title: The behaviour of monazite from greenschist facies phyllites to anatectic gneisses: An example from the Chugach Metamorphic Complex, southern Alaska.
abstract: Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U-Pb geochronology. However, linking monazite U-Pb ages with thePTevolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U-Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~ 550-650 °C and ≥ 3.4 kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U-Pb SHRIMP analysis of the second and third monazite generations yields ages of ~ 55-50 Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle.
authors.affiliation: Department of Earth Science, University of Graz, Universitätsplatz 2, A-8010 Graz, Austria.
authors.forename: Deta
authors.initials: D
authors.lastname: Gasser
authors.affiliation: Department of Earth Science, University of Graz, Universitätsplatz 2, A-8010 Graz, Austria.
authors.forename: Emilie
authors.initials: E
authors.lastname: Bruand
authors.affiliation: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.
authors.forename: Daniela
authors.initials: D
authors.lastname: Rubatto
authors.affiliation: Department of Earth Science, University of Graz, Universitätsplatz 2, A-8010 Graz, Austria.
authors.forename: Kurt
authors.initials: K
authors.lastname: Stüwe
keywords: Alaska; Allanite; Chugach Metamorphic Complex; Monazite; SHRIMP U–Pb geochronology