See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/249551225 Ophiolites in Earth history: Introduction Article in Geological Society London Special Publications · January 2003 DOI: 10.1144/GSL.SP.2003.218.01.01 CITATIONS READS 139 998 2 authors: Yildirim Dilek Paul T. Robinson Miami University China University of Geosciences 460 PUBLICATIONS 17,909 CITATIONS 578 PUBLICATIONS 12,255 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Early Mesozoic Tectonics and Magmatism in the Qinling Orogenic Belt, China View project Diamonds and Recycled Mantle View project All content following this page was uploaded by Yildirim Dilek on 28 December 2015. The user has requested enhancement of the downloaded file. Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 Ophiolites in Earth history: introduction YILDIRIM D I L E K 1 & P A U L T. R O B I N S O N 2 1Department of Geology, Miami University, Oxford, 0H45056, USA (e-mail: [email protected]) 2Department of Earth Sciences, Dalhousie University, Halifax, N.S. B3H 3J5, Canada Ophiolites record significant evidence for tectonic and magmatic processes from rift-drift through accrefionary and collisional stages of continental margin evolution in various tectonic settings. Structural, petrological and geochemical features of ophiolites and associated rock units provide essential information on mantle flow field effects, including plume activities, collision-induced aesthenospheric extrusion, crustal growth via magmatism and tectonic accretion in subductionaccretion cycles, changes in the structure and composition of the crust and mantle reservoirs through time, and evolution of global geochemical cycles and seawater compositions. Ophiolite studies over the years have played a major role in better understanding of mid-ocean ridge and subduction zone processes, mantle dynanlics and heterogeneity, magma chamber processes, fluid flow mechanisms and fluid-rock interactions in oceanic lithosphere, the evolution of deep biosphere, the role of plate tectonics and plume tectonics in crustal evolution during the Precambrian and the Phanerozoic, and mechanisms of continental growth in accretionary and coltisional mountain belts. Through multi-disciplinary investigations and comparative studies of ophiolites and modern oceanic crust and using advanced instrumentation and computational facilities, the international ophiolite community has gathered a wealth of new data and syntheses from ophiolites around the world during the last 10 years. The purpose of this book is to present the most recent data, observations and ideas on different aspects of 'ophiolite science' through case studies and to document the mode and nature of igneous, metamorphic, tectonic, sedimentological and/or biological processes associated with the evolution of oceanic crust in different tectonic settings in Earth's history. It comprises 32 papers collected in six sections on temporal relations anaongst ophiolite genesis, mantle plume events and orogeny in Earth history; Tethyan ophiolites in the AlpineHimalayan orogenic system; magmatic, metamorphic and tectonic processes in ophiolite genesis; hydrothermal and biogenic alteration of oceanic crust; mechanisms of ophiolite emplace- ment; and regional occurrences of ophiolites and their geodynamic implications. Ophiolites, mantle plumes and orogeny Ophiolite occurrences around the world are not a random geological phenomenon. Ophiolites with certain age groups in different orogenic belts characterize distinct ophiolite pulses, which mark times of enhanced ophiolite genesis and emplacement. Examining the geological record of motmtain-building episodes and related events, Dilek shows that ophiolite pulses overlap significantly with the timing of major collisional events during the assembly of supercontinents, their break-up and increased mantle plume activities that developed extensive large igneous provinces (LIPs). These global events have been involved in the Wilson cycle evolution of ancient ocean basins that in turn contributed to ophiolite genesis in diverse tectonic settings. Suprasubduction zone ophiolites represent anomalous oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Accelerated LIP formation associated with superplume activities may have facilitated both the generation and tectonic emplacement of ophiolites at global scales. These spatial and temporal relations suggest that ophiolite pulses, mantle plume activities and orogenic events have been closely linked through complex mantle dynamics in Earth history. Tethyan ophiolites in the AlpineHimalayan orogenic system Papers in this section present diverse data from Tethyan ophiolites and provide refined geodynamic models for their evolution. Flower & Dilek examine the processes of arc-trench rollback and forearc accretion, and present an 'actualistic' model for ophiolites based on recent observations of forearc evolution in western Pacific and Mediterranean marginal basins. Collision-induced mantle flow and 'slab-pull' forces may result in rapid From:DILEK,Y. & ROBINSONP. T. (eds) 2003. Ophiolitesin Earth History.Geological Society, London, Special Publications, 218, 1-8. 