U-Pb Monazite Dating in Orthogneiss Terrains -
Some Potential Problems and Possible Solutions

Gertrude Friedl Institut für Geologie & Paläontologie, Hellbrunnerstraße 34, A-5020 Salzburg, Austria

Fritz Finger Institut für Mineralogie, Hellbrunnerstraße 34, A-5020 Salzburg, Austria

The mineral monazite combines a number of features that are very attractive for U-Pb geochronology. These include the incorporation of high quantities of uranium and low
common lead, a low risk of old lead inheritance despite high closing temperatures of the crystal lattice for the U-Pb
system (approximately 725 ±25°C), and a particularly low susceptibility to low-T lead loss (Parrish, 1990).

The bulk of natural monazite forms in peraluminous granitoids during early stages of magmatic crystallisation. A number of U-Pb studies have shown that granite monazites typically yield concordant, low-error ages, that closely match the age of intrusion.

In peraluminous orthogneisses primary monazites are often preserved. However, surprisingly, these relictic
monazites have been used rarely in geochronology as yet. During the past years, our research has concentrated on the dating of monazites from orthogneiss terrains. Monazites from various amphibolite-facies granite gneisses from the Austrian Alps and the Austrian part of the Moldanubian unit have been studied. In addition to the conventional multi-grain isotope analysis, we have studied the Th-U-Pb systematics of the monazites by electron microprobe analysis. The main outcome of this research has shown that relictic
monazite grains are able to yield precise constraints on the formation ages of high-grade metamorphosed orthogneisses. However, some tricks are required.

For example, primary monazites from orthogneisses are often partially replaced by a rim of apatite (Ap), allanite (All) and epidote (Ep) of variable thickness, interfingering the monazite. This metamorphic "skin" adheres to the single monazite grains, even after heavy mineral separation. If not removed, these impurities lead to strong, unsystematic and difficult to interpret discordancies, involving metamorphic and recent lead loss, and complicate the dating results further by adding a high common lead content. Thus, when dating orthogneiss monazites, drastic mechanical abrasion of the selected monazite grains should be applied routinely. Also, the least deformed rocks should be collected for dating
purposes, because the replacement of monazite by Ap-All-Ep is usually less advanced in such samples.

It appears from microprobe analysis that primary
monazite relics have often experienced onlz minor lead loss during amphibolite-facies metamorphism, even in examples, where they are coated with a thick metamorphic Ap-All-Ep rim. However, lead loss in monazite frequently occurs around small thorite/uranothorite inclusions. These can be quite common in some monazite grains. Also, the inclusions themselves have often lost more than two thirds of their
radiogenic lead. This may seriously effect the analysis of the bulk fraction, because sometimes these inclusions have U contents as high as 50 wt% UO2. Therefore, careful microscopic selection of monazites, poor in such inclusions, is important in order to obtain ages with little discordance. This holds true also for monazite dating in granites.

Although it is well known from pelites, that monazites may grow at temperatures as low as 525°C (Smith and Barreiro, 1990), we have rarely found newly formed monazite in the studied orthogneisses. One important exception is the Moldanubian Dobra gneiss. This strongly deformed, high-T metamorphic rock of calc-alkaline composition, recrystallised at temperatures of about 800°C (Büttner and Kruhl, 1994). It contains small, newly grown monazites with concordant metamorphic ages (Gebauer and Friedl, 1994), but is apparently devoid of primary monazite. The metamorphic monazite grains of the Dobra gneiss typically contain small inclusions of quartz and plagioclase, a feature that is rare in monazites of magmatic origin.

Coexistence of a relictic magmatic and a newly grown metamorphic monazite generation was observed only in some cases and does not appear to be a very common feature in orthogneisses.


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