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Lithos 100 (2008) 1–13www.elsevier.com/locate/lithos

Editorial

Links between ophiolites and Large Igneous Provinces (LIPs) inEarth history: Introduction

1. Introduction

Plate tectonics cycle, driven by lithospheric subductionand surface cooling, is responsible for the melting andprimary differentiation of the Earth's mantle while alsointroducing chemical heterogeneity in the upper mantle.Oceanic crust generated at divergent plate boundariescommonly gets recycled into the mantle via subduction(Cloos, 1993), although the oceanic crust formed insubduction zone environments may become incorporatedinto continental margins through collisional and/oraccretionary orogenic events as in ophiolites. Themajority of the world's best preserved ophiolites appearto have formed in suprasubduction zone (SSZ) settings,where slab rollback, mantle flow in the arc wedge corner,subduction-induced mantle metasomatism, and upperplate extension collectively lead to oceanic crustformation (Pearce et al., 1984; Umino et al., 1990; Searleand Cox, 1999; Shervais, 2001; Ishikawa et al., 2002;Dilek and Flower, 2003; Beccaluva et al., 2005; Dileket al., 2007). Systematic studies of SSZ ophiolites andsuture zones in ancient and modern orogenic belts showthatmost of these ophiolites evolved in an older andwiderocean basin, following the initial collapse and consump-tion of its floor as a result of intra-oceanic subduction(Dilek and Flower, 2003; Harris, 2003; Garfunkel, 2006).Continued subduction, slab rollback, and upper plateextension and magmatism without any collisionalinterference would result in successive periods offorearc–protoarc splitting and hence in the formation ofnested oceanic crust formation with larger age ranges; themodern and recent examples of this scenario include theIzu–Bonin–Mariana (IBM) system (Stern and Bloomer,1992; Bloomer et al., 1995) and the northern Philippines(Encarnación, 2004; Yumul, 2007). The Jurassic ophio-lites in California (Stern and Bloomer, 1992; Godfrey andDilek, 2000), the Neo-Tethyan ophiolites in the Balkan

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Peninsula (Bortolotti et al., 2002; Saccani and Photiades,2004; Beccaluva et al., 2005; Dilek et al., 2007) and in theMediterranean region (Robertson, 2002; Dilek andFlower, 2003; Garfunkel, 2006) may be good ancientanalogues for this model, although passive margin-trenchcollisions ultimately arrested these arc-trench rollbackcycles and nested oceanic crust formation in the Neo-Tethyan domains and caused ophiolite emplacement inthe early stages of collisional orogens.

Times of enhanced ophiolite genesis and emplacementin Earth history appear to coincide with the timing ofmajor collisional events during the assembly of super-continents (basin collapse and closure), dismantling ofthese supercontinents via continental rifting, and wide-spread development of Large Igneous Provinces (LIPs)(Coffin and Eldholm, 1994; Yale and Carpenter, 1998;Dalziel et al., 2000; Coffin and Eldholm, 2001; Ernst andBuchan 2002; Courtillot and Renne, 2003; Coffin andEldholm, 2005; Ernst et al., 2005; Bryan and Ernst, inpress) of oceanic affinity (oceanic plateaus, ocean basinflood basalts, and related seamount chains), suggestingspatial and temporal reactions between these events atglobal scales (Fig. 1; Dilek, 2003a). The most discreteophiolite pulse during 180–140 Ma coincides with theformation and emplacement of the Tethyan, Caribbean,and some of the Circum-Pacific (Western Pacific andNorth American Cordillera) ophiolites. In the Tethyansystem this timing marks the collapse of restricted basinsbetween various Gondwana-derived subcontinents priorto the terminal closure of oceans and major continentalcollisions. In the Caribbean system this was the periodwhen LIPs-generated oceanic lithosphere was accreted tothe continental margins of northern South America andthe northern Caribbean Islands (Lapierre et al., 1997; Kerret al., 1998; Giunta et al., 2002).

The second important ophiolite pulse during the LateCretaceous follows the Mid-Cretaceous “superplume

Fig. 1. Histogram showing the occurrence of major ophiolite pulses, the life spans of supercontinents and major collisional-orogenic events that led totheir assembly, and formation of LIPs and giant dike swarms through time (only through the Neoproterozoic). Large Igneous Provinces include the‘classic’ flood basalts, oceanic plateaus and also those LIPs in which the plumbing system of dike intrusions, sills, and layered intrusions are exposed(cf. Ernst and Buchan, 2001; Bryan and Ernst, in press). Note the change in time scale about the Phanerozoic–Proterozoic boundary. Abbreviationsfor orogenic events (from youngest to oldest): Ar–Eu, Arabia–Eurasia collision; In–Eu, India–Eurasia collision; Al–Ur, Altaid–Uralian orogenies ofcentral Asia; Ap–Hy, Appalachian–Hercynian orogenies; Cld, Caledonian orogeny; Fmt, Famatinian orogeny; P–Af–Br, Pan–African−Brasilianoorogenies; Grn, Grenville and related orogenies. Period of ‘No Magnetic Reversals’ between 120 and 80 Ma concides with the mid-Cretaceous‘superplume’ event (Larson, 1991). After Dilek, 2003a (see the extensive citation in this paper for the data source used in the compilation of thisfigure).

