Abstract

Pumpellyite, seldom recognized as a detrital mineral, is a widespread detrital constituent of the Oligocene-Miocene Alpine foreland molasse. Electron-probe microanalyses show the presence of three different types of pumpellyite in the sediments: with low-Mg and low-Fe content; high-Mg and low-Fe content, and with high-Mg and high-Fe content. Their overall chemistry covers a wide compositional field which can be related to different metamorphic conditions throughout the Alps. The earliest appearance of pumpellyite was detected in the lower part of the Upper Oligocene. It is associated with index minerals from high-pressure rocks of the Penninic domain, which indicates a common parentage. In the Burdigalian upper-marine molasse of western and central Switzerland, pumpellyite was commonly encountered in a variety of sedimentary facies. The provenance of the majority of the grains can be traced to the meta-ophiolites of the Upper Pre-Alpine nappes and to those of the Platta happy (southeast Switzerland), respectively. There is no appreciable evidence of pumpellyite being eroded from the low-grade metamorphic "Taveyannaz" type of sandstone of the Helvetic domain, suggesting that it was still under burial during the Burdigalian.

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