1661-8726/08/01S127-29 Swiss J. Geosci. 101 (2008) Supplement 1, S127–S155 DOI 10.1007/s00015-008-1289-6 Birkhäuser Verlag, Basel, 2008 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 MICHAEL WIEDERKEHR1, 2, *, ROMAIN BOUSQUET 2, STEFAN M. SCHMID1 & ALFONS BERGER3 Key words: Lepontine dome, meta-sediments, Fe-Mg carpholite, Barrovian metamorphism, high-pressure metamorphism, Alpine tectonics ABSTRACT The Cenozoic-age metamorphic structure of the Alps consists of a through- phase) HP/LT metamorphism under blueschist facies conditions (350–400 °C going pressure-dominated belt (blueschists and eclogites) that strikes parallel and 1.2–1.4 GPa) was immediately followed by “cold” isothermal (or cool- to the orogen and was later truncated by two thermal domes characterised ing) decompression during D2 nappe-stacking (Ferrera phase). (2) Collision- by Barrow-type metamorphism (Lepontine dome and Tauern window). This related Barrovian overprint (500–570 °C and 0.5–0.8 GPa) postdates the D3 study documents for the first time that relics of Fe-Mg carpholite occur also nappe-refolding event (Domleschg phase) and represents a late heating pulse, within meta-sedimentary units that are part of the north-eastern Lepontine separated by D2 and D3 from the D1 high-pressure event. It occurred before structural and metamorphic dome, where so far exclusively Barrovian as- and/or during the initial stages of D4 (Chièra phase) representing a second semblages were found. They occur in meta-sediments of both Valais Ocean- nappe-refolding event. derived Lower Penninic Bündnerschiefer and structurally lower Europe-de- In discussing possible heat sources for the late Barrow-type heating pulse rived Sub-Penninic cover nappes and slices. These high-pressure units were it is argued that heat release from radioactive decay of accreted material may subsequently overprinted by a thermal event, as is documented by the growth play an important role in contributing much to heat production. Based on the of new minerals typical for Barrovian metamorphism. field evidence, we conclude that heat transfer was essentially conductive dur- We present evidence for a two-stage metamorphic evolution in the north- ing these latest stages of the thermal evolution. ern part of the Lepontine dome: (1) Early subduction-related syn-D1 (Safien 1. Introduction and the Tauern window in the Eastern Alps (Oberhänsli et al. 2004). The zoning of Alpine metamorphism is rather complex, evolv- Our area of investigation is located at the NE border of ing over a very long period of time before, during and after the Lepontine thermal dome. There, along strike of the tectonic the collision of Europe with Adria, i.e. from Late Cretaceous units, a remarkable metamorphic field gradient that ranges to Late Cenozoic times. Mapping of metamorphic facies in the from pressure-dominated blueschist facies in the NE to tem- Alps started with early pioneering studies based on the spatial perature-dominated Barrovian metamorphism in the SW is ob- distribution of index minerals and mineral assemblages (Wenk served within an amazingly short distance (< 10 km, Fig. 1 & 2). 1962; Niggli & Niggli 1965; Trommsdorff 1966; Frey 1969; Fox This allows for a clear correlation between the two metamor- 1975; Frey et al. 1980). Metamorphic maps at the scale of the phic events and structures that resulted from a polyphase de- Alpine orogen, showing the spatial arrangement of the differ- formation history. Hence, the area is well suited for studying ent metamorphic facies types, were repeatedly synthesised and spatial and temporal relationships between these two types improved (Ernst 1971; Niggli & Zwart 1973; Frey et al. 1999; of metamorphism, including their relative timing in respect to Oberhänsli et al. 2004). The Cenozoic-age metamorphic pat- discrete deformation phases linked to particular geodynamical tern is characterised by a pressure-dominated belt (blueschists stages. and eclogites) that strikes orogen-parallel but is interrupted by The availability of meta-sediments all along strike facilitates two thermal domes, the Lepontine dome in the Central Alps the reconstruction of the metamorphic and structural evolution 1 Geologisch-Paläontologisches Institut, Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland. E-mail: [email protected] 2 Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D-14476 Potsdam/Golm, Germany. 