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Ophiolites in Earth history: introduction

YILDIRIM DILEK 1 & PAUL T. ROBINSON 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 ment; and regional occurrences of ophiolites and and magmatic processes from rift-drift through their geodynamic implications. accrefionary and collisional stages of evolution in various tectonic settings. Ophiolites, mantle plumes and Structural, petrological and geochemical features of ophiolites and associated rock units provide Ophiolite occurrences around the world are not a essential information on mantle flow field effects, random geological phenomenon. Ophiolites with including plume activities, collision-induced certain age groups in different orogenic belts aesthenospheric extrusion, crustal growth via mag- characterize distinct ophiolite pulses, which mark matism and tectonic accretion in - times of enhanced ophiolite genesis and emplace- accretion cycles, changes in the structure and ment. Examining the geological record of motm- composition of the crust and mantle reservoirs tain-building episodes and related events, Dilek through time, and evolution of global geochemical shows that ophiolite pulses overlap significantly cycles and seawater compositions. Ophiolite stud- with the timing of major collisional events during ies over the years have played a major role in the assembly of , their break-up better understanding of mid-ocean ridge and sub- and increased mantle plume activities that devel- duction zone processes, mantle dynanlics and oped extensive large igneous provinces (LIPs). heterogeneity, chamber processes, fluid These global events have been involved in the flow mechanisms and fluid-rock interactions in evolution of ancient ocean basins oceanic , the evolution of deep bio- that in turn contributed to ophiolite genesis in sphere, the role of and plume diverse tectonic settings. Suprasubduction zone tectonics in crustal evolution during the Precam- ophiolites represent anomalous gen- brian and the Phanerozoic, and mechanisms of eration in subduction rollback cycles during the continental growth in accretionary and coltisional closing stages of basins prior to terminal continen- mountain belts. Through multi-disciplinary inves- tal collisions. Accelerated LIP formation asso- tigations and comparative studies of ophiolites and ciated with superplume activities may have modern oceanic crust and using advanced instru- facilitated both the generation and tectonic empla- mentation and computational facilities, the inter- cement of ophiolites at global scales. These spatial national ophiolite community has gathered a and temporal relations suggest that ophiolite wealth of new data and syntheses from ophiolites pulses, mantle plume activities and orogenic around the world during the last 10 years. The events have been closely linked through complex purpose of this book is to present the most recent mantle dynamics in Earth history. data, observations and ideas on different aspects of 'ophiolite science' through case studies and to document the mode and nature of igneous, meta- Tethyan ophiolites in the Alpine- morphic, tectonic, sedimentological and/or bio- Himalayan orogenic system logical processes associated with the evolution of oceanic crust in different tectonic settings in Papers in this section present diverse data from Earth's history. It comprises 32 papers collected in Tethyan ophiolites and provide refined geody- six sections on temporal relations anaongst ophio- namic models for their evolution. Flower & Dilek lite genesis, mantle plume events and orogeny in examine the processes of arc-trench rollback and Earth history; Tethyan ophiolites in the Alpine- accretion, and present an 'actualistic' Himalayan orogenic system; magmatic, meta- model for ophiolites based on recent observations morphic and tectonic processes in ophiolite gen- of forearc evolution in western Pacific and Medi- esis; hydrothermal and biogenic alteration of terranean marginal basins. Collision-induced man- oceanic crust; mechanisms of ophiolite emplace- tle flow and 'slab-pull' forces may result in rapid

From: DILEK, Y. & ROBINSONP. T. (eds) 2003. Ophiolites in Earth History. Geological Society, London, Special Publications, 218, 1-8. 0305-8719/03/$15 9The Geological Society of London 2003. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

