The geological map of Valmalenco
Autor(en): Trommsdorff, Volkmar / Montrasio, Attilio / Hermann, Jörg
Objekttyp: Article
Zeitschrift: Schweizerische mineralogische und petrographische Mitteilungen = Bulletin suisse de minéralogie et pétrographie
Band (Jahr): 85 (2005)
Heft 1
PDF erstellt am: 11.10.2021
Persistenter Link: http://doi.org/10.5169/seals-1650
Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. Das Veröffentlichen von Bildern in Print- und Online-Publikationen ist nur mit vorheriger Genehmigung der Rechteinhaber erlaubt. Die systematische Speicherung von Teilen des elektronischen Angebots auf anderen Servern bedarf ebenfalls des schriftlichen Einverständnisses der Rechteinhaber.
Haftungsausschluss Alle Angaben erfolgen ohne Gewähr für Vollständigkeit oder Richtigkeit. Es wird keine Haftung übernommen für Schäden durch die Verwendung von Informationen aus diesem Online-Angebot oder durch das Fehlen von Informationen. Dies gilt auch für Inhalte Dritter, die über dieses Angebot zugänglich sind.
Ein Dienst der ETH-Bibliothek ETH Zürich, Rämistrasse 101, 8092 Zürich, Schweiz, www.library.ethz.ch
http://www.e-periodica.ch Schweizerische Mineralogische und Petrographische Mitteilungen 85,1-13, 2005
The Geological Map of Valmalenco
Volkmar Trommsdorff1, Attilio Montrasio2, Jörg Hermann Othmar Müntener1-4, Peter Spillmann1-5 and Reto Giere6
Abstract
The 'Carta Geologica delta Valmalenco' (Montrasio et al., 2005) is the result of a thirty five-year cooperation between ETH and University of Zürich and the University and CNR-ltalia at Milano.The map covers an area of about 350 km2 with the mapping having been carried out by over fifty people, mostly as part of students' theses. The Valmalenco area contains one of the largest ultramafic masses of the Alps, which occurs together with a well preserved, Jurassic to Cretaceous ocean floor suite. These units are sandwiched in between nappes derived from the Adriatic microplate and the European plate. The ultramafic rocks are of subcontinental origin and preserve on their top a Permian, continental, crust-mantle transition. During Jurassic rifting these rocks were exposed at the Adriatic margin of the Piemontese-Ligurian ocean basin. Two orogenic cycles affected the Adriatic margin: the first is related to Late Cretaceous nappe stacking, whereas the second is due to the Tertiary continental collision. At the end of collision, the Malenco rocks were, in their southwestern part, intruded by the calc-alkaline, Oligocene, Bergell complex. The Bergell rocks present an exceptionally rich variety of magmatic phenomena and a well-defined contact aureole. The area affected by contact metamorphism is an outstanding natural laboratory to study metamorphic and metasomatic processes. Isograds of prograde metamorphism of serpentinite and of ophicarbonate rocks were mapped for the first time in the Valmalenco, and the results integrated into a consistent petrological model.
1. Introduction Between 1969 and 1970, three sheets of the Carta Geologica d"Italia were published at a scale In 1865, Gottfried Ludwig Theobald (1810-1869) of LIOO'OOO. These sheets, i.e. 7-18: Pizzo Berni- published the first coherent geological map of na-Sondrio (1970);9:Tirano (1969) and 8: Bormio Valmalenco, exactly 139 years before the present (1970) cover the whole Malenco area. The base map was printed in November 2004. Theobald's mapping in Valmalenco for these sheets was geological map comprised an area from the Swiss carried out at a scale of 1:25'000 by Montrasio and bv Engadine Valley down to the Valtellina Valley in Venzo and co-workers, who published an northern Italy. As topographic basis served sheet additional map in 1971 (Venzo et al., 1971). The XX, Sondrio-Bormio of the famous Dufour map, tectonic units given in these maps are essentially the which, at a scale of LIOO'OOO, was the first official same as in Staub's map, but a number of rock map of Switzerland. units had to be redefined, in particular for the A great step ahead was subsequently the crystalline basement rocks. Bernina map 1:50'000 by Rudolf Staub With Rudolf Staub's map and the Carta (published in 1946), which covers a major part of the Geologica d'Italia as a basis and with the introduction Malenco area and which was based upon litholo- of the concept of plate tectonics to Geology, the gical as well as on tectonic criteria. Many of the Malenco area received increasing attention. This crystalline rocks were given local names and not area contains one of the largest ultramafic masses pétrographie ones. Despite these shortcomings in the Alps, belonging to the Malenco unit, and a Staub's work remains a masterpiece ofAlpine well-preserved ocean floor suite, the Forno unit. geologic mapping and the effort of one man to map Both these units are sandwiched in between such an enormous area deserves high appreciation. crystalline nappes, respectively, derived from the Adriatic microplate and from the European plate.
1 Institut für Mineralogie und Pétrographie, ETH Zürich, CH-8092 Zürich. 2 IDPA-Istituto per la Dinamica dei Processi Ambientali, CNR Milano, Via Botticelli 23,1-20133 Milano. 3 Present address: The Australian National University, Miles Road, Acton Canberra, ACT 0200, Australia. 4 Present address: Institut für Geologie, Universität Bern, Baltzerstr. 1-3, CH-3012 Bern. Corresponding author
A calc-alkaline igneous complex, the Bergell In- the numbers given in the legend and correspondingly trusives, cross-cuts adjacent units during the on the map. For each tectonic zone, lithological Oligocène. The Valmalenco area has become an units are listed in their stratigraphie exceptional area for geological studies owing to its sequence. The various magmatic rocks of the Bergell large variety of rock types, including mantle peri- Intrusive are listed according to their temporal dotites, continental lower and upper crust, oceanic sequence of intrusion. crust and sedimentary rocks, all occurring As a result of the Alpine collision, continental together in a major suture zone of the Alpine upper crust with its sedimentary cover as well as orogeny. In order to understand the complex geologic continental lower crust and subcontinental mantle evolution of the Valmalenco area, detailed remapping are exposed in a restricted area together with of the area was required. oceanic crust and associated sedimentary rocks. Most of these rocks were deformed and metamorphosed during the Alpine orogeny. One goal of 2. Mapping procedure and criteria the mapping campaign was to reconstruct the original setting of these rocks. A schematic profile The map of Valmalenco 1:25'000 presented here of the reconstructed Lower Cretaceous paleogeo- (Montrasio et al., 2005) covers an area of about graphy of the Adriatic continental margin is 350 km2 and reaches from the Bernina mountains shown in the top left corner of the map. This at the Swiss-Italian border in the North to Son- reconstruction summarises the most important drio in the Valtellina Valley in the South, and from relationships between the major rock types (apart Val Poschiavo in the East to the area of Val Masi- from the younger Bergell intrusive rocks) of the no in the West. The map covers the entire exposure Valmalenco region (labelled with the same numbers of the Valmalenco ultramafic rocks. It is the as used in the legend and in the map) and result of a 35-year collaborative research between serves as a guide for a better understanding of the scientists from the Italian National Research pre-Alpine evolution of the rocks. In this profile, Center (CNR) and the University at Milano and the positions of future nappes are indicated, scientists from the Swiss Federal Institute of providing a link between the present day nappe stack Technology (ETH) and the University of Ziirich. Under and the reconstructed paleogeography. the guidance of the scientific coordinators, Due to the great structural complexity of the over fifty students were involved in the mapping region, strike and dip of the main foliation changes project, with Diploma and Ph.D. theses from over a very short distance. This makes it Ziirich and Tesi di Laurea from Milano, respectively. impractical to show such structural data on a map, All names of researchers involved in the which is already loaded with lithological details. detailed mapping campaign are listed on the map Instead, the main structural features are illustrated and the titles of their diploma and Ph.D. theses in a set of four cross sections at a scale of are given in the references. l:50'000.The traces of the cross-sections are The base mapping for the published sheet was indicated on the map. Section B-B' was extended outside performed throughout the area at a scale of the mapped area to the South to permit TIO'OOO. Some critical regions were re-mapped to correlation with section A-A' to the Insubric line. ensure consistency. The authors involved in the Additionally, the traces of major syn- and anti- scientific coordination and in compiling the map forms, which overprint the nappe stack, are shown supported the students and checked their work in on the tectonic map. Numerous late-stage faults the entire area. They also mapped the gaps with an offset of 10-100 m exist in the mapped remaining between individual thesis areas. After area. However, the various students have mapped their Ph.D. theses, the three compilers were these faults inconsistently and hence these structures employed for one year each to combine the individual are not reported on the map. base maps into a coherent 1:25'000 map sheet. For the mapping of the Quaternary, morphological Mapping has been based strictly on lithologie and stratigraphie criteria have been used. criteria, i.e. pétrographie, structural and litho- The limits of the glaciers correspond to the state stratigraphic observations. In the legend, individual of 1995, except for those of Vedretta di Scerscen lithological units are grouped in the order of inferiore and superiore, which reflect glaciation in their superposition and according to a tectonic 2002. The Quaternary has been given a consistent scheme, which is given in the lower left corner of picture by Lodovica Folladori. the map sheet. Boundaries between different Special symbols on the map show the locations tectonic zones are not shown on the main map. However, of active and abandoned mines and quarries, individual lithological units from the different because these localities have or had a profound tectonic units can easily be distinguished by economic impact on the population of Valmalenco. The Geological map ofValmalenco 3
On the other hand, localities, where collector- relics. A series of orthogneisses (14,19,29) of quality mineral specimens have been found, are presumably pre-Variscan age are associated with not shown; readers are referred to the excellent paragneisses and micaschists. A large body - the books by Bedogné et al. (1993,1995). M. Canale orthogneiss (28) - has been mapped The topographic basis of the map has been separately and is found in the southern part of the reproduced with the authorisation by swisstopo (nr. Bernina nappe. In the southern and eastern part BA 046150), as indicated on the map. The coordinates of the Margna nappe there are amphibole-bear- are the kilometer grids of the Swiss national ing gabbros and diorites (42) of alkaline affinity map.The map is fully digitised. that occasionally preserve magmatic textures and minerals, especially close to the type locality of Lago d'Arcoglio. No crystallisation age is available 3. The geological evolution of the Valmalenco for these gabbros.
