Ophiolite Genesis and Global Tectonics -Dilek & Furnes 2011

Geological Society of America Bulletin

doi: 10.1130/B30446.1 2011;123;387-411Geological Society of America Bulletin

Yildirim Dilek and Harald Furnes

fingerprinting of ancient oceanic lithosphereOphiolite genesis and global tectonics: Geochemical and tectonic

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ABSTRACT

Ophiolites, and discussions on their origin and signifi cance in Earths history, have been instrumental in the formulation, testing, and establishment of hypotheses and theories in earth sciences. The defi nition, tectonic ori-gin, and emplacement mechanisms of ophio-lites have been the subject of a dynamic and continually evolving concept since the nine-teenth century. Here, we present a review of these ideas as well as a new classifi cation of ophiolites, incorporating the diversity in their structural architecture and geochemi-cal signatures that results from variations in petrological, geochemical, and tectonic processes during formation in different geo-dynamic settings. We defi ne ophiolites as suites of temporally and spatially associated ultramafi c to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic envi-ronments. Their geochemical characteris-tics, internal structure, and thickness vary with spreading rate, proximity to plumes or trenches, mantle temperature, mantle fertility, and the availability of fl uids. Subduction-related ophiolites include suprasubduction-zone and volcanic-arc types, the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle, melting of the subducting sediments, and repeated episodes of partial melting of meta-somatized peridotites. Subduction-unrelated ophiolites include continental-margin, mid-ocean-ridge (plume-proximal, plume-distal, and trench-distal), and plume-type (plume-proximal ridge and oceanic plateau) ophio-

lites that generally have mid-ocean-ridge basalt (MORB) compositions. Subduction-related lithosphere and ophiolites develop during the closure of ocean basins, whereas subduction-unrelated types evolve during rift drift and seafl oor spreading. The peak times of ophiolite genesis and emplacement in Earth history coincided with collisional events leading to the construction of super-continents, continental breakup, and plume-related supermagmatic events. Geochemical and tectonic fi ngerprinting of Phanerozoic ophiolites within the framework of this new ophiolite classifi cation is an effective tool for identifi cation of the geodynamic settings of oceanic crust formation in Earth history, and it can be extended into Precambrian green-stone belts in order to investigate the ways in which oceanic crust formed in the Archean.

INTRODUCTION

Ophiolites represent fragments of upper mantle and oceanic crust (Dewey and Bird, 1971; Coleman, 1977; Nicolas, 1989) that were incorporated into continental margins during continent-continent and arc-continent collisions (Dilek and Flower, 2003), ridge-trench inter-actions (Cloos, 1993; Lagabrielle et al., 2000), and/or subduction-accretion events (Cawood et al., 2009). They are generally found along suture zones in both collisional-type (i.e., Alpine, Himalayan, Appalachian) and accretionary-type (i.e., North American Cordilleran) orogenic belts (Fig. 1) that mark major boundaries be-tween amalgamated plates or accreted terranes (Lister and Forster, 2009). The geological rec-ord of the evolution of ocean basins from the rift-drift and seafl oor spreading stages to the ini-tiation of subduction and fi nal closure (the Wil-

son cycle) is well preserved in most orogenic belts. Magmatism during each of these phases produces spatially and temporally associated, mafi c-ultramafi c to highly evolved rock assem-blages. These rock units, which have varying in-ternal structures, geochemical affi nities, and age ranges, and originally formed in different geo-dynamic settings, constitute discrete ophiolite complexes and can become tectonically juxta-posed in collision zones (Dilek, 2003).

In the Penrose defi nition (Anonymous, 1972, p. 24), an ophiolite is described as a distinctive assemblage of mafi c to ultramafi c rocks that includes, from bottom to top, tectonized perido-tites, cumulate peridotites, and pyroxenites over-lain by layered gabbros, sheeted basaltic dikes, a volcanic sequence, and a sedimentary cover; an ophiolite may be incomplete, tectonically dismembered, or metamorphosed. This original Penrose defi nition of ophiolites (Anonymous, 1972) is highly restrictive and does not refl ect the actual heterogeneity in ophiolite composition and occurrence, and therefore a more determin-istic approach to defi ning ophiolites and their ig-neous evolution is needed. In this paper, we fi rst review the evolution of the ophiolite concept before and after the formal Penrose defi nition, and we redefi ne an ophiolite in light of recent observations and diverse data sets from ophio-lites worldwide. We outline the signifi cance of ophiolite pulses in Earth history within a global tectonic framework and introduce a new and more comprehensive classifi cation of ophiolites based on their distinctive internal structures, geo-chemical signatures, and regional tectonics. We then present petrogenetic models for the forma-tion of different types of ophiolites and discuss the implications of this new ophiolite classifi ca-tion for the origin of Precambrian oceanic crust, particularly for some Archean greenstone belts.

For permission to copy, contact [email protected] 2011 Geological Society of America

Ophiolite genesis and global tectonics: Geochemical and tectonic fi ngerprinting of ancient oceanic lithosphere

Yildirim Dilek1, and Harald Furnes21Department of Geology, Shideler Hall, Miami University, Oxford, Ohio 45056, USA, and Faculty of Earth Sciences, China University of Geosciences at Wuhan, Wuhan 430074, Hubei Province, China2Department of Earth Science & Centre for Geobiology, University of Bergen, Bergen 5007, Norway

387

GSA Bulletin; March/April 2011; v. 123; no. 3/4; p. 387411; doi: 10.1130/B30446.1; 12 fi gures; 2 tables, Data Repository item 2011131.

E-mail: [email protected].


Dilek and Furnes

388 Geological Society of America Bulletin, March/April 2011

HISTORICAL BACKGROUND AND NEW DEFINITION OF OPHIOLITES

Early Ideas and Evolving Ophiolite Concept

The term ophiolite was fi rst used in 1813 by a French mineralogist, Alexandre Brongniart (17701847), in reference to serpentinites in mlanges; he subsequently redefi ned his defi ni-

tion of an ophiolite (Brongniart,1821) to include a suite of magmatic rocks (ultramafi c rocks, gabbro, diabase, and volcanic rocks) occurring in the Apennines. Gustav Steinmann (18561929) elevated the ophiolite term to a new concept by defi ning ophiolites as spatially as-sociated kindred rocks that originally formed as in situ intrusions in axial parts of geosynclines (Steinmann, 1927). Steinmann emphasized the common occurrence of peridotite (serpenti-

nite), gabbro, and diabase-spilite, in association with deep-sea sedimentary rocks in the Medi-terranean mountain chains and interpreted the origin of these rocks as differentiated magmatic units evolved on the ocean fl oor. He considered these rock assemblages to have developed from a consanguineous igneous process during the evolution of eugeosynclines. This interpretation subsequently led to the widely known notion of the Steinmann trinity.

mid-ocean-ridge

165E15W

7575E

Indonesian belt (Cenozoic)Western Pacific and Cordilleran belts (Paleozoic-Tertiary)Alpine - Himalayan belt (Jurassic - Cretaceous)Appalachian - Caledonian - Hercynian - Uralian & Central Asian belts (early Paleozoic)Tasmanides (Paleozoic)

Sunda Trench

Figure 1. Global distribution of major Phanerozoic orogenic belts and ophiolite age clusters on a north polar projection. Signifi cant examples of different ophiolite types with characteristic geochemistries are marked with symbols used in Figure 2. Modern mid-ocean ridges and subduction zones (marked by trenches) where contemporary oceanic lithosphere has been produced are also depicted. The two major arc-trench rollback systems, Izu-Bonin-Mariana and Tonga-Kermadec, are the sites of ophiolite and volcanic-arc generation, which undergo tectonic extension and trenchward-migrating magmatic construction. The collision zone between the NW Australian passive margin and the Sunda arc-trench system where the island of Timor has been emerging during the last ~5 m.y. represents the best modern analogue for ophiolite emplacement.


Ophiolite genesis and global tectonics

Geological Society of America Bulletin, March/April 2011 389

Although Steinmann considered peridotite, gabbro, diabase, and volcanic rocks in ophio-lites as comagmatic in origin, his observation that gabbroic and diabasic rocks were intru-sive bodies in the serpentinized peridotites is an extremely important one because it differs from the contemporary interpretation of the Penrose-type ophiolite. It implies that, at least in the Apennine ophiolites, the gabbros and volcanic rocks are younger than the peridotites. Steinmann also correctly interpreted the ophio-lites in the Northern Apennines as thrust sheets tectonically overlying the Tertiary sedimentary rocks in Tuscany (Steinmann, 1913). This inter-pretation led to the discovery of allochthonous nappe sequences in the Alpine-Apennine oro-genic system.

Thayer (1967) discussed the signifi cance of the consanguineous relationship between ultramafic and associated mafic rocks in alpine-type peridotites, which were defi ned by Benson (1926) earlier, and explained how the gabbro, diabase, and other leucocratic rocks in alpine-type peridotites could have originated from a single primary peridotitic magma. Jackson and Thayer (1972) subsequently dis-tinguished harzburgite-type versus lherzolite-type alpine peridotites. In this subgrouping, the harzburgite-type alpine peridotites represent the uppermost oceanic mantle, whereas the less-depleted lherzolite-type alpine peridotites correspond to the subcontinental mantle and/or to the deeper oceanic mantle, where partial melting is much less intense. Recent studies of ophiolites have shown that both harzburgite- and lherzolite-type peridotites may occur in ophiolites, and that they can be used to clas-sify ophiolite types and their inferred spread-ing rates of formation in an oceanic setting (Ishiwatari, 1985; Boudier and Nicolas, 1985; Nicolas and Boudier, 2003).

