From subduction to collision. a combined metamorphic, structural and geochronological study of polymetamorphic metasediments at the NE edge of the Lepontine dome (Swiss Central Alps)

This study analyses the tectono-metamorphic evolution of metasedimentary units belonging to the Valaisan and adjacent European domains at the north-eastern border of the Lepontine dome (Central Alps). The investigated area is characterized by a remarkable metamorphic gradient ranging from subduction-related HP/LT metamorphism in the NE to collision-related Barrovian overprint in the SW. Detailed structural fieldwork and petrological investigations including Raman spectroscopy of carbonaceous matter were carried out in order to reconstruct the tectono-metamorphic evolution on a larger scale. Furthermore, new 40Ar/39Ar dating of white mica and biotite reveal the timing of both subductionrelated high-pressure metamorphism and collision-related Barrovian overprint. The combination of all these investigations allows for deciphering a complete P-T-d-t path (pressure, temperature, deformation and time) of an area that occupies a key position in the Alpine orogenic belt for understanding the transition from subduction to collision. This study documents for the first time that relics of Fe-Mg carpholite indicating blueschist facies conditions occur also within metasedimentary units that are part of the north-eastern Lepontine dome where, so far, exclusively Barrovian assemblages were found. They occur in metasediments from both the Valaisan domain (Grava and Tomül nappes) and parts of the adjacent European domain (Peiden slices and Piz Terri-Lunschania unit). These high-pressure units were subsequently overprinted by a thermal event, as is documented by the growth of new minerals typical for Barrovian metamorphism. The investigated metasediments provide clear evidence for a bimodal P-T path in the north-eastern Lepontine dome characterized by the following polyphase metamorphic evolution: (1) Subductionrelated syn-D1 (Safien phase) HP/LT metamorphism under blueschist facies conditions (350-400 °C and 1.2-1.4 GPa) was established at 42-40 Ma, as revealed by 40Ar/39Ar dating of white mica associated with Fe-Mg carpholite; the early high-pressure event was followed by “cold” isothermal (or cooling) decompression during D2 nappe-stacking (Ferrera phase) for which an age of 36-33 Ma is inferred based on 40 Ar/39Ar dating of white mica replacing Fe-Mg carpholite. (2) Early collision-related greenschist facies overprint (350-425 °C) post-dating substantial decompression and associated D2 deformation was established at 32-29 Ma and affected both HP and LP metasediments. This metamorphic event clearly predates D3 deformation (Domleschg phase, ~25 Ma) as is evidenced by folded isotemperature contours. (3) Collision-related Barrovian overprint (500-590 °C and 0.5-0.8 GPa) represents a second and considerably younger (post 20 Ma) “isobaric” heating pulse only preserved in the SW part of the investigated area. Hence amphibolite facies metamorphism representing the mature stage of a colliding orogen is clearly separated by D2 and D3 deformations, as well as by an intervening greenschist facies event, from the D1 high-pressure stage. Amphibolite facies overprint occurred before and/or during the initial stages of D4 (Chièra phase), representing a second nappe-refolding event. This investigation revealed a significant time gap in the order of some 20 Ma between subductionrelated HP/LT metamorphism and collision-related MP/MT Barrovian overprint. This supports the notion of a polymetamorphic evolution associated with a bimodal P-T path. The results of this study argue that heat release from radioactive decay of vast amounts of accreted continental-derived basement nappes may play an important role in contributing much to heat production needed for amphibolite facies Barrow-type overprint. Based on field evidence, we conclude that heat transfer in the north-eastern Lepontine was essentially conductive during the latest stages of the thermal evolution.