Abstract
Gabbros of the Montgenèvre ophiolite (external Piémont zone, western Alps), with the mineralogical, geochemical and isotopic characteristics of an oceanic ridge, show evidence of pervasive high-temperature deformation before the intrusion of basalt dikes. This deformation is incipient in the layered troctolites immediately above a locally preserved paleo-Moho, and increases in intensity upward in the overlying olivine-poor gabbros producing an anastomosing system of mylonitic shear zones and several steep ultramylonite belts. The mylonites and ultramylonites developed by solid-state shearing of the gabbros at temperatures in excess of 800–850°C, allowing recrystallization of augite subgrains in the wings of ductilely deformed magmatic diopside-salite clasts followed by crystallization of brown amphiboles. On the basis of textural relationships it is proposed that syn-kine-matic partial melting within the sheared gabbros, at temperatures of 850° C or higher, generated augite- and pargasite-bearing leucodioritic magmas which evolved in narrow magmatic conduits and percolated through tec-tonites and actively deforming mylonite zones, to form cross-cutting veins in adjacent less deformed gabbros. The complex geometry of the structures and their kinematics, when restored to a stage before the onset of pre-Alpine and Alpine brittle tectonics, suggest that this tectono-metamorphic evolution started after rapid solidification of the gabbros. It is proposed that the gabbroic mass was pushed away from a spreading center due to the activity of extensional and transform shear zones, in response to spreading through continuous domal uplift of the underlying mantle peridotites from which, at deeper levels, new basaltic melt was extracted to produce off-axis basaltic volcanism. The “magmatic” Moho became thereby transformed into a “tectonic” moho, followed by uplift of peridotites to the ocean floor where they became overlain by ultramafic-derived sediments such as to form a “sedimentary” Moho. The scarce geochronological data suggest a long time span, of about 50–60 Ma, between magmatic crystallization of gabbros and diorites (212± 8 Ma) and the deposition of the oceanic cover (165–160 Ma), suggesting a slow-spreading paleoridge in the relatively narrow (<1000 km) Piemonte-Ligurian branch of the Tethys ocean.
Keywords
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Preview
Unable to display preview. Download preview PDF.
References
Beard, J. S., 1990. Dehydration melting of amphibolite at 1–7 kilobars. Trans. Am. Geophys. Union, 71: 1714.
Bernoulli, D. and Lemoine, M., 1980. Birth and early evolution of the Tethys: the overall situation. 26th Int. Geol. Congr. Paris, 1980, Coll. C5, 167–179.
Bertrand, J., Steen, D., Tinkler, C. and Vuagnat, M., 1980. The Mélange zone of the Col du Chenaillet Montgenèvre ophiolite, Hautes Alpes, France. Arch. Sei., Genève, 33: 117–138.
Bertrand, J., Courtin, B. and Vuagnat, M., 1981. Le massif ophiolitique du Montgenèvre Hautes Alpes, France, et province de Turin, Italie: Données nouvelles sur un vestige de manteau supérieur et de croûte océanique liguro-piémontais. Bull. Suisse Minerai. Pétrogr., 61: 305–322.
Bertrand, J., Courtin, B., and Vuagnat, M., 1982. Elaboration d’un secteur de lithosphère océanique liguro-piémontais d’après les données de l’ophiolite du Montgenèvre, Hautes Alpes, France, et province de Turin, Italie. Ofioliti, 7: 155–196.
Bertrand, J., Dietrich, V, Nievergelt, P., and Vuagnat, M., 1987. Comparative major and trace element geochemistry of gabbroic and volcanic rock sequences, Montgenèvre ophiolite, Western Alps. Schweiz.Mineral. Petrogr. Mitt., 67: 147–169.
Blake, M.C. and Jayko, A.S., 1990. Uplift of very high pressure rocks in the western Alps: evidence for structural attenuation along low-angle faults. In: F. Roure, P. Heitzmann and R.Polino (Editors), Deep structure of the Alps. Mem. Soc. géol. Fr., Paris, 156: 237–246.
Bodinier, J. L., 1988. Geochemistry and petrogenesis of the Lanzo peridotite body, western Alps. Tectonophysics, 149: 67–88.
Boudier, F. and Nicolas, A., 1977. Structural controls on partial melting in the Lanzo peridotites. In: H.J.B. Dick (Editor), Oregon Dept. Mineral. Ind., 96: 63–78.
Caby, R., Dupuy, C. and Dostal, J., 1987. The very beginning of the Ligurian Tethys: Petrological and geological evidence from the oldest ultramafic-derived sediments in Queyras, Western Alps, France. Eclog. geol. Helv., 80: 223–240.
