Article
Reference
Oceanic sedimentary processes and alpine metamorphic events in the Montgenèvre ophiolite, Western Alps
BERTRAND, Jean, NIEVERGELT, Peter, VUAGNAT, Marc
Abstract
Dans de précédentes études sur l'ophiolite du Montgenèvre, l'accent a souvent été mis sur la séquence des gabbros et certains des phénomènes, tels flaserisation, métamorphisme, hydrothermalisme, que l'on peut y observer. Cet article est consacré plus spécialement aux sédiments associés à l'ophiolite. Diverses roches clastiques doivent être distinguées de la série pélagique. Ainsi, des arénites volcaniclastiques forment des intercalations dans la séquence des laves en coussins alors que des ophicalcites et arénites a débris de serpentinite reposent stratigraphiquement sur les vestiges d'un paléo-plancher océanique serpentineux. La série pélagique constitue des unités tectoniques disposées l'une à la marge externe, l'autre à la marge interne du massif; elle s'observe probablement aussi en position originale sur son substratum ophiolitique. Connue sous le nom de "Série de Chabrière", elle montre la succession cherts à radiolaires-calcaires recristallisés - Formation de la Replatte (alternance schistes-calcaire). Dans le domaine Liguro-piémontais, cette série est interprétée comme la [...]
BERTRAND, Jean, NIEVERGELT, Peter, VUAGNAT, Marc. Oceanic sedimentary processes and alpine metamorphic events in the Montgenèvre ophiolite, Western Alps. Ofioliti , 1984, vol.
9, no. 3, p. 303-320
Available at:
http://archive-ouverte.unige.ch/unige:153369
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303
Ofioliti, 1984, 9 (3), 303-320 A R T I C L E S / A R T I C 0 L I
OCEANIC SEDIMENTARY PROCESSES AND ALPINE METAMORPHIC EVENTS IN THE MONTGENEVRE OPHIOLITE, WESTERN ALPS
J. Bertrand, P. Nievergelt and M. Vuagnat
Department of Mineralogy, University of Geneva, Switzerland
Key words: ophiolite, sedimentary cover, HP-LT metamorphism. Montgenevre, Western Alps.
Abstract
In previous studies on the Montgenevre ophiolite, attention has been focused on oceanic events particularly in the gabbro sequence (flaserisation, metamorphism, hydrothermal ism).
This paper is mainly concerned with the sediments associated with the ophiolite.
A elastic rock group may be distinguished from the pelagic sequence. Volcaniclast- ic arenites occur as intercalations in the pile of pillow lavas, whereas sediment- ary ophicalcites and serpentinitic arenites overlie stratigraphically a serpenti- nitic palaeo-ocean floor. The pelagic sequence is found as tectonic units at the frontal and internal margins of the massif and probably in original position on the ophiolitic substratum.
This sedimentary sequence called "serie de Chabriere" is made up of the well- known succession: radiolarian cherts-Calpionellid limestones-alternating limestones and shales (Formation de la Replatte), and is classically interpreted in the Ligu- ro-piemontais domain as the primary sedimentary cover of the oceanic lithosphere.
The members of this series are generally easily recognizable; however, they may show some lithological variations and transitions from one term to another. Of par- ticular interest are ophiolitic olistoliths and levels of ophiolitic detritus observ- ed in each member of this series.
The sedimentary record in the "serie de Chabriere" as in the arenites and other reworked formations gives ample evidence of oceanic tectonic events over a long time and furthermore demonstrates a heterogeneous ocean floor and various deposi- tional environments.
No simple pattern of metamorphic overprint is observed in the massif, neither in the above mentioned sedimentary formations nor in the ophiolites. If the flaser- gabbros have definitely undergone an oceanic metamorphism, the age and precise con- ditions of development of some prehnite-pumpellyite assemblages are more hypothetic- al. The growth of Na-amphibole, Na-pyroxene and lawsonite is of Alpine age and post- dates, where present, penetrative deformation. These minerals show random orienta- tions or occur as patchy or fibrous aggregates in suitable chemical domains; they developed on the preserved or slightly deformed original structure.
304
The erratic geographical distribution of these minerals and variations in the succession of formation Na-amphibole replacing Na-pyroxene or vice-versa may be explained by variable conditions during metasomatic processes and an incipient stage of Alpine metamorphism.
The "early" Alpine events were followed by thrusting of ophiolitic units with already different imprints of deformation and metamorphism. The transport to the present position could be an even later event.
Resume
Dans de precedentes etudes sur l 'ophiolite du Montgenevre, 1 'accent a souvent ete mis sur la sequence des gabbros et certains des phenomenes, tels flaserisa- tion, metamorphisrne, hydrothermalisme, que l 'on peut y observer.
Cet article est consacre plus specialement aux sediments associes
a
l 'ophioli- te. Diverses roches clastiques doivent etre distinguees de la serie pelagique.Ainsi, des arenites volcaniclastiques forment des intercalations dans 'la sequence des laves en coussins alors que des ophicalcites et arenites
a
debris de serpenti nite reposent stratigraphiquement sur les vestiges d'un paleo-plancher oceanique serpentineux. La serie pelagique constitue des unites tectoniques disposees l 'une a la marge externe, l 'autre a la marge interne du massif; elle s'observe probable- ment aussi en position originale sur son substratum ophiolitique. Connue sous le nom de "Serie de Chabriere", elle montre la succession chertsa
radiolaires-calcai- res recristallises - Formation de la Replatte (alternance schistes-calcaire). Dans le domaine Liguro-piemontais, cette serie est interpretee comme la couverture se- dimentaire primitive de la lithosphere oceanique. Les trois termes mentionnes se differencient generalement sans difficulte, toutefois certaines variations litho- logiques et transitions peuvent exister. Les olistolites ophiolitiques et horizons a debris ophiolitiques associes a cette serie sont particulierement interessants.Les caracteres observes tant dans la serie de Chabriere que dans les arenites et autres formations remaniees demontrent non seulement 1 'existence d'evenements tectoniques au cours de la phase oceanique, et ceci durant une longue periode, rnais aussi confirment 1 'heterogeneite du paleo-plancher oceanique et la diversite des environnements de depots.
L'ernpreinte du metamorphisme sur le massif du Montgenevre ne repond pas a un schema simple. S'il est definitivement admis que les flasergabbros ont subi un me- tamorphisme oceanique, par contre 1 'age et les conditions precises de formation de certaines parageneses du facies pumpellyite-prehnite demeurent plus hypotheti- ques.