0305-8719/03/$15 9 The Geological Society of London 2003. Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 2 Y. DILEK & P. T. ROBINSON arc-trench rollback pulses and associated extensional episodes (splitting of nascent volcanic proto-arcs), producing proto-ophiolites in arc-forearc settings. These ophiolites commonly include hightemperature metamorphic soles, boninitic rocks, juxtaposed refractory peridotites and high-temperature epidosites that are generally absent in mid-ocean ridge, normal arc and back-arc basin environments. As subduction rollback continues, arc-forearc complexes become increasingly heterogeneous, displaying significant internal age and structural discrepancies, a common feature both in the SW Pacific subduction zone environments and Tethyan ophiolites. When an arc-trench rollback cycle is terminated by a collision, heterogeneous forearc lithosphere is accreted as ophiolites in the initial stages of the evolution of collisional orogenic belts. This model demonstrates the apparent correspondence of subduction nucleation and mantle flow to plate collisions at regional and global scales. In a companion paper, Dilek & Flower explore the application of the arc-trench rollback and forearc accretion model to Neo-Tethyan ophiolites, specifically to the Mirdita (Albania), Troodos (Cyprus) and Semail (Oman) ophiolites. NeoTethyan oceans evolved as east-west-oriented basins separated by discrete continental fragments, which were rifted off from the northern edge of Gondwana beginning in the Triassic. Triassic rift assemblages containing within-plate-type alkaline basalt to transitional (T-MORB) and mid-ocean ridge basalt (MORB) are spatially associated with ophiolites in the eastern Mediterranean region and may represent the precursor of Late Triassic oceanic crust, which was subsequently consumed to produce the suprasubduction zone ophiolites. The three ophiolites examined here include a basement of typical 'oceanic' lithosphere intruded and overlain by boninitic (ultra-refractory) to calcalkaline series rocks that formed in a proto-arcforearc setting. This progression was a result of upper plate extension and further melting of previously depleted asthenosphere that occurred in response to successive stages of slab rollback. This igneous evolution of the ophiolites involved subduction initiation and one or more episodes of proto-arc splitting before the termination of slab rollback cycles as a result of trench-continent collisions. Miintener & Pieeardo examine the Lanzo and Corsica ophiolitic peridotites in the Alpine-Apennine mountain system that are interpreted as remnants of the Ligurian Tethys. The texture, geochemistry and petrology of these peridotites suggest that they represent exhumed subcontinental lithospheric mantle, which was modified and refertilized by migrating melts during opening of the embryonic Piedmont-Ligurian Ocean. Pervasive melt infiltration and melt-rock reaction produced gabbroic intrusions with a wide range of compositions characteristic of the melting column beneath mid-ocean ridges. These observations are critical to better understand the effects of melt percolation and impregnation in development of plagioclase-enriched peridotites. The Ligurian ophiolites clearly do not represent a typical Penrose-type, idealized oceanic crust. Bazylev et al. present mineral and bulk-rock chemistry data from the Jurassic Brezovica ultramafic massif (Serbia) in the Dinarides and show that its petrogenetic evolution involved two distinct magmatic stages. A suite of spinel harzburgites was produced during the first stage as a result of partial melting of the mantle and segregation of tholeiitic melts. Percolation of melt through these spinel harzburgites and melt-rock reaction produced dunites and refractory harzburgites during the second stage and generated highCa boninitic melt. The authors conclude that the second magmatic stage had to occur in a suprasubduction zone setting. Saccani et aL present new field and geochemical constraints from the Western Hellenides in Greece, documenting that initial stages of seafloor spreading and oceanic crust formation in the Pindos basin probably occurred in the Mid- to Late Triassic, earlier than previously thought. Pillow lavas from the Argolis Peninsula have MORB trace element characteristics and are divided into T-MORB and normal MORB (NMORB). These are the oldest unequivocally dated oceanic crust in the Hellenide sector of the Pindos Basin. Early Triassic rifting produced shoshonitic and calc-alkaline lavas derived from a mantle source that was previously contaminated by subduction components. Associated alkaline basalts were derived from ocean island basalt-type (OIB) mantle source. Mixing of mantle sources produced enriched MORB (E-MORB) and T-MORB, and then N-MORB lavas were erupted in Mid(?)