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

It remains controversial whether burial and exhumation in mountain belts represent episodic or continuous processes(1-20) 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 +/- 2.7 Myr. These results indicate that the Alpine high-pressure metamorphism did not occur as a single episode some 80-120 Myr ago(6,7,10,18), but lather that burial and exhumation represent continuous and relatively recent processes.