In the following, we provide a brief overview on 3.1.2. Late and post-Variscan intrusives the variety of main rock types that occur in the Valmalenco and highlight their geologic history. The northernmost part of the map comprises the For this purpose the oldest rocks are described highest peaks of the Valmalenco area culminating first and then progressively younger rocks and in the Piz Bernina (4048 m). These peaks are units are discussed. The number of the rock type formed by magmatic rocks, which intruded into as it appears on the map will be put in parenthesis old, perhaps Proterozoic (von Quadt et al., 1994) when discussed in the text. basement rocks. In the Bernina nappe, these A detailed description of most rock types and intrusive rocks are slightly deformed and display a of the general geology of the area is given in the limited Alpine metamorphic overprint.They form comments to the Swiss Geologic Map 1:25'000, two chemically distinct suites. An older (333 Ma; sheet Piz Bernina (Spillmann and Trommsdorff, von Quadt et al., 1994) calc-alkaline suite intruded 2005,2006), which covers the area adjacent to and during a late stage of the Variscan orogeny and North of the Valmalenco map. In addition, rock consists of diorites (22) and granodiorites (23) types of the Bergell Intrusion, and the surrounding (Rageth. 1984; Spillmann and Büchi, 1993). This units, have been described by Schmid et al. suite is partly cross-cut by syenites (24), leu- (1996), the explanatory paper for the Bergell map cogranites (25) and alkali granites (26), which of Berger (1996). document a younger (296 Ma, von Quadt et al., 1994), post-Variscan alkaline magmatic activity 3.1. The crystalline basement nappes (Spillmann and Biichi, 1993). Meta-rhyolites (21) have an Early Permian crystallisation age The rock types of the basement nappes have been (288 Ma, von Quadt et al., 1994; Biichi. 1987) and subdivided into late- to post-Variscan magmatic display an alkaline affinity. They are regarded as rocks and older, polymetamorphic rocks. The the subvolcanic to effusive members of the alkaline Suretta nappe is discussed separately as it is intrusive suite. Similar meta-rhyolites (16, entirely overprinted by contact metamorphism of 31, 34, 38) occur in other Austroalpine nappes. the Bergell intrusion. Based on lithological correlations to the Bernina meta-rhyolites and the lack of pre-Alpine 3.1.1. Polymetamorphic rocks metamorphism, these rocks are collectively assumed to be of post-Variscan origin. In the Margna and 'lire polymetamorphic rocks occur in the South- Sella nappes, granodiorites (36) and granites (35, alpine and the Austroalpine nappes as well as in 39) belonging to the calc-alkaline suite are mostly the Suretta nappe. They consist of paragneisses transformed into orthogneiss. Only in the core and micaschists (8,13,18.27,32,37,43,44,61 with of large intrusive bodies such as in the chain associated amphibolites (15) and rare marbles. between Piz Glüschaint and Piz Sella (36) or in the Occasionally, pre-Alpine metamorphic relics, Cime di Musella (35), the magmatic minerals presumably related to the Variscan orogeny, are and textures are preserved. Occasionally, basal- preserved within paragneisses. The best example of tic-andesitic dikes (20, 33) with alkaline affinity such rocks can be found at M. Palino where the cross-cut the Late Variscan intrusives. The dikes outcrops of the preserved pre-Alpine metamorphic are 2-3 m thick (and thus are exaggerated on the schists are large enough to be mapped (37). map) and can be followed over several hundred Garnet-bearing amphibolites and calcsilicate meters. rocks often preserve pre-Alpine metamorphic 4 V. Trommsdorff et al.
3.1.3. The Suretta nappe lenco peridotites, 59) by a Permian gabbro, which intruded at -30 km depth. The gabbro displays a In terms of paleogeography, the Penninic Suretta tholeiitic differentiation trend and consists of nappe represents the Briançonnais realm predominantly Mg-rich gabbroic cumulates with a (Schmid et al., 1990) and is exposed in the western flaser texture (54) and finer-grained, more part of the mapped area. It underlies both the For- differentiated Fe-Ti-rich gabbros (55; Hermann et al., no and the Malenco units, and is discordantly cut 2001). A detailed description of the weakly by the Bergell Intrusives. In the mapping area, the metamorphosed ultramafic rocks (59; spinel peridotites, rocks of the Suretta nappe occur primarily as garnet pyroxenites, dunites) is given in Müntener isolated xenoliths within Bergell tonalité and grano- and Hermann (1996), and the distribution diorite. These xenoliths, like those of the Forno of pre-Alpine structures and their relationship to unit, have various dimensions (10 cm to 100 m Alpine structures is given in Hermann and Müntener across), and their spatial arrangement is (1996). The gabbro intrusion caused partial systematic, indicating only minor displacement of the melting of the country-rock metapelites, and xenoliths during magma intrusion (Drescher- some of the resulting leucogranites reach a map- Kaden and Storz, 1926; Gyr, 1967; Gieré, 1984, pable size (58). U-Pb determinations on single 1985; Puschnig, 1996). A kilometer-sized mappa- zircons from differentiated gabbros yield an age ble block of Suretta basement and cover is of 281119 Ma. Zircons separated from leucogranites preserved at Cima di Vazzeda. Further to the South, yielded an upper intercept age of 278±2.6 Ma from Monte Disgrazia to Valle di Preda Rossa, the (Hansmann et al., 2001). Ion microprobe dating of Suretta nappe is preserved as a very thin band zircon from a kyanite-garnet-plagioclase granu- along the contact between Bergell tonalité and lite resulted in apparent ages of inherited cores Malenco unit (Gieré, 1985; Pfiffner, 1992; Weiss, between 520 and 3050 Ma and of three metamorphic 1992; Pfiffner and Weiss, 1994). This attenuated overgrowths at 280+5, 270±5, and 258±4Ma band runs parallel to the Preda Rossa shear zone, (Hermann and Rubatto, 2003). Monazite records a major syn-magmatic shear zone, which follows the same metamorphic ages within error. These and deforms the margin of the pluton (Berger and data indicate that granulite-facies metamorphism Gieré, 1995; Berger et al., 1996). lasted for at least 20 m.y. For the subsequent 30- Within the map area, biotite schist and gneiss, 50 m.y. neither pétrographie nor geochronological metapelite, and some amphibolite of alkali basaltic data could be obtained for the granulites of the composition are the predominant rock types in crust-mantle transition. It is possible that the the polymetamorphic Suretta basement (Wenk, crust-mantle transition was stable into late Triassic. 1974; Wenk et al., 1974; Gieré, 1985; Pfiffner and Weiss, 1994). The sedimentary cover (60) of the Within the core of the Margna nappe (see profile Suretta basement (61) comprises metaconglomer- C-C') an association of gabbros (40, 41) and ate and fine-grained quartzite, probably of Per- lower crustal granulites (44) occurs and is identical mo-Triassic age (Staub 1918, 1921), followed to the lower crustal section exposed in the stratigraphically by thick beds of pure and impure Malenco nappe. However, the Margna rocks calcite and dolomite marbles (Gyr, 1967; Bucher, display a much higher degree of Alpine metamorphic 1977; Gieré, 1985; Pfiffner and Weiss, 1994). The overprint and only locally the pre-Alpine presumably Triassic marble sequence also textures, and - even more rarely - the granulite- contains intercalations of green amphibolite, which facies minerals are preserved (Bissig and probably represents former tuffs, tuffites or sills Hermann, 1999). (Gieré, 1985). All Suretta rock types occurring in the area covered by the map exhibit a distinct 3.3. Rifting and denudation of Malenco mantle contact metamorphic overprint. and continental margin formation
