IMEDL 2004: Remnants of ancient margins in the Alps, part II

FIELD GUIDE

Additional field trips (not visited during IMEDL) but equally important for the understanding of ocean-continent transitions in the Eastern Central Alps

Remnants of the ancient Tethys margins preserved in the Davos (Totalp) and Margna-Malenco units in SE Switzerland and N- (Central Alps)

1 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 1 Stratigraphy of an ocean-continent-transition zone as determined from field observations from the south Pennine-Austroalpine boundary zone in (SE Switzerland). From Manatschal and Nievergelt, 1997.

Introduction The oceanic sediments overlying the ophicalcites include: (1) shales and breccias; (2) The close assemblage of serpentinites, basalts cm-bedded ribbon cherts (Radiolarite Formation, and radiolarites, first described by Gustav Middle to Upper Jurassic, Bill et al. (2001)); (3) Steinmann (1905) from the Arosa zone and the gray, thin-bedded limestones with greenish, shaly Platta , is distinctly different from typical interbeds (Calpionella or Aptychus Limestone, fast-spreading ridge associations and can be Berriasian); (4) black shales with dm-bedded fine- compared to the situation described from ocean- grained limestones (Palombini Formation, approx. continent-transitions (OCT) of present-day magma- Valanginian to Barremian) (Weissert and Bernoulli, poor rifted continental margins (Manatschal and 1985). The red shales occur only locally; they are Bernoulli, 1999). This association includes associated with debris-flow deposits, serpentinised peridotites, which were exhumed unfossiliferous and may be interpreted as along detachment faults and which are locally hydrothermal metalliferous sediments drived from overlain by extensional allochthons of continental rocks associated with mantle exhumation. The crust; in addition, minor gabbroic intrusions, radiolarites and the Calpionella Limestone contain tholeiitic flows, pillow lavas and pillow breccias breccias and graded sandstones including clasts of and a succession of oceanic sediments occur (Fig. both serpentinite and continental basement rocks 1). indicating local submarine relief at the time of The serpentinised peridotites are fragments of deposition. The facies evolution of the pelagic sub-continental mantle and record deformation sequence is determined by subsidence and under decreasing temperatures during extension and paleoceanographic changes, in particular by a exhumation leading to their final exposure at the pronounced drop of the calcite compensation depth Jurassic sea-floor. at the Jurassic-Cretaceous boundary accompanying Tectono-sedimentary breccias, the so-called the replacement of radiolarian-dominated planktic ophicalcites, occur along the surface of the assemblages by coccolith- and nannoconid- exhumed mantle rocks and are overlain by other, dominated ones. sedimentary breccias and/or pelagic sediments or basalts. The igneous rocks consist of gabbros intruding Sites visited the already serpentised mantle rocks at shallow In eastern Switzerland, fragments and depth 161 Ma ago and basaltic flows and pillow associated oceanic sediments are preserved in the lavas stratigraphically overlying the exhumed sub- south-Pennine Malenco-Forno unit, the Platta continental mantle rocks and the tectono- nappe and in the Arosa zone (Fig. 2). These units sedimentary breccias (Desmurs et al., 2001) (Fig. are sandwiched between the middle (Briançonnais) 1). Gabbros and basalts are derived from an and north Pennine units in the footwall and the asthenospheric MORB-type source and appear to Austroalpine in the hanging wall, which are document the onset of sea-floor spreading across an derived from the northwestern and the southeastern exhumed subcontinental mantle during the earliest margin respectively of the Alpine segment of the evolutionary stages of a slow-spreading ridge Tethys ocean. (Schaltegger et al., 2002).

2 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 2 Tectonic sketch map of the South-Pennine- Austroalpine boundary zone in Grisons. D:Davos, B: Bivio, SM: San Murezzan Modified after Froitzheim et al., 1994.

In the Arosa zone, the Totalp imbricate near Davos shows the clearest relationship between the basement of the ocean-continent transition and the overlying oceanic sediments (1st day). In the Malenco-Forno unit, a Permian crust-mantle boundary is preserved, which allows to study lower crustal and upper mantle rocks as well as their exhumation history during rifting (2th day).

Aims of the excursion Our excursion will focus on the tectonic evolution of the Alpine segment of the Tethys ocean (Liguria-Piemonte ocean). More particularly, we focus during the four days of excursion on: (1) the interplay between tectonic, sedimentary and diagenetic processes during the exhumation of mantle rocks at the seafloor (first day: Totalp), and (2) characteristics of subcontinental mantle and its relation to the pre-rift-lower continental crust (second day: Malenco).

Mantle exhumation processes: Totalp area (Davos)

Daniel Bernoulli Geologisches Institut der Universität Basel, Bernoullistrasse 32, CH-4056 Basel

Gianreto Manatschal CGS - EOST, CNRS-Université Louis Pasteur, F-67084 Strasbourg, e-mail : [email protected]

Othmar Müntener Geology Institute, University of Neuchâtel, Rue Emile Argand 11, CH-2007 Neuchâtel, e-mail : [email protected]

