Dating the allanite-monazite metamorphic reaction

Daniela Rubatto¹, Courtney Gregory¹, Emilie Janot² and Ioan Gabudianu- Radulescu³

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Institute of Geological Sciences, University of Bern, Bern, Switzerland
3 Institute for Geography and Geology, Copenhagen, Denmark

 

U-Pb geochronology is one of the most widely used methods for the investigation of the timing of crustal processes. One main limitation of the application of U-Pb geochronology to metamorphism is the lack of direct links between the age measured and the conditions at which the dated minerals formed. In this respect, dating metamorphic reactions that can be placed in pressure-temperature space is particularly appealing.

One metamorphic reaction between U-Pb minerals that has been widely reported is the mutual replacement of allanite and monazite (figure 1 and 2). With the recent set-up of a protocol for accurate dating of allanite by SHRIMP ion microprobe (Gregory et al. 2007) it has become possible to date this reaction in-situ. We applied this method to the two geological settings where allanite-monazite reactions are most commonly documented.

1) Monazite replacing allanite is observed in prograde greenschist to amphibolite facies metamorphism of metapelites in the Central Alps (Fig. 1). Petrographic observations and thermobarometric calculations allow placing the reaction at T = 560-580°C, whereas initial allanite formation occurred at T = 430-450°C. In-situ SHRIMP U-Th-Pb dating of allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constraints the time elapsed between 430-450°C and 560-580°C, which implies an average heating rate of 15-8 °C/My (Janots et al., in press).

2) During subduction-related metamorphism, the replacement of monazite by allanite is associated to increasing pressure. This is observed in the silvery micaschists of the Gran Paradiso Massif, Western Alps where microstructural relationships among major and accessory minerals indicate the following prograde sequence of U-Th bearing accessory minerals: florencite -> monazite -> allanite (Fig. 2). Thermobarometric calculations indicate that the allanite-bearing peak assemblage was stable at P = 2.3 ± 0.4 GPa and T = 570 ±30 °C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 34.5 ± 0.8 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.5 ± 0.9 Ma, implying a minimum duration of ~3 Ma for the Alpine subduction event (Gabudianu Radulescu et al., in press).

 

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Gregory C, Rubatto D, Allen C, Williams IS, Hermann J, Ireland T (2007) Allanite micro-geochronology: a LA-ICP-MS and SHRIMP U-Th-Pb study. Chemical Geology 245:162-182
Janots E, Engi M, Rubatto D, Berger A, Gregory C, Rahn M (in press) Metamorphic rates in collisional orogeny from in situ allanite and monazite dating. Geology
Gabudianu Radulescu I, Rubatto D, Gregory C, Compagnoni R (in press) The age of HP metamorphism in the Gran Paradiso Massif, Western Alps: a petrological and geochronological study of "silvery micaschists". Lithos