3.2. Preserved Permian continental Retrograde hydration of Malenco granulite crust-mantle transition assemblages was triggered by the influx of aqueous fluids and caused formation of amphiboles and The Malenco unit, which contains a preserved other hydrous minerals at 0.9 GPa and 650 °C Permian crust-mantle transition NW of Chiesa (Müntener et al., 2000). This hydration event and S of Chiareggio, is predominant in the map marks the starting point of a nearly isothermal area (Trommsdorff et al., 1993; Müntener and decompression that is related to the exhumation Hermann, 1996; Hermann et al., 1997; Hansmann of lower crustal rocks and the attached subcontinental et al., 2001). Lower crustal metapclitic rocks (58) Malenco mantle. Dating of early amphiboles are welded to upper mantle ultramafic rocks (Ma¬ by Villa et al. (2000) resulted in Late Triassic The Geological map ofValmalenco 5 to Early Jurassic ages. These ages indicate that starts with detritic rocks of presumably Upper hydration and concomitant exhumation mark the Permian to Triassic age, which are mapped as beginning of rifting, which ultimately led to the quartzite intercalated with marbles and mica- separation of the Adriatic and European continents. schists (12). These rocks are overlain by dolomite The exhumation of the lower crustal rock marble, locally accompanied by cornieules (11) association and the subcontinental Malenco mantle originating from a Triassic shallow marine shelf was initiated by an extensional fault system region. Quartz-rich calcschists (10) locally contain dipping towards the Adriatic continent (Spill- dolomite breccias and presumably represent mann, 1993; fJermann and Müntener, 1996). A Lower Jurassic syn-rift sediments documenting remnant of this fault system - the Margna Normal the fragmentation of the Triassic platform in an Fault - is exposed between the Fedoz Gabbro extensional setting. The Upper Jurassic to Lower (40) and the Margna Orthogneiss (39). This fault Cretaceous post-rift sequence consists of quartz- excised 10 to 20 km of mostly intermediate to lower schists with local Mn-rich horizons, calcite marbles crust and accommodated a displacement of -50 and calcschists (9). Lithologically similar kilometers (Hermann and Müntener, 1996; Bissig quartzschists, micaschists and quartzites (45, 46, and Hermann, 1999). Extensional low angle faulting 47, 48) - although metamorphosed to higher in the lower crust is linked to, and contemporaneous grade due to the Bergell intrusion - form the with, rift-related basins in the upper crust sedimentary cover of the Forno Unit metaba- (Spillmann, 1988. 1989, 1993; Zingg, 1988; Müntener salts (49). A schematic distribution of different and Hermann, 2001), as schematically sedimentary profiles along the ocean-continent shown in the paleogeographic reconstruction in transition is given in the Lower Cretaceous the top left corner of the map. Along the fault paleogeographic reconstruction. It is important system, ductile shear zones (Hermann and Müntener, to note that the Mesozoic sedimentary rocks that 1996; Müntener and Hermann, 2001) and later are listed in the legend in the Malenco Unit (51, breccias developed with components of crustal 52, 53) are not in stratigraphie contact with the and mantle rock, analogous to the situation along serpentinites, but rather represent an alloch- the Err-Platta nappes (Manatschal and Nie- thonous block of continental sedimentary rocks vergelt, 1997) and the Galicia margin (Boillot et tectonically emplaced on top of the exhumed al., 1995).The Malenco breccias (59) were named mantle rocks during rifting. Ur-breccia after the type locality Pass d'Ur. The exhumation of Malenco mantle was accompanied 3.5. Forno unit, and its relation to the by (a) low-temperature (<300 °C) serpentinisa- Malenco unit, ophicarbonates tion of Malenco ultramafic rocks with the formation of chrysotile (now included as relics in anti- The Forno unit consists of a suite of metamorphosed gorite, Mellini et al., 1987); (b) intrusion of basaltic mid ocean ridge-type basalts with locally dikes and sills of the Forno unit into the preserved pillow lavas, pillow breccias, and Fe- Malenco mantle; and (c) formation of ophicarbonate Cu-Zn mineralisations (49; Montrasio, 1973; Fer- rocks as breccias and debris flows on top and in rario and Montrasio, 1976; de Capitani et al., 1981; fracture zones of the Malenco mantle. Rodingiti- Peretti, 1985; Puschnig, 2000). Dikes of Forno sation of mafic dikes and ophicarbonate breccias basalts intrude the Malenco ultramafic rocks at Alpe bear a marine stable isotope signature (Burkhard Zocca (Ulrich and Borsien, 1996). A large amphi- and O'Neil, 1988; Benning, 1990; Pozzorini and bolite body SW of P. Cassandra represents a Früh-Green, 1996) indicating oceanic metasomatism metamorphosed sheeted dike complex of Forno and metamorphism. basalts, emplaced within the Malenco ultramafic rocks. Numerous smaller dikes associated with 3.4. Permo-Mesozoic sedimentary rocks the complex are mapped, and these dikes often display an intense rodingitisation, indicating The Permo-Mesozoic sedimentary rocks mark oceanic metasomatism of mafic rocks during serpen- the boundaries between the different basement tinisation of the peridotites (Puschnig, 2000). A nappes. These sedimentary rocks document the metasedimentary sequence of presumably Late formation of theTethys ocean basin due to the Jurassic to Early Cretaceous age is associated to separation of Africa and Europe during the Mesozoic. the Forno metabasalts and comprises meta- The sedimentary rocks can be subdivided into a quartzites with Mn-mineralisations (48), garnet- pre-rift, syn-rift and post-rift sequence, and their andalusite-biotite schists (47), quartzites (46) and ages are inferred from lithostratigraphic correlation calcareous micaschists (45). Most Forno rock with non-metamorphic Austroalpine types covered by the geological map exhibit a sedimentary rocks. The pre-rift sedimentary sequence strong contact metamorphic overprint. The in- 6 V. Trommsdorff et al.
