Stilpnomelan als alpinmetamorphes Produkt im Mont-Blanc-Granit
Stilpnomelane is known to be index-mineral of lowest metamorphic facies in the northern zone of the Swiss Alps. In Montblanc-region the first stilpnomelane due to alpine metamorphism will be found in the Montblanc-granite, whereas until now the adjacent metamorphic rocks in the north of granite have not yielded any stilpnomelane. This could be explained by chemical control in the metamorphic rocks or by instability of stilpnomelane because a new formed biotite has been found in the shearing-zones in the northern part of granite and stilpnomelane is disappearing.Metamorphic reactions in the very coarse granite are bound to the limits between constituents or to the interior of larger crystals. Stilpnomelane very often has been found in microcline situated near to biotite or chlorite of primary origin. Towards the southern part of granite stilpnomelane will be found in larger quantities and occasionally a new formed chlorite will appear. This evolution is accompanied by the appearance of larger quantities of patch-perthite and chessboard-perthite in the microcline-megacrysts.Probably a green biotite coexists with stilpnomelane, but majority of alpine biotite has been formed in a later phase and is accompanied by a strong change of quartz-fabric and disappearance of stilpnomelane.One might suppose, that the change of lowest greenschist-facies (stilpnomelane) to a higher degree (biotite) even over very short distances depends more on the influence of changing chemical composition in the coexisting aqueous-gaseous phases than on a real increase of pressure and temperature.