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event” and coincides with the breakup of Pangea, theclosure of Neo-Tethyan seaways, and the emplacement ofgiant dike swarms and LIPs (Fig. 1). Some of the Alpine–Apennine ophiolites that formed during the breakup ofPangea represent rift-related mafic–ultramafic assem-blages (i.e., exhumed subcontinental mantle fragments)and/or remnants of embryonic ocean floor (Dilek, 2003b,and the references therein). The enhanced LIP formation(comprising mainly of flood basalts and giant dikeswarms) and ophiolite generation in the Late Cretaceousseem to be linked in space and time through the increasedseafloor spreading rates, extensive oceanic plateauformation and widespread compression at convergentmargins (Larson, 1991; Vaughan, 1995; Dalziel et al.,2000; Dilek, 2003a; Vaughan and Scarrow, 2003). Spatialand temporal associations of rift volcanics showingwithin-plate alkaline basalt (WPB) to subalkaline tholei-itic basalt (akin toMORB) chemistry with passive marginsequences and ophiolites indicate that the plume activitiesand thermal anomalies in the mantle may have beenresponsible for the initial continental breakup, which ledto the opening of ocean basins and seafloor spreading (i.e.

Dilek and Rowland, 1993, and the references therein;Garzanti et al., 1999; Lapierre et al., 2004, 2006). Theemplacement of contemporaneous continental floodbasalts with similar geochemical signatures along someof the rifted continental margins supports this model(Garzanti et al., 1999; Song et al., 2001; Nikishin, 2002).This topic is further explored in the papers by Song et al.,Xiao et al., and Mo et al., in this issue.

Correlation of the ophiolite pulses with majororogenic events that led to the assembly of super-continents is particularly important during the Protero-zoic and Paleozoic (Fig. 1), although our data from thistime window of the Earth's history are rather limited.The collisional buildup of Rodinia around 1 Ga, thecollision of East and West Gondwana and the construc-tion of Pannotia (c. 700 and 600 Ma), Pan–African–Brasiliano orogenies (520–500Ma), Caledonian–Fama-tian orogenies (460–440 Ma), Appalachian–Hercynianorogenies (c. 300–270 Ma), and Altaid–Uralian oroge-nies in central Asia (c 240 Ma) are the most importantexamples (Dilek, 2003a). Most ophiolites that formedduring these orogenic events are the SSZ ophiolites

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representing subduction rollback cycles. We also see anaccelerated rate of production of LIPs and giant dikeswarms corresponding to the timing of major orogenicevents particularly in the Neo-Proterozoic and EarlyPaleozoic (Fig. 1). It is possible that major orogenicevents may have supplied cold subducted slabs into thelower mantle, triggering large upwellings and thermalanomalies feeding mantle plumes, which produced ex-tensive continental and oceanic LIPs (Moores et al., 2000;Nikishin, 2002). The ophiolite and LIP record for theArchean geology is not as nearly complete as the Pro-terozoic and Phanerozoic history, and hence the relateddiscussions are highly limited (Ernst and Buchan, 2002;Eriksson et al., 2004; Kusky, 2004). In fact, whether themodern plate tectonics might have operated in theArchean (particularly in the Mesoarchean and earlier) isstill debated (Hamilton, 1998; Stern, 2002; Polat et al.,2002; Hamilton, 2003; Smithies et al., 2004; Cawoodet al., 2006; Condie and Benn, 2006; Ernst, 2007b).

The record of oceanic Large Igneous Provinces (LIPs)for the past ca. 200Myr indicates that they have occurredat an average rate of about one per 20 Myr (Fig. 2; Ernst,2007a). On the other hand, few oceanic LIPs have beenidentified in the Proterozoic and Paleozoic (mainly asaccreted packages in orogenic belts). This change in rateof oceanic LIP identification is likely a result of poorpreservation potential of the oceanic LIPs during sub-duction, coupled with the absence of oceanic crust olderthan about 180 Ma. Some Phanerozoic and Precambrianophiolites and greenstone belts may be accreted scraps ofoceanic plateaus and/or volcanic sections of plateau-like thick oceanic crust (particularly in the Archean).Because the average rate of continental LIP productionthroughout the entire Proterozoic and Phanerozoicperiod (including within the last 200 Myr) was ap-proximately constant at about one per 20 Myr (Ernst andBuchan, 2002), we can infer that the oceanic LIP pro-duction rate was also broadly constant back to 2500 Ma.If we extrapolate the more recent oceanic LIP productionrate to earlier times, we can surmise that there are morethan 100 unrecognized oceanic events in the Proterozo-ic–Paleozoic time. Strategies for recovering this ‘lost’LIP oceanic record include: (1) recognizing whichophiolites and other accreted volcanic/plutonic packagesare of oceanic LIP origin (e.g. Coffin and Eldholm,2001), and (2) tracing giant dike swarms (up to 2500 kmin length) that radiate from ‘lost’ oceanic LIP centresonto formerly adjacent continental landmasses (Ernstand Buchan, 1997).

The contributions in this special issue of LITHOSexamine the LIP and ophiolite records through time todevelop the criteria for their recognition in the older rock

record, and to better understand the significance of theirmulti-varied evolution patterns in plate tectonic cycles.Some of these papers were presented in a thematicsession at the 2005 Fall Meeting of the AmericanGeophysical Union (San Francisco). Characterizatingthe origin of “ophiolites” in the older record may bepredictive regarding ore deposit potential. For instance,ophiolites of suprasubduction zone settings (such asTroodos) are associated with massive sulphide (mainlyCu) deposits, whereas LIPs, and presumably ophiolitesof LIP affinity, have potential to form Ni–Cu-PGEdeposits. A careful review of the Precambrian andPhanerozoic ophiolite record should aid in recoveringthe missing pre-Mesozoic record of oceanic LIPs. Themajority of the papers in this special issue haveextensive new petrological and geochemical data thatare interpreted within the regional structural and tectonicframework of the area investigated. This new informa-tion on modern oceanic crust and plateaus providesessential information on the mode and nature ofgeochemical and tectonic processes during their forma-tion that we can use in our examination of thePhanerozoic and Precambrian record of ophiolites andoceanic plateaus. It is our hope that this special issueshall make a strong contribution to our knowledge ofoceanic crust and oceanic plateau formation, thepossible spatial and temporal links between LIPs andmagma plumbing systems in divergent and convergentplate boundaries and between the different geochemicalreservoirs, and shall help us better understand thedynamics of plate tectonics in the Precambrian.

