Metamorphic rates in collisional orogeny from in situ allanite and monazite dating
- Emilie Janots1,*,
- Martin Engi1,
- Daniela Rubatto2,
- Alfons Berger1,
- Courtney Gregory2 and
- Meinert Rahn3
- 1Institut für Geologie, Universität Bern, Blatzerstrasse 3, CH-3012 Bern, Switzerland
- 2Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia
- 3Hauptabteilung für die Sicherheit der Kernanlagen (HSK), CH-5232 Villingen, Switzerland
Abstract
The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560–580 °C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (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) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fission track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.
Footnotes
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↵*Current address: Institut für Mineralogie, WWU Münster, Corrensstrasse 24, 48149 Münster, Germany
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↵GSA Data Repository item 2009004, methods and Tables DR1 (allanite U-Th-Pb data), DR2 (clinozoisite U-Th-Pb data), DR3 (monazite U-Th-Pb data, in APi0413), DR4 (monazite U-Th-Pb data, in MF315), DR5 (fission track data), and DR6 (sample coordinates), is available online at www.geosociety.org/pubs/ft2009.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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- Received 26 May 2008.
- Revision received 3 September 2008.
- Accepted 4 September 2008.
- © 2009 Geological Society of America