- to Late Triassic, suggesting that sea-floor spreading had reached a steady state. The authors cite the Red Sea as a modern analogue with along-strike chemical variations for the Pindos Basin. Sarkarinejad describes the internal structure of the Cretaceous Neyriz ophiolite in southern Iran, and presents structural and microstructural observations for the existence of a NW-trending palaeotransform fault zone within this Neo-Tethyan ophiolite. Fabric analysis of mylonitic rocks (including hornblende and plagioclase textures and chemistry) suggests that the plastic deformation of mafic-ultramafic rocks occurred at amphibolitefacies conditions within a dextrally slipping oceanic transform fault zone. The author infers that Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 INTRODUCTION the Neyriz transform fault separated ENE-trending spreading centre segments within a Neo-Tethyan basin. The last three papers in this section present diverse stratigraphic, petrological, geochemical and geochronological data from the YarlungTsangpo suture zone ophiolites in southern Tibet. Aitchison et aL define several discrete terranes along the suture zone and use their sedimentological and biostratigraphic data to constrain the timing of ophiolite formation and terrane accretion within this segment of the HimalayanTibetan orogenic belt. Different ages of ophiolitic assemblages from Xigaze, Jungwa and Zedong indicate that the suture zone may contain remnants of multiple (two?) island arc complexes that had evolved within the same branch of Neo-Tethys. Hrbert et aL report mineral chemistry data and petrological findings from mafic-ultramafic rocks of the Yarlung Tsangpo ophiolites. Mantle peridorites were exhumed from depths of more than 50 km and underwent 10-40% partial melting and melt percolation within a suprasubduction zone wedge. The Yarlung Tsangpo ophiolites represent a heterogeneous collage of arc, forearc and backarc oceanic lithosphere developed in a NeoTethyan basin south of the active continental margin of Eurasia. Malpas et aL present new geochronological data from the Yarlung-Tsangpo ophiolites and a refined geodynamic model for their evolution. The new sensitive high-resolution ion microprobe date of 126 Ma for the Dazhuqu massif indicates that the Xigaze ophiolite is significantly younger than the Loubusa ophiolite and Zedong island arc complex (c. 175 Ma). These findings are consistent with the geochemical interpretations of H6bert et al. Basaltic rocks from all ophiolites are composed of island arc tholeiites, and the peridotites show textural and chemical evidence for percolation of boninitic melts through the upper mantle at later stages of magmatism. The Yarlung-Tsangpo ophiolites may have formed at different times in suprasubduction zone environments and were subsequently juxtaposed during the collision of the Indian continental margin with the arc-trench system around 90-80 Ma. Magmatic, metamorphic and tectonic processes in ophiolite genesis The six papers in this section present processoriented case studies of oceanic crust evolution from the Appalachian, Cordilleran, Tethyan and Japanese ophiolites. Harper demonstrates that the extrusive sequence and sheeted dyke complex in the Jurassic Josephine ophiolite in CaliforniaOregon (USA) display chemical evidence for a 3 wide range in magma types and degree of fractionation. New discoveries of Fe-Ti-rich and Ti-poor (boninitic) magmas in the Josephine ophiolite illustrate its compositional complexity and provide new constraints on its tectonic environment of formation. The Fe-Ti lavas imply formation along a propagating rift, whereas the low-Ti lavas suggest a forearc environment of their origin. The Lau Basin is cited as a likely modern analogue because the available geochemical data from several environments within this modern back-arc basin are consistent with the new chemical data and interpretations from the Josephine ophiolite. Northern Tonga and the Andaman Sea may also be plausible analogues for the Josephine ophiolite. Sehroetter et al. examine the internal structure and stratigraphy of the Ordovician Thetford Mines ophiolite in Quebec (Canada). The discovery of a locally well-developed sheeted dyke complex, combined with other structural data, indicates that the Ordovician oceanic crust was developed at a stow-spreading centre, where faulting and magmatism were coeval, keeping pace with crustal extension. The boninitic affinity of cumulate rocks and lavas suggests that the Thetford Mines ophiolite probably formed in a forearc setting. This is one of the best-documented cases of well-established pre-collisional extensional tectonics in a palaeoforearc environment. Raymond et aL investigate the occurrence and petrogenesis of ultramafic rock bodies in the Southern Appalachian (USA) orogenic belt. These ultramafic rocks are part of