La croissance d'amphibole et de pyroxene sodiques et de lawsonite, d'age alpin, apparait posterieure aux deformations penetratives observees localement. Ces mine- raux, qui ne montrent en effet pas d'orientation privilegiee, s'observent aussi, dans des domains chirniques appropries, sous forme d'aggregats mouchetes ou acicu- laires developpes sur la structure originale intacte ou legerement deformee.
La distribution geographique aleatoire de telles parageneses de meme que les changements d'ordre de cristallisation de ces mineraux - amphibole sodique rempla- gant le pyroxene sodique et vice versa - pourraient s'expliquer par l 'existence de conditions physi co-chimiques variables lors des process us metasomati ques et des stades initiaux du metamorphisme alpin. L 'ecaillage en unites ophiolitiques deja affectees par differentes empreintes de deformation et de metamorphisme a pris place apres ces evenements alpins "precoces". Le transport de l 'edifice ophioliti que ainsi constitue
a
son emplacement actuel pourrait quanta
lui correspondre a un305
evenement plus tardif encore.
Introduction
The Montgenevre ophiolite is situated east of Brian~on on both sides of the French-Italian border. It covers about 30 km2 , exposing a sequence more than 500 meters thick. At its western limit, the ophiolite is thrusted on the Prepiemon- tais zone, which is thought to represent a transitional realm between the Brian-
~onnais and the Piemontais domains of the Tethys (Lemoine, 1971). In the east, the massif is thrust on the schistes lustres of the Piemontais domain. The con- tact in the northeast is less well defined due to the presence of ophiolitic len- ses in the underlying schistes lustres themselves.
The regional geological context of the Montgenevre ophiolite has been present- ed in several articles (Lemoine, 1961; 1964; 1971; Lemoine in Debelmas, 1979;
1983). In contrast to other ophiolitic bodies in the Piemontais domain, this mas- sif is moderately dismembered and is characterized by a weak Alpine metamorphic overprint. Thus, the comparison between the oceanic lithosphere and ophiolites, now interpreted as the equivalent of an ancient ocean floor, seems to be parti- cularly justified in this case. According to the current models, the Montgenevre massif represents a rather complete ophiolitic sequence, with serpentinites, gab- bros, pillow lavas and pelagic sediments (Pusztaszeri, 1969; Bertrand et al., 1981; 1982).
Several rock transformations have been attributed to an oceanic stage. Flaser- isation and amphibolitisation of some gabbros have been with certainty assigned to this stage (Mevel et al., 1978; Steen et al., 1980). For prehnite-pumpellyite as- semblages and greenschist type parageneses, the age of formation is, so far, not exactly known but Alpine metamorphism was generally considered responsible for the development of such mineral associations. Mineral parageneses of the high pressure- low temperature type, typical of the Alpine metamorphism in the Piemontais domain in this part of the Western Alps, were found towards the eastern margin of the Montgenevre massif. In ophiolites outcropping near the road Montgenevre-Cesana To- rinese below the backthrust Prepiemontais zone of Mont Chaberton, at Mont-Cruzeau (Koehn and Vuagnat, 1970), in Val Gimont (Zufferey, 1973) and near Lago Nero (MUller, 1978; Bertrand et al., 1982), lawsonite-blue amphibole-sodic pyroxene as- semblages have been found. Lately, Mevel et al. (1982) mentioned aegirinic pyro- xene and riebeckite from the western margin of the ophiolite. They described their specimen as a brecciated gabbro belonging to the gabbro zone west of the Chenaillet.
The aim of this article is to contribute to the distinction between oceanic and Alpine events that have affected the Montgenevre ophiolite. Therefore, the sediment- ary record may help us to recognize the oceanic metamorphic and tectonic history.
On the other hand, the study of metamorphic mineral parageneses and crystallization/
deformation relationships is useful to establish the succession of Alpine events.
The structure of the Montgenevre ophiolite
A model for the evolution of the Montgenevre ophiolite from its genesis to its incorporation into the Alpine orogene - and a schematic structural map have recent- ly been presented (Bertrand et al., 1982). Further field studies by one of the au- thors (P.N.) have allowed us to work out more clearly the structure of the massif.
Tectonic Map of the Montgenevre Ophiolite
Chabrler• Serles, with Olistoliths:
o Replalle Formation, alternating limestones and Shales
o Limestones, calcaires marmoreizns o Radiolarlan Cherts
Pillow lavas and Pillow Breccias Arenaceous lntercalations in
Pillow Lavas Gabbros
Serp•nlinites, with Albitnes
Im
M~lang• ZoneExternal and Internal kctonic Slices
Strongly deformed Pillow Lavas
LJR
Schistes lustr~s, some Calcschistes resembling Replatte Formation,"~
'
+/>;"
N
I
Roche des Clots - Grande Hoche S<rles.
former Gondran Series, Prqpiemontais
Road Favlt
Stream, Lake Thrust Bedding with Dip and Strike,
horizontal, ve.rtical, overturned 1 km
Fig. l - Tectonic map of the Montgenevre Ophiolite.
bJ Crysl•lhn1 Nis5ivu
~UltradavP>"lt1QiS
8 S1,.1bbtian~orina.is Q Briani;onnais
m:!JJ ,Prtpltmrn!al$'
[ZJ Schistts lvstrts
Regional tectonic map mainly after Lemoine (1969), Gidon (1977) and Debelmas and Kerckhove (1980).
w 0
"'
A
2500]
2000
c
le Janus R. de l'A!gle
2500]
2000
~ Chobriere Series, with Olistoliths:"'
~R iR) Replatte Formation
r;:;?;;l Pillow Lovas: <"> aphyric, <7>porphyric,
l28il ono Indication of Bedding ;strongly(D) O,N and non deformed \ N )
!~g] Pillow Breccias
Arenaceous !ntercololions in Pillow Sequence
M. de la Plane P. Rascia
P, N.
N
j~j00 Dolerites
r
l~s Schisfes Lustres
l!II!IfilJu
Serpentinites, with Ophicolcites:; 11 Pr6piE!montois 11Melange Zone Faults
External and Internal Tectonic Slices
Fig. 2 - Simplified cross sections. The north-eastern border zone is drawn schematically {compare Fig. 1 ).
s
wA
x
Sections of the Chabriere Series (not to scale)
Replatle Formation
Limestones Radlolari\es
B C
\"n"n" " " \_<J n f"")OO(}Oi-~
W of Cab. de Oouaniers SW of Cab. de Douan!ers R. Renard
lithologic Sections - Window of Val Gimont
Y ~nnnnnn nonnnn nn nnnnnn nnnnn< nnnn z
3 4
5
~a
9
D
Near Lago Nero
Uilillll
10 1112 13 14 15 I A
- 16 17 18
Between Bousson and Sagna Longa hypothetical
P. N.
...,,
0 00
309
To outline the structure of the massif (Fig. 1 and 2) only the major faults are shown. The thrusting direction of the main units is consistent, from east to west.