In his classic paper published in Crust of the Earth (Geological Society of America Spe-cial Paper 62), Hess (1955, p. 393) stated that Steinmanns ophiolite concept was confusing because it obscured critical relationships of its [ophiolite] various members to the tectonic cycle. Recognizing the importance of serpenti-nites and alpine-type peridotites in orogeny and mountain-building episodes, he argued that ser-pentinites and rocks of Steinmanns trinity are common in island arcs and that island arcs rep-resent an early stage in the development of an alpine-type of mountain system (p. 395). Hess was, therefore, advocating an island-arc origin of mafi c-ultramafi c rock assemblages and ser-pentinized peridotites found in orogenic belts. This was nearly 20 yr before Miyashiro (1973) made the fi rst formal and rather controversial call on the island-arc origin of the Troodos

ophiolite (Cyprus), connecting ophiolite genesis to subduction-zone processes.

Hess discussed in his 1962 paper that the main oceanic crustal layer (his layer 3) along the Mid-Atlantic Ridge was made largely of serpentinite (his Fig. 2, p. 603; Hess 1962), and that the seismic velocity of this layer would be highly variable, depending on the magnitude of serpentinization of the peridotite. He pro-posed that the interface between the oceanic crust (composed mainly of serpentinite) and the under lying perido tite with seismic veloci-ties of 7.4 km/s represented the Moho discon-tinuity. Since he had interpreted serpentinites as hydrated perido tites, Hess described the Moho beneath the Mid-Atlantic Ridge as an altera-tion front (phase transition) rather than a sharp boundary separating the igneous crust from the underlying mantle (his Fig. 7, p. 612). Although we now know that oceanic crust is not made of 70% serpentinite, marine geological and geo-physical studies have documented that the slow-spreading oceanic crust along the Mid-Atlantic Ridge has a highly heterogeneous lithological composition and thickness (Dick, 1989). For ex-ample, thin-crust domains along the ridge axis (i.e., magma-poor segment ends) consist of tec-tonically uplifted ultramafi c rocks with gabbroic intrusions and a thin basaltic cover (Cannat et al., 1995). This nonuniform thickness and the heterogeneous lithostratigraphy of the Mid-Atlantic Ridge crust are remarkably similar to Steinmanns description of the Ligurian ophiol-ites in the Apennines. It also largely corresponds to Hess characterization of oceanic crust devel-oped at the Mid-Atlantic Ridge. This Hess-type crust differs signifi cantly from Penrose-type oceanic crust in terms of its internal architec-ture, as discussed in the following.

The Dutch geologist de Roever (1957) re-interpreted the Steinmann trinity to result of mantle melting, producing the basaltic rocks on top and the residual ultramafi c rocks at the bottom. Subsequently, the Swiss petrolo-gist Vuagnat argued that the peridotite massifs in ophiolites were partial melting residues in the upper mantle (Vuagnat, 1964), because he thought that the overwhelming abundance of ultramafi c rocks in ophiolites compared to the small volumetric occurrence of gabbroic rocks could not simply be explained by differentiation of submarine outpourings of basaltic magma. It is important to note that these two papers by de Roever (1957) and (Vuagnat, 1964) mark in the literature the beginning of a signifi cant shift in Steinmanns cogenetic ophiolite concept and of a new paradigm in oceanic crustal evolution.

Recognition of extensional sheeted dike complexes, the existence of a refractory mantle unit represented by harzburgitic peridotites

with high-temperature deformation fabrics, fossil magma chambers in plutonic sequences, and the allochthonous nature of ophiolites by the mid-1960s was instrumental in the formula-tion of the ophiolite model and the ophioliteocean crust analogy within the framework of the new plate-tectonic theory. The ophiolite suite became an ideal analogue to explain the seismic velocity structure of modern oceanic lithosphere, as more seismic data became avail-able from modern ocean basins, particularly from the Pacifi c Ocean. Combined with obser-vations from the Troodos (Cyprus) and Semail (Oman) ophiolites in particular, the seismic velocity structure of modern oceanic crust and its inferred layer-cake pseudostratigraphy came to be known as the ophiolite model. This analogy was confi rmed at the fi rst Penrose Conference on ophiolites in 1972 (Anonymous, 1972), whereby an ideal ophiolite sequence was defi ned to have a layer-cake pseudostratig-raphy complete with a sheeted dike complex as a result of seafl oor spreading. Ophiolites were interpreted to have developed mainly at ancient mid-ocean ridges through this model. In a uniformitarian approach, ophiolite geolo-gists then started reconstructing the evolution of fossil oceanic lithosphere exposed on land as a product of paleomid-ocean ridges using the ophioliteocean crust analogy (Gass, 1968; Coleman, 1971; Moores and Vine, 1971; Cann, 2003, and references therein).

Geochemical studies challenged this view of a mid-ocean-ridge origin of ophiolites as early as the beginning of the 1970s, and suggested the association of magma evolution with sub-duction zones. Miyashiro (1973, p. 218) argued that about one-third of the analyzed rocks of the lower pillow lavas and sheeted dike rocks in the Troodos ophiolite follows a calc-alkalic trend, suggesting that the massif was cre-ated as a basaltic volcano in an island arc with a relatively thin ocean-type crust rather than in a mid-oceanic ridge. This was the fi rst formal proposal of a subduction-zone origin of the Troodos oceanic crust that questioned the ruling hypothesis of a mid-ocean-ridge setting of ophiolite genesis. Miyashiros geochemical argument on the island-arc origin of the Troodos ophiolite would start a major paradigm shift in the ophiolite concept in the wake of the plate-tectonic revolution. The subsequent scientifi c exchange in the form of discussions and replies to Miyashiros 1973 paper initiated a long-last-ing debate about the tectonic setting of ophio-lite genesis. Pearce (1975) proposed a marginal basin origin for the Troodos massif during the evolution of an incipient submarine island arc.

Findings from modern subduction-zone en-vironments in the western Pacifi c prompted


Dilek and Furnes


researchers to consider more rigorously the evolution of ophiolites in spreading environ-ments within the upper plate of subduction zones (Hawkins, 1977, 2003; Pearce, 2003). This development, which came about as a col-lective result of ophiolite studies on land and marine geological and geophysical investiga-tions in modern convergent margin settings in the oceans, led to the defi nition of supra-subduction-zone ophiolites in the early 1980s (Pearce et al., 1984). The forearc environment of the Izu-Bonin-Mariana arc-trench system is today one of the best studied (through deep-ocean drilling and submersible diving surveys) and best understood modern suprasubduction zones that we consider to be a contemporary suprasubduction-zone ophiolite factory (Fig. 1; Stern et al., 1989; Stern and Bloomer, 1992; Reagan et al., 2010; Dilek and Furnes, 2010). Sys tematic petrological and geochemical in-vestigations of world ophiolites throughout the 1980s and 1990s demonstrated the signifi cance of subduction-zonederived fl uids and melting history in development of ophiolitic magmas (Saunders and Tarney, 1984; Rautenschlein et al., 1985; Hbert and Laurent, 1990; Thy and Xenophontos, 1991; Beccaluva et al., 1994; Bdard et al., 1998; Dilek et al., 1999; Shervais, 2000; Dilek and Flower, 2003). Forearc, embry-onic arc, and backarc settings in suprasubduc-tion zones became the most widely accepted tectonic environments of origin.

New Defi nition of Ophiolites

The basic tenet of the 1972 Penrose defi ni-tion is that an ideal ophiolite has a layer-cake pseudostratigraphy with laterally persistent and horizontal contacts. The Mohorovicic disconti-nuity (Moho) is considered to be a petrologi-cal transition zone separating the crustal and upper-mantle rocks that have a melt-residua genetic relationship. Studies since 1972 have demonstrated, however, that most ophiolites have a dynamic evolution and display a later-ally discontinuous and vertically heterogeneous crustal architecture and varying geochemical characteristics due to multiple magmatic epi-sodes and different mantle sources during their igneous evolution. The fossil Moho also differs in character in ophiolites; in some, it represents a major tectonic discontinuity (i.e., detachment fault), whereas in some others, it is an altera-tion front. However, in some ophiolites it is a nearly 1-km-thick transition zone reminiscent of the Moho in slow-spreading young oceanic lithosphere (Dick et al., 2006). The diversity in the architecture and geochemical fi ngerprints observed in ophiolites refl ects differences in igneous and tectonic processes involved in the

formation of oceanic crust in different geo-dynamic settings.

We defi ne an ophiolite as an allochthonous fragment of upper-mantle and oceanic crustal rocks that is tectonically displaced from its pri-mary igneous origin of formation as a result of plate convergence. Such a slice should include a suite of, from bottom to top, peridotites and ultramafi c to felsic crustal intrusive and volcanic rocks (with or without sheeted dikes) that can be geochronologically and petrogenetically re-lated; some of these units may be missing in in-complete ophiolites. Ophiolite emplacement is a process that starts with displacement of oceanic lithosphere from its primary geodynamic en-vironment and ends with its incorporation into mountain belts during orogenesis (Coleman, 1971; Dewey, 1976; Searle and Cox, 1999; Gray et al., 2000; Wakabayashi and Dilek, 2003). Ophiolites are commonly emplaced on a passive continental margin (buoyant crust) and island arc or in an accretionary complex. The mag-matic and structural architecture of an ophio lite may refl ect a product and complex interplay of successive melting episodes and processes of magmatic differentiation, spreading rate and geometry, intra-oceanic faulting, and deforma-tion associated with tectonic extension, prox-imity to plumes or trenches, mantle temperature and fertility, and the availability of fl uids during its primary igneous evolution. Some ophiolites are stratigraphically overlain by pelagic (chert or limestone) and/or Fe-Mnrich hydrother-mal sedimentary rocks and are underlain by amphibolite-greenschist rocks related to their tectonic displacement and emplacement.

