Tracing two orogenic cycles in one eclogite sample by Lu-Hf garnet chronometry

摘要

Subduction of rocks into the mantle results in high-pressure metamorphism and the formation of eclogites from basaltic precursor rocks. At the Earth's surface, eclogites often occur as isolated fragments embedded in crustal rocks that lack evidence for high-pressure conditions. The high-pressure rocks are therefore often viewed as dismembered fragments that have been assembled and intercalated with rocks devoid of any high-pressure history at shallow crustal levels, forming a tectonic mélange. Such mélange models were supported by age discrepancies among high-pressure rocks from the Adula nappe (Central Alps), which was thought to represent a classic example of such a situation. Here we present Lu-Hf age data from two populations of the high-pressure mineral garnet, found within a single eclogite sample taken from Trescolmen, in the Central Adula nappe. We report a minimum Variscan age of 332.7 Myr and a maximum Alpine age of 38 Myr for the two populations. We suggest that the Trescolmen eclogite was subducted to mantle depth and subsequently exhumed, becoming part of a continental crust during the Variscan orogeny. Later, during the Alpine orogeny, the Adula nappe must have been subducted to-and exhumed from-mantle depth a second time, as one coherent unit. We conclude that the Adula nappe is not a mélange, and therefore, the crustal rocks that envelope the eclogites have also been subjected to high-pressure conditions through deep subduction during the Alpine event.