The relationships at the northeastern margin are somewhat obscured by a lack of good outcrops and by the presence of ophiolitic bodies in the schistes lustres.
The existence of a simple structural pattern of tectonic slices is obvious between Sammet des Anges and Cima le Vert, or even Punta Rascia. However, this pattern is modified in the field by late, probably Quaternary movements. Thus, between the Crete de Chouchar and Peyre Moutte, south of the Chenail1et and Cime de Saurel, huge masses of pillow lavas have slid step by step along semiconical fault planes.
The resulting scenery resembles a set of natural amphitheatres. Such an arrange- ment of faults, not explicitly shown in Fig. l and 2, is less spectacular but is also observed on the western, eastern and southern flanks of Val Gimont and between Cime le Vert and Bousson. Late major faults near lac Gignoux and Cime de Saurel that cut the previously existing structure of the massif, indicate sliding towards the north. Complex field relations in the afore mentioned areas are therefore in part due to late and extensive events of sliding. Nevertheless, the structural pattern of slices is again evident in the tectonic window of Val Gimont. Here, in addition, a zone of strongly deformed pillow lavas (Zufferey, 1973) underlying non-deformed pillow basalts (Fig. 3X, Y, Z) indicates without any doubt a complex history of emplacement for the Montgenevre ophiolite.
Good field observations in the northeastern part of Val Gimont are lacking be- cause of faulting and Quaternary deposits. But the relationship between the lava- rich western part and the serpentinite-rich eastern part of the massif is clearly visible between Cirne de Saurel and Cima le Vert.
The northern and part of the southern limits of the massif are marked by faults along which the ophiolitic compartment was downfaulted (Lemoine, 1964). The faults and the lithological boundaries along the north-eastern margin of the massif are strikingly parallel to the easterly limit of backthrusting of the Prepiemontais zone of Mont Chaberton onto the schistes lustres. The tectonic window in Val Gi- mont as well as contact relations - between Valley de la Cerveyrette and Lago Ne-
ro - of the ophiolite with the underlying units, support the picture of a huge ophiolite thrust on a complex basement. However, an internally coherent structural
Fig. 3 Stratigraphic sections of the Chabriere series with ophiolitic material at severa 1 1 eve ls .
The lithologic sections in Val Gimont indicate a transported metamorphism.
1. Siliceous and marly limestones. 2. Limestones ( "calcaire
a
trame rous- se") and shales. 3. Pale coloured (Calpionellid) limestones. 4. Radiolar- ites. 5. Pillow lavas. 6. Metamorphosed hematitic pillow lavas, brecciated pillows and pillow breccias. 7. Pillow breccias, hematitic brecciated pil- lows. 8. Gabbros. 9. Fe-Ti rich gabbros, deformed gabbros. 10. Gabbro brec- cias. 11. Serpentinites. 12. Ophicalcites. 13. Calcitic ophiolitic breccias.14. Chlorite schists. 15. Albitite breccias. 16. Major stratigraphic (Fig.
3A) and tectonic discontinuity. 17. Schistes lustres, calcschists in Val Gimont, probably including post-Triassic sediments of the Prepiemontais zone and the Replatte Formation. 18. Triassic dolomites of the Prepiemontais · zone.
310
pattern has been preserved in this huge ophiolitic body.
Before describing briefly the various units of the Montgenevre ophiolite, we must draw attention to the contacts between the lithological units. Actually, in most places, there is little deformation at these contacts. This, however does not
imply that these units belong to an undisturbed original sequence. The contact be- tween the gabbros and pillow lavas west of the Chenail let is considered tectonic because of the appearance of serpentinite slices in between. The Melange zone se- parates two units with different structural setti'ngs. The same is true for the con- tact marked by serpentinite west of Mont de la Plane. It is not proved that the pillow lavas overlying the gabbro in the lower part of Val Gimont display an origin- al succession because no absolutely convincing arguments have as yet been put for-
ward. ·
Description of the tectonic units
The Montgenevre ophiolite is thrust in the west on the Prepiemontais zone (or zone piemontaise externe). This latter consists of Triassic platform sediments, mainly dolomites, and of a limestone and limestone-shale sequence Liassic in age
(Serie du Gondran, Lemoine, 1971). Including even younger sediments in a compar- able stratigraphic section, this series has been renamed as serie Roche des Clots - Grande Hoche (Lemoine et al., 1978).
However, the precise age and tectonic position of a sequence of dark shales (Lemoine, 1971) between the true Prepiemontai s sediments and the Chabri ere series, near Sommet des Anges, are still unknown. Recently, Lemoine (in Debelmas, 1983) considered these shales as part of the Chabriere series. In this article, the dark shales are not distinguished from the Prepiemontais zone (cf. Fig.land 2).
The ophiolite itself is subdivided into several units for lithological and/or structural reasons (Fig. l and 2). In the most frontal and tectonically lowest part, the Chabriere series appears as a recumbent fold west of the Cabane de Doua- niers. This pelagic sequence of radiolarian cherts, pale coloured (Calpionellid) limestones, and Replatte Formation (alternating shales and "calcaire
a
trame rous- se") is considered to represent the original sedimentary cover of the oceanic li- thosphere. The existence of various ophiolitic olistoliths, mainly in the pale co- coured limestones, has already been mentioned (Bertrand et al., 1981; 1982).The external tectonic slices, which follow the Chabriere series, mark the thrusting of the ophiolite on this series. This thin zone of external slices con- sists of rather undeformed gabbros, serpentinites and pillow lavas, with intricate mutual relationships, outcropping west of Lac des Sarailles.
The adjacent serpentinite zone with albitite intrusions can be traced from north of Cabane de Douaniers southwards into the ravine of Peyre Moutte.
