High-Thorium Monazite-(Ce) Formed During Disequilibrium Melting of Metapelites Under Granulite-Facies Conditions

Gordon R. Watt
Geology & Cartography Division, School of Construction & Earth Sciences, Oxford Brookes University, Headington, Oxford, OX3 0BP, UK

Abstract: Monazite is a light rare-earth element (LREE)-bearing accessory phase common in felsic granitic rocks, and strongly influences LREE concentrations in granites and the chemistry of melts and residues formed during partial melting. Cheralite-rich (high-Th) monazite-(Ce) has been recorded as rims on partially dissolved cores in a suite of granulite-facies migmatites generated by disequilibrium melting. High-Th monazite has previously been recorded only in granitic-pegmatite occurrences. Subidiomorphic monazite-(Ce) from leucosome, melanosome and mesosome consists of complexly zoned cores (up to 300 µm in diameter) containing 5.17–9.87 wt.% ThO2, overgrown by essentially unzoned rims (containing up to 21.4 wt.% ThO2). Uranium contents are average for Th-rich monazite-(Ce) (0.22–3.17 wt.% UO2). Th enrichment (relative to LREE) in monazite-(Ce) rims is ascribed to the presence of fluorine-bearing melts (formed during the incongruent breakdown of biotite to produce a water-undersaturated melt), allowing the formation of REE-fluoride complexes in the melt, coupled with the increased charge balanced substitutions Th4+ + Si4+REE3+ + P5+ and Th4+ + Ca2+ ⇌ 2REE3+ into monazite-(Ce). Fractionation of Th and U (reflected by an increased in Th/U in rims relative to cores) may have occurred due to the removal by a CO2-rich fluid phase in the melt. These interpretations are consistent with elevated CO2 and F contents of granitic liquids produced during the water-undersaturated breakdown of biotite under granulite-facies conditions. Evidence for restricted monazite-(Ce)-melt equilibration and rapid melt removal is provided by the ubiquitous presence of partially embayed cores and unzoned rims.

Keywords: thorium • monazite • granulite facies • REE

Mineralogical Magazine; December 1995 v. 59; no. 397; p. 735-743; DOI: 10.1180/minmag.1995.059.397.14
© 1995, The Mineralogical Society
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