ferred age of the meta-sedimentary rocks is based Mn-mineralisations (63). This unit is possibly on lithological correlation to weakly metamorphosed derived from the ocean-continent transition of the Penninic sedimentary rocks covering the Briançonnais realm as evidenced by a similar Platta unit in the NW of the mapped area (see metasedimentary sequence in the Penninic Suret- tectonic map in the lower left corner). ta nappe (Montrasio, 1984; Gieré, 1985). A distinct boundary between the Malenco and Forno units cannot be located in the field. They 3.7. Bregaglia (5). Basaltic to andesitic dikes (7), which are crops out in the Valle Scerscen and forms the presumably related to the Bergell intrusion, are peaks of P. Malenco and P. Tremoggia, linking the distributed over a wide area in Valmalenco (Ban- Margna and the Sella crystalline basements (profile gerter, 1978; Gautschi and Montrasio, 1978). C-C'). The thickness of the Margna nappe As for the depth of intrusion, Reusser (1987) and the sedimentary syncline is drastically determined the solidus pressure of the Bergell reduced from the West to the East (profile C-C'), tonalité in the map area as 0.5 GPa and 60 km due to large-scale, post-thrusting low angle normal farther to the West as 0.75 GPa. These values correspond faulting and stretching of the nappe stack to a difference in depth of 9 km over which (Hermann and Müntener, 1992, Spillmann, 1993). the intrusion can be studied. Also in the West, In fact, not all of the present nappe boundaries Oberli et al. (2004) have shown that the tonalitic display exclusively thrust-related structures. melt persisted over a period of ~5 m.y., suggesting Along most of the nappe boundaries (Bernina- that it must have been a major controlling factor Corvatsch, Corvatsch-Sella, Bernina-Sella), that for the Oligocene evolution of the adjacent Insub- contain only relics of Mesozoic sedimentary rocks ric line. (see tectonic inset), shear zones with kinematic indicators that point to a top-to-the-E sense of movement are observed (Spillmann, 1993). These 4. Alpine deformation and metaniorphisin shear zones are interpreted as low-angle normal faults related to a Cretaceous extensional event Two Alpine orogenic cycles affected the former postdating nappe stacking. It is possible that the passive margin. An E-W directed thrusting and a sudden termination of the Sella nappe in the subsequent extension of Late Cretaceous age southern part of the mapped area is related to produced the South Penninic-Austroalpine nappe such a shear zone. stack (Hermann and Müntener, 1992; Spillmann. This first orogenic cycle is responsible for the 1993; Froitzheim et al., 1994; Handy et al., 1993; main Alpine metamorphism observed in the Handy, 1996; Villa et al„ 2000). Younger S-direct- Austroalpine and South-Penninic units.The metamorphic ed thrusts and E-W trending folds, related to the conditions increase from lower greenschist- Tertiary continent-continent collision, overprinted facies conditions in the Bernina nappe to epidote- the Cretaceous nappe stack (e.g. Handy et al., amphibolite conditions in the Margna nappe and 1993,1996; Froitzheim et al., 1994; Hermann and in the Malenco and Forno units. Maximum Müntener, 1992), but in general did not disrupt temperatures (Trommsdorff and Evans, 1974; Mellini the continuity of the Cretaceous structures in the et al., 1987;Trommsdorff and Connolly, 1996; Per- cross sections C-C' and D-D', respectively. The etti, 1988; Benning and Sidler, 1992) and pressures tectonic evolution of the South-Penninic and Aus- (Bissig and Hermann, 1999) are around 450-500 troalpine units differs from that of the underlying °C and 0.5 GPa. These values indicate that the middle Penninic units (Briançonnais: Tambo and rocks derived from the Adriatic margin of the Pie- Suretta nappes. Tertiary flysch), where only the montese ocean basin, now exposed in Val Malenco, Tertiary evolution has been preserved (Liniger were only moderately buried during Alpine and Nievergelt, 1990; Puschnig, 1996). metamorphism in the late Cretaceous (Villa et al., The main deformation in the Austroalpine and 2000). South-Penninic units is related to W-directed In the Malenco-Forno and the overlying thrusting and nappe stacking. The position of the Austroalpine units N-S shortening is documented in nappes was probably already predestined in the S-vergent folds and shear zones (attributed to a Cretaceous by fault-bounded blocks resulting first phase of backfolding; Sidler and Benning, from Jurassic rifting (see paleogeographic 1992; Hermann and Müntener, 1992). This profile). The Alpine deformation generally increases deformation may be linked to the formation of the with increasing depth in the nappe stack. In the main foliation in the underlying Tambo and uppermost Bernina nappe, deformation is Suretta units, which affected Tertiary flysches. concentrated close to the nappe boundaries or occurs However, the contrasting style of deformation in discrete shear zones within the nappe itself, indicates that the Austroalpine, together with the whereas in the lower units of the Bernina nappe, uppermost Penninic ophiolitic units, acted as an and in the Margna nappe and Malenco unit, only 'orogenic lid' suffering only minor deformation. rare high-grade metamorphic rocks or primary Crustal thickening was later followed by orogen- magmatic textures escaped pervasive Alpine parallel extension (Turba mylonite; Nievergelt et deformation. Alpine thrusting resulted in W-facing al., 1996; Froitzheim et al., 1994), which was sedimentary synclines that separate the basement strongly localised in the Austroalpine and South- blocks. The most prominent of these synclines Penninic units, but more penetrative in the Bri- 8 V. Trommsdorff et al. ançonnais domain (Liniger and Nievergelt, 1990; vertical stretching lineation (Berger et al., 1996). Huber and Marquer, 1996). A shear zone at the A vertical offset of several kilometers (tonalité base of the Mesozoic sedimentary rocks between up) has been inferred, and this deformation is Margna and Sella nappes (Btta. delle Forbici) is likely responsible for the verticalisation of the likely related to this deformation phase second backfolding in this area (Puschnig, 1998). (Hermann and Müntener, 1992). A potential southeast A major brittle fault - the Muretto line - can be directed extension of the Turba mylonite followed from the Passo di Muretto towards zone could be responsible for the drastic thinning Chiareggio, where it disappears (see tectonic of the Malenco unit at its southern border inset). The Insubric Line, a major Alpine tectonic (Puschnig, 1998).The Malenco unit is reduced to a boundary separating the Southalpine from the few hundred meters between the Suretta nappe Alpine nappes (see tectonic inset), crops out just and the Margna nappe at Sasso Arso and between N of Sondrio. the window of St. Anna and the Margna nappe (profile B-B'). Ongoing N-S convergence produced large- 5. Contact metamorphism and metasomatism scale, E-W trending open folds (attributed to a second phase of backfolding) and a pronounced Mapping of the mineral assemblages of ultrama- steepening of the nappe stack towards the South. fic and ophicarbonate rocks (59) in the contact This deformation phase produces prominent aureole of the SE Bergell tonalité (3) has led to an features of the map. It is responsible for the tectonic exceptionally well constrained picture of thermal window of Penninic rocks within the Austroalpine metamorphism. The contact aureole thus has be- j nappe stack. Traces of the axial planes of these come an excellent example of a natural laboratory. folds are shown on the tectonic scheme in the lower For the first time, reactions occurring during I left corner of the map and the general feature