The gabbro unit forming part of the western slope of the Chenail let is crosscut by numerous aphyric and porphyric diabase dykes and by albitite dykes. Further to the east, the remaining principal body of the Montgenevre ophiolite, as far as Val Gimont, is subdivided into three main pillow lava units. The Chenaillet unit is re- markable for its fine stratigraphic sequence of petrographically different pillow
lava flows, often separated by pillow breccias. Pillow lavas of the almost horizont- al Rocher de 1 'Aigle unit are more uniform but pillow breccias are again often found between the flows. The Mont de la Plane unit, mostly in vertical or even overturned position, shows a major stratigraphic unconformity represented by interpillow are- nitic sediments. A coarse dolerite sill with chilled margins intruded these sedi- ments. The stratigraphically lower part of the Mont de la Plane unit is character-
i zed by a number of thick pi 11 OI·/ brecci a layers especially towards Mont Fort du Boeuf, whereas above the unconformity pillow breccias are almost lacking.
311
The Grand Charvia-Cime de Saurel pillow lava sequence seems to be a continua- tion of the Mont de la Plane unit, displaying the same structural trend. But there is a slight change south of Collet Vert from more uniform to more heterogeneous pillow lavas, perhaps due to lateral or upward lithological variations in the vol- canic pile. However, possible interpillows thrusting could be easily overlooked in this area. The internal structure of the succession of pillow lavas between Grand Charvia, Cime de Saurel and Crete de Chouchar is less spectacular and in part also less well known. Some interpillow arenitic sediments were found.
Separating the Chenail let unit from the Rocher de 1 'Aigle and the Grand Charvia units, the Melange zone of the Col du Chenail let (Bertrand et al., 1980) is situat- ed at an important tectonic position. The northern and also probably the southern continuation of this zone is made up of gabbros, often flaserised.
The internal tectonic slices appear at the same tectonic level as their counter part in the West, between the main ophiolitic body and the underlying zone of pe- lagic sediments. The internal tectonic slices are composed of lenses of serpentin- ite and deformed and metamorphosed pillow lavas including a massive volcanic rock.
These basic rocks. contain postkinematic mineral parageneses of the high pressure- low temperature type and are comparable to the metamorphic volcanic rocks in the window of Val Gimont and in the Mont Cruzeau area.
The contact zone of the Montgenevre ophiolite with the schistes lustres in the east is subdivided into two parts. North of Lago Nero, predominantly serpentinites come into contact with the schistes lustres themselves containing several lenses of serpentinites with ophicalcites. The latter indicate the top of the original sea floor. Radiolarites, and perhaps also the Replatte Formation, might have strati- graphical ly overlain the ultrabasic substratum. South-west of Lago Nero, but in- cluding the huge mass of spilitic pillow lavas at Lago Nero itself, a zone charac- terized by the presence of numerous ophiolitic olistoliths, mainly pillow lavas, associated with the Replatte Formation outcrops.
At Mont Crouzeau, ophiolites and sedimentary rocks differ from those ot the Montgenevre ophiolite because of penetrative deformation and widespread blue am- phibole-sodic pyroxene assemblages.
Sediments associated with the ophiolite
The observation of ophiolitic olistoliths in the Chabriere series during earlier field work by B. Courtin and one of the authors (J.B.) caused additional interest in these sediments. Together with the results of our 1983 field studies, it seems appropriate to report some of our observations. Further work will be carried on.
The sediments discussed in the following are distinguished into two main groups:
1. a. Arenites intercalated in the volcanic sequence.
l. b. Serpentinitic arenites and sedimentary ophicalcites stratigraphically over- lying remnants of an ultrabasic palaeo-ocean floor.
2. Pelagic sediments, observed as independent tectonic units at the western and eastern margins of the Montgenevre massif, but also lying directly on relics of ophiolites in the schistes lustres domain (north of Lago Nero, Cruzeau).
312 Arenites
The volcaniclastic rocks within the pile of pillow lavas appear as layers of li- mited lateral extent, except at Mont de la Plane. Their thickness varies from some decimetres to a few metres at the most. These sediments are also observed between pillows or filling fractures and hollow cores of some pillows. A colour banding with alternating red, green, and beige is often emphasized by a recurrent graded bedding. Important hematite dissemination and first calcitization process might be attributed to the diagenetic stage. The grain size, though variable, is small and rarely exceeds 5 mm. The matrix is often micritic or sparry (probably recrystalliz- ed) calcite, but also calcitic/siliceous and, in some cases, rich in albite.
With the exception of lithic fragments from the pillow lavas, most of the detrit- al material is made up of mineral fragments. The following were recognized: clear pyroxenes, dusty deformed pyroxenes, brown and green amphiboles, some zoned amphi boles ranging from brown to green or colourless, chromium rich spinels and hydro- grossular. Detrital blue amphibole, sodic pyroxene or pumpellyite were never found.
The detrital minerals give information about the source region. Thus, apart from the debris coming from the pillow lavas, spinels derive from ultrabasic rocks, de- formed pyroxenes from oceanic tectonites and deformed gabbros, and amphiboles again from gabbros. Hydrogrossular is attributed to rodingites. Heterogeneity in the sour- ce region is therefore evident. The sediments indicate submarine erosion and accu- mulation processes in areas where volcanic activity had stopped for a while.
The best outcrops are located in the Mont de la Plane-Grand Charvia region, the most interesting being near Collet Guignard. In this place the growth of a meta- morphic blue amphibole rim around a colourless amphibole core was observed.
Serpentinitic arenites and sedimentary ophicalcites
Serpentinitic arenites and sedimentary ophicalcites indicate an in situ reworking, to different degrees, of an ultrabasic section of the palaeo-ocean floor. These se- diments were found in the Melange zone, and along the eastern margin of the ophio- lite, north of Lago Nero.
In the area of le Soureou, north of the Col du Chenaillet, it is possible, in spite of difficult outcrop conditions, to follow a brecciated massive serpentinite grading into a serpentinite breccia. The latter contains rounded and angular serpen- tinite and rare diabase fragments. The matrix is either rich in serpentinite debris or is micritic calcite. The continuous transition leads to a mostly beige serpenti- nitic arenite very similar at first sight to the arenites described earlier. This holds true macroscopically for the variety distinguished by red, centimetric, hema- tite-rich calcite bands; however the rock and mineral fragments are predominantly ultrabasic. In coarser grained serpentinitic sandstones or microbreccias, sparry calcite replaces serpentine minerals and sometimes even spinel. An important develop- ment of very fine grained to rarely well crystallized hydrogrossular is often ob- served in the matrix as well as in the fragments. This process might be correlated with other phenomena of calcium metasomatism reported from the Col du Chenaillet
(Bertrand et al., 1980).