3 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Excursion will focus on: Locally, peridotite mylonites indicate deformation at mantle depth which is possibly related to a pre- • Characteristics of subcontinental mantle Mesozoic event. Ar/Ar ages on phlogopites derived • Interplay between tectonic, sedimentary and from a pyroxenite of the Totalp serpentinite yielded diagenetic processes associated with the an age of 160±8 Ma (Peters and Stettler, 1987), exhumation of subcontinental mantle indicating that the mantle rocks were uplifted to • Ophicalcite-genesis about 10 km depth in the late Middle Jurassic. Initial serpentinisation accompanied exhumation Geological overview and deformation under decreasing temperatures The Arosa zone in southeastern Switzerland, to during Jurassic rifting. This evolution is which the Totalp serpentinite belongs, is a highly documented by serpentinite mylonites, deformed complex zone comprising imbricates derived from under greenschist-facies conditions, overprinted by an ancient ocean-continent transition and from the cataclasis and a post-deformational calcification adjacent distal continental margin (Fig. 2). The under still lower temperatures. Differences in Arosa zone is sandwiched between the middle hydrogen isotope ratios between serpentinites and Pennine (Briançonnais) and ophicalcites suggest that serpentinisation was which are derived from the northwestern and the initially restricted to deformation zones and southeastern margin of the Alpine segment of the continued during and after ophicalcite formation Tethys ocean (Fig. 3). Along the upper and lower and Alpine orogeny (Früh-Green et al., 1990). boundaries of the Arosa zone, as well as within the zone, tectonic melanges are widespread (Lüdin, “Ophicalcites” 1987). Within the Arosa zone, the Totalp imbricate Locally, the serpentinites are crossed by a near Davos shows the clearest relationship between system of calcite veins. According to Peters (1963), the basement of the ocean-continent transition and these veins are restricted to the stratigraphically the overlying oceanic sediments, which are the upper part of the serpentinites and to the theme of the excursion. In the Totalp imbricate, neighbourhood of the ophicalcites. Rocks generally Mesozoic igneous rocks are not observed. Alpine described as ophicalcites are breccias with metamorphism is of pumpellyite-prehnite grade unsorted, millimetre- to metre-sized clasts of fresh (Peters, 1963) which is also reflected in the or oxidised serpentinite, set in a calcitic matrix equilibration with Alpine metamorphic fluids of the stained red to pink by hematite or greenish-gray by oxygen isotopes in the calcite of the ophicalcites chlorite minerals and/or cemented by sparry calcite. and the overlying pelagic limestones (Früh-Green et The fabric varies considerably, from serpentinite al., 1990). host rock filled by red-stained limestones or white calcite with a typical cement fabric to clast- supported breccias in which the serpentinites were fragmented in situ (ophicalcite type I of Lemoine et al., 1987). The breccias are of polyphase origin, as shown by the different phases of fragmentation and generations of cementation and sediment infill (Bernoulli and Weissert, 1985). Clast- and matrix-supported breccias, locally containing clasts of Triassic dolomites and of Fig. 3 Schematic cross-section across the Totalp continental basement rocks, indicate lateral imbricate, D. Bernoulli and G. sediment transport, probably from nearby Manatschal (unpublished). extensional allochthons, by debris flow. This type of breccia includes the ophicalcites type II of Mantle rocks Lemoine et al. (1987). Macroscopically, the contact between The major part of the Totalp serpentinite is a serpentinite host rock and carbonate matrix or heterogeneous, variably serpentinised, amphibole- cement is often sharp, but, in places, it is obscured bearing spinel lherzolite with (garnet) pyroxenite by the replacement of serpentine and peridotite layers and locally phlogopite-hornblendite veins. minerals by calcite and by dynamic The pyroxenite layers are mostly parallel to the recrystallisation. In situ replacement of serpentine high temperature foliation. Temperature estimates minerals is documented by relics of pyroxene, in pyroxenites and lherzolites (two pyroxene chromian spinel or iron oxides floating in a red thermometry) are between 850 and 950°C and microsparitic limestone matrix that preserves the pressures of about 15kbar were calculated from original fabric of the rock. Hydrothermal activity is garnet-orthopyroxene barometry (Peters and probably also the source of the sulfide deposits Stettler, 1987), indicating equilibration of the within the and of ferromanganese Totalp peridotite in the spinel peridotite field.

4 IMEDL 2004: Remnants of ancient margins in the Alps, part II mineralisations in the oceanic sediments (Stille et Formation, approx. Valanginian to Barremian) al., 1989). (Weissert and Bernoulli, 1985). The red shales occur only locally; they are associated with debris- Post-rift sediments flow deposits, unfossiliferous and may be interpreted as hydrothermal metalliferous sediments The oceanic sediments overlying the derived from fault rocks associated with mantle ophicalcites include: (1) red shales; (2) cm-bedded exhumation. The Radiolarite Formation and the ribbon cherts (Radiolarite Formation, middle to Calpionella Limestone contain breccias and graded upper Jurassic); (3) gray, thin-bedded limestones sandstones including clasts of both serpentinite and with greenish, shaly interbeds (Calpionella or continental basement rocks indicating local Aptychus Limestone, Berriasian); (4) black shales submarine relief at the time of deposition. with dm-bedded fine-grained limestones (Palombini

Fig. 4 Topographic map of the Totalp area with excursion stops.

Description of the stops Excursion route Coordinates of the localities are from the From Davos by train to Klosters and then by topographic map of Switzerland (Landeskarte der cable-car to Gotschnagrat; from there by foot to the Schweiz, 1197, Davos, 1:25 000). Geological map: Parsennhütte, Parsennfurgga, Obersasställi, Cadisch and Leupold (1929). Totalpsee and back to Parsennhütte and Gotschnagrat and cable-car to Klosters. All Stop 1 (Panorama from Gotschnagrat, Fig. 11) outcrops are in the Totalp serpentinite of the Arosa Coord. 783'650/192'550 zone. On the excursion, we will see partially serpentinised mantle rocks deformed under Panoramic view over the and decreasing temperatures during Mesozoic rifting, Austroalpine nappes surrounding the Prättigau half- peridotite mylonites and several types of window. In the northwest to north, the intricately pyroxenites and their relationships to the overlying folded sediments of the north-Pennine Valais oceanic sediments. A topographic map of the trough ("Bündner Schiefer" and Prättigau Flysch, excursion area is given in Fig. 4. Jurassic to lower Eocene) are overthrust by the

5 IMEDL 2004: Remnants of ancient margins in the Alps, part II middle Pennine Falknis (below) and Sulzfluh In the north (near Pt 2422), the hill of Pt. 2442 (above) nappes. The upper Jurassic shallow-water exposes a partially overturned sequence of thin- limestones of the Sulzfluh nappe (Sulzfluh bedded ribbon cherts (Radiolarite Formation, Limestone) form the prominent reliefs of Middle to Upper Jurassic) and pelagic limestones Drusenfluh and Sulzfluh. The Sulzfluh nappe is (Calpionella Limestone, Lower Cretaceous) resting separated from the large Austroalpine basement stratigraphically on ophicalcites. Replacement of nappe of the Silvretta by the Arosa zone, in the serpentinite by calcite is documented by relics of background consisting of a tectonic melange of pyroxene floating in a red microsparitic limestone Mesozoic shales and flysch (below) and the matrix. Internal sediments in the in-situ fractured Madrisa zone, an overturned slice of Triassic rocks are partially dolomitised. The carbon isotope sediments, which originally was part of the values (δ13C = 2.2%o PDB) of these dolomites are Austroalpine Lechtal nappe. To the southwest, the compatible with dolomite formation by marine Arosa zone is represented by the complex south- waters. The basal part of the Radiolarite Formation Pennine Totalp imbricate (Fig. 3), consisting includes graded layers composed of debris of mainly of partially serpentinized lherzolites, ophicalcite testifying the pre-Alpine origin of the ophicalcites and oceanic sediments (above) and the ophicalcites. tectonically overturned lower Austroalpine Weissfluh imbricate, consisting of light grey Stop 4. (NW of Pt 2582) dolomites (Hauptdolomite Formation, Upper Coord. 780'425/190'200 Triassic), Middle Jurassic syn-rift breccias and post-rift sediments identical to those of the Totalp Partially serpentinised foliated lherzolites and imbricate. To the east are the crystalline basement cm-scale websterite dikes. At this locality, the rocks of the Silvretta nappe. Totalp peridotites are less serpentinised and most mantle minerals are preserved. The lherzolites show Walk along the Panoramaweg across Aptychus a high temperature foliation dipping steeply to the limestones and radiolarites including occasional southeast. The peridotites are fertile spinel turbitites with ophicalcite clasts to Parsennhütte and lherzolites consisting of olivine, clinopyroxene, further towards Parsennfurgga to Stop 2. orthopyroxene, spinel and occasionally titanian pargasite. The clinoyproxenes of the Totalp