2 Y. DILEK & P. T. ROBINSON arc-trench rollback pulses and associated exten- the embryonic Piedmont-Ligurian Ocean. Perva- sional episodes (splitting of nascent volcanic pro- sive melt infiltration and melt-rock reaction pro- to-arcs), producing proto-ophiolites in arc-forearc duced gabbroic intrusions with a wide range of settings. These ophiolites commonly include high- compositions characteristic of the melting column temperature metamorphic soles, boninitic rocks, beneath mid-ocean ridges. These observations are juxtaposed refractory and high-tem- critical to better understand the effects of melt perature epidosites that are generally absent in percolation and impregnation in development of mid-ocean ridge, normal arc and back-arc basin plagioclase-enriched peridotites. The Ligurian environments. As subduction rollback continues, ophiolites clearly do not represent a typical Pen- arc-forearc complexes become increasingly het- rose-type, idealized oceanic crust. erogeneous, displaying significant internal age and Bazylev et al. present mineral and bulk-rock structural discrepancies, a common feature both in chemistry data from the Jurassic Brezovica ultra- the SW Pacific subduction zone environments and mafic massif (Serbia) in the Dinarides and show Tethyan ophiolites. When an arc-trench rollback that its petrogenetic evolution involved two dis- cycle is terminated by a collision, heterogeneous tinct magmatic stages. A suite of spinel harzbur- forearc lithosphere is accreted as ophiolites in the gites was produced during the first stage as a initial stages of the evolution of collisional oro- result of partial melting of the mantle and segre- genic belts. This model demonstrates the apparent gation of tholeiitic melts. Percolation of melt correspondence of subduction nucleation and through these spinel and melt-rock mantle flow to plate collisions at regional and reaction produced and refractory harzbur- global scales. gites during the second stage and generated high- In a companion paper, Dilek & Flower explore Ca boninitic melt. The authors conclude that the the application of the arc-trench rollback and second magmatic stage had to occur in a supra- forearc accretion model to Neo-Tethyan ophiolites, subduction zone setting. specifically to the Mirdita (Albania), Troodos Saccani et aL present new field and geochem- () and Semail () ophiolites. Neo- ical constraints from the Western Hellenides in Tethyan oceans evolved as east-west-oriented Greece, documenting that initial stages of sea- basins separated by discrete continental fragments, floor spreading and oceanic crust formation in the which were rifted off from the northern edge of Pindos basin probably occurred in the Mid- to Gondwana beginning in the Triassic. Triassic rift Late Triassic, earlier than previously thought. assemblages containing within-plate-type alkaline Pillow lavas from the Argolis Peninsula have to transitional (T-MORB) and mid-ocean MORB trace element characteristics and are di- ridge basalt (MORB) are spatially associated with vided into T-MORB and normal MORB (N- ophiolites in the eastern Mediterranean region and MORB). These are the oldest unequivocally dated may represent the precursor of Late Triassic oceanic crust in the Hellenide sector of the Pindos oceanic crust, which was subsequently consumed Basin. Early Triassic rifting produced shoshonitic to produce the suprasubduction zone ophiolites. and calc-alkaline lavas derived from a mantle The three ophiolites examined here include a source that was previously contaminated by sub- basement of typical 'oceanic' lithosphere intruded duction components. Associated alkaline and overlain by boninitic (ultra-refractory) to calc- were derived from ocean island basalt-type (OIB) alkaline series rocks that formed in a proto-arc- mantle source. Mixing of mantle sources produced forearc setting. This progression was a result of enriched MORB (E-MORB) and T-MORB, and upper plate extension and further melting of then N-MORB lavas were erupted in Mid(?)