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

Abstract

It remains controversial whether burial and exhumation in mountain belts represent episodic or continuous processes1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20. 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 ago6,7,10,18, but rather that burial and exhumation represent continuous and relatively recent processes.