Hornblende K-Ar ages and the climax of Tertiary metamorphism in the Lepontine Alps (south-central Switzerland): an old problem reassessed

Abstract

Twenty-five new K-Ar ages for amphiboles from the Tertiary Barrovian-type metamorphic terrain of the Central Alps were determined. These results lead to the conclusion that the meso-Alpine metamorphism reached its peak condition and maximum expanse about 26 ± 3 m.y. ago. Along a profile of increasing grade of Alpine metamorphism in both basement and cover rocks, the following observations can be made: (1)Pre-Alpine hornblendes in the zone of greenschist-facies show ages ranging from 639 to 152 m.y. The implied open-system behavior may be related to the observed mineralogical instability of these hornblendes during the Alpine overprint. In contrast, two greenschist facies hornblendes which grew in Pennine Mesozoic rocks during the Alpine metamorphism yield ages of 23 ± 2 and 29 ± 3 m.y. (2)On both sides of the boundary marking the first occurrence of Alpine staurolite Pre- or Early-Alpine hornblende K-Ar ages are not fully reset unless the rocks were completely recrystallized. For one rock specimen which clearly exhibits two generations of hornblende, an age two times greater was obtained for the smaller crystals oriented parallel to the foliation than for the structurally younger hornblende porphyroblasts which overgrew the preexisting texture. (3)In the high-grade area of Alpine metamorphism only young Alpine ages between 23 and 28 m.y. were found for hornblendes which grew syn- to postkinematically with respect to the last penetrative deformation. One amphibolitic inclusion not affected by this deformation yielded a significantly older hornblende age of 32 ± 1 m.y. These results suggest that temperature-controlled Ar diffusion is not the main factor governing the K-Ar system of metamorphic hornblendes. We conclude that these hornblende ages should not be interpreted as cooling ages from a structurally ill-defined earlier event; instead they indicate that the high-grade metamorphism responsible for the well-known isograd pattern in the Lepontine area persisted until some 23 m.y. ago.