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Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites: Contributions to a Specialized Symposium of the VII EUG Meeting, Strasbourg, Spring 1993 PDF

214 Pages·1995·6.04 MB·English
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Preview Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites: Contributions to a Specialized Symposium of the VII EUG Meeting, Strasbourg, Spring 1993

MANTLE AND LOWER CRUST EXPOSED IN OCEANIC RIDGES AND IN OPHIOLITES Petrology and Structural Geology VOLUME6 Series Editor: ADOLPHE NICOLAS Department of Earth and Space Sciences, University of Montpellier, France The titZes published in this series are listed at the end of this voZume. Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites Contributions to a Specialized Symposium o[ the VII EUG Meeting, Strasbourg, Spring 1993 Edited by R.L.M. VISSERS Faculty oi Earth Sciences, Geodynamics Research Institute, Utrecht, The Netherlands and A. NICOLAS Laboratoire de Tectonophysique, Universite des Sciences et Techniques du Languedoc, Montpellier, France Springer-Science+Business Media, B.V. A c.1.p. Catalogue re cord for this book is available from the Library of Congress ISBN 978-90-481-4557-7 ISBN 978-94-015-8585-9 (eBook) DOI 10.1007/978-94-015-8585-9 Printed on acid-free paper All Rights Reserved © 1995 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1995. Softcover reprint of the hardcove 1s t edition 1995 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any me ans, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. Table of Contents Introduction R.L.M. Vissers and A. Nicolas 1 PART I: MARINE STUDIES An Ultramafic Lift at the Mid-Atlantic Ridge: Successive Stages of Magmatism in Serpentinized Peridotites from the 15°N Region. Mathilde Cannat and lohn F Casey 5 Gabbroie Dikelets in Serpentinized Peridotites from the Mid-Atlantic Ridge at 23°20'N P. Tartarotti, M. Cannat and C. Me el 35 Mafic and Ultramafic Intrusions into Upper Mantle Peridotites from Fast Spreading Centers of the Easter Micropiate (South East Pacific) M. Constantin, R. Hekinian, D. Ackermand and P. Stoffers 71 PART 11: OPHIOLITE STUDIES Plastic Deformation of Gabbros in a Slow-spreading Mesozoie Ridge: Example of the Montgenevre Ophiolite, Western Alps R. Caby 123 Pre-orogenic High Temperature Shear Zones in an Ophiolite Complex (Bracco Massif, Northern Apennines, Italy) Giancarlo Molli 147 A Detailed Study of Mantle Flow away from Diapirs in the Oman Ophiolite B. Ildefonse, S. Billiau and A. Nicolas 163 PART III: NUMERICAL MODELLING Non Steady-State Thermal Model of Spreading Ridges: Implications for Melt Generation and Mantle Outcrops Chan tal Tisseau and Thierry Tonnerre 181 Introduction This volume follows a Specialized Symposium on "Mantle denudation in slow spreading ridges and in ophiolites", held at the XII EUG Meeting in Strasbourg, spring 1993. During the meeting it was felt that the contribu- tions to the Symposium justified a volume presenting its main scientific achievements. The present title of the volume shows that the center of inter- est has slightly shifted with respect to the initial objective: in order to under- stand the processes involved in accretion taking place at oceanic ridges, it is crucial to study the interaction between uppermost mantle and lower crust. The approach favored here is that of petrological and structural analysis of oceanic rocks in present-day oceanic ridges combined with similar studies in ophiolites. Rock specimen collected by submersibles or dredge hauls in oceanic ridge environments provide a "ground truth". However, except for areas such as the MARK (Mid-Atlantic Ridge ne ar Kane fracture zone) where, thanks to multiple submersible dives, the local geology is known with aprecision even better than in many onshore ophiolites, mutual rela- tionships between uppermost mantle and lower crust are poorly known. In contrast, onshore ophiolites provide a necessary large-scale picture built up over many years of structural and petrological mapping. Systematic observation of mantle rocks near the Mid Atlantic Ridge (MAR) has yielded a surprise, as it was predicted to find an "ophiolite- type" of oceanic crust in these regions. By the late 80's it became under- stood that mantle denudation and exposure of mantle rocks at the ridge axis result from cyclic magmatic activity. During a magmatic stage, expan- sion is accommodated by injection of fresh basaltic material creating the "ophiolite-type" oceanic crust, whereas during a following amagmatic stage expansion re lies on tectonic stretching. Oceanic