prograde dehydration of serpentinite were of the deformation is visible in the N-S profiles derived from field observations, and the B-B' and D-D'. This deformation is cut by the corresponding isograds were mapped (Trommsdorff Blankenburg, Oligocène Bergell tonalité (32 Ma, Von and Evans, 1972). Along the first isograd, the re- ; 1992; Spillmann, 1993; Puschnig, 1996). The gional metamorphic assemblage serpentine + di- last regional deformation caused NE-SW trending opside is replaced by tremolite + olivine (-(-chlorite); folds (transverse folding). Interference of the coincident with this reaction, titanian clinohu- transverse folding with the second phase of back- mite reacts to form olivine + ilmenite (Trommsdorff folding forms a dome and basin structure (see and Evans, 1980). Along the second isograd, inserted tectonic map). The Lanzada window talc + olivine was formed at the expense of anli- exposing the Penninic Lanzada-Santa Anna unit is gorite. This isograd is easily recognised in the situated in such a dome structure (see profile A- field, and therefore, the high-temperature side of A', D-D'). On the other hand, the Margna klippen the isograd is marked in the map (59 with spotted exposed at Mt. Roggione and at Chiesa are situated pattern). Trommsdorff and Evans (1977) further in basins. The Bergell tonalité and its contact mapped isograds for the devolatilisation of aureole are partially overprinted by the transverse serpentine + carbonate rocks and derived a folding. However, the younger granodiorite corresponding phase diagram from their field observations. (30 Ma, Von Blankenburg, 1992) cuts this The phase diagram was later refined deformation phase (Puschnig, 1996). The observation (Trommsdorff and Connolly, 1990) and served to extract that the main structures are older than the Bergell thermodynamic data for some of the minerals. All intrusives is also confirmed by discordant, Eocene this information was then integrated into a thermal to Oligocene andesitic to basaltic dikes (7). These model (Trommsdorff and Connolly, 1996), dikes are largely undeformed, cross-cut the main which indicates that temperature increases from foliation of the country rocks, and are younger -350 °C outside the aureole to -570 °C at the contact. than the regional metamorphism (Nievergelt and Pressure of contact metamorphism was Dietrich, 1977), but they are overprinted by contact determined at -0.35 GPa, which is somewhat lower metamorphism within the aureole of the than the pressure derived for the Bergell tonalité Bergell tonalités (Gautschi and Montrasio, 1978; solidus (0.5 GPa. Reusser. 1987). Gautschi, 1980). The Bergell contact metamorphism was At the SE border of the Bergell, the Preda- accompanied by widespread metasomatic activity. Rossa shear zone occurs between the tonalité and In particular, the carbonate rocks of the upper Val the gneisses of the Suretta nappe (Berger and Sissone, which belong to the middle Penninic Gieré, 1995). The tonalité displays an intensive Suretta nappe, contain abundant hydrothermal subsolidus deformation in this area and shows a veins with variable mineral assemblages (Bucher, The Geological map ofValmalenco 9 1972,1977; Ried, 1994). Some of these veins document Berger, A. (1996): Geological-tectonic map of the Ber- rocks (Southern metasomatic of gell pluton and surrounding country high-temperature transport Switzerland, Northern Italy). Supplement to titanium, zirconium, rare earth elements and acti- Schweiz. Mineral. Petrogr. Mill. 76/3. nides (Gieré. 1986; Ried, 1994). Berger, A. and Gieré, R. (1995): Structural observations at the eastern contact of the Bergell Pluton. Schweiz- Mineral. Petrogr. Mitt. 75,241-258. A., Rosenberg, C. and Schmid, S.M. (1996): As¬ Acknowledgements Berger, cent, emplacement and exhumation of the Bergell pluton within the Southern Steep Belt of the Central and The authors would like to thank all the people Alps. Schweiz. Minerai Petrogr. Milt. 76,357-382. Institutions this First who contributed to mapping project. of Binaghi, F. (1984): Studio petrografico e strutturale della students all, the numerous diploma and doctorate are regione tra la media Val Malenco e la Val have compresa greatly acknowledged. The project could never di Togno, con particolare riguardo agli Gneiss del M. been completed wïthout their contributions. Mapping Canale (falda Bernina, Austroalpino inferiore) was generously supported by ETH Ziirich, the University (Prov. Sondrio).Tesi di Laurea, inedita; 1st. Mineral. of Milano, the Schweizerischer Nationalfonds (SNF). Petrogr. Université di Milano. di the municipality and CNR Italia. The Provincia Sondrio, Bissig, T. (1997): Geologische Untersuchungen im pen- of Sondrio. and the Comunità Montana Valtellina ninisch-ostalpinen Grenzbereich am Bergell Ost- di Sondrio have generously contributed to the printing rand (Alpe dell'Oro, Val Malenco, N-Italien). the swiss- costs of the map. We also would like to thank Unpublished Diploma thesis ETH Zürich. topo (Swiss Federal Office of Topography) for providing Bissig, T. and Hermann. J. (1999): From pre-Alpine ex¬ basis the We thank Re- the topographic of map. Eric tension to Alpine convergence: the example of the the usser and Martin Engi for corrections of manuscript. southwestern margin of the Margna nappe (Val least sincere thanks to the of Last but not our go people Malenco, N-Italy). Schweiz. Mineral. Petrogr. Mitt. 79, Valntalenco for their continuous hospitality, generosity 363-380. and friendship to all of us. The completion of this mapping Boillot, G., Beslier, M.O., Krawczyk, C.M., Rappin, D. project, which took more than 30 years until and Reston, T.J. (1995): The formation of passive publication, is overshadowed by the unexpected decease of margins: constraints from the crustal structure and the first author in June 2005. segmentation of the deep Galicia margin, Spain. In: Scrutton, R.A.et al. (eds):The tectonics, sedimentation and palaeoceanography of the North Atlantic References region. Geol. Soc. of London Spec. Puhl. 90,71-91. Borsien, G.-R. (1995): Petrographische Untersuchungen Aemisegger, B. (1976): Der Nordrand des Malenco-Ser- im oberen Val Malenco (Val Ventina, Provinz pentinites zwischen Piz Fora und Sassa d'Entova. Sondrio, N-Italien). Teil II: Alpe Zocca. Unpublished Unpublished Diploma thesis ETH Ziirich. Diploma thesis ETH Zürich. Avogadro, A. (1990): Studio geologico e petrografico del Broggi, L. (1983): Studio geologico-petrografico del plutone di Triangia (SO), con particolare riferimen- cristallino della falda Margna e delle serpentiniti del to ai rapporti con le metamorfici Austroalpine incas- complesso ofiolitico della Valmaienco nella Zona santi. Tesi di Laurea, inedita; 1st. Mineral. Petrogr. del Pizzo Scalino (Valmaienco - Sondrio). Sottotesi Université di Milano. di rilevamento inedita. Dip. Sei. Terra, Université di Bangerter, G. (1978): Strukturen und Metamorphose im Milano. Grenzgebiet des Malenco-Serpentins und der Mar- Broggi, L. (1983): II Ghiacciaio del Pizzo Scalino gna-Decke im Gebiet um den Monte Braccia. nell'Olocene e nel Tardo-Glaciale. Tesi di Laurea, Unpublished Diploma thesis University of Zürich. inedita; 1st. Mineral. Petrogr. Univ. Milano Baumer, M. 1993): Studio délia petrografia e délia geo- Bucher, K. (1972): Geologische-petrographische Unter¬ logia strutturale attorno al Monte Roggione (Val- suchungen am nördlichen Kontakt des Malencoserpentins malenco, Prov. Sondrio, Italia). Unpublished Diploma (Zentraler Teil). Unpublished Dipioma thesis thesis ETH Zürich. ETH Zürich. Bay, F. (1997): Strukturelle und petrographische Unter¬ Bucher, K. (1977): Hochmetamorphe Dolomitmarmore suchungen am Südrand des Malencokörpers. Valle und zonierte metasomatische Adern im oberen Val Airale, Prov. Sondrio, N-Italien. Teil III: Zone Cal- Sissone (Provinz Sondrio, Norditalien). PhD thesis denno. Unpublished Diploma thesis University of ETH Zürich, No. 5910. Zurich. Büchi, H.J. (1987): Geologie und Pétrographie der Bedogné, F., Montrasio, A. and Sciesa, E. (1993): 1 Min- Bernina IX: Das Gebiet zwischen Pontresina und erali delja Provincia di Sondrio. Valmaienco. Bettini, Morteratschgletscher. Unpublished Diploma thesis Sondrio, 275 pp. University of Zürich. Bedogné, F.. Maurizio, R.. Montrasio, A. e Sciesa, E. Burkhard, D.J.M. and O'Neil, J. (1988): Contrasting ser- (1995): I Minerali della Provincia di Sondrio e della pentinization processes in the eastern Central Alps. Bregaglia Grigionese. Val Bregaglia, Val Masino, Val Contrib. Mineral. Petrol. 