OPHIOLITE PULSES AND GLOBAL TECTONICS

The distribution of ophiolites in orogenic belts shows spatial and temporal patterns (Fig. 1), and the clusters of ophiolites with particular age ranges in different orogenic belts mark clear pulses, refl ecting peak times of ophiolite genesis and emplacement in Earth history (Fig. 2). Some of the main ophiolite pulses overlap in time with major orogenic events that led to the construction of supercontinents. Examples include the Fama-tinian (Fmt) and Caledonian (Cld; Baltica- Lau-rentia collision) orogens in the early Paleozoic, which collectively formed the Gondwana and Laurasia supercontinents, and the Appalachian-Hercynian (Ap-Hy) and Altaid-Uralian (Al-Ur) orogens later in the Paleo zoic, which built the Pangean supercontinent (Fig. 2; Moores et al., 2000). The sequential collisions of India (In-Eu) and Arabia (Ar-Eu) with Eurasia during the Neogene, after the emplacement of Neotethyan ophio lites and elimination of the Neotethyan sea-

ways by subduction, are part of the current as-sembly of a new supercontinent that has been taking place since the Paleogene.

Paleozoic ophiolites in the Appalachian-Caledonian orogenic belts (Fig. 1) developed in the Iapetus Ocean and its seaways between North America and Baltica-Avalonia (van Staal et al., 2009, and references therein). Ophiolites in Iberia, central Europe, and northwestern Africa evolved in the Rheic Ocean between Baltica-Avalonia and Gondwana continental masses (Nance et al., 2010; Murphy et al., 2010, and references therein). The Paleozoic ophiolites in the Uralides and the Altaids in central Asia are the remnants of the Pleionic Ocean, which evolved between the BalticaEastern Europe and Kazakhstan-Siberian continental masses (Brown et al., 2006; Windley et al., 2002; Xiao et al., 2004). The JurassicCretaceous ophiolites of the Tethyan Ocean systems extend from the Betic-Rif and Pyrenees in the west through the Alpine-Himalayan orogenic belts in the center to the Indonesian region in the east (Fig. 1; Hall, 1997; Pubellier et al., 2004; Bortolotti and Principi, 2005). The Phanerozoic ophio lites in these col-lisional orogenic belts (i.e., Appalachian, Caledo-nides, Uralides, and Altaids in central Asia, Betic-Rif and Pyrenees, Alpine-Himalayan) com-monly show mid-ocean-ridge basalt (MORB) to island-arc tholeiite (IAT) and boninitic geochem-ical affi nities (Varfalvy et al., 1997; Bdard et al., 1998; Spadea and DAntonio, 2006; Pag et al., 2009). The ophio lites in the accretionary-type Western Pacifi c and Cordilleran orogenic belts are slivers of abyssal peridotites and volcanic ocean islands, seamounts, and mid-ocean-ridge crust scraped off from downgoing plates, and they are commonly associated with accretionary mlanges and high-pressure metamorphic rocks (Cloos, 1982; Waka bayashi, 1999; Ernst, 2005; Ring, 2008; Hall, 2009; Cawood et al., 2009; Xiao et al., 2010).

The principal ophiolite pulses during the last 250 m.y. coincide with the emplacement of plume-related large igneous provinces (LIPs) and giant dike swarms (Ernst et al., 1995; Yale and Carpenter, 1998; Coffi n and Eldholm, 2001) and collectively mark supermagmatic events in Earth history (Fig. 2). The enhanced large igneous province formation and ophiolite gen-eration in the Late Jurassic and Cretaceous are particularly noteworthy (Vaughan and Scarrow , 2003). The evolution of the Tethyan and Carib-bean ophiolites overlapped with the Cretaceous super plume event (12080 Ma), which was responsible for the formation of oceanic plateaus in the Pacifi c and Indian Oceans, high global sea levels, and increased rates of seafl oor spreading (Larson, 1991). The JurassicCretaceous peri-Caribbean ophiolites (Fig. 1) include remnants




Ng PgTertiary

Cretaceous Triassic Permian Carb. Devonian Sil. Ord. Camb.Mesozoic Paleozoic

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ot st

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roj

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aler

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oi hpo

Jurassic

Central Asian ophiolites

Age (Ma)

Age (Ma)

Figure 2. Ophiolite pulses and the distribution of major orogenic belts with ophiolite occurrences during the Phanerozoic. A. Ophiolite pulses and the geographic distribution of Phanerozoic ophiolites through time. B. Distribution of representative examples of major ophio-lite types through time. C. Approximate time intervals for the lifespan of major supercontinents and their breakup, signifi cant orogenic events, and supermagmatic events represented by the emplacement of giant dike swarms and large igneous provinces (LIPs). The main pulses of ophiolite generation coincide with plate movements leading to the closure of ocean basins and continental collisions, large mag-matic events (with the production of large igneous provinces and giant dike swarms), and the breakup of supercontinents. Major orogenic events are (from youngest to oldest): Ar-EuArabia-Eurasia collision, In-EuIndia-Eurasia collision, Al-UrAltaid-Uralian orogenies of Central Asia, Ap-HyAppalachian-Hercynian orogenies, CldCaledonian orogeny, FmtFamatinian orogeny, P-Af-BrPan-AfricanBrasiliano orogenies. NgNeogene; PgPaleogene. For a list of different ophiolite types, see Table 1.


Dilek and Furnes


of proto-Caribbean oceanic crust and the Carib-beanColombian oceanic plateau (Kerr et al., 1998) and display a complex record of igneous activity associated with continental rifting, sea-fl oor spreading, the construction of an oceanic plateau, and the development of island arcs (Giunta and Oliveri, 2009; Kerr et al., 2009). The most prominent ophiolite pulse during the Mesozoic coincided with the breakup of Pangea through discrete episodes of continental rifting during the Late Triassic and Jurassic (Fig. 2; Dalziel et al., 2000).

A NEW CLASSIFICATION OF OPHIOLITES

The main ophiolite pulses appear to be tem-porally and spatially linked to some fi rst-order global tectonic and magmatic events. These global events and related mantle processes con-

trolled the development of different ophiolite types in different tectonic environments (Dilek, 2003). We list representative examples of the main ophiolite types, their ages, geographic lo-cation, and related references in Table 1. These ophiolite types are marked in Figures 1 and 2 with different symbols, indicating formation in different tectonic environments, as explained in the following section. In Table 2, we also list and explain a series of abbreviations in refer-ence to different ophiolite types and all the rel-evant geochemical terminology used in the next two sections and on the fi gures.

Tectonic Settings of Ophiolite Types

Continental margin (CM) ophiolites form during the early stages of ocean basin evolution, following initial continental breakup. These ophiolites are fragments of magma-poor, ocean-

continent transitions (OCT). Modern, in situ ocean-continent transitions include the Iberia and Red SeaWestern Arabia rifted margins (Fig. 1). Some classic examples of continental margin ophiolites include the Jurassic ophio-lites in the Northern Apennines (Ligurian) and the western Alps (Caby, 1995; Rampone et al., 2005; Manatschal and Mntener, 2009). These ophiolites consist of exhumed, subcontinental lithospheric mantle lherzolite directly overlain by basaltic lavas and intruded by small gab-broic plutons and rare mafi c dikes. The crustal rocks display normal (N) MORB geochemical signatures. Continental margin ophiolites cor-respond to the lherzolite-type (LOT) ophiolites of Ishiwa tari (1985) and Boudier and Nicolas (1985) and are the products of low degrees of melting of less-depleted subcontinental litho-spheric mantle and upwelling asthenosphere (Rampone et al., 2005).

TABLE 1. REPRESENTATIVE EXAMPLES OF MAIN OPHIOLITE TYPES, THEIR GEOGRAPHIC LOCATIONS, APPROXIMATE AGES, AND RELATED REFERENCES

Ophiolite Location Age (Ma) ReferencesContinental margin type1 Tihama Red Sea, Saudi Arabia 20 Coleman et al. (1972, 1977), Dilek et al. (2009)2 Ligurian Italy 200 Rampone and Piccardo (2000), Muntener and Piccardo (2003)

Manatschale and Muntener (2009)3 Ust-Belaya 1 NE Russia 310 Ishiwatari et at. (2003), Sokolov et al. (2003)4 Ust-Belaya 2 NE Russia 320 Ishiwatari et at. (2003), Sokolov et al. (2003)5 Nurali S Urals, Russia 410 Spadea et al. (2003)Mid-ocean-ridge type1A Macquarie Isl. SW Pacifi c 10 Kamentsky et al. (2000), Varne et al. (2000), Rivizzigno and Karson (2004)1B Taitao S Chile 10 Le Moigne et al. (1996), Guivel et al. (1999), Lagabrielle et al. (2000),

Shibuya et al. (2007)2 Khoy Iran 98-103 Ghazi and Hassanipak (2000), Hassanipak and Ghazi (2000)