dismembered Ordovician ophiolites, which probably formed in a slowspreading centre within a subduction zone setting. A suprasubduction zone environment of origin is supported by the existence of metadunites representing sublithospheric melt channels and zones of high melt flux. The authors suggest that the Taconic subduction zone that was responsible for the formation of the Southern Appalachian ophiolites may have been west-directed, rather than east-directed as previous models have inferred. Hirano et al. show that the Tertiary Mineoka ophiolite in central Japan had a multi-stage tectonic evolution prior to its emplacement onto the Japanese continental margin. It occurs near a trench-trench-trench triple junction and contains tholeiitic pillow basalts and dolerites, calc-alkaline plutonic rocks and alkali-basaltic sheet flows. The sea-floor spreading stage of the ophiolite probably occurred during the generation of an oceanic Mineoka Plate in the Eocene. Subduction of the Pacific Plate beneath the Mineoka Plate produced island arc volcanism during 40-25 Ma (second stage). Eruption of the within-plate-type alkali basalts (WPB) during the third stage occurred around 20 Ma, shortly before the emplacement of Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 4 Y. DILEK & P. T. ROBINSON the polygenetic Mineoka ophiolite onto the continental margin. The ophiolite was derived from the Mineoka Plate, not from the Philippine Sea or Pacific Plates as previous models suggest. The companion paper by Takahashi et aL examines the internal structure of the Mineoka ophiolite and reports three main phases of deformation recorded by ophiolitic rocks. The first deformation phase was manifested in obliquenormal faults and associated vein systems, and was associated with extensional tectonics at a palaeo-spreading centre. The second phase of deformation, characterized by thrust faults and strike-slip shear zones, was related to the emplacement of the ophiolite. The third phase of deformation is represented by transpressional dextral faults, manifestation of the modern tectonic regime in a trench-trench-trench triple junction. The last paper in this section, by Stakes & Taylor, documents the occurrence of large plagiogranite intrusions in the northern part of the Semail ophiolite (Oman) and their spatial and temporal association with the formation of massive sulphide deposits. Chemical, isotopic and field relations indicate that plagiogranite bodies near the overlying sheeted dykes formed through a complex process of combined assimilation and fractional crystallization, and recharge by injection of basaltic magma in open-system magma chambers. These plagiogranites were clearly late-stage magmatic products postdating the formation of the main ophiolitic crust and acted as shallow point sources of heat and metals for development of the overlying economic massive sulphide deposits. Hydrothermal and biogenic alteration of oceanic crust as recorded in ophiolites The four papers in this section examine the nature, mechanisms and products of hydrothermal and biogenic alteration of oceanic crust and their implications for geochemical cycles in Earth history. Gregory demonstrates that the hydrothermal alteration history of ophiolites has major implications for the isotopic evolution of seawater. Isotopic profiles through ophiolites (e.g. Semail) show completely different characteristics depending on the element involved (Nd, Sr and O) and its residence time in the ocean. Oxygen isotopes are perhaps the most useful indicators of geochemical cycles and seawater-rock interaction. The mean value of altered oceanic crust is close to its primary lso/160 ratio, which means that there must be complementary reservoirs of 1SOdepleted and -enriched rocks in the altered ocean crust. Ophiolites are particularly useful because they are pieces of oceanic lithosphere that have escaped recycling. Ophiolite studies show that oxygen isotopic composition of seawater resides at near steady-state conditions over Earth history. Gigu/~re et al. present mineral and oxygen isotope geochemistry data from gabbroic rocks of the North Arm Mountain massif in the Bay of Islands ophiolite in Newfoundland (Canada) to constrain the chronology and temperature conditions of fluid circulation with respect to the timing and nature of deformation as recorded in these lower-crustal rocks. With continued cooling of gabbroic rocks, amphibole compositions changed as temperatures of amphibole formation fell steadily. Early amphiboles show near igneous oxygen isotope compositions typical of MORB or backarc basin basalt (BABB). Seawater infiltration into the lower crust occurred along listric shear zones under low fluid/rock ratios during the initial stages of deformation and metamorphism. Further cooling facilitated brittle deformation and greater seawater penetration at depth with increased fluid/ rock ratios, as suggested by very low 61So values. Field