- to previously depleted asthenosphere that occurred in Late Triassic, suggesting that sea-floor spreading response to successive stages of slab rollback. had reached a steady state. The authors cite the This igneous evolution of the ophiolites involved Red Sea as a modern analogue with along-strike subduction initiation and one or more episodes of chemical variations for the Pindos Basin. proto-arc splitting before the termination of slab Sarkarinejad describes the internal structure of rollback cycles as a result of trench-continent the Cretaceous Neyriz ophiolite in southern Iran, collisions. and presents structural and microstructural obser- Miintener & Pieeardo examine the Lanzo and vations for the existence of a NW-trending palaeo- Corsica ophiolitic peridotites in the Alpine-Apen- transform fault zone within this Neo-Tethyan nine mountain system that are interpreted as ophiolite. Fabric analysis of mylonitic rocks (in- remnants of the Ligurian Tethys. The texture, cluding hornblende and plagioclase textures and geochemistry and petrology of these peridotites chemistry) suggests that the plastic deformation of suggest that they represent exhumed subcontinen- mafic-ultramafic rocks occurred at amphibolite- tal lithospheric mantle, which was modified and facies conditions within a dextrally slipping ocea- refertilized by migrating melts during opening of nic transform fault zone. The author infers that Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 3 the Neyriz transform fault separated ENE-trending wide range in magma types and degree of fractio- spreading centre segments within a Neo-Tethyan nation. New discoveries of Fe-Ti-rich and Ti-poor basin. (boninitic) in the Josephine ophiolite The last three papers in this section present illustrate its compositional complexity and provide diverse stratigraphic, petrological, geochemical new constraints on its tectonic environment of and geochronological data from the Yarlung- formation. The Fe-Ti lavas imply formation along Tsangpo zone ophiolites in southern Tibet. a propagating rift, whereas the low-Ti lavas sug- Aitchison et aL define several discrete terranes gest a forearc environment of their origin. The along the suture zone and use their sedimentologi- Lau Basin is cited as a likely modern analogue cal and biostratigraphic data to constrain the because the available geochemical data from timing of ophiolite formation and terrane ac- several environments within this modern back-arc cretion within this segment of the Himalayan- basin are consistent with the new chemical data Tibetan . Different ages of ophiolitic and interpretations from the Josephine ophiolite. assemblages from Xigaze, Jungwa and Zedong Northern Tonga and the Andaman Sea may also indicate that the suture zone may contain remnants be plausible analogues for the Josephine ophiolite. of multiple (two?) complexes that had Sehroetter et al. examine the internal structure evolved within the same branch of Neo-Tethys. and of the Ordovician Thetford Mines Hrbert et aL report mineral chemistry data and ophiolite in Quebec (Canada). The discovery of a petrological findings from mafic-ultramafic rocks locally well-developed sheeted dyke complex, of the Yarlung Tsangpo ophiolites. Mantle perido- combined with other structural data, indicates that rites were exhumed from depths of more than the Ordovician oceanic crust was developed at a 50 km and underwent 10-40% partial melting and stow-spreading centre, where faulting and magma- melt percolation within a suprasubduction zone tism were coeval, keeping pace with crustal exten- wedge. The Yarlung Tsangpo ophiolites represent sion. The boninitic affinity of cumulate rocks and a heterogeneous collage of arc, forearc and back- lavas suggests that the Thetford Mines ophiolite arc oceanic lithosphere developed in a Neo- probably formed in a forearc setting. This is one Tethyan basin south of the active continental of the best-documented cases of well-established margin of Eurasia. Malpas et aL present new pre-collisional in a palaeo- geochronological data from the Yarlung-Tsangpo forearc environment. ophiolites and a refined geodynamic model for Raymond et aL investigate the occurrence and their evolution. The new sensitive high-resolution petrogenesis of bodies in the ion microprobe date of 126 Ma for the Dazhuqu Southern Appalachian (USA) orogenic belt. These massif indicates that the Xigaze ophiolite is ultramafic rocks are part of dismembered Ordovi- significantly younger than the Loubusa ophiolite cian ophiolites, which probably formed in a slow- and Zedong island arc complex (c. 175 Ma). These spreading centre within a subduction zone setting. findings are consistent with the geochemical inter- A suprasubduction zone environment of origin is pretations of H6bert et al. Basaltic rocks from all supported by the existence of metadunites repre- ophiolites are composed of island arc tholeiites, senting sublithospheric melt channels and zones and the peridotites show textural and chemical of high melt flux. The authors suggest that the evidence for percolation of boninitic melts through Taconic subduction zone that was responsible for the at later stages of magmatism. the formation of the Southern