Khalatbari-Jafari et al. (2004)4 Masirah W Indian Ocean 150 Peters and Mercolli (1998), Peters (2000)5 Horo Kanai Central Hokkaido, Japan 165180 Ishiwatari et al. (2003)6 Kuyul 1 NE Russia 190 Sokolov et al. (2003)7 Kuyul 2 NE Russia 200 Sokolov et al. (2003)8 Kuyul 3 NE Russia 210 Sokolov et al. (2003)9 Nurali S Urals 405 Pertsev et al. (1997), Spadea et al. (2003)Plume type1A Loma de Hiero Venezuela 80 Giunta et al. (2002)1B Bolivar SW Colombia 80 Nivia (1996)2 Nicoya Costa Rica 8995 Kerr et al. (1997a, 1997b), Sinton et al. (1997), Hauff et al. (2000)3 Peri-Caribbean 1 Cuba, Puerto Rica, Hispaniola 105 Kerr et al. (1997a, 1997b), Giunta et al. (2006)4 Peri-Caribbean 2 Cuba, Puerto Rica, Hispaniola 125 Kerr et al. (1997a, 1997b), Giunta et al. (2006)5 Duarte Hispaniola 140 Lapierre et al. (1997, 1999), Giunta et al. (2006), Escuder Viruete et al. (2009)6 Loma La Monja Hispaniola 155 Escuder Viruete et al. (2009)7 Mino-Tamba 1 SW Japan 185 Ichiyama et al. (2008)8 Mino-Tamba 2 SW Japan 200 Ichiyama et al. (2008)Suprasubduction-zone type1 Zambales Philippines 4044 Yumul et al. (2000), Encarnacion (2004)2 Antique Panay, Philippines 7580 Dimalanta et al. (2006)3A Troodos Cyprus 9294 Batanova and Sobolev (2000), Dilek and Furnes (2009)3B Semail Oman 9295 Lippard et al. (1986), Hacker et al. (1996), Warren et al. (2005)

Dilek and Furnes (2009), Alabaster et al. (1982)3C Kizildag Turkey 9294 Tinkler et al. (1981), Erendil (1984), Bagci et al. (2005), Dilek et al. (1999)

Dilek and Thy (1998, 2009)4 Xigaze Tibet, China 120126 Aitchison et al. (2003), Malpas et al. (2003), Zhang et al. (2003)5 Sabah Northern Borneo 135140 Rangin et al. (1990), Mller (1991)6A Mirdita Albania 160 Beccaluva et al. (1994), Bortolotti et al. (2002), Saccani and Photiades (2005)

Dilek et al. (2007, 2008)6B Pindos Greece 160 Capedri et al. (1980), Saccani and Photiades (2005), Dilek and Furnes (2009)7 Cape Povorotny Far East Asia 230250 Sokolov et al. (2003)8 Yakuno SW Japan 270280 Ishiwatari (1985), Ichiyama and Ishiwatari (2004)

(continued)




Mid-ocean-ridge (MOR) ophiolites may form at plume-proximal (e.g., Iceland) and plume-distal mid-ocean ridges, trench-proximal mid-ocean ridges, or trench-distal backarc spreading ridges (Table 2). They generally have a Penrose-type structural architecture (particularly at the centers of ridge segments) and show N-MORB (e.g., Argolis-Pindos in Greece), enriched (E) MORB (e.g., Macquarie Island), and/or contaminated (C) MORB geochemical affi ni-ties. N-MORB and E-MORB ophiolites have compositions that are more depleted and more enriched, respectively, than primitive mantlederived magmas (Pearce, 2008). C-MORB ophiolites are crustally contaminated. The Taitao ophiolite in Chile (Fig. 1), which formed at a trench-proximal Chile Rise (Karsten et al., 1996), is a type example of C-MORB ophio-lite. It was emplaced into the South American continental margin as a result of a ridge-trench

collision (Anma et al., 2009). Mid-ocean-ridge ophiolites, in general, correspond to class II and III types in Miyashiros (1975) classifi cation of ophiolites based on the presence of tholeiitic and alkaline volcanic rocks.

Plume-type (P) ophiolites may form close to plume-proximal spreading ridges and as part of oceanic plateaus (e.g., Caribbean Plateau; Kerr et al., 2009). They have thick plutonic and volcanic sequences (Coffi n and Eldholm, 2001; Kerr et al., 2009), and show depleted (D-MORB) to enriched (E-MORB) trace-element patterns (Pearce, 2008).

Suprasubduction-zone (SSZ) ophiolites (e.g., Mirdita, Albania; Samail, Oman; Troodos, Cyprus; Fig. 1) form in the extending upper plates of subduction zones, as in the modern Izu- Bonin-Mariana and Tonga-Kermadec arc-trench rollback systems (Fig. 1; Hawkins, 2003; Reagan et al., 2010). They may evolve in ex-

tending, embryonic backarc to forearc environ-ments (BA-FA), forearc settings (FA), and both oceanic and continental backarc basins (OBA and CBA, respectively; Table 2). The Rocas Verdes ophiolites in southern Chile are the best examples of suprasubduction-zone continental backarc basin ophiolites (Saunders et al., 1979; Stern and de Wit, 2003). Suprasubduction-zone ophiolites commonly have a Penrose-type struc-tural architecture and may show a MORBIATboninitic geochemical sequence of igneous activity. Suprasubduction-zone forearc ophio-lites result from oceanic crust generation dur-ing the closure of ocean basins and mark major subduction initiation events (Casey and Dewey, 1984; Dilek and Furnes, 2010; Pearce and Rob-inson, 2010). The age range among their vari-ous ophiolitic subunits is commonly less than 10 m.y. (Dilek and Furnes, 2009). They cor-respond to the class I ophiolites of Miyashiro

TABLE 1. REPRESENTATIVE EXAMPLES OF MAIN OPHIOLITE TYPES, THEIR GEOGRAPHIC LOCATIONS, APPROXIMATE AGES, AND RELATED REFERENCES (continued)

Ophiolite Location Age (Ma) ReferencesSuprasubduction-zone type (continued)9 Magnitogorsk 1 S Urals, Russia 385400 Spadea and Scarrow (2000), Spadea et al. (2003)

Spadea and DAntonio (2006)10 Baimak-Buribai SW Urals, Russia 420 Spadea and Scarrow (2000)11A Trinity 1 California, USA 440 Brouxel et al. (1989), Metcalf et al. (2000)11B Solund-Stavfjord SW Norway 440 Furnes et al. (1982); Pedersen (1986), Dunning and Pedersen (1988)

Pedersen and Furnes (1991), Furnes et al. (1990, 2003, 2006)12 Kudi-Kunlun NW China 460470 Wang et al. (2001, 2002), Yang et al. (1996)13A Thetford Mines Canada 479 Hebert and Laurent (1989), Page et al. (2009), Schroetter et al. (2003)13B Bay of Islands Canada 484 Casey et al. (1985), Suhr (1992), Bedard and Hebert (1996)

Varfalvy et al. (1997), Kurth-Velz et al. (2004)13C Betts Cove Canada 489 Coish et al. (1982), Bedard et al. (1998), Bedard (1999)14A Karmy SW Norway 474493 Furnes et al. (1980), Pedersen (1986), Dunning and Pedersen (1988)

Pedersen and Hertogen (1990), Pedersen and Furnes (1991)14B Gulfjellet SW Norway 489 Furnes et al. (1982), Dunning and Pedersen (1988), Heskestad et al. (1994)14C Leka NW Norway 497 Prestvik (1974), Pedersen (1986), Dunning and Pedersen (1988)

Pedersen and Furnes (1991), Furnes et al. (1988, 1992)15 Lachlan SE Australia, Tasmania 495510 Spaggiari et al. (2003, 2004)Volcanic-arc type1 Itogon Philippines 30 Encarnacion (2004)2A Coast Range and

Great Valley 1California, USA 140 Shervais et al. (2004)

2B Zedong 1 Tibet, China 127140 Malpas et al. (2003)3A Coast Range and

Great Valley 2California, USA 155 Shervais et al. (2004), Hopson et al. (2008)

3B Zedong 2 Tibet, China 155162 Malpas et al. (2003)4A Smartville California, USA 155165 Saleeby et al. (1989), Dilek et al. (1990, 1991)4B Josephine Oregon and California, USA 162164 Saleeby et al. (1982), Harper and Wright (1984), Harper et al. (1994)

Harper (2003a, 2003b)5 DAguilar 1 E Australia 360 Spaggiari et al. (2003, 2004)6A DAguilar 2 E Australia 380 Spaggiari et al. (2003, 2004)6B Magnitgorsk 2 S Urals, Russia 370 Spadea et al. (2003)7A Magnitgorsk 3 S Urals, Russia 385 Spadea et al. (2003)7B Trinity 2 California, USA 385 Brouxel et al. (1989), Metcalf et al. (2000)Accretionary type1 Mineoka Central Japan 25 Hirano et al. (2003), Takahashi et al. (2003), Ogawa and Takahashi (2004)2 Tokoro Japan 60 Taira et al. (1988), Isozaki (1996)3 Peri-Caribbean 3 Hispaniola, Guatemala,

Aruba-Curacao, Central Cuba 8890 Donnelly (1989), Kerr et al. (1997b), Sinton et al. (1998)4 Tamba Japan 135 Nakae (2000), Koizumi and Ishiwatari (2006)5 Solonker 1 Central Asia 240 Xiao et al. (2003), Chen et al. (2009)6 Solonker 2 Central Asia 250 Xiao et al. (2003), Chen et al. (2009)7 Ganychalan 1 NE Russia 420 Sokolov et al. (2003)8 Ganychalan 2 NE Russia 440 Sokolov et al. (2003)9 Ganychalan 3 NE Russia 460 Sokolov et al. (2003)10 Ganychalan 4 NE Russia 480 Sokolov et al. (2003)11 Ganychalan 5 NE Russia 500 Sokolov et al. (2003v


Dilek and Furnes


TABL

E 2.