crust is then detached along movement zones leading to exhumation and exposure of the underlying mantle. In their respective papers, M. Cannat and J.F. Casey, P. Tartarotti and C. Mevel take advantage of the exposure of deep- seated rocks along the axis of the Mid Atlantic Ridge to describe the mag- matic and tectonic processes which affected these deep zones of the ridge. Obviously, magmatic and tectonic processes go hand in hand at fast spread- ing ridges and, at least on the large scale, there is no manifestly cyclic activ- 2 ity. Exposure of deep rocks in such cases is much more limited as in the Deeps of the Easter Micropiate where they are studied in this volume by M. Constantin and his co-workers. These three marine studies of present- day oceanic environments are followed in the volume by three onshore ophiolite studies, the first two by R. Caby and G. Molli dealing with west- ern Alps-Apennines ophiolites probably akin to slow-spreading ridges and therefore comparing best with the MAR, and the last one by B. Ildefonse and his co-workers focussed on part of the Oman ophiolite, probably a pre- decessor of the fast-spreading East Pacific Rise. Eventually, the volume closes with a numerical model study of cyclic oceanic activity, developed by C. Tisseau and T. Tonnerre, involving a non steady-state thermal model for the axial domain of spreading ridges in which accretion is simulated as the superposi- tion of seafloor spreading and thermal inputs which vary through time following magmato-tectonic cycles. Their model effectively reconciles the extreme cases of slow and fast spreading ridges. Essentially based on the analysis of the "real rocks", this volume deal- ing with mantle-crust interactions reinforces the well-known contrast between processes taking place at slow and fast oceanic ridges. We are greatly indebted to Paul van Oudenallen, Fred Trappenburg, Izaak Santoe and Brigitte Benders of the Audiovisual Service, Institute of Earth Sciences, Utrecht, for their high-quality technical support in the final stages of the preparation of this volume. R.L.M. Vissers and A. Nicolas Part I Marine Studies An Ultramafic Lift at the Mid-Atlantic Ridge: Successive Stages of Magmatism in Serpentinized Peridotites from the 15°N Region. MATHILDE CANNAT AND lOHN F. CASEY* URA 736, Laboratoire de Petrologie, UPMC, 4 pI. Jussieu, 75252 Paris Cedex 05, France *Department olGeosciences, University 01 Houston, Houston, TX 77204-5503, USA Abstract We use the mineralogy, textures and chemical diversity of a set of gabbroic to trondhjemitic sampIes intrusive into serpentinized ultramafics at 1soN along the Mid-Atlantic Ridge to constrain a model for the building of the lower crust at magma-starved ocean ridges, This model involves successive stages of magmatism within mantle rocks that are rising up from the as- thenosphere, and throughout the axial lithosphere, to ultimately form seafloor exposures, The relationships between gabbro chemistry and the de- gree of mantle melting, and the possible role of mechanical melt segregation to produce evolved magmas in this region of the Atlantic are also discussed. Introduction The number of documented exposures of serpentinized peridotites along the Mid-Atlantic Ridge has steadily increased in the last years, as more dredging and submersible cruise were carried on. The current interpretation is that emplacement of these rocks in the seafloor characterizes ridge re- gions suffering, or having suffered, a deficit in magma (Karson et al., 1987; Dick, 1989; Karson, 1991). Submersible surveys, sampIe studies and field data from ophiolitic massifs (Lagabrielle and Cannat, 1990; Boudier et al., 1989) suggest that, instead of being made of successive layers covering the mantle as proposed by participants to the 1972 Penrose Conference (Pen- rose, 1972), the crust formed at these magma-starved mid-ocean ridges may be made of discrete intrusions within mantle peridotites (Dick et al., 1989; Lagabrielle and Cannat, 1990; Mevel et al., 1991; Cannat et al., 1992; Cannat, 1993). Studies of a relatively sparse sampIe set has shown that these intrusions displaya wide range of compositions, suggesting extensive closed- system fractionation and some degree of chemical interaction with their host ultramafics (Cannat et al., 1992; Tartarotti et al., this volume). The investiga- tion of the mineralogy, chemical diversity and modes of emplacement of R.L.M. Vissers and A. Nicolas (Eds.), Mantle and Lawer Crust Exposed in Oceanic Ridges and in Ophiolites. 5-34. © 1995 Kluwer Academic Publishers.

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