The papers in this issue are collected in three mainsections. The first section contains three papers thatinvolve the theoretical aspects of the formation ofoceanic and continental basaltic volcanism and theiridentification (through geochemical fingerprinting andfield observations) and modeling. The next sectionincludes papers that examine the geological record,geochemistry, and geodynamics of various examples ofcontinental and oceanic plume-generated volcanism,mainly in Asia and Japan. The papers in the last sectionprovide extensive petrological and geochemical datafrom the Tethyan (Mesozoic) and Appalachian (EarlyPaleozoic) ophiolites and from a Mesoarchean green-stone belt in SW Greenland, and discuss theirtectonomagmatic evolution using regional tectonic andgeodynamic models.

2. Oceanic and continental basaltic volcanism

Identifying and fingerprinting oceanic basalts is avaluable tool in better understanding basalt petrogenesis

Fig. 2. Barcode and cumulative frequency diagram of LIP events (after Ernst, 2007a). Selected LIPs are labeled; NA, SA, EU, AF, AS, PA refer toNorth America, South America, Europe, Africa, Asia, Pacific Ocean, respectively. The average rate of preserved LIP production (on cumulativefrequency diagram) from 2500 to about 200 Ma is about 1 per 20 Myr, a rate which continues between 200 Ma and Present for Curve B (plotting onlythe continental record of LIPs). Curve A (which plots all the available LIPs during that interval) is steeper because it includes the full oceanic LIPrecord that is mostly lost during ocean closure, and therefore is greatly diminished and obscured in the pre-Mesozoic record.

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and the mantle's evolution and in reconstructing thegeodynamics of ancient ocean basins. Therefore,basaltic rocks in ophiolites and plume-related igneouscomplexes are important sources of information toexplore the plate tectonic evolution of the Earth,possibly back into the Archean. Using proxies forspecific petrogenetic processes, Pearce evaluates theexisting methods for fingerprinting oceanic basalts and

presents new ones that optimize the choice of elements.He points out that present methodologies for theidentification of oceanic basalts are limited in partbecause those discriminant diagrams based on immobileelements are problematic due to the “complexity ofoceanic volcanism and the common interactions be-tween plumes and ridges that cause basalt suites to crossdiscriminant boundaries”. Pearce demonstrates that two

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geochemical proxies are particularly significant in theclassification of oceanic basalts, the Th–Nb and Ti–Ybproxies. Modeling the sensitivity of the Th–Nb proxy tocrustal addition shows how magma–crust interactions,crustal recycling, and subduction processes may affectmagma compositions and drive them away from thediagonal MORB–OIB array, which commonly repre-sents the field of typical oceanic basalts. The Ti–Ybproxy is useful for deciphering the depth of melting andhence is sensitive to variations in mantle potentialtemperature and lithospheric thickness. Modeling thesensitivity of the Ti–Yb proxy to garnet meltingindicates that OIB originates by less melting withresidual garnet beneath thicker lithosphere, whereasoceanic plateau basalts originate from a higher degree ofmelting of fertile mantle beneath progressively thinnerlithosphere (particularly at volcanic rifted margins andoceanic plume–ridge interactions).

This proxy is particularly useful for further classify-ing Phanerozoic and Proterozoic MORB ophiolites intocontaminated (C-MORB), normal (N-MORB), enriched(E-MORB), and plume-influenced (P-MORB) sub-types, and thus for assisting the subdivision of ridgeenvironments into plume-distal vs. plume-proximalridges, ridge-subduction settings, subduction-distalback-arc basins, edges of LIPs, and incipient oceans atvolcanic and non-volcanic rifted margins. Pearce arguesthat while the Th–Nb proxy is useful in detecting theextent of crustal interactions in the petrogeneticevolution of the Archean basalts, the Ti–Yb proxy isineffective for fingerprinting Archean tectonic settingsbecause of higher than modern mantle potentialtemperatures and the insensitivity of the residual garnetoccurrence to lithospheric thickness in the hot Archeanmantle. He questions the applicability of modern-dayproxies to the Archean and warns that there areprecautions needed in interpreting Archean basalts ofunknown affinities.