Cannât, M., Mével, C. and Stakes D., 1991. Stretching of the deep crust at the slow-spreading Southwest Indian Ridge. Tectonophysics, 190: 73–94.
Carpena, J. and Caby, R., 1984. Fission-track evidence for Late Triassic oceanic crust in the French Occidental Alps. Geology, 12: 108–111.
Chapelle, B., 1990. La litosphère océanique de la Téthys ligure. Etude des magmatismes basiques et acides (massifs ophiolitiques du Montgenèvre et de Haute-Ubaye). Unpublished Thesis, Grenoble, France, 196 pp.
Dilek, Y. and Delaloye, M., 1992. Structure of the Kizildag ophiolite, a slow-spread Cretaceous ridge segment north of the Arabian promontory. Geology, 20: 19–22.
Ildefonse, B., Nicolas, A. and Boudier, F., 1993. Evidence from the Oman ophiolite for sudden stress changes during melt injection at oceanic spreading centers. Nature, 236: 673–675.
Ishiwatari, A., 1985. Alpine ophiolites: product of low-degree of melting in a transcurrent rift zone. Eart Planet. Sei. Lett. 76: 93–108.
Lagabrielle, Y., Polino, R. et al., 1984. Les témoins d’une tectonique intraocéanique dans le domaine tethysien. Analyse des rapports entre les ophiolites et leur couvertures métasédimentaires dans la zone piémontaise des Alpes franco-italiennes. Ofioliti, 9: 67–88.
Lemoine, M., 1980. Serpentinites, gabbros and ophicalcites in the Piemont-Ligurian domain of the Western Alps: possible indicators of oceanic fracture zones and associated serpentine protrusions in the Jurassic-Cretaceous Tethys. Arch. Sei. Genève, 33: 103–116.
Lemoine, M., Tricart, P. and Boillot, G., 1987. Ultramafic and gabbroic ocean floor of the Ligurian Tethys Alps, Corsica, Apennines: In search of a genetic model. Geology, 15: 622–625.
Lombado, B. and Pognante, U., 1982. Tectonic implications in the evolution of the western Alps ophiolite metagabbros. Ofioliti, 2/3: 371–394.
Mével, C, Caby, R. and Kienast, J.R., 1978. Amphibolite facies conditions in the oceanic crust: example of amphibolitized flaser-gabbro and amphibolites from the Chenaillet massif, Hautes Alpes, France. Earth Planet. Sei. Lett., 39: 98–108.
Nicolas, A., 1989. Structures of ophiolites and dynamics of oceanic lithosphère. Kluwer, Dordrecht, 367 pp.
Pupin, J. P., 1980. Zircon and granite petrology. Contr. Min. Petrol, 73: 207–220.
Hanmer, S. and Passchier, C.W., 1991. Shear sense indicators: a review. Geol. Surv. Canada Spec. Pap., 90–17: 72 pp.
Tricart, P. and Lemoine, M., 1983. Serpentite oceanic bottom in south Queyras ophiolites (French Western Alps): record of the incipient oceanic opening of the Mesozoic Ligurian Tethys. Eclogae geol. Helv., 76: 611–629.
Weissert, H.J. and Bernoulli, D., 1985. A transform margin in the Mesozoic Tethys: evidence from the Swiss Alps. Geol. Rundschau, 74: 665–679.
Wever de, P. and Caby, R., 1981. Datation de la base des schistes lustrés post-ophiolitiques par des radiolaires Oxfordien supérieur-Kimmeridgien moyen dans les Alpes cottiennes Saint Véran, France. C. Rend. Acad. Sei., 292: 467–472.
Wever De, P., Baumgartner, P. and Polino, R., 1987. Précisions sur la datation de la base des Schistes Lustrés postophiolitiques dans les Alpes Cottiennes. C. Rend. Acad. Sei., 305: 487–491.
Wolf, M.B. and Wyllie, P.J., 1990. Liquid morphology interConnectivity in solid amphibolite during dehydration melting at 10 kbar. Trans. Am.Geophys. Union, 71: 1714.
Author information
Authors and Affiliations
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 1995 Springer Science+Business Media Dordrecht
About this paper
Cite this paper
Caby, R. (1995). Plastic Deformation of Gabbros in a Slow-spreading Mesozoic Ridge: Example of the Montgenèvre Ophiolite, Western Alps. In: Vissers, R.L.M., Nicolas, A. (eds) Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites. Petrology and Structural Geology, vol 6. Springer, Dordrecht. https://doi.org/10.1007/978-94-015-8585-9_5
Download citation
DOI: https://doi.org/10.1007/978-94-015-8585-9_5
Publisher Name: Springer, Dordrecht
Print ISBN: 978-90-481-4557-7
Online ISBN: 978-94-015-8585-9
eBook Packages: Springer Book Archive