Sedimentary reworking of serpentinite could be assumed regarding several outcrops at the north-eastern limit of the massif (Fig. 3E). Along the road from Lago Nero to Bousson, before the branching to Cava di Marmo, there is a transition from serpen- tinite to ophicalcite. Further down, at the locality of La Penna, serpentinitic ele- ments are dispersed in probably supraophiolitic calcschists. At the abandoned quarry of Cava di Marmo, radiolarites containing ophiolitic debris are associated with the
313 adjacent serpentinite which shows various developments of ophicalcites. Such asso- ciations and presumed sedimentary successions are comparable to some of those de- scribed from the Apennines (Cortesogno et al., 1981). The processes of fonnation could be analogue to those put forward by Tricart et al. (1982) in the southern part of Queyras.
Pelagic sediments
The Chabirere series (Lemoine, 1971) is well known in the Cottian Alps, the Hau- te-Ubaye (Lemoine et al., 1970) and in the Queyras, where Upper Jurassic radiolar- ites have recently been dated (De Wever and Caby, 1981). This series parallels the trilogy radiolarites-Calpionella Limestones-Palombini schists from the Apenn1nes (Sestini, 1970).
The complete Chabriere series is represented only in the external zone of the ophiolite, west of Cabane de Douaniers (Fig. 3A). At the internal margin of the mas- sif, the Replatte Formation is found south of Lago Nero. Radiolarites and probably other supraophiolitic calcschists, resembling the Replatte Formation, are associat- ed with serpentinite bodies north of Lago Nero (Fig. 3E). At Mont Cruzeau, where radiolarites overlie spilitic pillow lavas, an original sedimentary succession on an ophiolitic substratum could be assumed. Nevertheless this should be handled with caution. Another original succession - the Replatte Formation overlying spilitic pillow lavas - is observed at Lago Nero. Here, however, field studies strongly sug- gest that the huge cliff of pillow lavas represents an impressive olistolith emplac- ed in a zone characterized by the presence of ophiolitic material in the Replatte
Formation (Fig. 3D). Within the Montgenevre ophiolite itself, no signs of the Cha- briere series are found. In the tectonic w1ndow at Val Gimont, assumed to display part of the structural configuration below the main massif, the Replatte Formation has been recognized {Fig. 3X, Y, Z). Its distinction from calcschists of the schi- stes lustres type or from assumed post-Triassic sediments of the Prepiemontais zone is admittedly difficult, except for the presence of the characteristic "calcaire
a
trame rousse".
Actually, the existence of ophiolitic olistoliths and of levels of ophiolitic debris in all the three terms of the Chabriere series is quite remarkable and is significant for palaeogeographical reconstructions.
Already mentioned in the Replatte Formation from the Cottian Alps (Lemoine and Tricart, 1979; Lemoine, 1980) and in the Chabriere series from the Montgenevre ophiolite (Bertrand et al., 1981; 1982), these olistoliths and other ophiolitic re- lics indicate an early structural pattern of the ocean floor. Tectonic processes and the subsequent emplacement of ophiolitic material in supraophiolitic sediments have lasted, at least, from Upper Jurassic to Lower Cretaceous. The different sedi- mentary rock types, including sedimentary ophicalcites, volcanic and serpentinitic arenites and pelagic sediments with olistoliths, demonstrate a variety of deposi- tional environments and processes. To explain the presence of olistoliths in the Chabriere series, we have to consider a juxtaposition of an ophiolitic ocean floor and of another section carrying the pelagic sediments and receiving the ophiolitic debris. The idea that the Chabriere series presently found at the external margin of the Montgenevre ophiolite once constituted the sedimentary cover of the latter seems, therefore, too simple.
Nature of ophiolitic elements in the Chabriere series
In the radiolarites at the western limit of the massif (Fig. 3A), ophicalcites
314
and calcitic ophiolitic breccias were recognized. The latter consist mainly of vol- canic, gabbroic and serpentinitic rock fragments showing signs of reworking and the effect of important calcitization. Some of the ophiolitic debris from this area are found in the central part of a fold. This material could correspond to relics of a decollement level of the sedimentary sequence from its original ophiolitic substra- tum (Lemoine, 1971). However, at Mont Crouzeau, serpentinites, ophicalcites, pillow lavas and pillow breccias occur as sedimentary fragments in the radiolarites that overlie the spilitic pillow lavas.
In the pale coloured (Calpionellid) limestones that outcrop only at the western limit of the ophiolite, olistoliths and other discontinuous sedimentary deposits of ophiolitic material are found at several levels (Fig. 38). The olistoliths are cen- timetric to more than metric in size and often are composite. The main constituents are strongly chloritized, massive to brecciated Fe-Ti gabbros. Other component rocks consist of hematite-rich albitite breccias, ophicalcites (some of which are apparent- ly reworked), serpentinites, chlorite schists and rare radiolarites. Finer grained and extending laterally from the larger olistoliths', calcitic ophiolitic breccias, gabbro breccias and ophicalcites may form discontinuous levels. As yet, no fragments of pillow lavas or pillow breccias were observed. However, some chlorite schists and debris of basic (fine grained) rocks in the calcitic ophiolitic breccia could be of such an origin. The presence of fragments of albitite breccias, of olistoliths made up of different blocks and of an ophicalcitic crust partly coating numerous olisto- liths, gives ample evidence of a polyphase sedimentary history.
In the south-eastern part of the massif near Rocher Renard and Lago Nero g. 3C and D), predominantly ophiolitic olistoliths, some of spectacular size, are associat- ed with the Replatte Formation. Blocks of pillow breccias prevail over those made up of pillow lavas and gabbros. Lenses of serpentinites, ophicalcites and calcitic gab- bro breccias are subordinate. In addition, one decimetric radiolarite fragment was found in a composite olistolith. Some cracks of split up olistoliths seem to be fill- ed with material from the Replatte Formation. The chaotic appearance of this outcrop zone is underlined by the presence of a secondary schistosity in the dark shales (in between the disrupted 1 ozenge-shaped limestones). Near Lago Nero the impressive cliff of pillow lavas, thought to represent a huge olistolith, is stratigraphically overlain by the Replatte formation. Towards the south-west, blocks of gabbro are found in the same ~ormation.
In contrast to the ophiolitic material observed in the pelagic sequence at the western limit of the massif, pillow breccias and pillow lavas predominate over gab- bros in the east; Fe-Ti gabbros in particular are very rare.
Oceanic processes
The new observations presented in this article help to complete the inventory of oceanic events that are recorded in the Montgenevre massif. Indications of metamor- phic and of tectogenetic processes during this stage have already been published (Mevel et al., 1978; Steen et al., 1980; Bertrand et aL, 1982).