OPH

IOLI

TE/O

CEAN

IC C

RUST

TY

PES,

THE

IR L

OCA

TIO

NS, A

ND R

EFER

ENCE

S TO

DAT

A SO

URCE

S, A

ND A

BBRE

VIAT

IONS

USE

D IN

THE

TEX

T AN

D TH

E FI

GUR

ES

Oph

iolite

/oce

anic

crus

t typ

eAb

brev

iatio

nsLo

catio

n

No.

anal

.Bo

wen

No.

anal

.M

ulti

No.

anal

.V/

Ti

No.

anal

.Th

-Yb-

NbRe

fere

nce

to d

ata

sour

ces

Cont

inen

tal m

argi

nCM

Inte

rnal

Lig

urid

es,It

aly

272

Otto

nello

et a

l. (19

84), R

ampo

ne et

al. (1

998)

Exte

rnal

Lig

urid

es, I

taly

2611

2619

Vann

ucci

et a

l. (19

93), M

ontan

ini et

al. (2

008)

North

Ape

nnin

e, It

aly

3939

Ferra

ra e

t al.

(1976

)Co

rsica

1313

Becc

aluv

a et

al.

(1977

)M

id-o

cean

ridg

eM

OR

Plum

e-pr

oxim

al m

id-o

cean

ridg

eM

OR

PPIc

elan

d11

937

6739

Sigv

alda

son

(1974

), Hem

ond e

t al. (

1993

)Pl

ume-

dist

al m

id-o

cean

ridg

eM

OR

PDM

acqu

arie

Isla

nd12

1212

12Ka

men

tsky

et a

l. (20

00)

Tren

ch-p

roxim

al m

id-o

cean

ridg

eM

OR

TPTa

itao

Peni

nsul

a, S

. Chi

le31

3131

9Le

Moi

gne

et a

l. (19

96), G

uivel

et al.

(199

9)No

rmal

mid

-oce

an ri

dge

basa

ltNM

ORB

Depl

eted

(in th

e inc

ompa

tible

eleme

nts)

mid

-oce

an-ri

dge

basa

ltDM

ORB

Enric

hed

(in th

e inc

ompa

tible

eleme

nts)

mid

-oce

an-ri

dge

basa

ltEM

ORB

Crus

tally

con

tam

inat

ed m

id-o

cean

-ridg

e ba

salt

CMO

RBTr

ansit

iona

l mid

-oce

an-ri

dge

basa

ltTM

ORB

Plum

eP

Gor

gona

Isla

nd, C

olom

bia

1010

Kerr

et a

l. (19

96a)

Wes

tern

Col

ombi

a85

1984

23Ke

rr et

al.

(1997

a)Ja

mai

ca17

1717

17Ha

stie

et a

l. (20

08)

Cura

cao,

Car

ibbe

an S

ea84

1119

19Kl

aver

(198

7), Ke

rr et a

l. (19

96b)

Oce

an-is

land

bas

alt

OIB

Supr

asub

duct

ion

zone

SSZ

Back

arc

to fo

rear

cSS

Z BA

-FA

Alba

nia

113

4610

245

Dile

k et

al.

(2008

)Cy

prus

56Ra

uten

schl

ein

et a

l. (19

85), A

ucla

ir an

d Lu

dden

(198

7),

Taylo

r (199

0)Tu

rkey

6140

6125

Dile

k an

d Th

y (19

98, 2

009)

Om

an13

415

113

57Li

ppar

d et

al.

(1986

), Eina

udi e

t al. (

2003

), God

ard et

al. (2

003)

Fore

arc

SSZ

FANe

wfou

ndla

nd47

2247

23Be

dard

(199

9)O

cean

ic ba

ckar

cSS

Z O

BAW

este

rn N

orwa

y80

280

2Fu

rnes

et a

l. (20

06, a

nd re

feren

ces t

herei

n)Co

ntin

enta

l bac

karc

SSZ

CBA

Sout

hern

Chi

le67

Elth

on (1

979),

Saun

ders

et al.

(197

9), St

ern an

d Elth

on (1

979),

Ster

n (19

80)

Volca

nic

arc

VALu

zon,

Phi

lippi

nes

5339

Evan

s et

al.

(1991

), Yum

ul e

t al.

(2000

)No

rth C

asca

des,

Was

hing

ton,

USA

66

6M

etzg

er e

t al. (

2002

)No

rthwe

ster

n Ca

liforn

ia,

USA

9322

9340

Harp

er (1

984),

Harp

er et

al. (1

988,

2003

a, 20

03b)

Sier

ra N

evad

a, C

alifo

rnia

, US

A4

Dile

k et

al.

(1991

)

Tota

l num

ber o

f ana

lyses

1902

283

1581

336




(1975) and harzburgite-type (HOT) ophiolites of Ishiwatari (1985) and Boudier and Nicolas (1985), which are the products of high degrees of melting of depleted, harzburgitic mantle. Both suprasubduction-zone oceanic backarc basin and continental backarc basin ophiolites form as a result of seafl oor spreading in ensi-matic and ensialic settings (respectively).

Volcanic-arc (VA) ophiolites form in ensi-matic arc settings (e.g., the Philippines, SE Asia; Sierra Nevada, California). They have a poly-genetic crustal architecture with a deformed, older oceanic basement, mafi c lower crust composed of gabbroic plutons and hypabys-sal intrusions, moderately to well-developed dioritic-tonalitic middle crust, andesitic to rhyo-litic extrusive rocks and dikes (locally sheeted) forming the upper crust, and volcaniclastic cover (locally subaerial). These crustal units display tholeiitic to calc-alkaline geochemi-cal signatures. Volcanic-arc ophiolites differ from suprasubduction-zone ophiolites based on their thicker and more fully developed arc crust with calc-alkaline compositions. The age range among various ophiolitic subunits in volcanic-arc ophiolites can be longer than 2030 m.y. (Dilek et al., 1991).

Accretionary-type ophiolites, occurring in subduction-accretion complexes of active mar-gins, contain fragments of any of the previ-ously outlined ophiolite types and are locally associated with pelagic-hemipelagic sedimen-tary rocks and trench-fi ll sediments that may have been deposited on them prior to and after their incorporation into the accretionary prism. These ophiolites may have diverse lithologi-cal assemblages, metamorphic grades, styles of deformation, and chemical affi nities with no genetic links between them, since they consist of tectonic slices of oceanic rocks scraped off from downgoing plates (e.g., Mineoka ophiolite in central Japan; Ogawa and Takahashi, 2004). They become progressively younger in age structurally downward within subduction-accre-tion complexes. We do not treat these ophiolites separately in our discussion here because they do not show a distinctive lithological construc-tion, and hence they lack a unique geochemical fi ngerprint.

Geochemical Fingerprinting of Ophiolite Types

We use a selection of diagrams to character-ize the geochemical signatures of some well-preserved examples of the types of ophiolites distinguished here. These diagrams are based on an extensive database (compiled from our own analytical work and the extant literature) that is summarized in Table 2. The literature we used

in our ophiolite classifi cation and geochemical-tectonic fi ngerprinting is presented in the GSA Data Repository.1

Since lavas and dikes in ophiolites are, in gen-eral, subject to various degrees of hydrothermal alteration and greenschist- to amphibo lites-facies metamorphism in intra-oceanic conditions, it is important to use elements that are relatively stable during such processes in order for us to determine their primary geochemical composi-tions. Several studies have been carried out on the element behavior of magmatic rocks that were variably altered and metamorphosed. In general, the mobility of an element relates to the water-rock interactions during reaction (e.g., Bickle and Teagle, 1992). Low-temperature ex-perimental studies of reaction between basalt and seawater have demonstrated minor leaching of Fe and Si and enrichment of Na and Mg; on the other hand, Al, Ti, and P are the least mobile elements, and Ca is variably depleted (Scott and Hajash, 1976; Seyfried et al., 1978). The trace elements Y, Zr, Nb, V, Cr, Co, Ni, rare earth ele-ments (REEs), Th, and Ta are generally rela-tively immobile (Coish, 1977; Hellman et al., 1979; Shervais, 1982; Seyfried and Mottl, 1982; Dickin and Jones, 1983; Dungan et al., 1983; Mottl, 1983; Staudigel and Hart, 1983; Seyfried et al., 1988; Gillis and Thompson, 1993). A study on the behavior of transition metals (Ti, V, Ni, Cr, Co, Cu, Zn, Fe, Mn) and Mg in meta basic rocks suggests relatively little mobility dur-ing medium to high degrees of metamorphism (Nicollet and Andriambololona, 1980). During hydrothermal alteration of basaltic pillow lavas, Ba shows variable alteration trends (Humphris and Thompson, 1978), and Pb becomes mod-erately to strongly depleted (Teagle and Alt, 2004). Alteration (palagonitization) of the glass rind of pillow lavas results in enrichment of K, Rb, and Cs, particularly the latter two (Hart, 1969; Staudigel and Hart, 1983). Therefore, we paid particular attention in constructing the geo-chemical diagrams presented here to use those elements that are relatively stable during hydro-thermal alteration.

