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Ophiolites in Earth history Introduction

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Ophiolites in Earth history: Introduction
Article in Geological Society London Special Publications · January 2003
DOI: 10.1144/GSL.SP.2003.218.01.01
2 authors:
Yildirim Dilek
Paul T. Robinson
Miami University
China University of Geosciences
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
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Ophiolites in Earth history: introduction
D I L E K 1 & P A U L T. R O B I N S O N
1Department of Geology, Miami University, Oxford, 0H45056, USA
(e-mail: dileky@muohio.edu)
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.
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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
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
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the Neyriz transform fault separated ENE-trending
spreading centre segments within a Neo-Tethyan
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
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
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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
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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
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
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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
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
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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.
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
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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
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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.