The occurrence of large igneous bodies associatedwith LIPs within short time spans in the Earth's historysuggests that extensive melting caused by significantchanges in physical conditions at depth must haveoccurred. These events are commonly interpreted tohave resulted from mantle plumes derived from deepthermal boundary layers, although recent studiessuggest that these melting anomalies can also be anatural result of non-rigid plate tectonics (e.g. Anderson,2005; Foulger, in press). Garfunkel examines theformation of continental flood volcanism, one of themost ubiquitous modes of LIPs magmatism commonlyfocused on rifted continental margins, in terms oftemperature and depth conditions of melting and the role

of continental lithospheric thickness in melt evolution.Assuming that melting results from adiabatic decom-pression in rising plume heads beneath the continentallithosphere, he constrains the melting depth to 120–150 km or deeper and the plume potential temperaturesto z300 °C higher than the ambient mantle adiabat(Tp=1350 °C). The MgO-rich (N20%) primary meltsthat are well above the solidus in the lithosphere interactwith and assimilate the fusible components of thelithospheric mantle, and they hence become modifiedduring their ascent. Further differentiation and assimi-lation may take place in shallow-depth magma cham-bers and/or due to circulation within plume heads andreactions with eclogite patches, adding to the diversityof magmas within single eruptive provinces. Nosignificantly high concentration of fusible/metasoma-tized lithospheric components is required during theevolution of continental flood basalt (CFB) magmatism,and no significant lithospheric stretching associatedwith continental rifting is necessary for CFB formationalthough magma intrusion weakens the lithosphere andassists initiation of rifting. CFB emplacement is,therefore, immediately followed by the formation ofvolcanic rifted margins. Comparison of continental LIPs(e.g. CFBs) and oceanic LIPs should consider, therefore,the role of magma differentiation through ascent andinteraction with the continental lithosphere during meltevolution of CFB volcanism.

Dobretsov et al. present a more elaborate modeling ofthermochemical plumes originating from the base of thelower mantle and offer the Siberian Traps as an exampleof an ancient thermochemical super plume event. Theauthors suggest that thermochemical plumes form at thecore–mantle boundary in the presence of heat flow fromthe outer core and with local chemical doping, whichresults from the reactions of lower mantle minerals (i.e.,perovskite and magnesiowustite) with hydrogen, meth-ane and/or components of a fluid phase derived from thecore. A plume starts its ascent when the lower mantlebegins to melt as the melting point of the thermalboundary layer at the core–mantle boundary becomeslower than the surrounding temperatures, and thediameter of this plume conduit remains nearly constantas it rises. A mushroom-shaped plume head developswhen the plume reaches a refractory layer (dunite–harzburgite) whose melting point is higher than the melttemperature in the conduit. The plume size, shape andevolution during its ascent are controlled mainly by meltviscosity, ascent time and velocity, temperature differ-ences in the conduit, and thermal power. Dobretsov et al.suggest that giant volumes of lavas and sills formingmajor continental flood basalt provinces and traps have

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originated from superplume events that commonlyhave three main stages of formation. The early phaseproducing variable picrites and alkali basalts corre-sponds to the initial stages of plume ascent (one orseveral independent plumes), the main phase generatingtholeiite plateau basalts follows the formation of thicklenses of mantle melts underplating the lithosphere, andthe final phase creating ultrabasic and alkaline lavas andintrusions coincides with the emplacement of differen-tiated mantle melts in the uppermost crust. The Permo–Triassic Siberian Traps in NE Russia display these majorstages of superplume magmatism. The granite–syeniteplutonic rocks and the coeval bimodal volcanics alongthe margins of the Siberian Traps represent themagmatic products of lower crustal melting (∼65–70 km at depth) caused by the plume's thermal effects.

3. LIP magmatism in Asia and Japan

The Permian Emeishan large igneous province inDongchuan of SW China is one of the best and well-studied examples of plume magmatism, and it providesan excellent laboratory to study the geochemicalevolution of magmas derived from plume activitiesand affected by plume-lithosphere interactions. Inaddition, the spatial and temporal associations of theEmeishan continental flood basalt sequence with thePaleo-Tethyan passive margin units and structuresindicate that the plume magmatism and continentalrifting here were intimately linked in space and time,offering a relatively complete geological record tofurther investigate this relationship. The next two papersin this issue involve the evolution of the Emeishan LIPand its association with the development of a Tethyanvolcanic rifted margin.

Song et al. describe the Emeishan continental floodbasalt sequence in Dongchuan and document theoccurrence of its two geochemically distinct volcanicunits. The upper tholeiitic basalt unit with high TiO2

contents (3.2–5.2 wt%), relatively high REE concentra-tions (La: 140–60 ppm, Sm: 12.5–16.5 ppm, Yb: 3–4 ppm), moderate Zr/Nb (4.3–10.2) and Nb/La ratios(0.6–0.9), and ɛNd(t) values of −9.4 to +2.3 representsthe products of partial melting of the Emeishan plumehead at a garnet stability depth (N80 km). The basaltephrite unit with high P2O5 (1.3–2.0 wt%), low REEconcentrations (e.g., La: 17–23 ppm, Sm: 4–5.3 ppm,Yb: 2–3 ppm), high Nb/La ratios (2.3–4.2), and verylow ɛNd(t) values (−11.1 to −10.6) most likely representmagmas derived from partial melting of a previouslymetasomatized, volatile-rich subcontinental lithosphericmantle when heated by the upwelling Emeishan mantle

plume. Depleted mantle model ages (TDM) of thesebasal tephrites (2.60–2.15 Ga) suggest the existence ofan Archean subcontinental lithospheric mantle beneathSW China.

