THE ORIGIN OF WHITESCHIST IN THE MONTE ROSA NAPPE, WESTERN ALPS

The origin of whiteschist is highly debated in the literature. In a recent paper by Ferrando (2012), for example, mantle fluids are suggested as the source of alteration, in many different tectonic scenarios. Here we present field, petrologic, and geochemical data to support a late magmatic hydrothermal alteration of a granite protolith. Subsequent - mostly closed system - high-pressure metamorphism resulted in a whiteschist mineralogy.

Whiteschists occur as 10-50 m bodies of talc-chloritoid-phengite - and local kyanite and garnet assemblage within metagranites in the Monte Rosa nappe. The well-exposed outcrops of the Mt. Rosa nappe allow a detailed observation of the complete transition from the unaltered metagranites to whiteschists. Combined structural and petrologic observations suggest that this chemical alteration is unrelated to any schistosity or shearing. The mineralogical zonation around the whiteschist display straight and locally lobate contacts typical for late, static hydrothermal alteration zones found in shallow granite environments. They resulted from the metasomatic alteration of the Permian granite. Rocks are enriched in MgO and depleted in CaO and Na₂O, in agreement with data presented by Pawlig et al. (2001).

In order to characterize the nature of the fluid, we determined the oxygen, hydrogen and carbon isotopic composition of the whiteschist, granites and of associated carbonate veins. Oxygen isotopic compositions are compatible with either fluids originating from serpentinites (Dessimoz, 2006) or late magmatic hydrothermal fluids. Carbon isotopes agree with juvenile compositions, and hydrogen compositions suggest near surface water. Sr-isotopes in whiteschist give model ages between 100-190 Ma and suggest a two-stage evolution (Pawlig, 2001).

The new data support the interpretation that whiteschist chemistry was established early on by late magmatic hydrothermal fluids in the Permian, while the whiteschist mineralogy crystalized at HP, inheriting the isotopic and whole rock composition.

References:

Dessimoz, M. (2006) Master thesis University Lausanne (CH)

Ferrando, S (2012) Terra Nova 24, 423-426

Pawlig, S (2001) PhD thesis University Mainz (G)

Pawlig, S et al., (2001), SMPM 81, 329-346