zone at 700800°C and78 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO2-rich fluid, whereas for the Val Strona diOmegna traverse, IvreaVerbano Zone, northern Italy, dehydrationtook place over a 34 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KClNaCl brines; metasomatism; granulite facies metamorphism; charnockiteenderbite; orthopyroxene; K-feldspar; biotite; hornblende |
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