relations suggest that late-stage trondhjemitic intrusions may have provided heat and convective circulation of hydrothermal fluids causing high-T alteration superimposed on earlier stage of lower-T alteration. These relations clearly show that successive episodes of hydrothermal alteration of fossil lower crust in the Bay of Islands ophiolite were entirely intra-oceanic in origin. Muehlenbachs et aL use the hydrothermal alteration history of the Ordovician SolundStavfjord Ophiolite Complex (SSOC) in western Norway to examine the oxygen isotope ratio of ancient seawater. Similar to most ophiolites, the SSOC shows enrichment of 180 in the lavas altered at low temperatures and depletion in the dykes and gabbros altered at higher temperatures; this is also compatible with the alteration profile of 5.9 Ma in situ oceanic crust drilled in Ocean Drilling Program Hole 504B south of the Costa Rica Rift. Ophiolites can reflect the isotopic composition of ancient seawater. There is no observable secular trend in the 6180 of seawater, and hence the mode and scale of seawatersea-floor interaction has not changed with time. The 6180 of sediments and fossils may not record true values but rather owe their compositions to isotopic resetting, warmer oceans or biased sampling of restricted basins. Thus, models of ancient climates and ocean volumes determined from such data may be incorrect. Furnes & Muehlenbachs examine the nature and extent of bioalteration in fossil oceanic crust with different ages. Bioalteration of volcanic glass has been demonstrated in in situ oceanic crust but is not yet well documented from ophiolites. The authors have looked for evidence of bioalteration Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 INTRODUCTION 5 in glassy pillow lavas from four major ophiolites: Cretaceous Troodos (Cyprus), Jurassic Mirdita (Albania), Ordovician Solund-Stavt~ord (western Norway) and early Proterozoic Jormua (Finland). Bioalteration may be recognized from textural evidence, organic remains, chemical fingerprints (C, N, S and P) and carbon isotopic signatures. Textural evidence in the form of coalesced spheres and tubes is present only in Troodos and Mirdita, the youngest of the ophiolites investigated. Some textural features in the SSOC resemble biogenerated textures, but rocks metamorphosed to amphibolite facies grade lack any evidence of bioalteration. Organic remains, in the form of twisted filaments, have been found only in Troodos. Probable organic carbon has been found in rocks from Troodos and the SSOC. Carbon isotope data in glassy samples are shifted to lower values and have a pattern very similar to that for in situ oceanic lavas. Evidence of bio-alteration appears to survive low-grade greenschist-facies metamorphism but is generally destroyed at higher grades of metamorphism. structural data from rocks beneath the ophiolite nappe suggesting that there was an earlier period of underthrusting-subduction beneath the Arabian continental margin prior to its formation and obduction. Therefore, emplacement of the Semail nappe cannot simply be linked to a single subduction zone dipping away from the continent during the evolution of the ophiolite. The age of eclogite metamorphism in the lower-plate rocks beneath the ophiolite nappe (Saih Hatat Window) is crucial in testing this and other existing models. Searle et al. dispute this model by Gray & Gregory and discuss whether all structures and metamorphism observed in northern Oman are related to a single, prolonged episode of ophiolite emplacement, lasted for c. 27 million years and associated with a subduction zone dipping away from the Arabian continent. Suprasubduction zone origin of the ophiolite, metamorphic sole generation and eclogite formation were all linked to this subduction zone. Clearly, more precise age dating of the highpressure rocks beneath the ophiolite is needed to resolve the current debate. Ophiolite emplacement: mechanisms and processes Regional occurrence of ophiolites and geodynamic implications Emplacement of ophiolites into continental margins is a first-order tectonic problem in plate tectonics and a significant phase in the evolution of orogenic belts. Ever since their recognition as on-land fragments of ancient oceanic lithosphere, mechanisms and processes involved in incorporation of ophiolites into continents have been a subject of discussion amongst researchers. The three papers in this section evaluate the existing models and ideas on ophiolite emplacement mechanisms with a focus on the Cretaceous Semail ophiolite in Oman. Wakabayashi & Dilek discuss the mechanisms and significance of subduction initiation and metamorphic sole formation in ophiolite emplacement and define four prototype ophiolites based on their emplacement mechanisms. Tethyan ophiolites are collisional-type emplaced over passive continental margins, whereas Cordilleran ophiolites are emplaced over subduction complexes through