lherzolites are rich in Na2O (up to 2.1 wt%) and Al2O3 (up to 7.6 wt%) (Peters, 1968) and are typical for subcontinental peridotites. Trace elements measured by Laser Ablation ICP-MS show nearly

flat chondrite normalized REE patterns with LaN ~2-4 and CeN/YbN >0.25. (O.Müntener, unpublished data). Pyroxenite dikes at a cm scale are always parallel to the high temperature foliation Fig. 5 Panoramic view from Gotschnagrat. and are boudinaged. They contain variable proportions of ortho- and clinopyroxene, Cr-rich Stop 2 (along track from Parsennhütte to spinel and minor amounts of titanian pargasite and Parsennfurgga) rarely phlogopite. Coord. 781'320/191'450 Stop 5 (SW of Pt 2582) Along the track from Parsennhütte to Coord. 780'425/189'915 Parsennfurgga, ophicalcitic breccias (ophicalcite I) are overlain by polymictic breccias including Garnet pyroxenite blocks. At this stop, the blocks of cataclastic continental basement rocks original outcrop has been planified for easier skiing. and gabbros with a high-temperature foliation,. The Garnet pyroxenite blocks are occasionally found origin and age of the gabbro fragments is uncertain within blocks of serpentinised lherzolites. The (Permian gabbro from continental basement (e.g. original orientation with respect to spinel peridotite Malenco) versus Jurassic gabbro associated with foliation (discordant or concordant is unclear onset of sea-floor spreading (e.g. Platta)). The (Peters, 1963). The garnet pyroxenites consist breccias are overlain by red shales and Calpionella mainly of pyropic garnet, hercynite, clino- and Limestone (lower Cretaceous). The succession is orthopyroxene and titanian pargasite. tightly folded around an east-west trending, Orthopyroxene-garnet barometry (at temperatures subhorizontal fold axis. between 850 and 950°C) indicates equilibration pressures of 15 to 18 kbars, indicating that Stop 3 (Obersasställi, Pt. 2442) lherzolites and pyroxenites equlibrated in the spinel Coord. 780'640/191'050 to 780'680/191'200. peridotite field.

6 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Stop 6 (W Totalpsee) Sedimentary lamination and geopetal infill show Coord. 780'570/190'200 that this type is a mechanically deposited sediment that later recrystallised. The second calcite type Calcite-replaced serpentinite mylonites and consists of clear sparry calcite crystals lining the discordant calcite-filled dikes and veins (ophicalcite walls of cracks and crevasses in the serpentinite I). At this locality, the serpentinites of the Totalp breccias. It also fills the remaining voids in slice show different syn- and post-tectonic fabrics geopetally filled sheltered cavities and is interpreted related to deformation under decreasing as a true cement. Both forms of calcite occur in temperatures during exhumation of the sub- various combinations. In some cases, deposition of continental mantle rocks. Mylonitic shear zones cut internal sediment was preceded by precipitation of across older serpentinite mylonites. Tremolite an early cement, and vice versa; however, the red associated with the mylonites suggests deformation limestones are restricted to the upper part of the under low-grade metamorphic conditions. The ophicalcites. The breccias are of polyphase origin, mylonites are in turn deformed by brittle fracturing as shown by the different phases of fragmentation and cataclasis and are partially replaced by calcite. and generations of cementation and sediment infill. Clear sparry calcite and red calcite microsparite Lithification of the breccias and cementation of also fill discordant veins and dikes. pockets and fractures must have occurred during an early stage of diagenesis as cemented breccias Walk over serpentinised peridotite talus along occur as composite clasts in younger breccias. track too Stop 7. Walk down along scattered outcrops of Stop 7 (along track from Totalpsee to Calpionella Limestone to Parsennhütte and to Stop Parsennhütte) 9. Coord. 781'650/190'800 Stop 9 (ESE of Parsennhütte) Deformed peridotites to peridotite mylonites. Coord. 782'300/191'340 They occur as larger masses as well as veins of a few centimeters, with relatively sharp contacts to A north-vergent syncline with an east plunging the surrounding foliated spinel lherzolites. The axis exposes oceanic sediments stratigraphically bright yellow weathering color and serpentine veins overlying the exhumed mantle rocks. Type-I cutting the peridotite mylonites in characteristic ophicalcites are overlain by type-II ophicalcites, tablets are the most striking features in the outcrop. matrix-supported breccias with clasts of Porphyroclasts of spinel, clinopyroxene or serpentinite and ophicalcite I embedded in a reddish deformed and rotated orthopyroxene, sometimes limestone matrix. Scarse pebbles of oolitic several centimeters long, are surrounded by a dolomites, probably of Triassic age, indicate lateral mylonitic fabric of olivine, orthoyproxene, sediment transport, probably by debris flows. The clinopyroxene, spinel and occasionally titanian ophicalcites are overlain by dark-coloured and red, pargasite. The age of this mylonitic foliation is non-calcareous shales interpreted as metalliferous, unknown, but probably not related to the hydrothermal sediments. Up-section, they are exhumation of the Totalp peridotite, as followed by red, thin-bedded ribbon cherts equilibration temperatures are indistinguishable (Radiolarite Formation) and pelagic limestones from the surrounding peridotites and pyroxenites (Calpionella Limestone). In the Calpionella (Peters and Stettler, 1987). Limestone some dm-thick breccias with clast of continental basement rocks are intercalated Stop 8 (valley SE of Parsennfurgga) Coord. 781'300/191240

Outcrops and blocks in the float show sedimentary and diagenetic fabrics in ophicalcites type I. Non-fragmented or tectonically brecciated serpentinites grade rapidly through a narrow zone of host rocks, cut by different generations of dikes, into complex breccias dominated by a carbonate matrix. Two different types of calcite can be distinguished in these ophicalcites: the first is a limestone, usually red, which under the microscope shows an equigranular fabric of anhedral calcite crystals with hematitic pigments between them. This limestone carries variable amounts of serpentinite clasts and pyroxene mineral grains.