In Bowen diagrams (Fig. 3) demonstrating the compositional variability in upper-crustal units (lavas and dikes), the subduction-related suprasubduction-zone and volcanic-arc ophio-lites show larger variability in SiO2 and TiO2 at given MgO contents than the subduction-unre-lated continental margin, mid-ocean-ridge, and plume ophiolites. The highest variability with respect to these two elements is represented by

the suprasubduction-zone backarc- to forearc-type ophiolites, whereas the suprasubduction-zone forearc-type ophiolites show invariably low TiO2 (Fig. 3B). The largest spread in MgO is exhibited by the subduction-unrelated plume-type ophiolites (Fig. 3A). In MORB-normalized multi-element diagrams, the con-tinental margin, mid-ocean-ridge, and plume ophiolites display fl at patterns between V and Zr, and an increase toward the most incompati-ble elements (i.e., Ba, Rb, Cs; Fig. 4A). In the same multi-element diagrams, the patterns of the suprasubduction-zone and volcanic-arc ophiolites display much larger variability; they are generally enriched in the most incompati-ble, nonconservative elements (Cs, Rb, Th) and show generally negative Ta and Nb and positive Pb and Sr anomalies (Fig. 4B).

In a Ti-V discrimination diagram (Shervais, 1982), the continental margin, mid-ocean-ridge, and plume ophiolites straddle the fi eld defi ned by the ratios between 20 and 50, typical of mid-ocean-ridge basalts (Fig. 5A), whereas the suprasubduction-zone and volcanic-arc ophio-lites show a wider scatter of Ti/V ratios between 50 (Fig. 5B). However, the subtypes of both the subduction-related and subduction-unrelated ophiolites demonstrate pronounced differences in their Ti-V distributions. For the subduction-unrelated types, the Ti-V data of the lavas and dikes for the plume subtype hardly overlap with those of the continental margin and mid-ocean-ridge trench-proximal subtypes (Fig. 5A). Similarly, for the subduction-related ophiolite types, the mafi c lavas and dikes of the suprasubduction-zone forearc subtype ex-clusively plot in the boninite fi eld and do not overlap with those of the suprasubduction-zone oceanic backarc basin subtype (Fig. 5B). By far, the suprasubduction-zone backarc to forearc subtype shows the largest range in the Ti-V dia-gram (Fig. 5B). This dispersion of Ti/V ratios is a result of a large geochemical range from boninite and island-arc tholeiite to MORB mag-mas that occur in subduction-infl uenced igne-ous systems (Shervais, 1982; Dilek et al., 2007; Dilek and Furnes, 2009).

In the Nb/Yb versus Th/Yb diagram (Pearce, 2008), the lavas and dikes of the continental margin, mid-ocean-ridge, and plume ophiolites plot within the mantle array (Fig. 6A), whereas those of the suprasubduction-zone and volcanic-arc ophiolites show a signifi cant shift away from this mantle array, toward the subduction-related Mariana arc fi eld (Fig. 6B). These fi ve elements (Ti, V, Th, Yb, Nb), which we have used in dis-criminating possible tectonic settings of ophio-litic magma generation, are most immobile during metamorphism and alteration; therefore, they are most reliable as proxies to differentiate

1GSA Data Repository item 2011131, Data source for geochemistry and tectonics of different ophio-lite types used in Tables 1 and 2, and for Figures 36, is available at http://www.geosociety.org/pubs/ft2011.htm or by request to [email protected].


Dilek and Furnes


between subduction-related and other magmas (Shervais, 1982; Pearce, 2008), particularly when utilized together with other informative geochemical techniques and fi eld-oriented re-gional tectonic constraints.

Geochemical characterization of different types of ophiolites allows us to distinguish two

major groups, one related to or least infl uenced by subduction-zone processes and the other un-related to subduction zones. The suprasubduc-tion-zone ophiolites that formed in backarc and incipient arcforearc tectonic environments (e.g., Mirdita, AlbaniaDilek et al., 2007, 2008; Troodos, CyprusRobinson et al., 2003;

Pearce and Robinson, 2010), in a forearc set-ting (e.g., Betts Cove, CanadaBdard, 1999), and as a volcanic arc (e.g., Smartville, Califor-niaDilek et al., 1991) display the most pro-nounced variations in geochemical patterns. On the other hand, trench-distal backarc ophiolites that formed in oceanic or continental settings,

Figure 3. Bowen diagrams show-ing the relationships between MgO-SiO2 and MgO-TiO2 for subduction-unrelated ophiolites (i.e., continental margin, plume, and mid-ocean-ridge types) (A1 and A2), and subduction-related ophiolites (i.e., volcanic-arc and supra subduction-zone [SSZ] types) (B1 and B2). The mid-ocean-ridge type (MOR) is subdivided into three sub-types, i.e., plume-proximal (PP), plume-distal (PD), and trench-proximal (TP). The supra-subduction-zone type (SSZ) is subdivided into four subtypes, i.e., backarc to forearc (BA-FA), forearc (FA), oceanic backarc (OBA), and continental backarc (CBA). Data sources (listed in the GSA Data Repository [see text footnote 1]): Continental margin typeFerrara et al. (1976), Becca luva et al. (1977), Ottonello et al. (1984), Vannucci et al. (1993), Rampone et al. (1998), Montanini et al. (2008). Plume typeKerr et al. (1996a, 1996b, 1997), Hastie et al. (2008). Mid-ocean-ridge types, includ-ing PP subtypeSigvaldason (1974), Hemond et al. (1993); PD subtypeKamenetsky et al. (2000); TP subtypeLe Moigne et al. (1996), Guivel et al. (1999). Volcanic-arc typeYumul et al. (2000), Evans et al. (1991), Metzger et al. (2002), Harper (1984), Harper (2003a, 2003b), Harper et al. (1988), Dilek et al. (1991). Supra-subduction-zone types, includ-ing BA-FA subtypeDilek et al. (2008), Lippard et al. (1986), Einaudi et al. (2003), Godard et al. (2003), Auclair and Lud-den (1987), Rautenschlein et al. (1985), Taylor (1990), Dilek and Thy (1998, 2009), Y. Dilek (personal observation, 1998). FA subtypeBdard (1999); oceanic backarc basin-subtypeFurnes et al. (2006, and references therein); and continental backarc basin-subtypeSaunders et al. (1979), Stern and Elthon (1979), Stern (1979, 1980), Elthon (1979).

A1. Subduction-unrelated

400 5 10 15 20 25 30 0 5 10 15 20 25 30

0 5 10 15 20 25 30 0 5 10 15 20 25 30

50

60

70

MgO (wt. %)

SiO

2 (w

t. %)

Cont. margin Plume MOR (PP)MOR (PD)MOR (TP)

A2. Subduction-unrelated

0

1

2

3

TiO

2

Cont. margin Plume MOR (PP)MOR (PD)MOR (TP)

B1. Subduction-related

40

50

60

70

MgO (wt. %)

SiO

2 (w

t. %)

Volc. arcSSZ (BA-FA)SSZ (FA)SSZ (OBA)SSZ (CBA)

B2. Subduction-related

0

1

2

3

MgO (wt. %)

TiO

2Volc. arcSSZ (BA-FA)SSZ (FA)SSZ (OBA)SSZ (CBA)

(wt. %

)(w

t. %)

MgO (wt. %)




e.g., the Solund-Stavfjord ophiolite in West Norway (Furnes et al., 2006) and the Rocas Verdes ophiolites in the southernmost Andes, Chile (Saunders et al., 1979; Stern and De Wit, 2003), show weaker geochemical evidence of subduction. The groups of ophiolites that are entirely unrelated to subduction processes are the continental margin, mid-ocean-ridge, and plume ophiolites.

PETROGENESIS OF OPHIOLITE TYPES IN DIFFERENT TECTONIC SETTINGS

Figure 7 depicts the petrogenesis of subduc-tion-related and subduction-unrelated types of ophiolites in different tectonic settings. The petro-genesis of a subduction-unrelated continental margin ophiolite involves slow exhumation and

limited partial melting of subcontinental mantle lherzolite (Fig. 8A) and upwelling astheno-sphere in response to lithospheric extension and continental rifting (Fig. 7A1; Rampone et al., 2005; Piccardo et al., 2009). Multiple intrusions of MORB-type magma form small olivine gab-bro pods and dikes (Fig. 8A) and cause basaltic eruptions on the seafl oor (Figs. 7A2 and 8B). Extensional tectonics and associated faulting

A. Subduction-unrelated

0.1

1

10

100

Cs Rb Ba Th U Ta Nb K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Sc Co Cr Ni

Roc

k/M

ORB

Cont. marginPlumeMOR (PP)MOR (PD)MOR (TP)

B. Subduction-related

0.1

1

10

100

Cs Rb Ba Th U Ta Nb K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Sc Co Cr Ni

Roc

k/M

OR

B

SSZ (BA-FA)SSZ (FA)SSZ (OBA)Volc. arc (MORB-like)Volc. arc (IAT-bon)

Figure 4. Mid-ocean-ridge-basalt (MORB)normalized multi-element diagrams, showing average values for subduction-unrelated (A) and subduction-related (B) ophiolites. IATisland-arc tholeiite; bonboninite. Different types and subtypes of ophiolites are explained in Figure 3. Normalizing values (in ppm) are: Cs (0.007), Rb (0.56), Ba (6.3), Th (0.12), U (0.047), Ta (0.13), Nb (2.33), K (1079), La (2.5), Ce (7.5), Pb (0.3), Pr (1.32), Sr (90), P (314), Nd (7.3), Zr (74), Hf (2.05), Sm (2.63), Eu (1.02), Gd (3.68), Ti (7614), Tb (0.67), Dy (4.55), Y (28), Ho (1.01), Er (2.97), Tm (0.456), Yb (3.05), Lu (0.455), V (300), Sc (40), Co (40), Cr (275), and Ni (100). The elements have been placed in order of their relative incompatibility with spinel-lherzolite mantle (after Pearce and Parkinson, 1993). Data sources (listed in the GSA Data Repository [see text footnote 1]): Continental margin typeMontanini et al. (2008); plume typeKerr et al. (1996b, 1997), Hastie et al. (2008); mid-ocean-ridge types, including plume-proximal subtypeHemond et al. (1993); plume-distal subtypeKamenetsky et al. (2000); trench-proximal subtypeLe Moigne et al. (1996), Guivel et al. (1999); volcanic-arc typeHarper (2003b); suprasubduction-zone types, including BA-FA subtypeDilek et al. (2008), Dilek and Thy (1998), Y. Dilek (personal observation, 1998); FA subtypeBdard (1999); and oceanic backarc basin subtypeH. Furnes (personal observation, 1997).