Tectonic processes, contemporaneous with the creation of the oceanic crust, are particularly obvious in the gabbro sequence. The presence of petrographically inho- mogeneous gabbros, some varieties even resembling magmatic breccias, together with flaserisation reflect a highly dynamic environment. The existence of early tectonic movements is illustrated by intrusions of albitite dykes into gabbros and serpen-
tinites. Finally, possible serpentinite protrusions in the Melange zone (Bertrand et al., 1980) might belong to the same category of phenomena. Thus, early tectonic
movements are made responsible for the heterogeneity of the palaeo-ocean floor.
The observed diversity in sediments and sedimentation processes confirms this idea.
315
Oceanic transformation processes have definitely been established in numerous studies on material collected from present day oceans and from ophiolites. Sever- al of these processes were recognized in the Montgenevre massif. Some gabbros were metamorphosed to amphibolites (Mevel et al., 1978; Steen et al., 1980). Non- isochemical transformations are widespread and indicated by spilitization of sub- marine pillow lava flows and by albitization processes. Hydrothermal alterations are obvious in some gabbros (Bertrand et al., 1981) and in hematitic pillow lavas and pillow breccias (Vuagnat, 1966). Further signs of hydrothermal or similar pro- cesses in the Alps were recently presented (Lemoine et al., 1982). The presence of serpentinite detritus in the arenites is a proof of early serpentinization. Ear- ly Ca-metasomatism in rodingites and ophispherites is demonstrated by the presence of detrital hydrogrossular in arenites. A similar metasomatic process might also be responsible for the postdepositional grov1th of a Ca-rich garnet phase in ser- pentini tic areni tes.
The sediments studied record a long history of a dynamic ocean floor. Early tec- tonic processes induced or at least made easier the reworking of parts of the oceanic lithosphere. The same processes may account for the different depositional events and for various types of deposits, from ophicalcites to ophiolitic breccias and arenites. The formation of composite olistoliths requires a polyphase mecha- nism. It remains to explain more precisely how these blocks viere emplaced. Compar- ed viith the structure of present day oceans, the importance of faults, including transform faults, has to be stressed. Implications of such oceanic tectonic pro- cesses to the sedimentary record have recently been discussed for ophiolites in the Cottian Alps (Lagabrielle et al., 1982; Tricart et al., 1982).
Alpine (post-oceanic) processes
The Montgenevre ophiolite has almost entirely escaped the penetrative metamor- phic overprint associated vlith blueschist facies typical of the Piemontais zone in this part of the Western Alps. But the presence of lawsonite, blue amphibole and Na-pyroxene assemblages has so far been reported from the eastern and lately as viell as from the viestern part of the massif (see introduction and below). To understand this distribution and the apparent lack of these minerals in the main ophiolitic body, it is advisable to consider in detail the tectonic pattern of the massif. The Montgenevre ophiolite s. str. shows only a few signs of Alpine defor- mation. In contrast, the underlying tectonic units including remnants of ophiolit- ic rocks and associated sediments are in great part schistose and folded, the bas- ic volcanics often being extensively spilitized.
New data modify somewhat the distribution, just mentioned, of metamorphic miner- al assemblages in the Montgenevre massif. The widespread occurrence of blue amphi- bo le and Na-pyroxene in the Chabri ere series at the western margin of the ophio 1 i te is quite remarkable. These minerals grow preferentially in the ophiolitic material incorporated into the radiolarites and pale coluured limestones, but also in other chemically suitable environments. Tvio of the three known albitite dykes in the gab- bro zone west of the Chenail let contain blue amphibole and Na-pyroxene. Blue amphi- bole rims around colourless actinolitic-tremolitic cores viere found in an ophicalc- ite of the Col du Chenail let and in an arenite south-east of Collet Guignard.
The presence of lavisonite in the plagioclase of many gabbros in the massif was
316
revea1ed by X-ray studies (Martin, to be published). In the Rocher Renard zone, blue amphibole and Na-pyroxene occur in the ophio1itic materia1 and lawsonite in the associated shales. These three minerals were found in the lenses of schistose basic volcanics outcropping below the backthrust Prepiemontais zone near the road Montgenevre-Cesana Torinese, below the main ophiolitic body east of Spagna Longa, in the internal tectonic slices and in the window of Val Gimont (Zufferey, 1973).
Na-pyroxene and lawsonite were described in the rather undeformed pillow lava se- quence near Lago Nero (MUller, 1978). Finally, at Mont Cruzeau, the three minerals of high pressure-low temperature type have been observed in the volcanic rocks, the radiolarites showing only growth of sodic amphibole and pyroxene (Koehn and Vuagnat, 1970).
In spite of the previously mentioned occurrences of blue amphibole, Na-pyroxene and lawsonite within the main ophiolitic body, a distinction must be made between the latter and the underlying tectonic units when one discusses metamorphic events.
An often penetrative deformation, a strong spilitization of the volcanic rocks and a widespread growth of blue amphibole, Na-pyroxene and lawsonite assemblages in the under1ying units are the main arguments for such a distinction. Nevertheless, all the units of the massif seem to have been once in their history subjected, in different degrees, to a metamorphism approaching high pressure-low temperature fa- ci es conditions.
From microscopical observations of crystallization/deformation relationships of metamorphic minerals and from field observations, the following succession of Al- pine events has been established:
Penetrative deformation. Where developed, the schistosity is well visible in pil- low lavas and pillow breccias. Moreover, the sediments of the Chabriere series and the schistes lustres show polyphase folding.
Crystallization of blue amphibole, sodic pyroxene and lawsonite assemblages. The growth of these minerals is almost exclusively postkinematic, or at least post main-deformation. A well-defined succession of formation is difficult to set up.
Lawsonite often occurs in chlorite-rich parts without any textural relation to the other minerals, and blue amphibole may replace sodic pyroxene and vice versa, but all these three minerals probably belong to the same metamorphic event. In unde- formed rocks (e.g. albitites), the age of formation for the sodic amphibole and so- dic pyroxene is less certain.
Thrusting of the tectonic units and transport on the Prepiemontais zone. Thrusting of tectonic units with different imprints of deformation and metamorphism is shown at the eastern margin of the massif and particularly in the window of Val Gimont.