99,498-506. Codera e Valle Spluga. Bettini, Sondrio, 300 pp. Casati, F. (1997): 11 Tardiglaciale nel settore orientale Benning, L.G. (1990): Metamorphose und Struktur im dell'Alta Valmaienco. Tesi di Laurea inedita. Ist. Sei Malenco-Serpentinit und in der Margna- und Sella- Naturali, Université di Milano. Decke, zwischen Pass d'Ur und Pizzo Scalino (Val Caciano, G.I. (1993): Le variazioni Oloceniche del Ghi¬ Malenco, Prov. di Sondrio, Italien) (Teil 2). Unpublished acciaio di Caspoggio. Tesi di Laurea inedita. Ist. Sei. Diploma thesis ETH Zürich. Naturali, Université di Milano. Benning, L.G. und Sidler, D.M. (1992): Pétrographie der Cazzaniga, S. (1997): II Tardiglaciale nel settore centrale Margna- und der Sella-Decke und des Malenco- dell'Alta Valmaienco. Tesi di Laurea inedita. Ist. Sei. Serpentinites zwischen Pass d'Ur und Pizzo Scalino Naturali, Université di Milano. (Val Malenco, Provinz Sondrio, Italien). Schweiz- Cornelius, H.-P. (1915): Geologische Beobachtungen in Mineral. Petrogr. Mit!. 72,213-224. den italienischen Teilen des Albigna-Disgrazia- 10 V. Trommsdorff et ai. massivs. Ceol. Rundschau VI, 3,166-177. implications for mobility ofTi, Zr and REE. Contrib. de Capitani L., Ferrario, A. and Montrasio, A. (1981): Mineral. Petrol. 93,459^170. Metallogeny of the Malenco metaophiolithic complex, Gieré, R.. ed. (1996): Transition from Penninic to Aus¬ Central Alps. Ofioliti 6.87-100. troalpine units in the Bergell Alps. Schweiz. Minerai Diethelm, K.-H. (1984): Geologie und Petrog. aphie des Petrogr. Mitt.. Special Volume 76/3. Bergell-Ostrandes II. Unpublished Diploma thesis Gigliotti, L.T. (1997): Il Tardiglaciale nel settore occi¬ ETH Zürich. dentale dell'Alta Valmalenco Tesi di Laurea inedita. Diethelm, K.-H. (1989): Petrographische und geoche- Ist. Sei. Naturali, Università di Milano. mische Untersuchungen an basischen Gesteinen der Guerra, S. (1992): Studio geologico-petrografico e strut- Bergeller Intrusion (Graubünden, Schweiz/Provinz turale dell'alta Val di Togno (Val Malenco, Prov. Sondrio, Norditalien). PhD thesis ETH Zürich, No Sondrio). Tesi di Laurea inedita. 1st. Mineral. Petrogr. 8855. Università di Milano. Drescher-Kaden, F.K., and Storz, M. (1926): Ergebnisse Gyr.T. (1967): Geologische und petrographische Unter¬ petrographisch tektonischer Untersuchungen im suchungen am Ostrand des Bergeller Massivs. Ph.D. Bergeller Granit. Neues Jahrbuch Mineralogie 54 thesis University of Zürich. Abt. A, 284-291. Handy, M.R.. Herwegh, M. and Regli, R. (1993).Tekto- Engi, M. (1973): Strukturen und Metamorphose im nische Entwicklung der westlichen Zone von Kontaktbereich des südwestlichen Malenco. Samedan (Oberhalbstein, Graubünden, Schweiz). Unpublished Diploma thesis ETH Zürich. Eclogue Geolog. Helv. 86.785- 818. Ferrario, A. and Montrasio, A. (1976): Manganese ore Handy, M.R., Herwegh, M., Kamber, B.S.,Tietz. R. and deposits of Monte del Forno. Its stratigraphie and Villa, I.M. (1996). Geochronologic, petrologic and structural implications. Schweiz. Mineral. Petrogr. kinematic constraints on the evolution of the Err- Mi«. 56,377-386. Platta boundary, part of a fossil continent-ocean Folladori, L. (1992): Variazioni Oloceniche dei Ghiacciai suture in the Alps (eastern Switzerland). Schweiz- di Scerscen Superiore e di Scerscen Inferiore. Tesi di Mineral. Petrogr. Mitt. 76,453^174. 1st. Laurea inedita. Sei Naturali, Università di Milano. Handy, M.R. (1996): The transition from passive to ac¬ Folladori, L. (1997): Global change. Studio delle variazi¬ tive margin tectonics: a case study from the Zone of oni climatiche ed ambientalfOloceniche nelle Alpi Samedan (eastern Switzerland). Geologische Rundschau Lombarde, mediante indagini geomorfologiche e 85,832-851. glaciologiche. In: "1000 giorni di ricerca il Lombar- Hansmann. W. Müntener, O. and Hermann, J. (2001): U- dia. Relazioni finali delle borse di formazione 1994/ Pb zircon geochronology of a tholeiitic intrusion and 1996". A cura di Elisabetta Tromellini. Libri delta associated migmatites at a continenal crust-mantle Fondazione Lombardia per VAmbiente 30, 100-123. transition, Val Malenco, Italy, Schweiz. Mineral. Folladori, L. (1999): Come i ghiacciai della Valmalenco Petrogr. Milt. 81,239-255. hanno registrato le variazioni climaticoambientali. Hermann, J. (1991): Geologische Untersuchungen süd¬ Pub il ghiacciaio conservare informazioni sulla lich der Bernina-gruppe, Oberstes Val Malenco. chimica dell'atmosfera? In: "Studi geografici e geo- Prov. Sondrio, N-Italien,Teil I. Unpublished Diploma logici in onore di Severino Belloni a cura di thesis ETH Zürich. Giuseppe Orombelli - Glauco Brigati-Genova 1999. Hermann, J. (1997). The Braccia gabbro (Malenco, Fornera, T. (1996): Ricerche strutturali e petrografiche Alps): Permian intrusion at the crust to mantle interface lungo il margine méridionale del corpo ultramafico and Jurassic exhumation during rifting. PhD del Malenco (Valle Airale. provincia di Sondrio, thesis ETH Zürich, No. 12102. Nord-ltalia). Parte II: Zone Mastabia. Unpublished Hermann, J. and Müntener, O. (1992): Strukturelle Ent¬ Diploma thesis ETH Zürich. wicklung im Grenzbereich zwischen dem penni- Froitzheim, N„ Schmid, S.M. and Conti, P. (1994): Re¬ nischen Malenco-Ultramafitit und dem Unterost- peated change from crustal shortening to orogen- alpin (Margna- und Sella-Decke). Schweiz. Mineral. parallel extension in the Austroalpine units of Petrogr. Mitt. 72,225-240. Graubünden. Eclog. geol. Helv. 87,559-612. Hermann, J. and Müntener, O. (1996): Extension-related Gautschi, A. (1976): Geologisch-petrographische Unter¬ structures in the Malenco-Margna-system: implications suchungen am Südrand des Malencoserpentinites for paleogeography and consequences for rifting (Prov. Sondrio, Nord-ltalien). Unpublished Diploma and Alpine tectonics. Schweiz. Mineral. Petrogr. thesis ETH Zürich. Mitt. 76.501-520. Gautschi, A. (1980): Metamorphose und Geochemie der Hermann, J., MUntener, O.,Trommsdorff, V., Hansmann basischen Gesteine des Bergeller Ostrandes. PhD W. and Piccardo, G.B. (1997): Fossil crust to mantle thesis ETH Zürich, No. 6672. transition, Val Malenco (Italian Alps). J. Geopliv. Gautschi, A. und Montrasio, A. (1978): Die andesitisch- Res. 102,20123-20132. basaltischen Gänge des Bergeller Ostrandes und Hermann, J., Müntener, O. and Günther, D. (2001): Dif¬ ihre Kontaktmetamorphose. Beziehung zur Regional- und ferentiation of mafic magma in a continental crust- Schweiz. Mineral. Petrogr. Mitt. 58, 329- to-mantle transition zone. / Petrol 42,189-206. 