accretionary processes. Emplacement of ridge-trench intersection (RTI) ophiolites occurs through complex processes resulting from interaction of a spreading ridge and a subduction zone. Macquarie Island-type ophiolite represents oceanic crust exposed as a result of shifts in plate boundary configurations (i.e. spreading ridge segments converting into a diffuse transpressional plate boundary). Gray & Gregory review emplacement models for the Semail ophiolite in Oman and present The papers in this section involve the regional occurrence of ophiolite belts on different continents and provide new petrological, geochemical and geochronological data and syntheses to better constrain their geodynamic evolution. Harris explores the spatial, temporal, geological and geochemical patterns of ophiolites in the Indonesian and New Guinea region (ING) in the first paper. ING is a repository of island arcs, marginal basins, continental fragments and ophiolites amalgamated by repeated plate boundary reorganizations. Major plate boundary reorganizations in the ING region coincide with global plate motions and there is a strong correlation in space and time between ophiolite genesis and collisional events. Opening of basins and suprasubduction zone generation of ophiolites are likely to have been 'enhanced' by extrusion of aesthenosphere escaping collisional zones in the region. Ophiolites forming in these suprasubduction zone environments display age and compositional heterogeneity, indicating their composite nature. Milsom examines the New Caledonia region in the SW Pacific to determine the spatial relations between forearc ophiolites and their volcanic arc systems. Repeated episodes of collisional events, postcollisional faulting and magmatism, and sea-floor spreading appear to have displaced and separated forearc tectonic assemblages from their respective volcanic arc systems in the New Caledonia-New Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 6 Y. DILEK & P. T. ROBINSON Guinea region. This complex history may be responsible for the apparent lack of volcanic arc edifices associated with other forearc ophiolites (e.g. Troodos in Cyprus) around the world. Spaggiari et al. provide an overview of the Neoproterozoic to Cambrian ophiolites of the Tasmanides in eastern Australia, and examine the differences in their emplacement styles and tectonic settings. Eastern Australian ophiolites fall into Tethyan- and Cordilleran-type categories depending on their relationship to 'continental basement', and they appear to have developed in various suprasubduction zone environments (arc, forearc, back-arc) along the eastern Gondwana margin. Their age progression and geochemistry, combined with regional structural and tectonic constraints, suggest that they evolved in a complex rifled arc-back-arc system during 530-485 Ma, and that the collapse of this system into the continental margin of East Gondwana resulted in their emplacement. This event might have been related to far-field stresses associated with the collisional assembly of greater Gondwana in the early Palaeozoic. Zhang et ai. summarize the regional distribution, ages and inferred tectonic settings of ophiolites in China. The Chinese ophiolites fall into four major age groups, Proterozoic, early Palaeozoic, late Palaeozoic and Mesozoic-Cenozoic, and they mainly occur along suture zones separating different tectonic blocks. They have a m61ange character in general and display structural and metamorphic evidence for multiple episodes of collisional events. The majority of the Chinese ophiolites are compositionally heterogeneous, containing mixtures of island arc tholeiite and boninite with lesser amounts of MORB and OIB. Palaeo-Tethyan ophiolites mostly have MORBtype rocks and may have formed in small intracontinental basins. Spadea et al. investigate the pyroxene and amphibole compositions of various mantle peridorites, particularly the Nurali and Mindyak massifs in the Southern Uralides in Russia. The Ural Mountains are a fold mountain system that records a Late Paleozoic arc-continent collision along the eastern European palaeomargin of Baltica. The Main Uralian Fault marks the related suture zone that consists of a mrlange composed of arc fragments and dismembered ophiolites. The Nurali and Mindyak peridotites have several anomalous features for abyssal peridotites: fertile composition; internal zoning from lherzolite to dunite to harzburgite; anomalous crust-mantle transition with amphibole-bearing, plagioclase-free, ultramafic cumulates; lack of associated crustal section; and intrusion of late (400Ma) gabbro-diorite plutons. These peridotite bodies underwent multi- stage igneous events including porous flow, and rock-melt interaction involving pyroxene dissolution and plagioclase precipitation. They thus show some similarities to peridotites of subcontinental mantle and/or continent-ocean transition zone mantle. The authors present two