7 IMEDL 2004: Remnants of ancient margins in the Alps, part II

2nd day: Lago Pirola (Malenco complex) A pre-rift continent-mantle boundary and its exhumation during rifting

Othmar Müntener Geology Institute, University of Neuchâtel, Rue Emile Argand 11, CH-2007 Neuchâtel, e-mail : [email protected]

Jörg Hermann Australian National University ANU, Canberra, e-mail : [email protected]

Volkmar Trommsdorff Institute for Mineralogy and Petrography, ETH Zürich,

Tectonic setting Cretaceous nappe stack (e.g. Fig. 4; Hermann and Müntener, 1992; Handy et al., 1993 and Together with the lower Austroalpine Bernina 1996; Froitzheim et al., 1994) but generally did Sella and Margna nappes, the Malenco nappe not disrupt the continuity of the Cretaceous preserves the remnants of the southeastern margin of structures in cross sections (Fig. 7). The tectonic the Liguria-Piemonte segment of the Alpine Tethys evolution of the South-Penninic and (Müntener and Hermann, 2001) (Fig. 6). The Austroalpine units differs from that of the Austroalpines nappes include the remnants of the underlying middle Pennine units (Briançonnais: distal Adriatic margin and were thrust onto the Tambo and Suretta unit, Tertiary flysch) where Malenco nappe during late Cretaceous time (Villa et only a Tertiary evolution has been observed. al., 2000). The Malenco nappe, originally situated The main deformation in the Austroalpine oceanward of the Margna nappe represents the ocean- and south Pennine units is related to west- continent transition including serpentinised directed thrusting and nappe-stacking. Maximum peridotites, basaltic volcanics and oceanic sediments temperatures (Trommsdorff and Evans, 1974; (Fig. 6). In addition, a Permian continental crust- Mellini et al.,1987; Trommsdorff and Connolly, mantle transition zone forms part of the Malenco 1996) and pressures (Bissig and Hermann, 1999) nappe. Extensional allochthons, derived from the are around 450°C and 0.5 GPa. This indicates, distal continental margin, were emplaced on the that the rocks derived from the Adriatic margin exhumed mantle rocks. Malenco and Austroalpine of the Piemontese ocean basin, now exposed in nappes are now part of a late Cretaceous west- Val Malenco, were only moderately buried directed thrust wedge, which was probably associated during Alpine metamorphism in the late with subduction along the eastern border of Adria. In Cretaceous (Villa et al., 2000), because they contrast to most of the ophiolites in the western Alps, remained in the wedge above the more intra- those of the eastern Alps, including the Malenco- oceanic subduction zone. Thickening of the crust Forno unit lack a high-pressure metamorphic was followed by extension documented in top-to- overprint. Therefore, the Malenco complex has been the-east shear zones. This extension is probably interpreted to belong to the hanging wall of a south- related to gravitational collapse (Froitzheim et directed subduction zone, which apparently first al., 1994) developed within the ocean. In the Malenco-Forno and the overlying Austroalpine units N-S shortening is documented Alpine deformation and metamorphism in south vergent folds and shear zones (first Two orogenic cycles affected the former phase of backfolding; e.g. Fig. 4). However, this passive margin. E-W directed nappe stacking and deformation caused the main foliation in the subsequent extension both occurred during the underlying Tambo and Suretta units and affected Late Cretaceous (Hermann and Müntener, 1992; Tertiary flysches. The contrasting style of Spillmann, 1993; Froitzheim et al., 1994; Handy deformation indicates that the Austroalpine, et al., 1993 and 1996; Villa et al., 2000) (Fig. 7). together with the south Pennine units, acted as an Younger N- and S-directed thrusts and E-W ‘orogenic lid’ suffering only minor deformation. trending folds (Fig. 8) related to the Tertiary Crustal thickening was later followed by orogen- continent-continent collision overprinted the parallel extension (Nievergelt et al., 1996;

8 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Froitzheim et al., 1994), which was strongly Evans, 1974; Trommsdorff and Connolly, 1996). localised in the Austroalpine and South Pennine The stable regional metamorphic paragenesis units but more penetrative in the Briançonnais consist of antigorite + diopside + olivine + domain (Liniger and Nievergelt, 1990; Huber and chlorite + magnetite. The transition from Marquer, 1996). diopside to tremolite marks the first mappable Ongoing N-S convergence produced large isograd. The second isograd is defined by the scale, E-W trending open folds (second phase of breakdown of antigorite to olivine and talc. Both backfolding) and caused verticalisation of the isogrades are parallel to the tonalite contact (see nappe stack towards the south (Fig. 8). This Fig. 2 in Trommsdorff and Connolly, 1996). deformation is cut by the Oligocene Bergell Thermal modeling indicates that temperatures tonalite (32 Ma, Von Blankenburg, 1992; increased from about 350°C outside the aureole Spillmann, 1993; Puschnig, 1996 ). to about 570°C at the contact (Trommsdorff and The last regional deformation caused NE-SW Connolly, 1996). trending folds (transverse folding). Interference Deformation postdating the granodiorite of the transverse folding with the second phase of intrusion occurred along major fault zones backfolding forms a dome and basin structure, (Engadine Line, Insubric Line; Fig. 2). Uplift of well visible on a map scale (Fig. 6). The Bergell the Alpine nappe stack N of the Insubric line tonalite and its contact aureole are partially (~20 km in the Central Alps, ~10km in the overprinted by this deformation. However, the Valtelline area) led to final exhumation of the younger granodiorite (30 Ma, Von Blankenburg, rocks and is responsible for the sharp contact to 1992) cuts the transverse folding (Puschnig, Alpine unmetamorphosed rocks of the Southern 1996). The intrusion of the Bergell tonalite Alps (Fig. 2). caused a ~3km wide contact aureole in the Malenco ultramafic rocks (Trommsdorff and

9 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 6 Schematic tectonic map of the Malenco area. Inset in the upper right corner shows location of the area at the border between southeastern Switzerland and Northern Italy. Axial surface planes of major folds are indicated. Abbreviations are D, Monte Disgrazia; B, Monte Braccia; C, Chiesa; Ch, Chiareggio; CF, Campo Franscia; S, Pizzo Scalino; M, Monte Motta; R, Monte Roggione; Z, Alpe Zocca; V, Val Ventina; E.L. Engadine Line.

10 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 7 Reconstructed nappe pile of the south Pennine-Austroalpine units in Val Malenco (N-Italy) Müntener and Hermann (2001) redrawn after Spillmann (1993) and Hermann and Müntener (1996).

Fig. 8 N-S profiles across the Malenco region. The dominant structure is caused by large, open syn- and antiforms of the second backfolding event. Note the steep plunge of the nappe stack towards the Insubric Line, that separates the Alpine unmetamorphosed from the Central Alps.

