From subduction to collision: thermal overprint of HP/LT meta-sediments in the north-eastern Lepontine Dome (Swiss Alps) and consequences regarding the tectono-metamorphic evolution of the Alpine orogenic wedge

The Cenozoic-age metamorphic structure of the Alps consists of a through-going 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 Ocean-derived Lower Penninic Bundnerschiefer 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 degrees C and 1.2-1.4 Gila) was immediately, followed by cold isothermal (or cooling) decompression during D2 nappe-stacking (Ferrera phase). (2) Collision-related Barrovian overprint (500-570 degrees 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 (Chiera phase) representing a second nappe-refolding event. In discussing possible beat 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 oil the field evidence, we conclude that heat transfer was essentially conductive during these latest stages of the thermal evolution.