The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism

• Published in: Nature (London) Vol. 387; no. 6633; pp. 586 - 589
• Main Authors: Duchêne, S., Blichert-Toft, J., Luais, B., Télouk, P., Lardeaux, J.-M., Albarède, F.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 05.06.1997; Nature Publishing; Nature Publishing Group
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochronology; Geology; Hafnium; Humanities and Social Sciences; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Isotope geochemistry. Geochronology; letter; Lutetium; Mass spectrometry; Metamorphism; Minerals; multidisciplinary; Science; Science (multidisciplinary); Studies; Sesia-Lanzo Zone; Alps; Western Alps; Mesozoic; Paleogene; chain silicates; eclogite; isochrons; Sm/Nd; mica; garnet; mass spectroscopy; burial; phengite; pyroxene; sheet silicates; silicates; dating; Upper Cretaceous; Cenozoic; clinopyroxene; nesosilicates; Alpine Orogeny; Europe; Tertiary; lutetium; whole rock; Cretaceous; metamorphic rocks; high pressure; high-grade metamorphism; Phanerozoic; hafnium
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/42446

It remains controversial whether burial and exhumation in mountain belts represent episodic or continuous processes 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 . Regional patterns of crystallization and closure ages of high-pressure rocks may help to discriminate one mode from the other but, unfortunately, metamorphic geochronology suffers from several limitations. Consequently, no consensus exists on the timing of high-pressure metamorphic events, even for the Alps—which have been the subject of two centuries of field work. Here we report lutetium–hafnium (Lu–Hf) mineral ages on eclogites from the Alps as obtained by plasma-source mass spectrometry. We find that the Lu/Hf ratio of garnet is particularly high, which helps to provide precise ages. Eclogites from three adjacent units of the western Alps give (from bottom to top) diachronous Lu–Hf garnet ages of 32.8 ± 1.2, 49.1 ± 1.2 and 69.2 ± 2.7Myr. These results indicate that the Alpine high-pressure metamorphism did not occur as a single episode some 80–120Myr ago 6 , 7 , 10 , 18 , but rather that burial and exhumation represent continuous and relatively recent processes.