Prograde and retrograde thermal histories from the central Swiss Alps

The "Lepontine" phase of metamorphism in the central Alps is a result of thermal relaxation following continental collision and overthrusting. The timing of the peak remains poorly known, largely because the existing chronological data relate to cooling or to phases for which the pressure and temperature of growth is only poorly constrained. Here we report U sbnd Pb, Sm sbnd Nd and Rb sbnd Sr analyses of garnets from the central Swiss Alps, as well as major element data on garnets and other phases. The data shows that the climax of the metamorphism was diachronous across grade, from 30 Ma in the lower grade northwest to 27 Ma in the higher grade in the southeast. Consideration of these data in conjunction with existing chronological constraints suggests a period of about 10 Ma between the attainment of the metamorphic climax in greenschist facies and the upper amphibolite facies rocks. The time constraints obtained here yield a garnet growth interval of 2.9 ± 1.5 Ma, corresponding to a growth rate of 0.2 cm Ma ^-1, a heating rate of 12°C Ma ^-1 and a burial rate of 2.9 km Ma ^-1. The strain rate obtained from measurement of the rotation of the garnet relative to the matrix fabric during growth is ∼ 1.9 × 10 ^-14s ^-1. This strain rate, as well as the heating and burial rates, are in the range of values derived in a similar fashion from other Barrovian metamorphic terrains. Consideration of these data in conjunction with published cooling ages leads to cooling rates of about 20°C Ma ^-1 for the Steinental samples but much higher rates for the Bellinzona area (> 100°C Ma ^-1). In both cases it is necessary to invoke tectonic exhumation rather than isostatic recovery and erosion to explain the observed temperature-time trends. In the case of the Bellinzona area, the most likely cause of the very rapid cooling rates is late backthrusting over the Insubric Line.