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 II: Phosphate Mineral Chemistry

Abstract

This 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+ + (Y + REE)3+ = 2 Ca2+ and Si4+ + (Y + 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 the (13–30·1 mol % ThSiO4). These are the product of breakdown and subsequent mobilization of the lanthanides and actinides from monazite-(Ce) in the metapelite layers into the metabasite layers at the start of granulite facies metamorphism.