The Malenco continental crust-mantle granulites form part of an ancient continental transition zone crust-mantle transition zone (Müntener and Hermann, 1996; Hermann et al., 1997; Hansmann Field relations et al., 2001). A tholeiitic gabbro intrusion displays An undisturbed fossil lower crust to upper intrusive contacts to both lower crustal granulite mantle section is preserved and now exposed and the underlying mantle rocks. The intrusive along the boundary between the Penninic and complex ranges from Mg- to Fe-rich gabbronorites Austroalpine nappes in Val Malenco, Italy. and Fe-Ti and quartzdiorite dikes (Müntener and Detailed field, petrographic, petrologic and Hermann, 1996; Hermann et al., 2001). Bulk rock isotopic investigations of the high-grade and mineral major and trace element chemistry metamorphic rocks revealed that lower crustal

11 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 9 Geologic map of the Lago Pirola – Monte Braccia area (modified from Müntener and Hermann 1996). reveals that some trace element rich gabbros represent dikes (Müntener and Hermann, 1996; Ulrich and frozen liquids whereas others are cumulates formed Borsien, 1996). from highly differentiated residual liquids. The subcontinental mantle rocks are Differentiation of the gabbro complex is mainly composed in large parts of serpentinised driven by fractional crystallisation of pyroxenes and lherzolites, which formed the residues of partial plagioclase, resulting in a tholeiitic differentiation melting (Müntener, 1997). Only in the area of Mt trend (Hermann et al., 2001). The lower crustal Braccia (Fig. 9) fresh amphibole-bearing spinel metapelites experienced extensive partial melting lherzolites are preserved. The peridotite varies in which produced migmatites and residues rich in modal clinopyroxene from 3 to 15 %, garnet and kyanite. The density of the restites equals clinopyroxenes have high Al2O3 and Na2O or exceeds that of the underlying mantle rocks contents and spinels commonly show low (Hermann et al., 1997). Two different types of Cr/Cr+Al ratios. The lherzolites are cut by at leucosomes were found: (1) In the immediate vicinity least three different generations of igneous rocks of the gabbroic rocks small granitic bodies were ranging from an oldest suite of olivine-spinel found. Further away leucogranitic dikes have been websterites that form the characteristic layering observed which crosscut highly differentiated gabbro of the peridotite, followed by a younger suite of

12 IMEDL 2004: Remnants of ancient margins in the Alps, part II corundum-bearing garnet clinopyroxenites, to the (Fig. 10). More precise ages were obtained by youngest suite of discordant dunite, cumulate dunite, zircons from granite bodies in the metapelites, related clinopyroxenite and gabbroic dikes. The latter which yield an upper intercept age of are part of the Braccia gabbro complex. Locally, 278±2.6Ma. These ages strongly suggest that phlogopite-hornblendite dikes are found. emplacement of the gabbroic rocks and partial melting in the lower crust are coeval and of Peak Metamorphic conditions Permian age. Recent zircon and monazite The peak metamorphic paragenesis in the ionprobe dating of the granulites refined this metapelites consists of garnet, kyanite, biotite, quartz, picture and provided additional constraints on ilmenite/rutile, and feldspars. Phase relations and the timing of granulite facies metamorphism mineral chemistry suggest peak metamorphic (Hermann and Rubatto, 2003). Zircons from the pressures of about 1.0 GPa and a temperature of 800- lower crustal metapelitic granulites display 850°C (Müntener et al., 2000). Intercalated inherited cores with apparent ages ranging from metacarbonates containing the paragenesis calcite, 520 to 3050 Ma and three metamorphic wollastonite, garnet, quartz and clinopyroxene overgrowths with ages of 280±5, 269±3 and indicate minimum temperatures of 750°C at a 258±4. Monazites record the same three pressure of 1.0 GPa (Hermann 1997). A maximum metamorphic ages at 280±5, 270±5 and 258±4 temperature estimate of 870° was deduced by using Ma. These data indicate that granulite facies Zr-saturation temperatures in a granitic body close to metamorphism induced by underplating of mafic the gabbro intrusion (Hansmann et al., 2001). Phase rocks lasted for about 20 Ma. relations in mantle and gabbroic rocks are consistent with the results from the lower crustal rocks. Retrograde metamorphism

Age constraints

Fig. 10 Age constraints of the Braccia gabbro and leucogranites. The ages of 281±19 and 278±2.6 Ma overlap and strongly suggest that igneous underplating and partial melting of the lower crust are coeval (modified from Hansmann et al., 2001).

U-Pb determinations on single zircons from differentiated gabbros yield an age of 281±19Ma

13 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 11 P-T diagrams for the mafic, ultramafic and in the deep crust and upper mantle. Exhumation pelitic rocks of the Malenco area (adapted of the crust-mantle complex began with the onset from Müntener et al. 2000). (A) Compiled p- of continental rifting during Early Jurassic. T diagram for the Braccia gabbro. The Hydrous phases partially replaced the granulite reaction curves A and B qualitatively show facies mineral assemblages in all rock types and the transition from ol+pl through spinel to indicate a fluid influx at about 0.9 GPa and garnet granulite. (B) Petrogenetic grid for a 650°C (Müntener et al., 2000). Continued

portion of the system CaO-FeO-MgO-Al2O3- hydrous recrystallisation is accompagnied by SiO2-H2O. Dashed curves correspond to near-isothermal decompression. Dating of olivine-absent reactions. Curves are labeled amphiboles formed during this hydration yield by the appropriate reaction equations, with late Triassic to Early Jurassic ages (Villa et al., the high-temperature assemblage to the 2000). This indicates that decompression is right. The breakdown of ortho-pyroxene to related to Jurassic rifting, which led to the olivine+talc±chlorite±tremolite instead of separation of the Adriatic and European anthophyllite indicates relatively high continents. pressure during retrograde hydration. (C) P- T diagram for Malenco metapelites. Jurassic situation Thermometers: 1) Grt-Ilm; 2) Grt-Bt: 3) In the Western part of the Malenco unit, the