Appalachian ophio- The Yarlung-Tsangpo ophiolites may have formed lites may have been west-directed, rather than at different times in suprasubduction zone environ- east-directed as previous models have inferred. ments and were subsequently juxtaposed during Hirano et al. show that the Tertiary Mineoka the collision of the Indian continental margin with ophiolite in central had a multi-stage tec- the arc-trench system around 90-80 Ma. tonic evolution prior to its emplacement onto the Japanese continental margin. It occurs near a trench-trench-trench triple junction and contains Magmatic, metamorphic and tectonic tholeiitic pillow basalts and dolerites, calc-alkaline processes in ophiolite genesis plutonic rocks and alkali-basaltic sheet flows. The sea-floor spreading stage of the ophiolite probably The six papers in this section present process- occurred during the generation of an oceanic oriented case studies of oceanic crust evolution Mineoka Plate in the Eocene. Subduction of the from the Appalachian, Cordilleran, Tethyan and Pacific Plate beneath the Mineoka Plate produced Japanese ophiolites. Harper demonstrates that the island arc volcanism during 40-25 Ma (second extrusive sequence and sheeted dyke complex in stage). Eruption of the within-plate-type alkali the Jurassic Josephine ophiolite in - basalts (WPB) during the third stage occurred Oregon (USA) display chemical evidence for a around 20 Ma, shortly before the emplacement of Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

4 Y. DILEK & P. T. ROBINSON the polygenetic Mineoka ophiolite onto the con- escaped recycling. Ophiolite studies show that tinental margin. The ophiolite was derived from oxygen isotopic composition of seawater resides the Mineoka Plate, not from the Philippine Sea or at near steady-state conditions over Earth history. Pacific Plates as previous models suggest. Gigu/~re et al. present mineral and oxygen The companion paper by Takahashi et aL isotope geochemistry data from gabbroic rocks of examines the internal structure of the Mineoka the North Arm Mountain massif in the Bay of ophiolite and reports three main phases of defor- Islands ophiolite in Newfoundland (Canada) to mation recorded by ophiolitic rocks. The first constrain the chronology and temperature condi- deformation phase was manifested in oblique- tions of fluid circulation with respect to the timing normal faults and associated vein systems, and and nature of deformation as recorded in these was associated with extensional tectonics at a lower-crustal rocks. With continued cooling of palaeo-spreading centre. The second phase of gabbroic rocks, amphibole compositions changed deformation, characterized by thrust faults and as temperatures of amphibole formation fell stea- strike-slip shear zones, was related to the emplace- dily. Early amphiboles show near igneous oxygen ment of the ophiolite. The third phase of deforma- isotope compositions typical of MORB or back- tion is represented by transpressional dextral arc basin basalt (BABB). Seawater infiltration into faults, manifestation of the modern tectonic re- the lower crust occurred along listric shear zones gime in a trench-trench-trench triple junction. under low fluid/rock ratios during the initial stages The last paper in this section, by Stakes & of deformation and metamorphism. Further cool- Taylor, documents the occurrence of large plagio- ing facilitated brittle deformation and greater sea- granite intrusions in the northern part of the water penetration at depth with increased fluid/ (Oman) and their spatial and rock ratios, as suggested by very low 61So values. temporal association with the formation of mas- Field relations suggest that late-stage trondhjemi- sive sulphide deposits. Chemical, isotopic and tic intrusions may have provided heat and con- field relations indicate that plagiogranite bodies vective circulation of hydrothermal fluids causing near the overlying sheeted dykes formed through a high-T alteration superimposed on earlier stage of complex process of combined assimilation and lower-T alteration. These relations clearly show fractional crystallization, and recharge by injection that successive episodes of hydrothermal alteration of basaltic magma in open-system magma cham- of fossil lower crust in the Bay of Islands ophiolite bers. These plagiogranites were clearly late-stage were entirely intra-oceanic in origin. magmatic products postdating the formation of the Muehlenbachs et aL use the hydrothermal main ophiolitic crust and acted as shallow point