Dilek and Furnes


and shearing may cause tectonic brecciation of the lavas (Fig. 8C).

Oceanic crust formation at oceanic spreading axes involves decompression melting of uprising asthenosphere and focused upward ascent of the melt into a melt lens and associated crystal mush

zone (Fig. 7A1). Magma injection into a narrow, ~250-m-wide region (Rubin and Sinton, 2007) above the melt lens causes crustal accretion via diking and eruption on the seafl oor along the ridge axis. Lavas and dikes have compositions more depleted in incompatible trace elements

than magmas generated from primitive mantle. Locally, melts derived from incompatible- elementenriched mantle sources may segre-gate and rise to form off-axis intrusions and to feed near-ridge, E-MORB lavas. Studies of core samples from modern ocean ridges have shown that variations in rates of magma supply and the thermal structure beneath the spreading axis con-trol the mode of magmatic accretion and the ar-chitecture of oceanic crust produced. A low and episodic supply of magma to a slow-spreading ridge creates a cold environment in which ex-tensional faulting and crustal attenuation accom-pany seafl oor spreading. Amagmatic extension can result in exhumation of serpentinized upper-mantle peridotite on the seafl oor, and highly thinned lower crust and sheeted dikes (Fig. 7A2; Cannat et al., 1995; Dick et al., 2006). On the other hand, a voluminous supply of magma and the existence of a crustal melt lens at shallow depth (Fig. 7A1) beneath fast-spreading ridges create a hot environment, in which continuous magma emplacement keeps pace with seafl oor spreading. Contemporaneous extension and dik-ing produce Penrose-type oceanic crust under-lain by a



by rapid slab rollback leading to extension and seafl oor spreading in the upper plate (Fig. 7B1). In the subduction initiation stage, magma is fi rst produced by decompressional melting of deep and fertile lherzolitic mantle and produces the earliest crustal units with MORB-like compo-

sitions (Figs. 8D8F). Fluids derived from the subducted slab have little infl uence on melt evolution at this early stage. The subsequent phases of melting are strongly infl uenced by slab de hydra tion and related mantle metasoma-tism, melting of subducting sediments, repeated

episodes of partial melting of metasomatized perido tites, and mixing of highly enriched liquids from the lower fertile source with re-fractory melts in the melt column beneath the extending protoarc-forearc region (Fig. 7B1). Melt aggregation, mixing, and differentiation

A. Subduction-unrelated

0.01

0.1

1

10

Nb/Yb

Th/Y

b

Cont. marg.PlumeMOR (PP)MOR (PD)MOR (TP)

Marianaarc-basin

Cont. crust

B. Subduction-related

0.01

0.1

1

10

1001010.10.01

1001010.10.01

Nb/Yb

Th/Y

b

SSZ (BA-FA)SSZ (FA)SSZ (OBA)Volc. arc

Marianaarc-basin

Cont. crustOIB

E-MORB

N-MORB

Figure 6. Geochemical data from the subduction-unrelated (A) and subduction-related (B) ophiolite types and their subtypes (see Fig. 3 for explanation) plotted in Nb/Yb-Th/Yb discrimination diagram (after Pearce, 2008). OIBocean-island basalt; E- and N-MORBenriched and normal mid-ocean-ridge basalt. Data sources (listed in the GSA Data Repository [see text footnote 1]): Continental margin typeVannucci et al. (1993), Rampone et al. (1998), Montanini et al. (2008); plume typeKlaver (1987), Kerr et al. (1997), Hastie et al. (2008); mid-ocean-ridge types, including plume-proximal subtypeHemond et al. (1993); plume-distal subtypeKamenetsky et al. (2000); trench-proximal subtypeLe Moigne et al. (1996), Guivel et al. (1999); volcanic-arc type: Metzger et al. (2002), Harper (2003a, 2003b); suprasubduction-zone types, including BA-FA subtypeDilek et al. (2008), Einaudi et al. (2003), Godard et al. (2003), Dilek and Thy (1998), Y. Dilek (personal observation, 1998); FA sub-typeBdard (1999); oceanic backarc basin subtypeH. Furnes (personal observation, 1997)


Dilek and Furnes


can take place at many levels within this melt column, and repeated melting of the hydrated mantle leaves behind a highly depleted, olivine- and orthopyroxene-rich source. This subarc-forearc melting column produces island-arc tholeiite magma that is emplaced into and forms lavas overlying crustal units with MORB-like compositions. Rising temperatures in the mantle wedge, triggered by increased asthenospheric diapirism and lateral fl ow of hot mantle (the slab edge effect of Pearce and Robinson, 2010) and further infl ux of slab-derived fl uids result in shallow partial melting of the ultrarefractory peridotites (harzburgites), forming Mg- and silica-rich, hydrous, boninitic melts. Replace-ment of the primary olivine by orthopyroxene (opx) grains in the peridotites and the pres-ence of hydrous minerals (i.e., amphibole), as observed in most of the suprasubduction-zone ophiolites, indicate that the orthopyroxenite forms by the reaction of the preexisting olivine with these boninitic melts (Umino and Kushiro, 1989; Dilek and Morishita, 2009; Morishita et al., 2010). The orthopyroxenite thus repre-sents a reaction product between the migrating melt and the host peridotite in the upper mantle, whereas the harzburgite is the residual, depleted peridotite of the partial melt that produced the orthopyroxenite (Fig. 8G). It is likely, therefore, that geochemical features of boninitic melts are acquired as a result of interaction of migrating melts with depleted peridotites in the mantle wedge (Varfalvy et al., 1997). The harzburgite-dunite-orthopyroxenite suite in the upper-mantle peridotites of suprasubduction-zone ophiolites are melting residues and melt migration path-ways in the mantle wedge during the incipient stage of arc construction. Boninitic dikes and lavas commonly represent the youngest rock units crosscutting and overlying the earlier-formed igneous suites in suprasubduction-zone forearc ophiolites (Figs. 8H and 8K8L). Supra-subduction-zone ophiolites hence generally display a characteristic, sequential evolution of MORB to island-arc tholeiite to boninitic igne-ous activity, which manifests itself in a vertically and laterally well-developed chemostratigraphy (Fig. 7B2; Dilek and Furnes, 2009), as also observed in the modern Izu-Bonin-Mariana forearc system (Reagan et al., 2010).

The initial stage of construction of a volcanic-arc ophiolite involves basic magma. With continued subduction and infi ltration of arc magmas, the hydrated mafi c crust is partially melted to form tonalitic magmas, and this to-nalitic crust grows in thickness as the volcanic arc matures (Fig. 7B1). Residual mafi c crust can be transformed into peridotitic restite, and consequently the Moho becomes a fossil melt-ing front (Tatsumi et al., 2008). Volcanic-arc

ophiolites thus consist of an older oceanic litho-spheric foundation overlain by a mature arc suite, complete with gabbroic plutons and mas-sive diabase in the mafi c lower crust, dioritic to tonalitic middle crust, and andesitic to rhyolitic lavas, dike intrusions, and pyroclastic and vol-cani clastic rocks in the upper crust (Fig. 7B2). The construction of a volcanic arc is a result of prolonged subduction (~2040 m.y.) not termi-nated by colliding continental blocks, as is the case in the evolutionary history of suprasubduc-tion-zone ophiolites (Dilek and Flower, 2003).

Sheeted dikes (Figs. 8I8J) are tabular intru-sions of magma fl owing laterally and vertically along fractures produced by spreading-related tensile stresses, and they form along a narrow axial zone beneath central rifts along ocean ridges and above subduction zones. The exis-tence of sheeted dikes in ophiolites is conven-tionally interpreted as strong evidence for the origin of ancient oceanic crust now exposed on land by seafl oor spreading (Gass, 1990; Moores and Vine, 1971) and is generally regarded as an essential component of an ophiolite. However, the generation of a sheeted dike complex re-quires a delicate balance between the rates of spreading and magma supply for a sustained period such that suffi cient melt is produced to keep pace with extension in the rift zone (Rob-inson et al., 2008). In the upper plates of sub-duction zones, the extension is a consequence of the rate of slab rollback exceeding the rate

of plate convergence, whereas the magma sup-ply is related to the temperature profi le and the abundance and nature of fl uids in the mantle wedge, the age and lithological makeup of the subducting slab, and the history and extent of melting in the mantle source (Kincaid and Hall, 2003; Robinson et al., 2008). It is rare for the balance between spreading and magma supply rates to be maintained in a suprasubduction-zone setting of oceanic crust formation. In the absence of this balance, a sheeted dike com-plex will not form fully, or even at all, and may instead be replaced by magmatic infl ation and the emplacement of plutons, underplating the extrusive sequence (where the rate of magma supply exceeds the spreading rate), or by amagmatic tectonic attenuation of the oceanic crust (where the spreading rate exceeds the rate of magma supply). This phenomenon may explain the scarcity of sheeted dike complexes in nearly 90% of the world ophiolites (Robin-son et al., 2008), and should be considered in interpretations of the architecture of putative ancient oceanic crust, particularly in Archean greenstone belts.