Grt+Ky+H2O->Ctd+St+Qtz and 4) ultramafic rocks are cut by ophicarbonate zones Pl+Ky+H2O->Pg+Czo+Qtz); Barometers: and by mid-ocean-ridge-like basaltic dikes of the 5) Ky-Sil, 6) Fe-Mg in Grt, 7) GASP 8) Forno unit (Fig. 13). Additional evidence for GRIPS; 9) Stability field of czo+pg. final exhumation of the Malenco complex is documented by ophicarbonate rocks, which formed on top of the denuded mantle within fractures and as debris flows containing platform sediment and serpentinite components. The widespread serpentinisation of the Malenco ultramafic rocks occurred during oceanic hydration on the ocean floor. Chrysotile included in antigorite indicates temperatures lower than 300°C for the oceanic hydration. Rodingitisation of mafic dikes and ophicarbonate breccias bears a marine stable isotope signature (Burkhard and O’Neill, 1988; Pozzorini and Früh-Green, 1996) indicating metamorphism and metasomatism took place in an oceanic environment. The Forno unit itself consists of metamorphosed basalt with locally preserved pillow lavas and breccias Fig. 12 Pressure-temperature-time diagram for the (Montrasio, 1973), which are overlain by a Malenco crust-mantle complex, after metasedimentary sequence of presumably Müntener et al. (2000), Villa et al. (2000) Jurassic age (Peretti, 1985). In Jurassic time, the and Hansmann et al. (2001). Black arrow Margna, Malenco and Forno units formed the indicates the Permian to Jurassic retrograde ocean continent transition from the Adriatic metamorphic evolution, while the dashed continental margin to the Piemont Ligurian arrow indicates the burial and exhumation ocean (Fig. 13), with a geometry similar to the related to the Alpine cycle. Geotherms Platta nappe further to the north (Manatschal and corresponding to different surface heat flows Nievergelt, 1997) and the present-day margin of (in mW/m2) are taken from Chapman (1986). Galicia (e.g. Boillot et al., 1995). However a complication arises from the Two stages of retrograde metamorphism followed position of the Platta nappe, which is sandwiched (Fig. 11): Mineral parageneses in garnet-kyanite between two lower Austroalpine units (Fig. 14). gneiss, metagabbro and metaperidotite record a first This is a crucial problem in any palinspastic stage of near-isobaric cooling under anhydrous reconstruction and depends on how one conditions. The near-isobaric cooling probably interprets the pinching out of the Platta reflects slow relaxation of a high geothermal gradient ultramafic rocks southeast of the Engadine line after the gabbro intrusion. The stabilised crust-mantle (Schmid et al., 1990). One hypothesis put transition of about 30 km thickness then persisted forward originally by Trümpy (1975) is that over a period of about 30 to 50 Ma into the Late Margna-Sella formed a microcontinent. Later, Triassic. The Malenco crust-mantle section thus Froitzheim and Manatschal (1996), Handy provides strong constraints on the pre-rift conditions (1996) and Froitzheim et al. (1996) interpreted

14 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 13 Paleotectonic reconstruction across the Malenco ocean-continent transition (adapted from Müntener and Hermann 2001).

Margna-Sella as an extensional allochthon resulting Müntener and Hermann (2001) proposed that the from detachment faulting during the advanced stages Platta domain does not exist in the south and the of rifting, implying oceanic crust between the Bernina-Margna-Malenco domain represented Margna-Sella and the Austroalpine (e.g. Froitzheim the Adriatic passive continental margin in Late and Manatschal 1996, their Fig. 10). However, Jurassic times (Fig. 13). Such a geometry can studies of the Alpine tectonic evolution (Spillmann, best explain why cross sections along continent- 1993; Liniger, 1992) have shown that there is no ocean transitions (e.g. the Err-Platta and Margna- ophiolitic suture between the lower Austroalpine Malenco domain) vary laterally (see cross Bernina and Margna/Sella nappes east of the point sections of Froitzheim and Manatschal, 1996; where the Platta nappe pinches out (Figs. 6 and 7). Handy, 1996; Hermann and Müntener, 1996). An alternative hypothesis therefore suggested that the lower Austroalpine crystalline units represent a continuous segment of basement rocks in the southern part of the study area (Fig. 14) and that the arrangement of continental and oceanic units reflects the original paleogeography of a segmented passive continental margin. Consequently, Schmid et al. (1990) and later Spillmann (1993) proposed that the pre-Alpine extension direction and Alpine thrusting are not strictly parallel in this area and that the southernmost part of the Platta nappe became sandwiched between lower Austroalpine nappes during Cretaceous thrusting. Fig. 14 Schematic palaeogeography (map view) of the ocean-continent transition We suggest, that the fragmentation of the conti- along the Adriatic continental margin. nental margin was caused by rifting and that in the Note tectonic klippen of continental northern part (Fig. 14) the ocean-continent transition rocks (extensional allochthons) is formed by the Bernina-Err-Platta domain. overlying the exhumed subcontinental However, so far there is no field evidence south of mantle (adapted from Müntener and Piz Corvatsch that the Margna-Sella domain Hermann, 2001). represented an extensional allochthon in the sense of Froitzheim and Manatschal (1996). Consequently,

15 IMEDL 2004: Remnants of ancient margins in the Alps, part II

Fig. 15 Topographic map of the Pirola area in the Malenco nappe with excursion stops . Description of the stops Excursion route Coordinates of the localities are from the topographic map of Switzerland (Landeskarte From Chiareggio (1610 m) by foot along a trail to der Schweiz, 278, Disgrazia, 1:50’000). Lago Pirola. All outcrops are in the Malenco complex and show lower crustal metasedimentary and igneous Stop 1 (Alpe Pirola) rocks and the heterogeneity of former subcontinental Coord. 780'650/131'000 mantle. Return by foot to Chiareggio either via Val Ventina (steep descent on old moraine material from Petrology of granulite facies marbles the Ventina Glacier) to Rifugio Porro/Gerli and to Along the foot path from Chiareggio to Alpe Chiareggio or descent along the trail same trail from Pirola. Large blocks of granulite facies marbles Lago Pirola to Chiareggio. A topographic map of the and calcsilicate rocks can be found at Alpe excursion area is shown in Fig. 15. Pirola. Coarse grained quartz-free calcite marbles consist of olivine, diopside, phlogopite ±spinel, Excursion will focus on: ±humite. Strongly retrogressed samples contain dolomite+antigorite. In silica saturated marbles • Crust-mantle boundary, underplating, exhumation variable proportions of diopside, grossular, of subcontinental mantle calcite, quartz, wollastonite, ±phengite, • lower crust in an ocean-continent transition ±kalifeldspar, ±plagioclase, ± clinozoisite may be found. Rocks overprinted by retrograde amphibolite facies metamorphism are dominated

16 IMEDL 2004: Remnants of ancient margins in the Alps, part II by tremolite and clinozoisite-bearing assemblages. lower crustal granulites can be seen. A Fe-Ti The paragenesis wollastonite + grossular + quartz + gabbro dike crosscuts the granulites. This dyke is calcite indicates minimum temperatures of ~ 750°C at made of greenschist facies minerals but has an 1 GPa, consistent with thermodynamic calculations identical chemistry to the primary Fe-Ti gabbroic on olivine marbles. The variety of assemblages is dykes of Alpe Braccia. Dykes of gabbros in consistent with the assumption that bulk rock peridotites and pelitic granulites are evidence for chemistry of the protolith dominates the granulite an intact crust-mantle section in Val Malenco. facies paragenesis in individual samples and that the The surrounding rocks are made of coarse fluid chemistry was internally controlled. In contrast, grained garnet gneisses and a few intercalations the pre-Alpine retrograde assemblages are consistent of granulite facies marbles mainly preserving with an externally controlled low XCO2 fluid. their pre-Alpine structure. In the garnet gneisses all transitions from incipient melting (migmatite) Walk along trail over the contact between lower to coalescing and pooling of leucogranites can be crustal granulites and metagabbros to the stone huts found. Despite an intense static Alpine below the wall of Lago Pirola metamorphic overprint many pre-Alpine relics are preserved, among them are garnet (partially Stop 2 (Lago Pirola) or completely chloritised), kyanite (retrogressed Coord. 780'750/130'650 to paragonite± phengite ± chloritoid), and knobs of blue quartz, the color being caused by

