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Monazite-epidote reaction in amphibolite grade blackwall rocks

Institut für Mineralogie, Petrologie und Geochemie, Universität Freiburg, D-79104 Freiburg, Germany
^** Now at Department of Geological Sciences, Central Washington University, Ellensburg WA, USA

^* E-mail: rodney.grapes{at}minpet.uni-freiburg.de

Composite monazite-epidote grains in a staurolite schist protolith have reacted to form apatite, chlorite + thorite, with REE enrichment of the epidote during amphibolite facies hydrothermal conditions of blackwall formation, Kvesjöen, east-central Norway. The simplified breakdown reaction is,

with additional thorite (and a possible Y-oxide phase). The breakdown reaction is explained in terms of areas of utilisation (apatite) and destruction (thorite + chlorite) of the monazite phosphate structure in relation to several interrelated factors: coupled substitutions mainly involving P⁵⁺, REE³⁺, Th⁴⁺, U⁴⁺, Si⁴⁺, Ca²⁺, possibly (Fe,Al)³⁺; dissolution (monazite) - reprecipitation (REE-poor apatite) with removal of REE by a fluid boundary layer. X-ray element distribution mapping shows that REE's, Y, Th and U derived from monazite alteration probably remained within the boundaries of the original composite monazite-epidote grains. Magnesium, Fe, Mn, Cl (mainly in chlorite), F in apatite were introduced to the reaction site by way of a hydrous fluid; REE's were largely fixed in the unreacted epidote corona (e.g. along grain boundaries); P, Th, U essentially remained at the reaction site in apatite and thorite. Radial cracks associated with REE-epidote and reacted epidote-monazite enclosed in corundum and staurolite of the blackwall resulted from volume expansion due to retrograde uptake of H₂O (10 wt.%) into areas of inferred structurally damaged REE-epidote and/or formation of apatite + chlorite + thorite at monazite-epidote grain contacts. Decrease in bulk rock Ca and enrichment in Al during blackwall development favoured preservation of epidote rather than monazite, and inhibited growth of new epidote.

Key-words: Blackwall, staurolite schist, monazite-epidote stability, hydrothermal reaction, REE distribution, radial cracking.