the cenozoic-age metamorphic structure of the Alps consists of a throughgoing pressure-dominated belt (blueschists and eclogites) that strikes parallel to the orogen and was later truncated by two thermal domes characterised by barrow-type metamorphism (Lepontine dome and tauern window). this study documents for the first time that relics of Fe-Mg carpholite occur also within meta-sedimentary units that are part of the north-eastern Lepontine structural and metamorphic dome, where so far exclusively barrovian assemblages were found. they occur in meta-sediments of both Valais Oceanderived Lower Penninic bündnerschiefer and structurally lower Europe-derived sub-Penninic cover nappes and slices. these high-pressure units were subsequently overprinted by a thermal event, as is documented by the growth of new minerals typical for barrovian metamorphism.We present evidence for a two-stage metamorphic evolution in the northern part of the Lepontine dome: (1) Early subduction-related syn-D1 (safien phase) HP/Lt metamorphism under blueschist facies conditions (350-400 °c and 1.2-1.4 GPa) was immediately followed by "cold" isothermal (or cooling) decompression during D2 nappe-stacking (Ferrera phase). (2) collisionrelated barrovian overprint (500-570 °c and 0.5-0.8 GPa) postdates the D3 nappe-refolding event (Domleschg phase) and represents a late heating pulse, separated by D2 and D3 from the D1 high-pressure event. It occurred before and/or during the initial stages of D4 (chièra phase) representing a second nappe-refolding event.In discussing possible heat sources for the late barrow-type heating pulse it is argued that heat release from radioactive decay of accreted material may play an important role in contributing much to heat production. based on the field evidence, we conclude that heat transfer was essentially conductive during these latest stages of the thermal evolution.
Introductionthe zoning of Alpine metamorphism is rather complex, evolving over a very long period of time before, during and after the collision of Europe with Adria, i.e. from Late cretaceous to Late cenozoic times. Mapping of metamorphic facies in the Alps started with early pioneering studies based on the spatial distribution of index minerals and mineral assemblages (Wenk 1962;Niggli & Niggli 1965;trommsdorff 1966;Frey 1969; Fox 1975;Frey et al. 1980). Metamorphic maps at the scale of the Alpine orogen, showing the spatial arrangement of the different metamorphic facies types, were repeatedly synthesised and improved (Ernst 1971;Niggli & Zwart 1973;Oberhänsli et al. 2004). the cenozoic-age metamorphic pattern is characterised by a pressure-dominated belt (blueschists and eclogites) that strikes orogen-parallel but is interrupted by two thermal domes, the Lepontine dome in the central Alps and the tauern window in the Eastern Alps (Oberhänsli et al. 2004).Our area of investigation is located at the NE border of the Lepontine thermal dome. there, along strike of the tectonic units, a remarkable metamorphic field gradient that rang...