Ophiolites and Oceanic Lithosphere Ophiolites and Oceanic Lithosphere edited by I. G. Gass, S. J. Lippard & A. W. Shelton Department of Earth Sciences, The Open University Milton Keynes 1984 Published for The Geological Society by Blackwell Scientific Publications Oxford London Edinburgh Boston Melbourne Published by DISTRIBUTORS Blackwell Scientific Publications USA Osney Mead, Oxford OX2 0EL Blackwell-Mosby Book Distributors 8 John Street, London WC1N 2ES 11830 Westline Industrial Drive 9 Forrest Road, Edinburgh EHI 2QH St Louis, Missouri 63141 52 Beacon Street, Boston, Massachusetts 02108, USA Canada 99 Barry Street, Carlton, Victoria 3053, Australia Blackwell-Mosby Book Distributors 120 Melford Drive, Scarborough Ontario M 1B 2X4 First published 1984 Australia Blackwell Scientific Book Distributors 31 Advantage Road, Highett 1984 The Geological Society. Authorization to Victoria 3190 photocopy items for internal or personal use, or the internal or personal use of specific clients, British Library Cataloguing in Publication Data is granted by The Geological Society for libraries and other users registered with the Copyright Ophiolites and oceanic lithosphere.--(Geological Clearance Center (CCC) Transactional Reporting Service, provided that a base fee of $02.00 per Society special publications, ISSN 0305-8719; No. I3) copy is paid directly to CCC, 21 Congress Street, 1. Submarine geology Salem, MA 01970, USA. 1. Gass, I.G. II. Lippard, S.J. III. Shelton,A.W. 0305-8719/84 $02.00. IV. Series 551.46'08 QE39 ISBN 0-632-01219-6 Printed in Great Britain by Alden Press Ltd, Oxford Contents Page Preface: GASS, I. G., LIPPARD, S. J. & SHELTON, A. W .......................................................... 1 I. NATURE AND FORMATION OF OCEANIC LITHOSPHERE MAGMA CHAMBERS: PRODUCTS AND PROCESSES ORCUTT, J. A., MCCLAIN, J. S. & BURNETT, M. Evolution of the ocean crust: results from recent seismic experiments ............................................................................................... 7 FISK, M. R. Depths and temperatures of mid-ocean-ridge magma chambers and the composition of their source magmas ............................................................................... 17 FLOWER, M. F. J, Spreading-rate parameters in ocean crust: analogue for ophiolite? ........... 25 SMEWING, J. D., CHR1STENSEN, N. I., BARTHOLOMEW, I. D. & BROWNING, P. The structure of the oceanic upper mantle and lower crust as deduced from the northern section of the Oman ophiolite ................................................................................................................ 41 GREGORY, R. T. Melt percolation beneath a spreading ridge: evidence from the Semail peridotite, Oman ............................................................................................................. 55 PALLISTER, J. S. Parent magmas of the Semail ophiolite, Oman ........................................... 63 BROWNING, P. Cryptic variation within the Cumulate Sequence of the Oman ophiolite: magma chamber depth and petrological implications .................................................................. 71 ELTHON, D., CASEY, J. F. & KOMOR, S. Cryptic mineral-chemistry variations in a detailed traverse through the cumulate ultramafic rocks of the North Arm Mountain massif of the Bay of Islands ophiolite, Newfoundland ......................................................................... 83 FRACTURE ZONES WHITE, R. S. Atlantic oceanic crust: seismic structure of a slow-spreading ridge .................. 101 CYAGOR II GROUP. Intraoceanic tectonism on the Gorringe Bank: observations by submersible ...................................................................................................................... 113 HONNOREZ, J., MI~VEL, C. & MONTIGNY, R. Occurrence and significance of gneissic amphibolites in the Vema fracture zone, equatorial Mid-Atlantic Ridge ........................ 121 KARSON, J. A. Variations in structure and petrology in the Coastal Complex, Newfoundland: anatomy of an oceanic fracture zone ............................................................................... 131 MANTLE STRUCTURES NICOLAS, A. & RABINOWlCZ, M. Mantle flow pattern at oceanic spreading centres: relation with ophiolitic and oceanic structures ............................................................................. 147 LAVAS AND SEDIMENTS MALPAS, J. & LANGDON, G. Petrology of the Upper Pillow Lava suite, Troodos ophiolite, Cyprus ............................................................................................................................. 155 BOYLE, J. F. & ROBERTSON, A. H. F. Evolving metallogenesis at the Troodos spreading axis 169 ISOTOPE STUDIES AND METAMORPHISM ELTHON, D., LAWRENCE, J. R., HANSON, R. E. & STERN, C. Modelling of oxygen-isotope data from the Sarmiento ophiolite complex, Chile .................................................................. 