Rb-Sr age determinations on micas and total rocks from the Alps

Abstract

Rb-Sr age determinations on 19 samples, micas and total rocks, from the Swiss and Italian Alps were made. The special techniques for the determination of young ages by this method are described. The main contributions of our age results to the problems of Alpine geology are: The last main recrystallization phase in the central part of the Alps has been dated by age determinations on recrystallized rocks from the lowest nappe units. The age results are in the range from 16 to 21 m. y., 16 m. y. in the nappe region and the Gotthard massif farther north, 21 m. y. in the root region to the south of the 16-m. y. region. The interval of 5 m. y. is explained by a different time of uncovering after one metamorphic phase, rather than as having been caused by two different phases. This uncovering is believed to have been an uplift in the south, causing quick erosion there. Gliding of the overlying pile of nappes to the north could have been connected with this uplift. Several samples from the Alps give Rb-Sr ages older than the Tertiary Alpine metamorphism: a granite, 193±21 m. y. by total rock analysis, and a muscovite, 295±14 m. y., from a pegmatite from the Hercynian Aare and Tavetsch massifs which were metamorphosed during the Alpine orogeny. Furthermore, we found an age of 267±11 m. y. on the biotite from the pre-Alpine Habkern granite which occurs only in exotic boulders embedded in soft sediments. Two micas and a total rock from the Austroalpine Silvretta nappe gave Paleozoic ages, 306±13 m. y., 293±12 m. y., and 356±21 m. y. According to geologists the crystalline complex of the Silvretta, which overlies fossiliferous Mesozoic sediments, was thrust as a block over deeper nappe units during the Alpine orogeny, which did not greatly influence the internal structure of the Silvretta nappe. Both the young overthrust and the low grade of Alpine metamorphism were confirmed by our measurements.