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Palaeobotanical and geochronological evidence for the Alpine age of the metamorphism in the Sesia-Zone |
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| Authors: | Dr B Scheuring Dr H Ahrendt Dr J C Hunziker A Zingg |
| Institution: | 1. Botanische Anstalt, Sch?nbeinstr. 6, CH-4056, Basel 2. Geologisch-Pal?ontologisches Institut, Berliner Str. 28, D-34, G?ttingen 3. Mineralogisch-Petrographisches Institut, Sahlistr. 6, CH-3012, Bern 4. Institut für Kristallographie und Petrographie der ETH, Sonneggstr. 5, CH-8006, Zürich |
| Abstract: | For a long time the age of the last metamorphism of the Sesia-zone was considered to be Hercynian or older. Basement inclusions in basic volcanics were the main argument for this interpretation. The Trachyandesites — Andesites of the Sesia-zone were regarded as Permian, analogoues to the widespread Permian volcanics of the Southern Alps. Recently, plant remnants have been found in tuffitic interlayers of the Sesia volcanics. These fossils have been described as palaeozoic plants, in contradiction to structual observations and to the numerous radiometric data acquired in the region. To check the age of the fossils a detailed palaeobotanical study was carried out. Our samples contained a very modern flora of definitely Tertiary age. Not one palaeozoic fossil was detected. The Tertiary age found on palaeobotanical evidence proved to be between 29 and 33 m. y. on the basis of total rock K-Ar ages. The last high pressure metamorphism of the Sesia-zone occurred between 90 and 60 m. y. as has been shown by radiometric ages on micas. The external part of the zone was overprinted by the Lepontine phase of metamorphism in greenschist facies 38 m. y. ago. |
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