Probably worth pointing out that Emilie Janots et al. and Frank Spear independently modeled allanite-monazite stabilities in metapelites (Janots E. et al., 2007, CMP, 154, 1-14; Spear, FS, 2010, Chem Geol, 279, 55-62). Both studies found that for a specified bulk composition the field for coexisting allanite + monazite is quite narrow.
Matt
> One thing I can say for sure - I have only seen one case of monazite in a rock that contained either allanite or titanite. That one case was in a granulite gneiss from western North Carolina in which large (300-600 micron) 1100 Ma Grenvillian monazites were spectacularly sheared and recrystallized in a 280 Ma Alleghanian mylonite-forming event. The Th-rich monazite reacted to form allanite + apatite + thorite/huttonite during the shearing and hydration. Otherwise, in 100% of my observations you get either monazite or allanite/titanite, but not both. This North Carolina example was also interesting in that monazite is in general a rare thing in Grenville-age rocks of eastern North America, from the Green Mountains south to the Blue Ridge of the southern Appalachians, either metamorphic gneisses or granitoids. This whole terrane appears to be singularly depleted in Th, U and LREEs, and in many cases the rare monazites one finds are so low in Th and U that they have almost strikingly low Pb, given their age.
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Dr. Matthew J. Kohn, Professor
Department of Geosciences
Boise State University
1910 University Dr.; MS1535
Boise, ID 83725-1535
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http://earth.boisestate.edu/mattkohn/
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