Xiao et al. report on the geology and geochemistry ofthe Permo–Carboniferous volcanic units in the Xiaruo–Tuoding area of the Yunnan Province in SW China andexplore their genetic relation with the Emeishan LIP.These volcanic rocks are composed of OIB-like basalts,characterized by high TiO2 values (2.2–3.55 wt%),moderate MgO contents (4.15–6.49 wt%) and Mgnumbers (0.37–0.50), high-Ti/Y ratios (N450), LILEenrichment, and ɛNd(t) values of −1.43 to +1.90, that aresimilar to the upper tholeiitic basalt unit of the Emeishancontinental flood basalt sequence. The Xiaruo–Tuodingvolcanic rocks, more than 2 km in thickness, are part of avolcanosedimentary unit of a passive margin sequenceassociated with the opening of the Jingshajiang Ocean (aPaleo-Tethyan basin). The authors suggest that theinitial rifting that led to the opening of this basin wascaused by the Emeishan mantle plume, which was activealong the western margin of the Yangtze Craton during300–260 Ma. They argue that the Emeishan floodbasalts represent the last magmatic episode of a long-lived mantle plume, which might have operated inpulses.

The Permian was a time of widespread plume-relatedmagmatism, both terrestrial and marine, suggesting alarge-scale super plume event in the Permian. Ichiyamaet al. document the occurrence and petrogenesis of thePermian greenstones in the Jurassic Mino–Tambaaccretionary complex in SW Japan. The Permianvolcanic rocks in the Funafuseyama–Haiya nappewithin the Mino–Tamba complex are associated withpelagic limestone, chert and mélange units in amudstone matrix and consist of three geochemicallydistinct series. The most voluminous lavas intercalatedwith Lower Permian chert and limestone are composedof low-Ti volcanics with slightly more enrichedgeochemical and isotopic signatures than MORB. Thelavas of a transitional series show more enrichedgeochemical signatures than MORB, and their isotopiccharacteristics are divided into enriched and depletedones. A younger series, consisting of sills andhyaloclastites within a Middle Permian chert and dikesintruding the transitional series, has high-Ti and MgOcontents and is characterized by enrichment in incom-patible trace elements and an isotopic compositioncomparable to HIMU-type basalt. The authors suggestthat the low-Ti series lavas were produced by partialmelting of a shallow mantle plume head below a thickoceanic lithosphere in the Early Permian, while the

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transitional series formed simultaneously along themargins of the mantle plume head. The younger, high-Ti series rocks were the products of smaller degree ofpartial melting of a deep mantle plume tail in the MiddlePermian. Collectively, the Permian volcanic units in theMino–Tamba accretionary complex were the productsof an intra-oceanic superplume event and may havebeen part of a large oceanic plateau subsequentlyaccreted to the Eurasian continental margin.

Possible links (in space and time) between plumemagmatism and continental rifting and volcanic riftedmargin evolution are further explored in the easternHimalaya of south Tibet by Zhu et al. in the next paper.These authors document the internal stratigraphy andpetrogenesis of the Lower Cretaceous Cona mafic rocksand suggest that these geochemically diverse volcanicassemblages were the magmatic products of variabledegrees of partial melting of distinct mantle sources,resulted from the interactions between the incubatingKerguelen plume and the continental lithosphere.Basaltic flows and diabasic dikes and sills of Group 1in the Cona mafic sequence have high TiO2 and P2O5

contents and OIB-like trace element patterns with arelatively large range of ɛNd(t) values (+1.84 to +4.67)and may have formed from magmas derived from partialmelting of enriched garnet–clinopyroxene peridotite.Gabbroic sills and intrusions of Group 2 show lowerTiO2 and P2O5 contents and “depleted” N-MORB-liketrace element patterns with relatively higher, homoge-neous ɛNd(t) values (+5.67 to +6.37) in comparison toGroup 1 rocks and likely formed from magmas derivedfrom the partial melting of spinel–lherzolite. Group 1and Group 2 units have been dated at 144.7±2.4 Ma and131.1±6.1 Ma, respectively. Group 3 basaltic lavasintercalated with the Upper Jurassic–Lower Cretaceouspelitic rocks constitute a transitional sequence betweenthe first two groups as suggested by their flat to slightlyenriched trace element patterns. Zhu et al. propose thatthe formation of Group 1 units in the Cona Sequencewas related to the incubating Kerguelen plume—continental lithosphere interactions, whereas the forma-tion of the younger Group 2 units was related to theinteraction between an anhydrous continental litho-spheric mantle and a rising depleted asthenosphere,which was enriched by a “droplet” originated from theKerguelen plume. The transitional units of Group 3 maybe attributed to thermal erosion of the continentallithospheric mantle that led to its partial melting and tothe subsequent, long-term incubation of a magmachamber at shallow crustal levels. Thus, according tothese authors, the Kerguelen plume may have played anactive role in the continental breakup of Greater India,

eastern India, and NWAustralia, in a far more extensivefashion than previously considered.

4. Phanerozoic and Archean ophiolites and modelsfor ancient oceanic crust

Ophiolites have been the subject of considerableinterest since their recognition as on-land fragments ofancient oceanic lithosphere formed at divergent andconvergent plate boundaries. Since they are the onlyremaining vestiges of the Early Jurassic and olderoceanic crust, ophiolites are particularly important tounderstand the igneous, metamorphic, hydrothermal,sedimentological, biological, and structural processesinvolved in magmatic accretion, seafloor spreading andtectonic emplacement of the early Phanerozoic andPrecambrian oceanic crust. They record significantevidence for the evolution of oceanic crust from rift–drift through the accretionary and collisional stages ofcontinental margin evolution in various tectonic settingsand different stages of the Wilson cycle evolution ofancient ocean basins (Dilek and Robinson, 2003). Thepapers in this section present several case studies fromthe Phanerozoic and Precambrian ophiolite record, anddocument the structural, magmatic, and metamorphicevolution of ancient oceanic lithosphere in suprasub-duction zone environments.