alteration history of the Ordovician Solund- sources of heat and metals for development of the Stavfjord Ophiolite Complex (SSOC) in western overlying economic massive sulphide deposits. Norway to examine the oxygen isotope ratio of ancient seawater. Similar to most ophiolites, the SSOC shows enrichment of 180 in the lavas Hydrothermal and biogenic alteration of altered at low temperatures and depletion in the dykes and altered at higher temperatures; oceanic crust as recorded in ophiolites this is also compatible with the alteration profile The four papers in this section examine the nature, of 5.9 Ma in situ oceanic crust drilled in Ocean mechanisms and products of hydrothermal and Drilling Program Hole 504B south of the Costa biogenic alteration of oceanic crust and their Rica Rift. Ophiolites can reflect the isotopic implications for geochemical cycles in Earth composition of ancient seawater. There is no history. Gregory demonstrates that the hydrother- observable secular trend in the 6180 of seawater, mal alteration history of ophiolites has major and hence the mode and scale of seawater- implications for the isotopic evolution of seawater. sea-floor interaction has not changed with time. Isotopic profiles through ophiolites (e.g. Semail) The 6180 of sediments and fossils may not record show completely different characteristics depend- true values but rather owe their compositions to ing on the element involved (Nd, Sr and O) and isotopic resetting, warmer oceans or biased sam- its residence time in the ocean. Oxygen isotopes pling of restricted basins. Thus, models of ancient are perhaps the most useful indicators of geo- climates and ocean volumes determined from such chemical cycles and seawater-rock interaction. data may be incorrect. The mean value of altered oceanic crust is close Furnes & Muehlenbachs examine the nature to its primary lso/160 ratio, which means that and extent of bioalteration in fossil oceanic crust there must be complementary reservoirs of 1SO- with different ages. Bioalteration of volcanic glass depleted and -enriched rocks in the altered ocean has been demonstrated in in situ oceanic crust but crust. Ophiolites are particularly useful because is not yet well documented from ophiolites. The they are pieces of oceanic lithosphere that have authors have looked for evidence of bioalteration Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 5 in glassy pillow lavas from four major ophiolites: structural data from rocks beneath the ophiolite Cretaceous Troodos (Cyprus), Jurassic Mirdita nappe suggesting that there was an earlier period (Albania), Ordovician Solund-Stavt~ord (western of underthrusting-subduction beneath the Arabian Norway) and early Jormua (Finland). continental margin prior to its formation and Bioalteration may be recognized from textural . Therefore, emplacement of the Semail evidence, organic remains, chemical fingerprints nappe cannot simply be linked to a single subduc- (C, N, S and P) and carbon isotopic signatures. tion zone dipping away from the continent during Textural evidence in the form of coalesced spheres the evolution of the ophiolite. The age of and tubes is present only in Troodos and Mirdita, metamorphism in the lower-plate rocks beneath the youngest of the ophiolites investigated. Some the ophiolite nappe (Saih Hatat Window) is crucial textural features in the SSOC resemble bio- in testing this and other existing models. Searle generated textures, but rocks metamorphosed to et al. dispute this model by Gray & Gregory and amphibolite facies grade lack any evidence of discuss whether all structures and metamorphism bioalteration. Organic remains, in the form of observed in northern Oman are related to a single, twisted filaments, have been found only in Troo- prolonged episode of ophiolite emplacement, dos. Probable organic carbon has been found in lasted for c. 27 million years and associated with a rocks from Troodos and the SSOC. Carbon iso- subduction zone dipping away from the Arabian tope data in glassy samples are shifted to lower continent. Suprasubduction zone origin of the values and have a pattern very similar to that for ophiolite, metamorphic sole generation and eclo- in situ oceanic lavas. Evidence of bio-alteration gite formation were all linked to this subduction appears to survive low-grade greenschist-facies zone. Clearly, more precise age dating of the high- metamorphism but is generally destroyed at higher pressure rocks beneath the ophiolite is needed to grades of metamorphism. resolve the current debate.