Here, undeformed pillow lavas overlie a slice of strongly deformed and metamorphos- ed basic volcanic rocks (Fig. 3X, Y, Z). Thrusting of the tectonic units within the Montgenevre ophiolite s.str. might have been facilitated by a structural pat- tern inherited from the oceanic stage. Relationships at the contacts between the tectonic units give no further information, partly because weak or no deformation is observed. Therefore, it seems quite evident that thrusting took place preferen- tially along lithological boundaries and without producing much deformation. This in turn makes it difficult to decide whether the interna1 thrusting, the thrusting of the main ophiolitic body on its complex basement, and finally the transport on- to the Prepiemontais zone were achieved during one or more tectonic phases. The constant thrusting direction in the whole massif might indicate such a single phase event. If this is true, then the deformed and metamorphosed units in the complex basement, e.g. in Val Gimont, represent examples of transported metamorphism.
Further comment on the ophiolite emplacement will be given in the concluding re-
317 marks. A late major tectonic phase in the Montgenevre region is the backthrusting of the Prepiemontais zone eastwards onto the schistes lustres (Lemoine, 1969;
1971).
Recrystallizations and retrograde mineral growth are difficult to place in the succession of late Alpine events. Several phases of recrystallization of calcite in the pale coloured limestones and in the ophicalcites, as well as of quartz in radiolarites were observed. A transformation of blue amphibole-sodic pyroxene as- semblages to albite can be studied in Val Gimont. In the same mineral association, the growth of purnpellyite is also clearly post main deformation, and therefore Al- pine. Post blue amphibole recrystallizations of calcite, and also replacements by this latter mineral, are widespread in the calcitic ophiolitic breccias in the frontal Chabriere series. But these reequilibration processes happened before the latest brittle or locally penetrative deformations, which correspond to the final manifestations of Alpine orogeny in the area studied.
Concluding remarks
The existence of a heterogeneous palaeo-ocean lithosphere, the remnants of which form the Montgenevre ophiolite, is confirmed by the geological history recorded in the sediments studied. In contrast to the lithologies observed in the main ophio- litic body, the dominance of Fe-Ti rich gabbros and spilitized volcanic rocks form- ing the olistoliths has to be stressed. Therefore, these two rock associations re- present, strictly speaking, different sections of the oceanic lithosphere. Tecton- ic movements, responsible for a varied palaeo-topography, lasted over a long time as is indicated by the probably related emplacement of olistoliths. The periodic character of such an activity is demonstrated by the intermittent deposition of ophiolitic material in the pelagic sequence.
Mineral associations of an Alpine high pressure-low temperature type metamorphism have been found locally throughout the entire massif. The occurrence and development of these minerals are particularly remarkable in the ophiolitic olistoliths in the Chabriere series at the western margin. A riebeckite-aegirinic pyroxene assemblage has also been reported by Mevel et al. (1982) from the western part of the massif.
But the exact location and the geological context of the studied specimen are not clear from their description.
The blue amphibole-sodic pyroxene associations developed in different mineralo- gical environments (albitites, ophicalcites, Fe-Ti gabbros, pillow lavas, etc.).
Up to now, physical and chemical conditions of formation are difficult to assign to each case. In regard to the various occurrences, it seems possible that pene- trative deformation and metasomatic processes sometimes had a catalysing effect.
Moreover, ferric iron certainly has promoted the growth of blue amphibole and Na- pyroxene. A retrograde transformation of the high pressure-low temperature para- geneses into al bite was observed in the volcanic rocks from the window of Val Gi- mont. This transformation might be contemporaneous with the recrystallization of calcite and quartz in the Chabriere series.
The differences in degree of deformation and in expression of Alpine metamor- phism between the main ophiolitic body and its complex basement remains a puzzling fact. This obviously implies a different geological evolutionary history for these units before they finally joined each other in their present day position.
A firmly established correlation between metamorphic mineral growth and events of deformation and transport concerning all the tectonic units would help us to work out the history of the Montgenevre ophiolite emplacement in more detail. In
318
spite of the present lack of such information, some other conclusions can be drawn regarding the adjacent areas. From geological and tectonic maps (Lemoine, 1969;
Gidon, 1977 and Fig. 1) it is easily recognizable that the Prepiemontais zone in the Mont Chaberton and in the Rochebrune massifs, respectively north and south of the ophiolite, is thrust back onto the schistes lustres. It is less clear why the Montgenevre massif should have escaped this important tectonic phase. The Cervey- rette fault, running partly along the vallee de la Cerveyrette and traced further to the north-east (Lemoine, 1964; Gidon, 1977 and Fig. 1) is thought to represent the southern limit of a downfaulted compartment. Such a down faulting has often been considered explanatory of the weak metamorphic imprint of the Montgenevre mas sif which would represent a preserved segment of an upper tectonic level. However, the new observations presented in this article, particularly in the complex base- ment of the ophiolite s.str., indicate that this hypothesis is too simple and that other facts should be taken into consideration. As a matter of fact, the underly- ing complex basement zone and the schistes lustres with a higher degree of meta- morphism were affected by the same displacement.
In summary, the Montgenevre ophiolite s.str. is moderately dismembered and shows weak manifestations of Alpine detormation and metamorphism. This is in contrast to its basement zone and to the observations made on ophiolites in other areas of the Piemontais domain. The ophiolite s.str. together with its basement zone is thrust- ed in the west on the Prepiemontais zone. East of Val Gimont, the Prepiemontais zone no longer underlies the ophiolitic massif and there is a thrust contact with the schistes lustres.
Acknowledgments
This article has benefited from the support of the Swiss National Research Foundation (Project no. 2.504-0.82). We wish to express our sincere thanks to Mrs.
J. Berthoud for typing the manuscript.
Bertrand J., Steen D., Tinkler C. and Vuagnat M., 1980. The Melange zone of the Col du Chenaillet (Montgenevre Ophiolite, Hautes-Alpes, France). Symposium on tectonic inclusions and associated rocks in serpentinites. Geneve 1979. Arch.
Sci. Geneve, 33: 117-138.
Bertrand J., Courtin B. and Vuagnat M., 1981. Le massif ophiolitique du Montge- nevre (Hautes-Alpes, France, prov. de Turin, Italie): Donnees nouvelles sur un vestige de manteau superieur et de la croute oceanique liguro-piernontais. Be- richt. Schweiz. Mineral. Petrogr. Mitt., 61: 305-322.
Bertrand J., Courtin B. and Vuagnat M., 1982. Elaboration d'un secteur de litho- sphere oceanique liguro-piemontais d'apres les donnees de l 'ophiolite du Mont- genevre (Hautes-Alpes, France et province de Turin, Italie). Ofioliti (2/3):
155-196.
