ABSTRACT

The authigenic mineralogy of the Eocene-Oligocene Taveyannaz Sandstone of the Alps and equivalents in France and Italy has been correlated with the coal-rank of the associated humic material. A relationship between fixed carbon (FC), isochemical mineral zonation and illite crystallinity was established as tabulated below:

Table

Evidence suggests that chemical equilibrium was approached. Therefore a given mineral zone and illite crystallinity is a reliable coal-rank/temperature indicator. Paragenetic, chemical and coal-rank data suggest that the crystallization of the hydrated calcium-silicates and phyllosilicates was mainly controlled by temperature; the formation of the different calcium-silicates resulted from reactions between the clayey matrix and calcium rich solutions. None of the mineral zones represents a metamorphic product of the foregoing stage(s). According to geochronological and mineralogical data, the metamorphism of the Taveyannaz Sandstone took place some 40 million years ago, during the Lepontine Metamorphic Event. The complete spectrum of metamorphic grades was generated by this event, ran ing from the low temperature end (the subject of this paper) up to and including the amphibolite stage as found in the Ticino Culmination of the Central Alps.