343. Hermann, J. and Rubatto, D. (2003): Relating zircon and R. Gerber, (1966): Der Malencoserpentinit östlich des monazite growth zones: age and duration of granu- Passo d'Ur. Unpublished Diploma thesis ETH lite facies metamorphism in the Val Malenco lower Zürich. crust./. Metam. Geol. 21,833-852. Gieré, R. (1984): Geologie und Pétrographie am Ost¬ Honegger, K. (1977): Der Malencoserpentinit und seine rand des Bergeller Plutons. Unpublished Diploma Rahmengesteine östlich der Kette Pizzo Cassandra- thesis ETH Zürich. Pizzo Rachele. Unpubl. Diploma thesis ETH Zürich. Gieré, R. (1985): Metasedimente der Suretta-Decke am Huber, R.K. and Marquer. D. (1996).Tertiary deforma¬ Ost- und Südostrand der Bergeller Intrusion: Litho- tion and kinematics of the southern part of the Tam- stratigraphische Korrelation und Metamorphose. bo and Suretta nappes (Val Bregaglia, Eastern Swiss Schweiz. Mineral. Petrogr. Mitt. 65,57-78. Alps). Schweiz. Mineral. Petrogr. Mitt 76,383-397. Gieré, R. 1986): Zirconolite, allanite and hoegbomite in Lardelli.T. (1981): Die Tonale Linie im unteren Veltlin. a marble skarn from the Bergeil contact aureole: PhD thesis University of Zürich. The Geological map ofValmalenco Liniger, M. und Nievergelt. P. (1990). Stockwerktek- peridotite in embryonic ocean basins: trace element tonik im südlichen Graubünden. Schweiz Mineral. and Nd-isotopic evidence and implications for crust- Petrogr. Mitt. 70.95-101. mantle relationships. Earth Planet. Sei. Lett. 221. Locati.U. (1990): Studio petrografico e petrochimicodel 293-308. plutone diTriangia (SO). Tesi di Laurea, inedita; 1st. Nievergelt, P. und Dietrich. V. (1977): Die andesitisch- Mineral. Petrogr. Université di Milano. basaltischen Gänge des Piz Lizun (Bergell). Mariconti, M. (1990): Studio geologico-strutturale della Schweiz. Mineral. Petrogr. Mitt. 57,267-280. Val Torreggio (SO) e studio dei metagabbri Nievergelt, P., Liniger, M., Froitzheim, N. and Ferreiro dell'Arcoglio. Tesi di Laurea, inedita: 1st. Mineral. Mählmann, R. (1996): Early to Mid Tertiary crustal Petrogr. Université di Milano. extension in the central Alps: The Turba mylonite Meciani, L. (1990): Studio geologico-strutturale della zone (Eastern Switzerland). Tectonics 15,239-340. Val Torreggio (SO) e studio dei metagabbri Oberli, F., Meier, M., Berger, A., Rosenberg, C, and Gi¬ dell'Arcoglio. Tesi di Laurea, inedita; Ist. Mineral. ere, R. (2004): U-Th-Pb and 230Th/23sU disequilibrium Petrogr. Université di Milano. isotope systematics: Precise accessory mineral Mellini, M„ Trommsdorff, V. and Compagnoni, R. chronology and melt evolution tracing in the Alpine (1987): Antigorite polysomatism: Behaviour during Bergell intrusion. Geochirn. Cosmochirn. Ada 68. progressive metamorphism. Contrib. Mineral. Petrol. 2543-2560. 97. 147-155. Oterdoom, W.H. (1975): Geologische-petrographische Mercolli, I. (1973): II margine sud-occidentale delle ser- Untersuchungen des Malenco-Serpentins im Val pentiniti delle Valle Malenco. Unpublished Diploma Scerscen. Unpublished Diploma thesis ETH Zürich. lliesis ETH Zürich. Paglia, C. (1996): Ricerche strutturali e petrografiche Moiolo. A. 1981 ): Studio petrografico e strutturale della lungo il margine méridionale del corpo ultramafico regione ad ovest di Torre S. Maria (media Valmalenco), del Malenco. Parte I: Zona Cassandra. Unpublished con particolare riferimento all'unita Bernina. Diploma thesis ETH Zürich. Tesi di Laurea, inedita; Ist. Mineral. Petrogr. Université Pastore, M.L. (1983): Studio geologico-petrografico del di Milano. cristallino della falda Margna e delle serpentiniti del Montrasio, A. (1963): Studio petrografico e geologico complesso ofiolitico della Valmalenco nella Zona del versante sinistro della Val Lanterna (Val Malenco del Pizzo Scalino (Valmalenco - Sondrio) Sottotesi - Sondrio). Tesi di Laurea, inedita; Ist. Mineral. di rilevamento inedita. Dip. Sei. Terra, Université di Petrogr. Université di Milano. Milano. Montrasio, A. (1973). Strutture a pillow nelle anfiboliti Pastore, M.L. (1983): II Ghiacciaio di Fellaria del M. Forno (Pennidico medio - Alpi Retiche). Atti nell'Olocene e nel Tardo-Glaciale.Tesi di Laurea Accacl. Naz. Lincei, Rend. Cl. Sei. fis, mat. nat., 54, inedita. Dip. Sei.Terra, Université di Milano. 114-123. Peretti, A. (1983): Geologie und Pétrographie der For- Montrasio, A. (1984): Tire "Lanzada-Scermendone noserie. Piz dei Rossi. Unpublished Diploma thesis Zone": An ophiolitic unit of continental affinity in University of Zürich. the Southern Rhaetic Alps (Prov. Sondrio - Italy). Peretti, A. (1985): Der Monte del Forno Komplex am Schweiz. Mineral. Petrogr. Mitt. 64,111-129. Bergell-Ostrand: Seine Lithostratigraphie, alpine Montrasio, A. and Trommsdorff, V. (1983): Guida Tektonik und Metamorphose. Eclogae geol. Helv. 78, all'escursione del massiccio di Val Masino-Bregaglia 23-48. Valmalenco occidentale,Sondrio. Mem. Soc. Geol. It. Peretti, A. (1988): Occurrence and stabilities of opaque 26,421-434. minerals in the Malenco-Serpentinite (Sondrio, Montrasio, A.,Trommsdorff, V., Hermann, J., Müntener, Northern Italy). PhD thesis ETH Zürich, No. 8740. O. and Spillmann, P. (2005): Carta Geologica della Pfeifer, H.R. (1972): Metamorphose und Deformation Valmalenco. Supplement to Schweiz. Mineral. Petrogr. am Nordrand des Malenco-Serpentinkörpers westlich Mitt. 85/1. des Pass d'Ur (Nord-Italien). Unpublished Müntener, O. (1991): Geologische Untersuchungen süd¬ Diploma thesis ETH Zürich. lich der Berninagruppe, Oberstes Val Malenco, Prov. Pfiffner, M. (1992): Geologische Untersuchungen süd¬ Sondrio, N-Italien, Teil II. Unpublished Diploma lich und süd-westlich des Monte Disgrazia im Valle thesis ETH Zürich. di Scermendone und im Valle di Preda Rossa, Val Müntener, O. (1997): The Malenco peridotites (Alps): Masino, Prov. Sondrio, Italien. Teil 1. Unpublished Petrology and geochemistry of subcontinental mantle Diploma thesis ETH Zürich. and Jurassic exhumation during rifting. PhD thesis Pfiffner, M. and Weiss, M. (1994): Strukturelle und pet- ETH Zürich, No. 12103. rographische Untersuchungen im Grenzbereich Müntener, O. and Hermann, J. (1996): The Valmalenco Penninikum-Unterostalpin am Südostrand des lower crust - upper mantle complex and its field Bergell-Plutons (Val Masino, Italien). Schweiz. Mineral relations (Italian Alps). Schweiz. Minerai. Petrogr. Petrogr. Mitt. 74,245-264. Mitt. 76.475-500. Pika, J. (1976): Geologisch-petrographische Untersuch¬ Müntener, O., Hermann, J. and Trommsdorff, V. (2000): ungen am südwestlichen Kontakt des Malencoserpentins. Cooling history and exhumation of lower crustal Unpublished Diploma thesis ETH Zürich. granulites and upper mantle (Malenco, Eastern Pozzorini, D. (1996): Stable isotope investigations of Central Alps)./ Petrol. 41,175-200. ophicarbonate rocks, Bergell aureole, Valmalenco: Müntener, O. and Hermann, J. (2001): The role of lower Constraints on fluid-rock interaction. PhD thesis crust and continental upper mantle during formation ETH Zürich, No. 11436. of nonvolcanic passive margins. Evidence from Pozzorini, D. and Früh-Green, G.L. (1996): Stable iso¬ the Alps. In: Non-volcanic rifting of continental margins: tope systematics of the Ventina ophicarbonate zone, A comparison of evidence from land and sea, Bergell contact aureole. Schweiz. Mineral. Petrogr. edited by R.C.L. Wilson, R.B. Withmarsh, B. Taylor, Mitt. 76,549-564. and N. Froitzheim. Geol. Soc. London Spec. Publ. Puschnig, A.R. (1996): Regional and emplacement relat¬ 187,267-288. ed structures at the northeastern border of the Ber- Müntener, O., Pettke, T., Desmurs, L., Meier, M. and gell-intrusion (Monte del Forno. Rhetic Alps). Schaltegger. U. (2004): Refertilization of mantle Schweiz. Mineral. Petrogr. Mitt. 76,399-420. 