Ophiolite emplacement: mechanisms and Regional occurrence of ophiolites and processes geodynamic implications Emplacement of ophiolites into continental mar- The papers in this section involve the regional gins is a first-order tectonic problem in plate occurrence of ophiolite belts on different conti- tectonics and a significant phase in the evolution nents and provide new petrological, geochemical of orogenic belts. Ever since their recognition as and geochronological data and syntheses to better on-land fragments of ancient oceanic lithosphere, constrain their geodynamic evolution. Harris ex- mechanisms and processes involved in incorpora- plores the spatial, temporal, geological and geo- tion of ophiolites into continents have been a chemical patterns of ophiolites in the Indonesian subject of discussion amongst researchers. The and New Guinea region (ING) in the first paper. three papers in this section evaluate the existing ING is a repository of island arcs, marginal models and ideas on ophiolite emplacement me- basins, continental fragments and ophiolites amal- chanisms with a focus on the Cretaceous Semail gamated by repeated plate boundary reorganiza- ophiolite in Oman. Wakabayashi & Dilek discuss tions. Major plate boundary reorganizations in the the mechanisms and significance of subduction ING region coincide with global plate motions initiation and metamorphic sole formation in and there is a strong correlation in space and time ophiolite emplacement and define four prototype between ophiolite genesis and collisional events. ophiolites based on their emplacement mechan- Opening of basins and suprasubduction zone gen- isms. Tethyan ophiolites are collisional-type em- eration of ophiolites are likely to have been placed over passive continental margins, whereas 'enhanced' by extrusion of aesthenosphere escap- Cordilleran ophiolites are emplaced over subduc- ing collisional zones in the region. Ophiolites tion complexes through accretionary processes. forming in these suprasubduction zone environ- Emplacement of ridge-trench intersection (RTI) ments display age and compositional heterogene- ophiolites occurs through complex processes re- ity, indicating their composite nature. Milsom sulting from interaction of a spreading ridge and a examines the New Caledonia region in the SW subduction zone. -type ophiolite Pacific to determine the spatial relations between represents oceanic crust exposed as a result of forearc ophiolites and their volcanic arc systems. shifts in plate boundary configurations (i.e. Repeated episodes of collisional events, post- spreading ridge segments converting into a diffuse collisional faulting and magmatism, and sea-floor transpressional plate boundary). spreading appear to have displaced and separated Gray & Gregory review emplacement models forearc tectonic assemblages from their respective for the Semail ophiolite in Oman and present volcanic arc systems in the New Caledonia-New Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

6 Y. DILEK & P. T. ROBINSON

Guinea region. This complex history may be stage igneous events including porous flow, and responsible for the apparent lack of volcanic arc rock-melt interaction involving dissolu- edifices associated with other forearc ophiolites tion and plagioclase precipitation. They thus show (e.g. Troodos in Cyprus) around the world. some similarities to peridotites of subcontinental Spaggiari et al. provide an overview of the mantle and/or continent-ocean transition zone Neoproterozoic to Cambrian ophiolites of the mantle. The authors present two explanations for Tasmanides in eastern , and examine the the origin of these peridofite massifs in the South- differences in their emplacement styles and tec- ern Uralides: (1) the anomalous features (for tonic settings. Eastern Australian ophiolites fall abyssal pefidotites) reflect modification of normal into Tethyan- and Cordilleran-type categories de- MORB peridotites formed beneath a spreading pending on their relationship to 'continental base- axis by large volumes of island arc melts; or (2) ment', and they appear to have developed in the peridotites were originally part of subcontinen- various suprasubduction zone environments (arc, tal mantle, which underwent modification by forearc, back-arc) along the eastern Gondwana dominantly tholeiitic melts causing plagioclase margin. Their age progression and geochemistry, precipitation. combined with regional structural and tectonic Ishiwatari et aL discuss the petrological diver- constraints, suggest that they evolved in a complex sity and origin of ophiolites in Japan and Far East rifled arc-back-arc system during 530-485 Ma, Russia, and distinguish highly depleted mantle and that the collapse of this system into the (DH) massifs in them. These ophio- continental margin of East Gondwana resulted in lites range in age from Early Palaeozoic to their emplacement. This event might have been Cenozoic and are tectonically underlain by blues- related to far-field stresses associated with the chist-bearing rocks and accretionary complexes collisional assembly of greater Gondwana in the that are generally younger in age. The majority of early Palaeozoic. the ophiolites probably formed intra-oceanic is- Zhang et ai. summarize the