explanations for the origin of these peridofite massifs in the Southern Uralides: (1) the anomalous features (for abyssal pefidotites) reflect modification of normal MORB peridotites formed beneath a spreading axis by large volumes of island arc melts; or (2) the peridotites were originally part of subcontinental mantle, which underwent modification by dominantly tholeiitic melts causing plagioclase precipitation. Ishiwatari et aL discuss the petrological diversity and origin of ophiolites in Japan and Far East Russia, and distinguish highly depleted mantle harzburgite (DH) massifs in them. These ophiolites range in age from Early Palaeozoic to Cenozoic and are tectonically underlain by blueschist-bearing rocks and accretionary complexes that are generally younger in age. The majority of the ophiolites probably formed intra-oceanic island arc systems, as their petrological and geochemical characteristics suggest, and were incorporated into the Eurasian continental margin through repeated episodes of Mariana-type nonaccretionary subduction zone processes over time. There is little in the English literature on the ophiolite complexes of NE Asia. Sokolov et aL present new data on the age, structure and composition of ophiolites in the West Koryak fold belt in Far East Russia. The region consists chiefly of a variety of accreted terranes of different age and character. The ophiolites fall into two main categories. Palaeozoic ophiolites are primarily oceanic (MORB) in character and are viewed as fragments of the Panthalassa Ocean. Mesozoic ophiolites typically have an SSZ signature. In general, the ophiolites become younger towards the Pacific Ocean in the east. Accretionary prisms contain terrigeneous m61anges similar to those of the Shimanto Belt of SW Japan. Stern & De Wit describe the geology and geochemistry of the Mesozoic Rocas Verdes ophiolites in the southernmost Andes (South America) and show that these ophiolites evolved in a Late Jurassic-Early Cretaceous intra-arc basin along the southern edge of Gondwana. Primary crosscutting relations of ophiolitic dyke swarms with the surrounding crystalline basement rocks of the Andean magmatic arc indicate that Rocas Verdes basin was an ensialic small ocean that opened up by 'unzipping' from the south to the north, synchronously with the onset of seafloor spreading in the South Atlantic at c. 132 Ma. Thus the Rocas Verdes ophiolites provide a unique Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 INTRODUCTION opportunity to investigate the mode and nature of igneous, metamorphic and tectonic processes associated with continental rifting, sea-floor spreading and tectonic collapse of a back-arc basin in an Andean-type active continental margin. Finally, Dilek & Ahmed present an overview of the Proterozoic ophiolites in the Arabian Shield and discuss their significance in Precambrian tectonics. The Arabian Shield ophiolites range in age from c. 870 Ma to c. 627 Ma and display a record of rift-drift, sea-floor spreading and collision tectonics during the evolution of the East African Orogen in the aftermath of the break-up of Rodinia. Ophiolites in the western part of the shield were part of ensimatic are terranes, which were sutured through a series of collisional events. Younger ophiolites in the eastern Arabian Shield were incorporated into accretionary complexes through offscraping and collisional events during continued subduction, similar to the accretionary history of those Phanerozoic ophiolites in NE Asia as reported by Sokolov et al. The youngest ophiolites in the shield (Nabitah-Hamdah fault zone ophiolites) are post-collisional in origin and they represent Ligurian-type oceanic crust developed in an intracontinental para-rift basin. The Arabian shield ophiolites are clearly diverse in origin and provide a great opportunity to investigate oceanic and juvenile crust evolution in the latest Precambrian. Concluding r e m a r k s Ophiolites are critical windows into Earth history to examine the mode and nature of and the interplay between various igneous, metamorphic, sedimentological, hydrothermal and tectonic processes during generation of oceanic lithosphere. They also provide essential information on the mechanics and kinematics of mountain building episodes, as their incorporation into continental margins involved major tectonic events in orogenesis. New data and observations presented in different papers in this book clearly show that there is not a single, unique tectonic environment of ophiolite formation, and that ophiolites are diverse in origin, representing fragments of fossil oceanic lithosphere formed in various tectonic settings and in different stages of Wilson cycle evolution of ancient ocean basins. Most ophiolites are heterogeneous in lithological make-up, internal architecture and alteration history, indicating that their formation involved complex and multiple phases of magmatism, metamorphism and tectonism. Precise radiometric, isotopic and biostratigraphic age dating is needed to better constrain the timing of different evolutionary phases in ophiolite generation. 