12 V. Trommsdorff et al. Puschnig, A.R. (1998): The Forno unit (Rhetic Alps): oberen Val Malenco. Unpublished Diploma thesis Evolution of an ocean floor sequence from rifting to ETH Zürich. Alpine Orogeny. Ph.D. Thesis, ETH Zürich, No. Spillmann. P. (1989): Struktur und Metamorphose der 12702. Margna-Decke im obersten Valmalenco. Schweiz.. Puschnig, A.R. (2000): The oceanic Forno unit (Rhetic Mineral. Petrogr. Mitt. 69,147-150. Alps): field relations, geochemistry and paleogeo- Spillmann, P. (1993): Die Geologie des penninisch-ost- graphic setting. Eclog. geol. Helv. 93,103-124. alpinen Grenzbereichs im südlichen Berninagebirge. Rageth, R. (1984): Intrusiva und Extrusiva der Bernina- PhD thesis, ETH-Zürich. Nr. 10175. Decke zwischen Morteratsch und Berninapass. Spillmann, P. and Büchi, H.J. (1993): The Pre-Alpine Schweiz. Mineral. Petrogr. Mitt. 64, 83-109. Basement of the Lower Austroalpine Nappes in the Raz, U. (1987): Metamorphose und Strukturen im Gebiet Bernina Massif (Grisons, Switzerland; Valtellina. des Scerscen Inferiore. Unpublished Diploma thesis Italy). In: von Raumer, J.F. and Neubauer, F. (eds.): ETH Zürich. The Pre-Mesozoic Geology in the Alps. Springer Reber, D. (1995): Strukturelle und petrographische Verlag, Heidelberg, 457- 467. Untersuchungen im oberen Val Malenco (Val Ventina, Spillmann, P. und Trommsdorff, V. (2005): Blatt Nr. 1277 Provinz Sondrio, N-Italien).Teil 3. Unpublished Piz Bernina. - Geol. Atlas der Schweiz 1:25'000. Diploma thesis University of Zürich. Karte 119, mit Erläuterungen. Bundesamt für Wasser Reusser, E. (1987): Phasenbeziehungen im Tonalit der und Geologie, Bern. Bergeller Intrusion (Graubünden, Schweiz/Provinz Spillmann. P. und Trommsdorff, V. (2006): Blatt 1277 Piz Sondrio, Italien). Ph.D. Thesis, ETH Zürich, No. Bernina. - Geol. Atlas der Schweiz 1:25'000, 8329. Erläuterungen zu Blatt 119. Ried, F. (1994): Titanmobilität: Metasomatische titan¬ Staub, R. (1918): Geologische Betrachtungen am Berg- reiche Adern am Kontakt von Dolomitmarmoren eller Massiv. Vjschr. natf. Ges. Zürich, 63, 1-18. zur Bergeller Intrusion. PhD thesis ETH Zürich, No. Staub, R. (1921): Geologische Karte der Val Bregaglia 10637. (Bergell). Geologische Spezialkarte der Schweiz Nr. Riklin, K. (1977): Geologisch-petrographische Untersu¬ 90,1:50'000. Schweiz. Geol. Kommission. chungen am Bergell-Ostrand, nördlich der Valle Sis- Staub. R. (1946): Geologische Karte der Berninagruppe sone. Unpublished Diploma thesis ETH Zürich. und ihrer Umgebung im Oberengadin, Bergell, Val Riklin. K. (1978): Strukturen und Metamorphose im Malenco, Puschlav und Livigno. 1:50'000. Spezialkarte Bereich der südlichen Muretto-Linie. Schweiz. Mineral. Nr. 118, Schweiz. Geol. Komm. Petrogr. Mitt. 58,345-356. Theobald, G. (1866): Geologische Beschreibung der Riva, M. (1984): Studio petrografico e strutturale della südöstlichen Gebirge von Graubünden. Beitr. geol. regione compresa tra la media Val Malenco e la Val Karte Schweiz 3. 1. Serie: inkl. Blatt XX der geol. di Togno, con parlicolare riguardo agli Gneiss del M. Karte Schweiz LIOO'OOÖ, 1865. Canale (falda Bernina, Austroalpino inferiore) Trommsdorff, V. and Evans, B.W. (1972): Progressive (Prov. Sondrio).Tesi di Laurea, inedita; Ist. Mineral. metamorphism of antigorite schist in the Bergell Petrogr. Université di Milano. tonalité aureole (Italy). Am. J. Sei. 272,423-437. Rossi, F.M. (1976): Geologia, metamorfosi e strutture Trommsdorff, V. and Evans, B.W. (1974): Alpine meta¬ delle regione del Pizzo Scalino. Unpublished Diploma morphism of peridotitic rocks. Schweiz. Mineral. thesis ETH Zürich. Petrogr Mitt. 54,333-352. Schmid, S.M., Rück. P. and Schreurs, G. (1990): The sig¬ Trommsdorff, V. and Evans, B.W. (1977): Antigorite- nificance of the Schams nappes for the reconstruction ophicarbonates: contact metamorphism in Valmalenco of the paleotetonic and orogenic evolution of (Italy). Contrib. Mineral. Petrol. 64,301-312. the Penninic zone along the NFP 20 East Traverse. Trommsdorff, V. and Evans, B.W. (1980): Titanian hy- Mém. Soc. Geol. France 156,263-287. droxyl-clinohumite: formation and breakdown in Schmid, S.M, Berger, A., Davidson, C., Gieré, R., Her¬ antigorite rocks (Malenco. Italy). Contrib. Mineral. mann, J., Nievergeit, P., Puschnig, A.R. and Rosenberg, Petrol. 72,229-242. C. (1996): The Bergell pluton (Southern Trommsdorff, V. and Connolly, J.A.D. (1990): Constraints Switzerland, Northern Italy) overview accompanying a on phase diagram topology for the system CaO- geological-tectonic map of the intrusion and Mg()-SiO,-lTO-CO,. Contrib. Mineral. Petrol. 104,1-7. surrounding country rocks. Schweiz. Mineral. Petrogr. Trommsdorff, V. and Connolly, J.A.D. (1996): The ultra- Mitt. 76,329-355. mafic contact aureole about the Bregaglia (Bergell) Schumacher,T. (1975): Geologisch-petrographische tonalité: isotherms and a thermal model. Schweiz. Untersuchungen des Malenco-Serpentinits und der Minerai Petrogr Mitt. 76 537-547. Margnagesteine bei Chiareggio. Unpublished Trommsdorff, V. and Nievergeit, P. (1983): The Bregaglia Diploma thesis ETH Zürich. (Bergell) Iorio Intrusive and its field relations. Mem. Sciesa, E. (1987): Studio geologico-petrografico e miner- Soc. Geol. Ital. 26,55-68. alogico della "Zona Lanzada-Scermendone" (Val Trommsdorff, V., Piccardo, G.B. and Montrasio, A. Màsino - Val Malenco). Tesi di Laurea, inedita; 1st. (1993): From magmatism through metamorphism to Mineral. Petrogr. Università di Milano. sea floor emplacement of subcontinental Adria Sidler, D.M. (1990): Strukturen und Metamorphose in lithosphère during pre-Alpine rifting (Malenco, Italy). der Margna- und Sella-Decke und im Malenco- Ser- Schweiz. Minerai Petrogr Mitt. 73,191-203. pentinit zwischen Pass d'Ur und Pizzo Scalino (Prov. Ulrich, T. (1995): Petrographische Untersuchungen im di Sondrio, Italien) (Teil 1). Unpublished Diploma oberen Val Malenco (Val Ventina, Provinz Sondrio, thesis University of Zürich. N-ltalien).Teil I: Lago Pirola. Unpublished Diploma Sidler, D.M. and Benning, L. G. (1992): Die Entwicklung thesis ETH Zürich. der Strukturen im Südosten der Margna-Decke und Ulrich, T. and Borsien, G.R. (1996): Fedozer Metagab- des Malenco-UItramafitits (Provinz Sondrio, bro und Forno-Metabasalt (Val Malenco, Norditalien): Italien). Schweiz. Mineral. Petrogr. Mitt. 72,57-69. Vergleichende petrographische und geoche- Spillmann, P. (1988): Struktur und Metamorphose in der mische Untersuchungen. Schweiz. Minerai Petrogr. Margna-Decke und im Malenco-Serpentinit im Mitt. 76. 521-535. The Geological map ofValmalenco 13 Venzo, S.. Crespi, R., Schiavinatio, G. and Fagnani, G. Wenk, H.R. (1974): Two episodes of high-grade meta- (1971 ): Carta geologica-petrografica dell Alpi insub- morphism in the northern Bergell Alps. Schweiz. riche Valtellinesi tra la Val Masino e la Val Malenco Mineral. Petrogr. Mitt. 54,555-565. (Sondrio). In: Mem. Delia Soc. It. Scienze Nat. e del Wenk, H.R, Wenk, E„ and Wallace, J.C. (1974): Meta- Museo di Storia Naturale- Milano 1; vol 19. morphic mineral assemblages in pelitic rocks of the Villa, I.M., Hermann, J., Müntener, O. and Trommsdorff, Bergell Alps. Schweiz. Mineral. Petrogr. Mitt. 54, V. (2000):39Ar/40Ar dating of multiply zoned amphibole 507-554. generations (Malenco, Italian Alps). Contrih. Weiss, M. (1992): Geologische Untersuchungen südlich Mineral Petrol. 140,363-381. und südwestlich des Monte Disgrazia im Valle di Villa, L. (1982) Studio geologico della regione coinpresa Scermendone und im Valle di Preda Rossa, Val tra la Val Torreggio ed il Passo Scermendone (Val Masino, Prov. Sondrio, Italien.Teil II. Unpublished Malenco - Val Masino, Sondrio), con particolare rif- Diploma thesis ETH Zürich. erimento alia 'Zona Lanzada-Scermendone'.Tesi di Zingg, H.-P. (1988): Struktur und Metamorphose in der Laurea, inédita; 1st. Mineral. Petrogr. Université di Margnadecke und im Malencoserpentinit im oberen Milano. Valmalenco (Italien). Unpublished Diploma thesis von Blanckenburg, F. (1992): Combined high precision ETH Zürich. chronometry and geochemical tracing using accessory minerals: applied to the Central-Alpine Bergell intrusion. Chern. Geol. 100,19-4-0. von Quadt, A.; Grünenfelder, M. and Büchi, H.J. (1994): U-Pb Zircon ages from different igneous rocks of the Bernina nappe system. Schweiz. Mineral. Received 19 March 2005 Petrogr. Mitt. 74,373-382. Accepted in revised form 13 February 2006