Alpine deformation of metagabbros micrometre-sized TiO2 inclusions. Metamorphic The outcrops are formed by a schistose chlorite- conditions for the metapelitic primary actinolite-clinozoisite-albite rock which is an Alpine assemblage garnet + kyanite + plagioclase + recrystallised metagabbro originating from the quarz + ilmenite ± biotite point to pressures of Permian Braccia gabbro. The primary flaser type ~1.0 GPa and temperatures around 800°C texture is partially preserved, with (Hermann et al., 1997; Müntener et al., 2000). chlorite+amphibole growing on former pyroxenes and The high amount of former garnet and kyanite clinozoisite and albite on former plagioclase. The suggests that most of the rocks have a restitic main foliation is deformed by the first and second character. Garnet often exceeds 30 vol % phase of backfolding with steeply N-NE dipping axial resulting in an original density of the pelitic planes and by the transverse folding with steeply W granulite of at least 3.3 g/cm3, similar to mantle dipping axial planes. Superposition of these structures rocks. results in a variety of spectacular fold interference patterns visible along the main footpath. Stop 4 (SE of Lago Pirola) A few tens of meters to the east of the stone huts, Coord. 781'470/130'300 the pre-Alpine flaser structure in the gabbro is preserved in a lens weakly affected by Alpine Malenco mantle rocks deformation. Amphibole and clinozoisite are not These glacier-polished outcrops form an area oriented indicating static growth during Alpine of about one km2 and consist in large parts of metamorphism. Several layers of former pyroxenites serpentinised, layered ultramafic rocks with cpx- are still visible. The orientation of the flaser texture is rich websterite and cpx-poor lherzolite to in agreement with kyanite lineations in the pelitic harzburgite. The main mass of the ultramafic granulites and with spinel lineations in the peridotites rocks is a massive, magnetite-bearing chlorite- indicating a common high temperature deformation of olivine diopside serpentinite. However, around all rock types. Leucogranites that formed by partial Mte Braccia further to the southeast, several melting of metapelitic xenoliths within the gabbros lenses of fresh spinel peridotite and chlorite- are discordant to the gabbro flaser texture indicating amphibole peridotite can be found. The that the gabbro intrusion and partial melting of the serpentinites are nearly unaffected by Alpine pelites are closely linked. U-Pb dating of zircons of a deformation permitting to have insight into the nearby leucogranite resulted in Permian ages complexity of pre-Alpine subcontinental mantle. (Hansmann et al., 2001) indicating that partial melting The dominant pre-Alpine structure is a mineral of xenoliths and intrusion of the gabbro are coeval. stretching lineation trending SE-NW and a weak foliation. In the Ssouthwestern part of the Walk along highly deformed metagabbros on a outcrop, this lineation is parallel to the websterite trail to the foot of Mte Senevedo. layers which steeply dip towards SW. Depending on the grade of pre-Alpine deformation these Stop 3 Foot of Mte Senevedo websterites form boudins of cm to 10s of meters Coord. 781'450/130'650 in size. In a few blocks, however, they branch and form interconnected dykes indicating an Following along the footpath to the East, the igneous origin for the websterites. Towards NE, deformed contact between the Braccia gabbro and the layers turn to a horizontal position before

17 IMEDL 2004: Remnants of ancient margins in the Alps, part II dipping towards NE providing evidence for a large- enclosing peridotite and spinel websterite. These scale antiform (Fig. 16). The mineral lineation, observations indicate that the dunites are however, remains constant and the weak foliation younger than the folded layered peridotite. probably represents the axial surface foliation of this Younger deformation (exhumation related?) is folding. Dunite zones are not deformed by this mantle preserved by dunite boudins which locally antiform and some of the dunite bodies are discordant display strong mylonitic textures. to the layered peridotite. On a large scale close to Mt Braccia (Fig. 40), dunites form anastomozing zones

Fig. 16 Panoramic view of the Malenco mantle rocks from Lago Pirola (adapted from Müntener and Hermann 1996). A-A’: Weakly deformed lenses of granular peridotite are embedded in a tectonite foliation. Spinel websterites and amphibolitised garnet clinopyroxenites are parallelised in this foliation. Towards the left of the profile the peridotite tectonites are strongly serpentinized. B-B’: Field relationships between the layered peridotites and dunites south of Lago Pirola. The layered peridotites exhibit weak open folding indicated by spinel websterites. On the southwestern side of the antiform the dunites are subparallel to the banding defined by spinel websterites. On the northeastern side, however, the dunites are clearly discordant to spinel websterites. The spinel websterite layers form an open antiform. Note that dunite is discordant to the spinel websterites in the eastern part of the cross section and nearly parallel in the western part indicating that the dunites are younger than the open fold of the mantle rocks.

The most important rock types are: amphibole±chlorite). In many places a (1) Serpentinised peridotite: The main mass of clustering of pyroxene+spinel can be the ultramafic rocks is a massive, magnetite- found. This cluster texture probably bearing chlorite-olivine diopside serpentinite. indicates breakdown products of former The serpentinite is cut by different types of garnet, or represents extensive exsolution veins, consisting of (a) olivine, chlorite, of Al-phases from an Al-rich pyroxene magnetite, titanian clinohumite and diopside; precursor. Websterites completely and (b) pure chlorite veins. The latter overprinted by Alpine metamorphic possibly represent altered phlogopite-Ti- assemblages are characterised by amphibole veins (Müntener, 1997). abundant white diopside (pseudomorphic (2) Spinel Websterite: Spinel websterites consist after primary clinopyroxene) in a chlorite of cm-sized bright green clinopyroxene, Cr- ± serpentine ± titanian clinohumite Al spinel and orthopyroxene (replaced by matrix.