185 STAKES, D. S., TAYLOR JR, H. P. & FISHER, R. L. Oxygen-isotope and geochemical characterization of hydrothermal alteration in ophiolite complexes and modern oceanic crust ................................................................................................................................. 199 THIRLWALL, M. F. & BLUCK, B. J. Sr-Nd isotope and chemical evidence that the Ballantrae 'ophiolite', SW Scotland, is polygenetic .......................................................................... 215 MENZIES, M. A. Chemical and isotopic heterogeneities in orogenic and ophiolitic peridotites 231 AHMED, Z. & HALL, A. Petrology and mineralization of the Sakhakot-Oila oohiolite. vi Contents II. EMPLACEMENT (OBDUCTION) OF OPHIOLITES OPHIOLITE EMPLACEMENT AND OBDUCTION SPRAY', J. G. Possible causes of upper mantle decoupling and ophiolite displacement .......... 255 CASEY, J. F. & DEWEY, J. F. Initiation of subduction zones along transform and accreting plate boundaries, triple-junction evolution, and forearc spreading centres--implications for ophiolitic geology and obduction .............................................................................. 269 OGAWA, Y. 8z NAKA, J. Emplacement of ophiolitic rocks in forearc areas: examples from central Japan and Izu-Mariana-Yap island arc system ................................................... 291 SEARLE, U. P. 8g. STEVENS, R. K. Obduction processes in ancient, modern and future ophiolites ......................................................................................................................... 303 WOODCOCK, N. H. & ROBERTSON, A. H. F. The structural variety in Tethyan ophiolite terrains ............................................................................................................................ 321 REGIONAL STUDIES COLLEY, H. An ophiolite suite in Fiji? ................................................................................... 333 DAVIES, H. L. & JAQUES, A. L. Emplacement of ophiolite in Papua New Guinea ............... 341 MILSOM, J. The gravity field of the Marum ophiolite complex, Papua New Guinea ............. 351 COLEMAN, R. G. Ophiolites and the tectonic evolution of the Arabian Peninsula ................. 359 WADGE, G., DRAPER, G. & LEWIS, J. F'. Ophiolites of the northern Caribbean: A reappraisal of their roles in the evolution of the Caribbean plate boundary ..................................... 367 STURT, B. A., ROBERTS, D. & FURNES, H. A conspectus of Scandinavian Caledonian ophiolites ......................................................................................................................... 381 HALL, R. Ophiolites: figments of oceanic lithosphere? ........................................................... 393 ROTHERY, D. A. The role of Landsat Multispectral Scanner (MSS) imagery in mapping the Oman ophiolite ................................................................................................................ 405 Preface The proposal that ophiolites are on-land fragments of oceanic lithosphere has been with us for over 20 years and has been widely accepted by the Earth Sciences' community. Despite this, many oceanographers are reluctant to use ophiolite data. Their argument (and it comes largely from the geological rather than geophysical oceanographers) is that 'Even if ophiolites are on-land fragments of oceanic lithosphere, they must be atypical for otherwise they would have been subducted and not obducted'. So they conclude that, although the study of ophiolites is perfectly acceptable in its own right, its results should not be used in the investigation of present-day, in-situ oceanic lithosphere--i.e, the invoking of 'reversed uniformitarianism' is not acceptable. However using uniformitarian principles to interpret ophiolites from present-day oceanic lithosphere studies is equally problematical for (i) petrological/geochemical studies of ocean-floor rocks are based on widely spaced boreholes of limited depth and dredge-haul sampling, (ii) geophysical data can only detect large-scale phenomena and are open to various interpretations, and (iii) direct observations from submersibles are still very limited. Such circumstances should have drawn the two groups of workers closer together. We believe that the deliberate ignoring of ophiolite evidence has, at best, slowed down our understanding of processes by which oceanic lithosphere is produced. Two examples to illustrate this will suffice: in the early 1970s, studies on ophiolite metamorphism indicated that hot seawater passed through the uppermost 4 km of the oceanic crust