Corona formation around monazite and xenotime during greenschist-facies metamorphism and deformation
Authors: Hentschel Felix , Janots Emilie , Trepmann Claudia A. , Magnin Valerie , Lanari Pierre 
Published: October 2020

Abstract. Epidote/allanite–fluorapatite coronae around monazite and xenotime
are investigated in Permian pegmatites deformed under greenschist-facies
conditions during Alpine tectonometamorphism in the Austroalpine basement,
Eastern Alps. The aim was to evaluate the replacement reactions involved in
the formation of a corona microstructure, its age and relation to
deformation. In the corona core, monazite and xenotime single crystals show
domains with different composition and age. Monazite (Mnz1) and xenotime
(Xen1) dating by electron microprobe (EPM) reveals an age of 250–287 Ma,
consistent with the Permian magmatic age of the pegmatites. These are partly
replaced by secondary monazite (Mnz2) and xenotime (Xen2) compositions
yielding younger Mesozoic (170–210 Ma) and Alpine (30–120 Ma) ages. The
same crystallographic orientation of the primary and secondary monazite and
xenotime indicates interface-coupled dissolution–precipitation reactions.
Allanite U–Th–Pb dating by laser ablation inductively coupled mass
spectrometry in the corona revealed an age of 60±6 Ma, interpreted
as the age of corona formation. The coronae around monazite consist of an
inner zone of equant fluorapatite grains surrounded by prismatic allanite,
which are surrounded by epidote enriched in heavy rare earth elements (HREEs)
and REE-poor epidote grains. Compared to coronae around monazite,
fluorapatite has higher REE contents and no allanite occurs in the coronae
surrounding the xenotime. General reactions for monazite and xenotime
breakdown can be written as follows: Mnz1+(Si,Ca,Al,Fe,F)fluid→Mnz2+LREE-Ap+Aln+HREE-Ep+Ep+(Th,U)O2+(Th,U)SiO4,Xen1+(Si,Ca,Al,Fe,F)fluid→Xen2+HREE-Ap+HREE-Ep+Ep+(Th,U)O2. The amount of replacement (judged by the relative proportions of monazite
and fluorapatite) is low for monazite included in tourmaline but high within
the mylonitic foliation. This dependence on the degree of replacement on the
local surrounding microfabric indicates that fluid availability along grain
boundaries in the matrix and cracks controlled reaction advancement, allowing
the elementary mass transfer required for corona formation (e.g. input of
Ca, Al, Si, Fe, F). The oblate shape of the coronae aligned within the
foliation of the pegmatites and the deflected foliation around the coronae,
without an outer rim of prismatic epidote showing signs of deformation,
indicate that the main stage of corona formation took place during
deformation and reactions were still ongoing after the main stage of
deformation. The corona microstructure documents replacement reactions of a
single reactant into multiple distinct mineral growth zones by dissolution
and precipitation processes at non-isostatic, greenschist-facies conditions,
which prevailed in the area to the north of the Defereggen–Antholz–Vals
shear zone between the middle Cretaceous and the Oligocene. These reactions
ceased before being completed, and REE gradients within single grains within
the corona and on the thin-section scale are preserved, which suggests
restricted and/or episodic transport of REE in the fluid phase and/or
availability of fluid.