Dilek et al. present new geochemical data and apetrogenetic model from the Jurassic Mirdita ophiolite(Albania). Together with other coeval ophiolites in theBalkan Peninsula the Mirdita ophiolite constitutes acritical link between the slightly older MORB ophiolitesin the Alps–Appennines to the west and the younger(Cretaceous) suprasubduction zone ophiolites in theAnatolian and Himalayan orogenic belts to the east. TheMiddle Jurassic Tethyan ophiolites in the BalkanPeninsula show a geochemical dualism with bothMORB and SSZ signatures displayed by their crustaland mantle units. Their internal architecture ranges fromhighly attenuated, thin (∼3 km) Hess-type oceanic crustanalogous to oceanic core complexes (Ildefonse et al.,2007) to thick (∼12 km), Penrose-type idealized andcomplete oceanic crust, which is reminiscent of fast-spreading oceanic lithosphere (Dilek et al., 1998; Dilek,2003b). The occurrence of these different geochemicalaffinities with significantly different crustal architec-tures within the same ophiolite belts has been a subjectof debate over the years, and different petrogenetic andtectonic models have been proposed.

Dilek et al. show that the shift from MORB to SSZaffinities in the Mirdita ophiolite is both lateral andstratigraphic (vertical), indicating changes in the melt

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compositions and mantle sources through time withinthe same tectonic setting. The 3–4 km-thick westernMirdita ophiolite (WMO) consists of lherzolite perido-tites, gabbros, and extrusive rocks mainly composed ofbasalt and basaltic andesite. These volcanic rocksdisplay MORB affinities with Ti and Zr contentsdecreasing upsection in a 600 m-thick sequence inparallel with low-Ti and HREE abundances, and Cs andBa enrichments detected in the uppermost basalticandesites. The ɛNd(t) values (+8 to +6.5) remain nearlyconstant throughout the WMO extrusive sequence.These geochemical features suggest that WMO magmaswere derived from partial melting of fertile MORB-typemantle, and that they were increasingly influenced bysubduction processes in their evolution. The ∼12 km-thick eastern Mirdita ophiolite (EMO) contains harz-burgitic peridotites, gabbros, plagiogranites, sheeteddikes, and a thick (1.1 km) extrusive sequencecomposed of basaltic to basaltic andesitic pillow lavasin the lower 700 m, and andesitic, dacitic andrhyodacitic sheet flows in the upper 400 m. Basalticand basaltic andesite rocks in the lower sequence havelower Ti and Zr contents than but similar ɛNd(t) values(+7.5 to +6.5) to those of the WMO lavas and showvariable enrichment in subduction-enriched incompati-ble elements. The more felsic volcanics in the uppersequence have low Ti and ɛNd(t) values (+6.5 to +3.0)and large variations in their Ba, K and Pb contents,suggesting that the mantle source of these rocks wasvariably enriched in Th by melts derived fromsubducted sediments. The occurrence of boniniticlavas higher up in this sequence also indicates that thelate-stage magmas of the EMO were produced frompartial melting of highly depleted mantle.

The authors argue that this lateral (from west to east)and vertical transition from MORB to SSZ magmatismin the Mirdita ophiolite was a result of an eastward shiftin the protoarc–forearc magmatism, keeping pace withslab rollback in this direction. The Mirdita and otherwestern Hellenic ophiolites (i.e., Pindos, Vourinos,Othris) formed during the closing stages of a marginalbasin (Pindos basin), which had evolved between theApulian and Pelagonian subcontinents within theTethyan realm.

The occurrence of Jurassic island arc volcanism in adifferent Tethyan domain in the Balkan Peninsula,currently exposed in the eastern Rhodope of Bulgaria, isdocumented by Bonev and Stampfli in the next paper.Mafic extrusive rocks and the associated greenschistrocks are found in a mélange-like, low-grade Mesozoicunit in the eastern Rhodope and SW Trace (Greece) anddefine an early-Middle Jurassic island arc system that

developed as part of the Vardar basin to the south ofthe SerboMacedonian-Rhodope continental fragment.The island arc rocks are composed of low-Ti tholeiiticbasalts and boninitic-like calc-alkaline volcanics (basal-tic andesites and andesites) and metamorphosed pyro-clastic rocks. These rocks are intruded by dioritic dikesshowing a boninitic affinity. Mafic lavas and pyroclasticrocks show LILE enrichment, flat REE patterns with aslight LREE depletion, negative Nb anomalies andpositive ɛNd(i) values ranging from +4.87 to +6.09.These geochemical features suggest a depleted MORB-like mantle source, which was modified by subduction-derived LILE-enriched components. Late-stage dioriticdikes have lower ɛNd(i) values (−2.61) and are slightlymore Pb radiogenic, suggesting the involvement ofcrustal contamination and/or recycled sediments in theirmagma genesis.

This MORB to IAT to boninitic evolution of theJurassic eastern Rhodope mafic assemblages is analo-gous to the geochemical evolution of the Mirditaophiolite in Albania and appears to be a commontectonomagmatic pattern of incipient island arc devel-opment in the Tethyan domains (Ishikawa et al., 2002;Arai et al., 2006; Dilek et al., 2007). The authors suggestthat the Jurassic island arc magmatism in easternRhodope was a result of the southward subduction ofthe Meliata–Maliac ocean floor that led to the openingof the Vardar back-arc basin by the Late Jurassic.

