Harlov and Forester reported in 2002 on K-feldspar-plagioclase-quartz mineral textures as well as biotite and hornblende compositions and compared 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.
The Part II of the study explores the origin and geochemical evolution of apatite, monazite, and xenotime along two metamorphic traverses. The first, from the Kigluaik Mountains, Seward Peninsula, Alaska, consists of a localized (85 cm) orthopyroxene-clinopyroxene-bearing dehydration zone. The second consists of orthopyroxene ± clinopyroxene- bearing granulite facies metabasite layers interlayered with metapelites over a 3-4 km traverse, along the Val Strona, Ivrea-Verbano Zone, Northern Italy (IVZ).
In both dehydration zones small Th- and U-poor inclusions of monazite and/or xenotime occur in the apatite. These inclusions are metasomatically induced and nucleated within the apatite via the coupled substitutions Na+ + (Υ + REE)3+ = 2 Ca2+ and Si4+ + (Υ + REE)3+ = P5+ + Ca2+. These are not present in apatite from the original amphibolite facies gneiss. Apatite, in both dehydration zones, also shows a relative increase in both F and Cl compared with apatite from the amphibolite facies zone. Granulite facies metabasites in the IVZ also contain isolated monazite grains, which range from uniform to complexly zoned in Th (13-30.1 mol% ThSiO4). These are the product of the breakdown and subsequent mobilization of the lanthanides from monazite-(Ce) in the metapelite layers into the metabasite layers at the start of granulite facies metamorphism.
Part II: https://i2mconsultants.com/downloads/corp/Harlov-Forster-2002SewAlas-ItalyartII.pdf
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