regional distribu- land arc systems, as their petrological and geo- tion, ages and inferred tectonic settings of ophio- chemical characteristics suggest, and were lites in China. The Chinese ophiolites fall into incorporated into the Eurasian continental margin four major age groups, Proterozoic, early Palaeo- through repeated episodes of Mariana-type non- zoic, late Palaeozoic and -Cenozoic, accretionary subduction zone processes over time. and they mainly occur along suture zones separat- There is little in the English literature on the ing different tectonic blocks. They have a m61ange ophiolite complexes of NE Asia. Sokolov et aL character in general and display structural and present new data on the age, structure and compo- metamorphic evidence for multiple episodes of sition of ophiolites in the West Koryak fold belt in collisional events. The majority of the Chinese Far East Russia. The region consists chiefly of a ophiolites are compositionally heterogeneous, con- variety of accreted terranes of different age and taining mixtures of island arc tholeiite and boni- character. The ophiolites fall into two main cate- nite with lesser amounts of MORB and OIB. gories. Palaeozoic ophiolites are primarily oceanic Palaeo-Tethyan ophiolites mostly have MORB- (MORB) in character and are viewed as fragments type rocks and may have formed in small intra- of the Panthalassa Ocean. Mesozoic ophiolites continental basins. typically have an SSZ signature. In general, the Spadea et al. investigate the pyroxene and ophiolites become younger towards the Pacific amphibole compositions of various mantle perido- Ocean in the east. Accretionary prisms contain rites, particularly the Nurali and Mindyak massifs terrigeneous m61anges similar to those of the in the Southern Uralides in Russia. The Ural Shimanto Belt of SW Japan. Mountains are a fold mountain system that records Stern & De Wit describe the geology and a Late Paleozoic arc-continent collision along the geochemistry of the Mesozoic Rocas Verdes eastern European palaeomargin of Baltica. The ophiolites in the southernmost (South Main Uralian Fault marks the related suture zone America) and show that these ophiolites evolved that consists of a mrlange composed of arc in a Late Jurassic-Early Cretaceous intra-arc fragments and dismembered ophiolites. The Nurali basin along the southern edge of Gondwana. and Mindyak peridotites have several anomalous Primary crosscutting relations of ophiolitic dyke features for abyssal peridotites: fertile composi- swarms with the surrounding crystalline basement tion; internal zoning from to to rocks of the Andean magmatic arc indicate that harzburgite; anomalous crust-mantle transition Rocas Verdes basin was an ensialic small ocean with amphibole-bearing, plagioclase-free, ultrama- that opened up by 'unzipping' from the south to fic cumulates; lack of associated crustal section; the north, synchronously with the onset of sea- and intrusion of late (400Ma) -diorite floor spreading in the South Atlantic at c. 132 Ma. plutons. These bodies underwent multi- Thus the provide a unique Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 7 opportunity to investigate the mode and nature of Some ophiolites contain peridotites that may igneous, metamorphic and tectonic processes as- represent exhumed subcontinental lithospheric sociated with continental rifting, sea-floor spread- mantle. It is particularly interesting that this ing and tectonic collapse of a back-arc basin in an appears to be the case for those ophiolitic assem- Andean-type active continental margin. blages in the and Apennines, where the Finally, Dilek & Ahmed present an overview of ophiolite concept was born and first developed the Proterozoic ophiolites in the Arabian Shield through keen observations by influential research- and discuss their significance in Precambrian ers such as Alexandre Brogniart (1740-1847) and tectonics. The Arabian Shield ophiolites range in (1856-1929). The existence of age from c. 870 Ma to c. 627 Ma and display a these subcontinental lithospheric mantle perido- record of rift-drift, sea-floor spreading and colli- tites suggests that some ophiolites may record the sion tectonics during the evolution of the East initial stages of rift-drift evolution of small ocean African Orogen in the aftermath of the break-up basins in Earth history. Detailed petrological stud- of Rodinia. Ophiolites in the western part of the ies of some of the peridotite massifs (i.e. Miinten- shield were part of ensimatic are terranes, which er & Pieeardo; Spadea et aL) indicate that were sutured through a series of collisional events. pervasive melt migration through these ultramafic Younger ophiolites in the eastern Arabian Shield rocks resulted in extensive melt-rock reaction, were incorporated into accretionary complexes precipitation of plagioclase-enriched peridotites through offscraping and collisional events during and generation of gabbroic intrusions during the continued subduction, similar to the accretionary early stages of oceanic lithosphere formation. history of those Phanerozoic ophiolites in NE Asia Late-stage and off-axis(?) magmatism that pro- as reported by Sokolov et al. The youngest duced large plagiogranite-trondhjemite intrusions ophiolites in the