18 IMEDL 2004: Remnants of ancient margins in the Alps, part II

(3) Amphibolite (ex garnet clinopyroxenite): The Malenco-Forno units are the Suretta nappe garnet clinopyroxenite in this outcrop is (visible at Cime di Vazzeda) which probably completely retrogressed to amphibolite represents the European continent. consisting of pargasite, ± chlorite, ± titanite ± diopside. Fresh samples from the Mt Braccia Several classic concepts of Alpine geology area consist of Al-rich clinopyroxene, garnet, are well visible from Torrione Porro: Ti-rich pargasite and rare corundum, the latter containing micrometer-sized Cr-Al spinel Nappe stacking inclusions. The sequence of basement rocks and (4) Dunite: These rocks are easy to recognise in Mesozoic sediments permits to define a series of the field because of a distinct red-brown nappes in the Malenco region. They are well weathering color and a fine grainsize on exposed in the Bernina massif and include from weathered surfaces. Despite an intense Alpine bottom to the top: the Malenco, Margna, Sella metamorphic overprint, olivine and Cr-Al and Bernina nappes. Towards the North the view spinel are partially preserved although many is dominated by 3 peaks – Pizzo Tremoggia, of the dunites are serpentinized to more than Pizzo Malenco and Sassa d’Entova - which 50%. Field relations with the surrounding provide a cross section through the sedimentary peridotites suggest that the dunites are syncline linking the lower Austroalpine Sella and replacive, they form by dissolution of Margna nappes. The sequence from Pizzo pyroxene and precipitation of olivine ± Cr- Tremoggia to Sassa d’Entova consists of a more rich spinel. or less symmetrical metasedimentary sequence consisting of Triassic dolomite marbles (peak of Stop 5 (Torrione Porro) Pizzo Tremoggia), Lower Jurassic siliceous Coord. 780'650/130'350 calcschists, metaradiolarites including horizons of Mn-mineralisations (Peak of Pizzo Malenco), Panoramic overview of the Penninic-Austroalpine lower Jurassic siliceous calcschists and boundary, the continental crust-mantle transition, the intercalated dolomite marbles (Peak of Sassa oceanic Forno unit and the Bergell intrusives. The d’Entova). In the lower part of these mountains is geodynamic cycles of Jurassic rifting and Alpine made of basement rocks of the Margna nappe collision and exhumation assembled all important (including lower and upper crustal rocks) and the rock types of the lithosphere in a small area. At Mt. deepest part is formed by schistose antigorite Braccia, peridotites are preserved which represent the serpentinite of the Malenco nappe. lithospheric mantle. Attached to the subcontinental mantle are lower crustal gabbros (Lago Pirola) and Extensional faulting in the deep crust lower crustal granulites (Mt. Senevedo). Upper crust Further to the west, below Monte del Oro and consisting of polymetamorphic paragneisses and Pizzo Muretto, the internal structure of the distal amphibolites cut by intrusive granitoid rocks crop out Margna nappe is exposed. The Margna nappe is a in the Austroalpine nappes of the Bernina massif. composite basement unit consisting of granulitic These basement nappes are covered by a sedimentary lower crust intruded by (presumably Permian) sequence of Permo-Triassic to Lower Cretaceous in tholeiitic gabbros identical to the Braccia gabbro age. Remnants of the Piemonte Ligurian oceanic crust and of Variscan upper crust with a Permo- were found around Mte. del Forno. Alpine intrusives Mesozoic sedimentary cover. These two rock of the Bergell massif are found around Mte Sissone. associations represented the lower and upper The large difference in altitude from Sondrio (300 crust prior to rifting, respectively, and were m a.s.l.) to Piz Bernina (4049 m) permits insights into assembled during rift-related thinning of the the 3-dimensional relations between different tectonic continental crust. The contact between upper units. The Southern Alps, which are not affected by crustal and lower crustal rocks as exposed today Alpine metamorphism, are situated South of the in the Margna nappe forms two large recumbent Valtellina valley and are separated by the Insubric folds within the latter (Fig. 17). The significance Line from the Alpine metamorphosed Austroalpine of this contact for pre-Alpine extension becomes units. Austroalpine and Southalpine represent the clear only when the lower Austroalpine nappe Adriatic continental crust. The Penninic Malenco- stack (Fig. 17) is retrodeformed into its pre- Forno units are remnants of the Piemont-Ligurian collisional orientation (Fig. 13). Restoring the ocean that separated Adria and Europe in Mesozoic original geometry of this shear zone results in a times. The Malenco serpentinite covers more than low-angle shear zone dipping below the Adriatic 100 km2 and occurs mainly in the lower parts of the continent (Hermann and Müntener, 1996; Malenco valley but also form the peak of the Mt. Müntener and Hermann, 2001). Based on the Disgrazia (3678m a.s.l.). The serpentinites are mined ‘pressure gap’ of at least 5 kbar between upper for industrial and house building purposes, especially crustal granitoid rocks and lower crustal around the area of Chiesa. Tectonically below the granulites we conclude that rifting excised 15 to

19 IMEDL 2004: Remnants of ancient margins in the Alps, part II

20 km of mostly intermediate crust. Thus this shear schistose serpentinite, with a diameter ranging zone accommodated extensional exhumation of the from millimeters to a few meters, embedded in a pre-rift lower crust to a depth of 10 km or less. matrix of carbonate, i.e. calcite and/or dolomite. The ophicarbonate breccias show properties of Tertiary intrusions and contact metamorphism debris flows forming in fractures within denuded In the partially glaciated outcrops east of Monte subcontinental mantle. In situ fragmentation of Sissone and in Val Sissone, intrusive relations antigorite serpentinite are probably similar to between the nappe stack and the Tertiary Bergell jigsaw breccias observed in less metamorphosed hornblendites, tonalites and granodiorites are ophicarbonates in the Platta nappe. Oxygen and beautifully exposed. A prominent generation of hydrogen isotopes of minerals from the pegmatites cuts contact metamorphic mafic rocks on ophicarbonates do not display a systematic trend the slope of Monte del Forno. In the ultramafic rocks with increasing temperature suggesting the near the huts of Alpe Zocca across the Ventina valley absence of isotopic equilibrium during contact a tremolite-olivine-antigorite assemblage dominates metamorphism (Pozzorini and Früh-Green, 1996; indicating that Alpe Zocca is situated within the first Abart and Pozzorini, 2000). The relatively isograde defined by the transition of diopside to constant carbon isotope compositions of the tremolite (Trommsdorff and Evans, 1972; Ventina ophicarbonate matrix are comparable to Trommsdorff and Connolly, 1996). West of Alpe marine carbonate signatures (Pozzorini and Früh- Zocca towards Val Sissone the next higher Green, 1996). assemblage, talc + olivine can be observed (indicated by the reddish colour compared to the greenish color Back to Lago Pirola, Alpe Pirola and of antigorite serpentinites. The highest point in the Chiareggio on marked foot path, or on the trail to southwest (Monte Disgrazia 3678m a.s.l.) is made of Val Ventina (steep descent on old moraine olivine+talc rocks. material of the Ventina Glacier) to Rifugio Porro/Gerli and to Chiareggio. Ophicarbonate outcrops in Val Ventina Just below the end of the Ventina Glacier beautifully exposed ophicarbonate rocks can be seen. The ophicarbonate rocks consists of blocks of

20 IMEDL 2004: Remnants of ancient margins in the Alps, part II

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