Cortesogno L., Galbiati B. and Principi G., 1981. Descrizione dettagliata di al- cuni caratteristici affioramenti di brecce serpentinitiche dell a Liguria orien- tale ed interpretazione in chiave geodinamica. Ofioliti, 6: 47-76.
Debelmas J., 1979. Alpes, Savoie et Dauphine. Guides geologiques regionaux. Ed.
Masson, Paris.
Debelmas J., 1983. Alpes du Dauphine. Guides geologiques regionaux. Ed. Masson, Paris.
319
Debelmas J. and Kerckhove C., 1980. Les Alpes franco-ita1iennes. 26eme C.G.I. (Pa- ris) France. Introduction
a
1a geologie du Sud-Est, G 18 (4): 22-58.De Wever P. and Caby R., 1981. Datatior. de 1a base des schistes lustres post-ophio- 1itiques par des radio1aires (Oxfordien superieur-Kimmeridgien moyen) dans les Alpes Cottiennes (Saint-Veran, France). C.R. Acad. Sc. Paris (II), 292: 467- 472.
Gidon M., 1977. Carte geo1ogique simplifiee des Alpes occidentales du Leman
a
Di- gne, au 1:250.000. B.R.G.M. Ed. Didier-Richard.Koehn P. and Vuagnat M., 1970. Sur la presence du facies "schistes
a
glaucophane"dans les roches du Mont-Cruzeau (Province de Turin, Italie). C.R. Soc. Phys. Hist.
Nat., Geneve, r..s., 5:59-64.
Lagabrie1le Y., Nervo R., Polino R. and Dutto F., 1982. Sedimentary cover of some ophiolites of the Cottian Alps. Ofioliti, 2/3: 339-350.
Lemoine M., 1961. La marge externe de la fosse piemontaise dans les Alpes occiden- tales. Rev. Geogr. Phys. Geol. Dyn., 4: 163-180.
Lemoine M., 1964. Sur un faisceau d'accidents transversaux aux zones brian~onnaise
et piemontaise
a
la latitude de Brian~on. C.R. Acad. Sci. Paris, D., 259: 846- 847.Lemoine M., 1969. Carte geologique de Brian~on, au 1:80.000, 3eme edition. Ed.
B.R.G.M.
Lemoine M., 1971. Donnees nouvelles sur la serie du Gondran pres de Brian~on. Re- flexions sur les problemes stratigraphiques et paleontologiques de la zone pie- montaise. Trav. Lab. Geol. Fae. Sci. Grenoble, 47: 181-201.
Lemoine M., 1980. Serpentinites, gabbros and ophicalcites in the Piemont-Ligurian domain of the Western Alps: possible indicators of oceanic fractures zones and associated serpentinite protrusions in the Jurassic-Cretaceous Tethys. Symposium on tectonic inclusions and associated rocks in serpentinites. Geneve, 1979.
Arch. Sci. Geneve, 33: 103-115.
Lemoine M., Arnaud-Vanneau A., Arnaud H., Letolle R., Mevel C. and Thieuloy J.P., 1982. Indices possibles de paleo-hydrothennalisme marin dans le Jurassique et le Cretace des Alpes occidentales (ocean tethysien et sa marge continentale eu- ropeenne): essai d'inventaire. Bull. Soc. Geol. France, 24 (3): 641-647.
Lemoine M., Bourbon M. and Tricart P., 1978. Le Jurassique et le Cretace prepie- montais
a
l 'Est de Brian~on (Alpes occidentales) et 1 'evolution de la marge eu- ropeenne de la Tethys: donnees nouvelles et consequences. C.R. Acad. Sc. Paris, 286: 1237-1240.Lemoine M., Steen D. and Vuagnat M., 1970. Sur le probleme stratigraphique des ophiolites piemontaises et des roches sedimentaires associees: observations dans le massif de Chabriere en Haute-Ubaye (Basses-Alpes, France). C.R. Soc.
Phys. Hist. Nat. Geneve, 5: 44-59.
Lemoine M. and Tri cart P., 1979. Une partie des schistes et des ophiolites du Quey- ras (Alpes occidentales frangaises) resultent-ils de sedimentation et d'ecrou- lements au pied d'un escarpement de faille oceanique? C.R. Acad. Sc. Paris, D, 288: 1655-1658.
Martin R.F. (in press). Albitization in the Montgenevre ophiolitic complex, West- ern Alps: patterns of metastability (presente
a
Rennes, juillet 1983). Bull.Soc. Fr. Cristall. Min.
Mevel C., Caby R. and Kienast J.R., 1978. Amphibolite facies conditions in the oceanic crust: example of amphibolitized flaser-gabbro and amphibolite from the Chenaillet ophiolite massif (Hautes-Alpes, France). Earth Planet. Sci. Lett., 39: 98-108.
320
Mevel C., Kienast J.R. and Leikine M., 1982. Decouverte et signification de l 'as- sociation metamorphique aegyrine-riebeckite dans un gabbro ophiolitique du Che- nail let (Alpes occidentales). C.R. Acad. Sc. Paris, II, 295: 809.
MUller J.-P., 1978. Etude de la region du Lago Nero (Montgenevre, Hautes-Alpes, province de Turin, Italie). Departement de Mineralogie, Universite de Geneve, travail i nedit.
Pusztaszeri L., 1969. Etude petrographique du massif du Chenaillet (Hautes-Alpes, France). Schweiz. Mineral. Petrogr. Mitt., 49:· 425-466.
Sestini G. (Editor), 1970. Development of the Northern Apennines geosyncline. Se- dim. Geol., 4, 3/4: 203-642.
Steen D., Vuagnat M. and Wagner J.-J., 1980. Early deformations in Montgenevre gabbros. Coll. Intern. CNRS no. 212. Ass. mafiques-ultramafiques dans les oro- genes (Grenoble 1977), p. 97-103.
Tricart P., Bourbon M. and Lagabrielle Y., 1982. Revision de la coupe Peouvou-Ro- che Noire (zone piemontaise, Alpes franco-italiennes): brechification synsedi- mentaire d'un fond oceanique ultrabasique. Geologie Alpine, 58: 105-113.
Vuagnat M., 1966. Les coussins eclates du Lago Nero (Massif du Montgenevre, prov.
de Turin) et le probleme des breches ophiolitiques (note preliminaire). C.R.
des seances, Soc. Phys. Hist. Nat., Geneve, n.s., 1/3: 163-167.
Zufferey J., 1973. Etude petrographique du val Gimont (massif du Montgenevre). De- partement de Mineralogie, Universite de Geneve, travail inedit.
Received, March 23, 1984 Accepted, October 5, 1984