7 Some ophiolites contain peridotites that may represent exhumed subcontinental lithospheric mantle. It is particularly interesting that this appears to be the case for those ophiolitic assemblages in the Alps and Apennines, where the ophiolite concept was born and first developed through keen observations by influential researchers such as Alexandre Brogniart (1740-1847) and Gustav Steinmann (1856-1929). The existence of these subcontinental lithospheric mantle peridotites suggests that some ophiolites may record the initial stages of rift-drift evolution of small ocean basins in Earth history. Detailed petrological studies of some of the peridotite massifs (i.e. Miintener & Pieeardo; Spadea et aL) indicate that pervasive melt migration through these ultramafic rocks resulted in extensive melt-rock reaction, precipitation of plagioclase-enriched peridotites and generation of gabbroic intrusions during the early stages of oceanic lithosphere formation. Late-stage and off-axis(?) magmatism that produced large plagiogranite-trondhjemite intrusions into the pre-existing oceanic crust was responsible for extensive hdyrothermal alteration and mineralization in some ophiolites (Semail, Oman, Stakes & Taylor; Bay of Islands, Newfoundland, Gigu6re et al.). These intrusive bodies provided the local heat source that set up convective circulation of high-temperature fluids reacting with the host rocks and precipitating in due course epidosites and economic massive sulphide deposits. These spatial and temporal links between late plagiogranite intrusions and alteration-mineralization indicate that magmatism in oceanic crust generation is commonly episodic and multi-stage. Mantle dynamics and heterogeneity at regional and global scales appear to have played a critical role in the evolution of small ocean basins (mostly back-arc and/or marginal basins) and their lithosphere. Collision-induced mantle extrusion and flow strongly affected arc-trench rollback mechanisms, melt flow patterns and thermal state in subduction environments that collectively controlled ophiolite-forming processes (Dilek & Flower; Flower & Dilek). Some ophiolites and related tectonic units (i.e. rift assemblages as precursors to ophiolite generation) display geochemical evidence for mantle source(s), which were contaminated by previous subduction events in the region (e.g. Saccani et al.). These observations and interpretations from ophiolites, coupled with isotopic signatures of oceanic basalts, suggest that the mantle is heterogeneous at all scales mainly as a result of subduction of sediments, hydrothermal alteration of oceanic crust and melting-induced differentiation. Emplacement of ophiolites in collisional orogenic belts involves underplating of less-dense Downloaded from http://sp.lyellcollection.org/ by guest on December 24, 2015 8 Y. DILEK & P. T. ROBINSON crustal material beneath displaced oceanic lithosphere in subduction zone environments. The arrival of, and attempted partial subduction of, passive continental margins and/or island arc complexes at oceanic trenches provides the necessary physical conditions for this type of ophiolite emplacement. In accretionary-type orogenic belts (such as in Japan, Far East Asia and late Mesozoic-Cenozoic western North American Cordillera), continued consumption of ocean floor at active continental margins facilitates progressive ophiolite emplacement through tectonic incorporation of stranded slabs of oceanic crust, abyssal peridotites and seamounts into the subduction-accretion complexes. These kinds of ophiolites (defined as 'Cordilleran' by Wakabayashi & Dilek) are commonly spatially associated with blueschist-bearing tectonostratigraphic units and subduction m61anges. 'Ophiolite science' is a dynamic, evolving and interdisciplinary enterprise that is at its best View publication stats through international collaboration. Future international ophiolite studies, focusing on: (1) careful and systematic documentation of primary (seafloor spreading and/or igneous accretion stage) and secondary (emplacement and post-emplacement) structures within different ophiolitic subunits and of contact relations between them; (2) precise and systematic radiometric and isotopic dating of igneous and metamorphic rocks in ophiolites, and biostratigraphic dating of overlying sedimentary cover and underlying m~lange units; (3) isotopic analysis of ophiolite peridotites to delineate the mantle composition and signatures of their melt source, and mantle domains; and (4) combined geochemical, petrological and structural studies of ophiolites and associated tectonic units to differentiate tectonic settings of their origin and evolution, will help us better understand the Earth history and the processes involved in its evolution through time.