The Jurassic Othris ophiolite in Greece is part of theWestern Hellenide ophiolite belt, occurring west of thePelagonian subcontinent. Barth et al. report on thegeochemistry and petrogenesis of upper mantle perido-tites in the Othris ophiolite. The peridotite samplescollected from the western part of the ophiolite (FournosKaïtsa and western Katáchloron sub-massifs) arecompositionally similar to high-Ti basalts and basalticandesites of the Agoriani mélange in the northwesternpart of Othris and indicate a MORB-type mantle source.These samples also show close resemblance to theintermediate-Ti basalts found in the same mélange thathave geochemical characteristics between typical low-Tiisland arc tholeiites and high-Ti MORB. The authorssuggest that these peridotites likely experienced initialmelting in the garnet stability field followed bymoderate degrees of anhydrous near-fractional meltingin the spinel stability field beneath a mid-ocean ridgespreading axis. The plagioclase peridotites here were theproducts of impregnation of harzburgites with afractionating, MORB-type melt. The peridotite samplesfrom the Metalleio, Eretria, and western Katáchloronsub-massifs in Othris are highly depleted and havesignificantly low concentrations of Al2O3 and HREE,

9Editorial

and are compositionally similar to the very low-Tibasaltic andesites and andesites found in the Agorianimélange and to the boninitic lavas in the AgriliaFormation in the southern part of the Othris ophiolite.These peridotites have enriched LREE contents andrepresent residual upper mantle rocks that underwent inthe presence of slab-derived fluids hydrous melting ofmantle sources that had previously experienced extrac-tion of MORB-type melts.

These observations and interpretations suggest thatMORB-type magmatism was followed closely in timeby SSZ-type magmatism. This mantle evolution of theJurassic Othris ophiolite is similar to that of theCretaceous Oman ophiolite, as recently documentedby Arai et al. (2006) based on the chromian spinelgeochemistry of its upper mantle peridotites. Barth et al.proposed that this spatially and temporally closeassociation of the inferred MORB and SSZ magmatismrecorded in the Othris ophiolite may be explained byintra-oceanic thrusting and forced subduction initiationat or near a mid-ocean ridge. The implication of thismodel is that there seems to be an evolutionary pathfrom MORB to IAT to boninitic magmatism through avery short time period above a west-dipping subductionzone during the development of the Othris ophiolite.This pattern is consistent with the coeval Mirditaophiolite in the same belt farther north.

Pagé et al. discuss the mantle petrology andmineralogy of the Early Ordovician (∼480 Ma) Thet-ford Mines ophiolite in the southern Québec Appala-chians (Canada) and present a petrogenetic model for itsevolution in a forearc tectonic setting. The mantlesequence in the Thetford Mines ophiolite consistsmainly of foliated harzburgite (V5–6% cpx) cut bydunitic (±chromitite cores) and orthopyroxenitic veinsand dikes, and are subdivided into two sections based ondifferences in the mineral chemistry, textures, andstructural fabrics. The eastern section, Duck LakeBlock (DLB), has peridotites with secondary granulartextures and two sets of high-T °C ductile foliations: anearlier foliation sub-perpendicular to the Moho butsubparallel to the sheeted dike orientation, and anoverprinting younger foliation subparallel to the Moho.The earlier foliation represents asthenospheric flowassociated with upwelling beneath a spreading axis,whereas the younger foliation is a high-T °C lithospher-ic fabric (crust–mantle shear) associated with seafloorspreading. The DLB mantle rocks may have experi-enced ∼27–38% partial melting and have interactedwith an impregnating melt through both channeled anddiffuse porous flow that was facilitated by the verticalfabric.

The western mantle section, the Caribou MountainBlock (CMB), contains mainly porphyroclastic harzbur-gites with a strong mylonitic foliation that is orientedparallel to the spreading-related extensional faults in thecrust. These harzburgites include pods of dunite andchromitite and orthopyroxenite veins, indicating re-equilibration withmagma percolating through channeledflow that was facilitated by the subhorizontal lithospher-ic fabric. The existence in the mantle rocks of olivine andpyroxenes with high Mg #s and spinel and pyroxeneswith high Cr #s suggests extensive partial melting, whichwas responsible for the production of boninitic magmasthat generated the lavas and cumulate rocks in theThetford Mines ophiolite. The authors suggest that themagmatic evolution of the ophiolite occurred in anextended forearc setting of a subduction zone, which wasdipping eastward (in the present coordinate system) andaway from the Laurentian continental margin.

The origin and tectonic evolution of Archeangreenstone belts are subject to debate partly because itis often difficult to establish primary igneous featuresand geochemical affinities of these highly deformed andmetamorphosed rocks, but also because whether themodern plate tectonics operated in the Archean toproduce oceanic crust and ophiolites is still hotlycontested (Hamilton, 1998, 2003; Stern, 2005; Cawoodet al., 2006; Ernst, 2007b). Polat et al. report on thegeochemical makeup and origin of Mesoarchean mafic–ultramafic rocks in the Ivisaartoq greenstone belt(∼3075 Ma) in SW Greenland and suggest that theserocks collectively represent a remnant of a dismemberedMesoarchean SSZ ophiolite. The oceanic rocks includepillow basalts and ultramafic lavas, gabbros, minordiorites and serpentinized peridotites, spatially associ-ated with sulfide-rich siliceous volcaniclastic sedimen-tary rocks. The large ɛNd(i) values (+2 to +6) of thecrustal rocks suggest a long-term LREE-depleted mantlesource(s) (i.e. the source of modern N-MORB) for theMesoarchean Ivisaartoq ophiolite. However, the LREE-enriched and Nb-depleted (relative to Th and La) traceelement patterns of the majority of these rocks areconsistent with a subduction zone origin of theirmagmas. Therefore, the authors propose that theMesoarchean Ivisaartoq ophiolite may have had atectonic evolution similar to that of its Phanerozoiccounterparts. Following the initiation of an intra-oceanicsubduction zone, an N-MORB-like sub-oceanic deplet-ed mantle source was emplaced in a sub-arc mantlewedge position, where it was metasomatized by slab-derived hydrous fluids and melts. Extensive partialmelting of this subduction-modified mantle at shallowdepths beneath the forearc region was responsible for

10 Editorial

the production of LREE-enriched and HFSE-depletedmagmas. The inferred subduction zone origin of theIvisaartoq greenstone belt implies that modern platetectonics did operate in the Mesoarchean, and perhapseven earlier (Polat et al., 2002; Furnes et al., 2007).

