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The current role of rare-earth-element- (REE-) and actinide-bearing accessory minerals as both geochronological markers and recorders of geochemical processes is reviewed in this paper. The minerals covered include the most common REE- and actinide-bearing accessory minerals found in high-grade rocks—i.e., monazite, xeno-time, apatite, huttonite, thorite, zircon, allanite, and titanite. The goal of this review is to describe the most recent research developments regarding these minerals and their interaction with each other as well as their role as geochronometers. These results are then applied to two cross sections of lower crust from Rogaland–Vest Agder, SW Norway, and Tamil Nadu, S India (granulite and amphibolite facies) with regard to seeing how REE-accessory minerals relate to each other as a function of metamorphic grade. In either traverse the same relationships are seen between monazite, fluorapatite, allanite, and titanite. Namely, in the amphibolite-facies zone, the REE are hosted by titanite and allanite, whereas in the clinopyroxene-in transition zone between the amphibolite-facies and granulite-facies zones, monazite is the stable REE-bearing phase in the region of the orthopyroxene-in isograd. In the granulite-facies zone the REE are hosted by fluorapatite as opposed to monazite. These similarities suggest that mineral hosting of REE follows certain general trends, which are a function of metamorphic grade, whole rock chemistry, and intergranular fluid chemistry.

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