ALPINE AND PRE-ALPINE SUBDUCTION EVENTS IN POLYCYCLIC BASEMENTS OF THE SWISS ALPS

Coupling macrostructural and microstructural observation with metamorp hic petrology of polymetamorphic mafic rocks occurring in basement roc ks of the Central Alps clearly provides evidence that some part of thi s basement underwent at least two different and independent events of high-pressure metamorphism. The older eclogitic event occurred at midd le to high temperature and must have been pre-Alpine because late Vari scan plutons crosscut the high-pressure structures. The younger event occurred at high-pressure and low-temperature conditions and is Tertia ry in age. The separation of Alpine from pre-Alpine eclogite-facies ev ents is discussed for several Alpine nappes.