The emplacement of oceanic lithosphere onto conti-nental margins to form ophiolites is a first-order tectonicproblem and is inherently related to the tectonic settingof oceanic crust formation and the relative motions of thebounding continents (see Wakabayashi and Dilek, 2003for an overview and other relevant references). Tethyan-type ophiolites tectonically resting on the passive marginsequences of microcontinents are underlain by thinslivers (∼ few 100 m) of high-grade rocks formingmetamorphic soles. These soles contain thrust sheets ofhighly-strained (locally mylonitic) metabasic and meta-pelitic rocks, whose pressure-temperature conditions ofmetamorphism are consistent with high-T °C metamor-phism beneath sub-ophiolitic mantle. Typical soles showinverted metamorphic field gradients and an invertedocean crustal sequence such that the high-gradecomponents transition downwards from metagabbrosto metabasalts to metamorphosed pelagic sedimentaryrocks (Jamieson, 1986; Hacker and Mosenfelder, 1996).Although widely accepted models suggest that high-grade metamorphism and the sole formation occur at theinception of subduction (Williams and Smyth, 1973;Nicolas and LePichon, 1980; Spray, 1984; Jamieson,1986; Hacker, 1991; Searle and Cox, 2002), where andhow this subduction starts and whether this inferredsubduction may have been related to the igneousevolution of ophiolites remain controversial questions(see the recent debate on the Semail ophiolite of Omanby Boudier and Nicolas 2007; Warren et al., 2007). Howthese metamorphic soles were eventually exhumed to thesurface from depths of ∼25 km (∼8 kbar) in asubduction zone (as inferred from the P-T conditions ofthe formation of amphibolites, Warren et al., 2007) alsoremains an unresolved problem. Therefore, systematicstudies of metamorphic soles beneath ophiolites provideimportant constraints on the emplacement tectonics ofancient oceanic crust into continental margins.

Elitok and Drüppel report on the geochemistry ofmetamorphic sole rocks beneath a Tethyan ophiolite inSW Turkey and propose a tectonic model for theformation of its sole. The Cretaceous Beysehir–Hoyranophiolite tectonically overlying the Anamas–Aksekicarbonate platform of the Tauride belt is underlain by a∼150 m-thick metamorphic sole, which includes fromtop to the bottom pyroxene amphibolite, amphibolite,epidote amphibolite, calcschist and quartzite, showingan inverted metamorphic field gradient. The authors

estimate the average P-T conditions of metamorphism ofthe amphibolites at 630–770 °C and 6±1.5 kbar,corresponding to a burial depth of 18–20 km in asubduction zone. The geochemical signatures of the sub-ophiolitic amphibolites indicate two dominant groups ofprotoliths, within-plate alkaline basalts and tholeiiticisland arc basalts. Isolated diabasic dike swarms,intruding both the metamorphic sole rocks and theoverlying upper mantle peridotites, show trace elementpatterns indicating that dike magmas were derived frompartial melting of a variously enriched and subduction-metasomatized, heterogeneous mantle source(s). Somedike rocks occur as blocks in the sub-ophiolitic mélangebeneath the sole, but there are no dikes observed to occuras intrusions in the mélange. These dikes are common inthe entire Tauride ophiolite belt and are interpreted tohave intruded before the emplacement of the ophiolitesonto the northern edge of the Tauride carbonate platform(Dilek et al., 1999). The magma source for these dikesmay have been below the Benioff zone and from anasthenospheric window through the subducted slab(artifact of slab breakoff). The tectonic model presentedin the paper provides additional constraints on theemplacement direction and kinematics of the Taurideophiolites and on the protoliths of the sole rocks.

Acknowledgements

We thank our colleagues and members of theinternational scientific community for their help withtimely and thorough reviews of the papers in this issue.We extend our sincere thanks and gratitude to PatriciaMassar, Joanna Aldred, and Tim Horscroft in the EarthSciences Department of Elsevier Science B.V. for theirhelp and diligent work during the preparation andproduction of the Special Issue. We also thank theEditor-in-Chief, Professor Steve Foley, for his invitationto produce this special issue and for his help with theeditorial handling of the processed papers.

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13Editorial

Yumul Jr., G.P., 2007. Westward younging disposition of Philippineophiolites and its implication for arc evolution. Island Arc 16,306–317.

Yildirim DilekDepartment of Geology, Miami University, Oxford,

OH 45056, USAE-mail address: dileky@muohio.edu.

Corresponding author.

Richard ErnstErnst Geosciences, 43 Margrave Avenue, Ottawa, ON,

Canada K1T 3Y2Department of Earth Sciences, Carleton University,

Ottawa, ON, Canada K1S 5B6E-mail address: Richard.Ernst@ErnstGeosciences.com.