shield (Nabitah-Hamdah fault into the pre-existing oceanic crust was responsible zone ophiolites) are post-collisional in origin and for extensive hdyrothermal alteration and miner- they represent Ligurian-type oceanic crust devel- alization in some ophiolites (Semail, Oman, oped in an intracontinental para-rift basin. The Stakes & Taylor; Bay of Islands, Newfoundland, Arabian shield ophiolites are clearly diverse in Gigu6re et al.). These intrusive bodies provided origin and provide a great opportunity to investi- the local heat source that set up convective gate oceanic and juvenile crust evolution in the circulation of high-temperature fluids reacting latest Precambrian. with the host rocks and precipitating in due course epidosites and economic massive sulphide depos- its. These spatial and temporal links between late Concluding remarks plagiogranite intrusions and alteration-mineraliza- Ophiolites are critical windows into Earth history tion indicate that magmatism in oceanic crust to examine the mode and nature of and the generation is commonly episodic and multi-stage. interplay between various igneous, metamorphic, Mantle dynamics and heterogeneity at regional sedimentological, hydrothermal and tectonic pro- and global scales appear to have played a critical cesses during generation of oceanic lithosphere. role in the evolution of small ocean basins (mostly They also provide essential information on the back-arc and/or marginal basins) and their litho- mechanics and kinematics of mountain building sphere. Collision-induced mantle extrusion and episodes, as their incorporation into continental flow strongly affected arc-trench rollback me- margins involved major tectonic events in orogen- chanisms, melt flow patterns and thermal state in esis. New data and observations presented in subduction environments that collectively con- different papers in this book clearly show that trolled ophiolite-forming processes (Dilek & there is not a single, unique tectonic environment Flower; Flower & Dilek). Some ophiolites and of ophiolite formation, and that ophiolites are related tectonic units (i.e. rift assemblages as diverse in origin, representing fragments of fossil precursors to ophiolite generation) display geo- oceanic lithosphere formed in various tectonic chemical evidence for mantle source(s), which settings and in different stages of Wilson cycle were contaminated by previous subduction events evolution of ancient ocean basins. Most ophiolites in the region (e.g. Saccani et al.). These observa- are heterogeneous in lithological make-up, internal tions and interpretations from ophiolites, coupled architecture and alteration history, indicating that with isotopic signatures of oceanic basalts, suggest their formation involved complex and multiple that the mantle is heterogeneous at all scales phases of magmatism, metamorphism and tecton- mainly as a result of subduction of sediments, ism. Precise radiometric, isotopic and biostrati- hydrothermal alteration of oceanic crust and melt- graphic age dating is needed to better constrain ing-induced differentiation. the timing of different evolutionary phases in Emplacement of ophiolites in collisional oro- ophiolite generation. genic belts involves underplating of less-dense Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

8 Y. DILEK & P. T. ROBINSON crustal material beneath displaced oceanic litho- through international collaboration. Future inter- sphere in subduction zone environments. The arri- national ophiolite studies, focusing on: (1) careful val of, and attempted partial subduction of, passive and systematic documentation of primary (sea- continental margins and/or island arc complexes at floor spreading and/or igneous accretion stage) oceanic trenches provides the necessary physical and secondary (emplacement and post-emplace- conditions for this type of ophiolite emplacement. ment) structures within different ophiolitic subu- In accretionary-type orogenic belts (such as in nits and of contact relations between them; (2) Japan, Far East Asia and late Mesozoic-Cenozoic precise and systematic radiometric and isotopic western North American Cordillera), continued dating of igneous and metamorphic rocks in consumption of ocean floor at active continental ophiolites, and biostratigraphic dating of overlying margins facilitates progressive ophiolite emplace- sedimentary cover and underlying m~lange units; ment through tectonic incorporation of stranded (3) isotopic analysis of ophiolite peridotites to slabs of oceanic crust, abyssal peridotites and delineate the mantle composition and signatures seamounts into the subduction-accretion com- of their melt source, and mantle domains; and (4) plexes. These kinds of ophiolites (defined as 'Cor- combined geochemical, petrological and structural dilleran' by Wakabayashi & Dilek) are commonly studies of ophiolites and associated tectonic units spatially associated with blueschist-bearing tecto- to differentiate tectonic settings of their origin and nostratigraphic units and subduction m61anges. evolution, will help us better understand the Earth 'Ophiolite science' is a dynamic, evolving and history and the processes involved in its evolution interdisciplinary enterprise that is at its best through time.