3 Institut für Geologie, Universität Bern, Baltzerstrasse 3, CH-3012 Bern, Switzerland. *Corresponding author: Michael Wiederkehr. E-mail: [email protected] From subduction to collision S127 S 128 730 735 740 745 750 M. Wiederkehr etal. Wiederkehr M. Luzern 5 km ow 185 nd n a p p e s wi e in d a g Penninic n E Basal Thrust Ilanz A a r S i l v r e t t a Valzeina- 180 H e l v e t i c synform GotthardSteep Belt Adula Northern Simano Tambo Suretta Platta o n t i n e p 175 e Maggia L Southern Steep Belt Bergell Adamello Val Lumnezia Safiental Insubric line Luzern Mte Lunschania- Safien Rosa Maggiore anitform N 25 km 170 Ivrea Lago Giudicarie line Vrin P. Beverin A Valsertal P. Tomül 165 G o t t h a r d 165 P.Aul P. Medel Vals P.Terri 160 160 Val Luzzone P. Scopi Lukmanier Splügen A d u l a 155 155 Leventina - Olivone European margin / Sub-Penninic units Valaisan Briançonnais Helvetic domain Piz Terri - Basement and Gipfelzone Unit Grava Unit cover nappes Chièra- Lucomagno Piz Terri - synform Ilanzer Verrucano Schuppenzone Tomül Unit Quaternary 150 P.Molare Triassic, Lower/upper Ophiolites Flims landslide undifferentiated Valser slices Basement Aul Unit Structures Simano Scopi Unit nappes D3 syn-/ antiform Peidener D4 syn-/ antiform Valle di Blenio A’ Schuppenzone 700 705 710 715 720 725 Fig. 1. Tectonic sketch map of the study area showing the main geographic localities mentioned in the text as well as traces of axial planes of major D3 and D4 folds, and the main occurrences of index minerals and mineral assemblages found in the meta-sedimentary units (light-grey: Sub-Penninic/European units, dark-grey: Lower Penninic/Valaisan units; symbols are explained in Figure 2). The tectonic map of the Central Alps in the upper left is after Schmid et al. (2004), the frame shows the location of the study area. Letters A–A' mark the trace of the composite cross section shown in Figure 3. The dashed line delineates the cut-out shown in greater detail in Figure 2. in the working area. These meta-sediments contain widespread that predominantly consist of pre-Mesozoic igneous and meta- occurrences of Fe-Mg carpholite within an orogen-parallel HP/ sedimentary rocks, and cover nappes forming an orogen-paral- LT-metamorphic belt in eastern Switzerland (Grisons), char- lel belt of Mesozoic meta-sediments. The cover nappes are not acterised by blueschist facies conditions (Goffé & Oberhänsli interrupted by oblique tectonic contacts (thrusts or faults) and 1992; Oberhänsli 1994; Oberhänsli et al. 1995; Bousquet et al. they overlie the pre-Mesozoic basement units or nappes (Gott- 2002). Approaching the Lepontine dome, the same meta-sedi- hard “Massif”, Leventina-Lucomagno and Simano nappes). ments become increasingly affected by a temperature-domi- These Sub-Penninic nappes are structurally overlain by Lower nated, Barrovian metamorphic event, as is documented by am- Penninic cover nappes that originated from the Valais Ocean, phibolite facies mineral assemblages characterised by garnet, largely consisting of Mesozoic meta-sediments referred to as biotite, staurolite and kyanite (Chadwick 1968; Frey 1969; Fox Bündnerschiefer. The front of the Adula nappe complex only 1975; Engi et al. 1995; Frey & Ferreiro Mählmann 1999). No evi- reaches the southern rim of the working area. The occurrences dence is available, so far, that this part of the Lepontine dome, of oceanic remnants that are imbricated with typical continental characterised by this Barrow-type MP/MT metamorphism, crustal rocks in the overlying Misox Zone (e.g. Partzsch 1998), could have been previously also affected by HP/LT metamor- and according to some authors also within the Adula-Cima phism. Lunga nappe complex itself (e.g. Trommsdorff 1990, and ref- This tectono-metamorphic study primarily aims to docu- erences therein), indicate that the Adula nappe complex con- ment this transition from HP/LT blueschist facies metamor- tains slivers from the continent-ocean transition between the phism in the east to amphibolite-grade Barrow-type metamor- European margin and the Valais Ocean (lithospheric mélange; phism within the Lepontine dome further west. This will allow Trommsdorff 1990). The Penninic Basal Thrust represents an deducing whether these two contrasting types of metamorphism early-stage first-order thrust along which the Valaisan Bünd- evolved at the same time but differently in the different parts nerschiefer were originally thrust onto the Europe-derived of the study area, or alternatively, whether they evolved during Sub-Penninic units. However, this thrust was subsequently iso- consecutive stages of the evolution of the Alpine orogen. In the clinally refolded and hence penetratively overprinted by later second case the pressure-dominated metamorphism represents structures. The tectonic units, subdivided following the schemes an early stage related to subduction, followed by a tempera- proposed by Schmid et al. (2004) and Berger et al. (2005), are ture-dominated event, as proposed by Bousquet et al. (2008). mapped in Figures 1 and 2, as well as in cross section view The latter, i.e. a two-stage metamorphic evolution, was also (Fig. 3). In the following they are further described. proposed for more southerly located parts of the Lepontine dome. However, the question whether the Barrow-type over- 2.1. Sub-Penninic basement nappes print is associated with a second and discrete heating pulse that followed high-pressure metamorphism (e.g.