Continental margin, mid-ocean-ridge, and plume ophiolites may show pronounced varia-tions in trace-element abundances, particularly for the most incompatible elements, which may be related to both different degrees of melting and mantle fertility, but which do not defi ne any partic-ular geochemical evolutionary trend (Fig. 7A3).

Figure 7 (on following page). Tectonic settings and processes of subduction-unrelated (A1) and subduction-related (B1) ophiolite types, columnar sections depicting simplifi ed structural architecture of the ophiolite types (A2B2), and generalized changes in element concentration during their evolution (A3B3). Note that the scale varies from the crust to the mantle in B1. Panels A3 and B3: For subduction-unrelated types (continental margin [CM], mid-ocean-ridge [MOR], and plume [P]), there is no distinct, regular change with time. There may be large (for the most incompatible elements) to moderate (less incom-patible to compatible elements) changes in the element concentrations, as indicated by the vertical arrows. For the subduction-related ophiolites, there is a distinct element change from the youngest to the oldest components of the ophiolites. The blank horizontal arrows pointing in opposite directions in B3 indicate that the compositions of mid-ocean-ridge ba-salt (MORB)like to island-arc tholeiite (IAT) to boninitic may change to lower or higher contents of the elements indicated. Abbreviations: A1 (CM-type): U. Crustupper crust; L. Crustlower crust; Serp. perid.serpentinized peridotite; A1 (P-type): Cont.con-tinent; B1: MORBmid-ocean-ridge basalt; IATisland-arc tholeiite; BONboninite. A2 (CM type): Serpt. perd.serpentinized peridotite; Serp. brecciaserpentinized breccia; Ppillow lava; Lhzlherzolite; Ol-gabbroolivine gabbro; A2 (MOR type): Interm.intermediate; Neovolc.neovolcanic; TZtransition zone; MMoho; DFde-tachment fault. A2 (P type): Gbgabbroic to komatiitic intrusions; ultr. sillultramafi c sill; picr. bas.picritic basalt; plw brecciapillow breccia. B2 (suprasubduction-zone type): MORB, IAT, BON; same as in B1; And.andesitic lava; Trndj. Ntrondhjemite intrusions. B2 ( volcanic-arc type): Rhy.rhyolite; And. lavaandesitic lava; Gran./ton.granite/tonalite plutons; Gbgabbro; Didiorite; DMdepleted mantle; L, M, and HREElight, middle , and heavy rare earth elements.




Dep

th (k

m)

010203040

100 50 0 50 100

010203040

km

PostriftSynrift sediments

Dep

th

1000

2000

100

4000

Dep

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1000

2000

100

4000

0

nonconservative

Backarc

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dikes

Subcontinentalmantle (Lhz)

Sea level (SL)Serp. breccia/

ophicalcite Chert

CM type

massivelava

SL

UndifferentiatedOcean Crust

Plume source

P typeMOR type

SL

TZ

DF

0.3 km

Fast Interm. Slow

0.5 km 0.5 km Depleted mantle

P

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SLSSZ type

TimeIATMORB-like

daciteBon.

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Rhy.

Gran./ ton.

DM DMStronglydepleted mantle

SL

VA typevolcaniclastic/

pyroclastic rocks

10 km

Basalt lava

A2

A1

A3

B1

B2

B3

Subduction - unrelated ocean crustContinental margin (CM) type

Mid-ocean ridge (MOR) type

Plume (P) type

CM - MOR - P types SSZ - VA types

Subduction - related ocean crustSuprasubduction - Zone (SSZ) type

Volcanic arc (VA) type

Shallowintrusion

0.5km

Serpt. perd.

Depleted mantle

M

15 km

Depleted mantle

GbDi

Ol-gabbro

Neovolc.zone

youngpluton

plw breccia

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600C300C

1200C900C

Partialmelt.zone

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Trndj Gb

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20 km

10km

120km

120 km

AsthenosphereSubcontinental

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Figure 7.


Dilek and Furnes


A B

C D

E F

Lherzolite

Olv-gabbrodikes

Pillow lava

D1

D2

D1D2

Layeredgabbro

Gabbro

Dikes

Dike

Figure 8 (on this and following page). Field photos from continental margin and various suprasubduction-zone ophiolites, depicting their in-ternal structure and the crosscutting relationships of different ophiolitic subunits. (A) Lherzolitic peridotites of the Jurassic In-Zecca ophio-lite (continental margin type) in the Ligurian ophiolites (eastern Corsica) intruded by irregular olivine gabbro dikes and veins. (B) Pillow lavas with normal mid-ocean-ridge basalt (N-MORB) geochemical affi nities, resting directly on serpentinized peridotites of the In-Zecca ophiolite. (C) Tectonically brecciated pillow lavas (in B), showing cataclastic shearing in and around the pillow-shaped fl ows. (D) Layered gabbro rock in the 493 Ma Karmy ophiolite (suprasubduction-zone backarc to forearc [BA-FA] type) in western Norway intruded by ba-saltic dikes (D1) with MORB affi nities that are in turn crosscut by boninitic dikes (D2). (E) Sheeted dikegabbro transition zone (Karmy ophiolite), where leucocratic gabbros and basaltic dikes show mutually intrusive relationships in a Penrose-type crustal pseudostratig-raphy. (F) Pillow lavas with island-arc tholeiite (IAT) geochemical affi nities in the Karmy ophiolite crosscut by an island-arc tholeiite dike.




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Dilek and Furnes


Suprasubduction-zone and volcanic-arc ophio-lites show a characteristic geochemical evo-lution. In the early stages of their formation, magmas are MORB-like, but during repeated episodes of melting, their mantle source be-comes progressively depleted in the most incompatible elements. The geochemical evolu-tion of suprasubduction-zone and volcanic-arc ophiolitic magmas is characterized by low abun-dances of incompatible elements (Cs, Rb, Ba, U, Ta Nb, and light [L] REEs) in basaltic andesites, andesites, and dacites, which commonly occur in the upper parts of their extrusive sequences, and in young crosscutting dikes in sheeted dike complexes. With repeated melting, the residual mantle source is progressively enriched in oli-vine and orthopyroxene, the principal hosts of compatible elements such as Ni, Co, Cr, and Sc.

At a later stage in the magmatic evolution of suprasubduction-zone ophiolites, there is a change from depletion to enrichment in incom-patible element contents in the younger igneous rocks relative to MORB; the more incompatible an element is, the more pronounced its enrich-ment becomes in many suprasubduction-zone ophiolite lavas. This phenomenon suggests that the mantle source undergoes enrichment of highly mobile elements during or before the ex-traction of MORB-like magmas from it. It is the nonconservative, highly incompatible elements, Cs, Rb, Th, and U, that show the most pro-nounced change from depletion to enrichment during the late-stage evolution (Fig. 7B3); the other highly incompatible but conservative ele-ments, such as Ta and Nb, remain depleted (e.g., Pearce and Parkinson, 1993). Pb and Sr seem to be enriched at an earlier stage than the other non-conservative incompatible elements, and these elements, particularly Pb, increase in concentra-tion from the island-arc tholeiite magmatic stage to the fi nal boninite activity (Fig. 7B3).

Enrichment of the source mantle in slab-derived, nonconservative elements is a com-plex process that may involve fl uids released from altered oceanic crust and its sedimentary cover and felsic magmas generated by partial melting of subducted sediments (Pearce and Parkinson, 1993; Hawkesworth et al., 1997; Macdonald et al., 2000; Elburg et al., 2002). Thus, during the generation of subduction-related ophio lites, two dominant, contempora-neous processes oper ate to continuously modify the source region and are responsible for the typical trace-element patterns of the magmas produced: (1) Repeated episodes of partial melt-ing progressively deplete the mantle source in incompatible elements and enrich it in compat-ible elements. Inhomo geneities in the mantle source and variable degrees of partial melting could also result in variable concentrations of

incompatible elements in the magmas produced. (2) The mantle melt source becomes enriched in highly incompatible, nonconservative elements (particularly Cs, Rb, Ba, Th, U) transported in subduction-derived fl uids and/or felsic melts.

Application to Precambrian Greenstone Belts

We have selected three Precambrian green-stone belts ranging in age from Paleoprotero-zoic (Jormua, Finland) to Neoarchean (Wawa, Canada) and Paleoarchean (Isua, Greenland), for the purpose of comparing the published geo-chemical data for the volcanic and subvolcanic rocks of these sequences with the Phanerozoic ophiolite types as classifi ed herein.

Isua Supracrustal BeltThe mafi c-ultramafi c units of the ca. 3.8 Ga

Isua supracrustal belt in Greenland occur in two major tectonostratigraphic units, namely the un-differentiated amphibolites (UA) and Garben-schiefer amphibolites (GA) (e.g., Nutman et al., 1984, 1997; Rosing et al., 1996; Komiya et al., 1999; Furnes et al., 2007, 2009). The undifferen-tiated amphibolites unit contains all major litho-logical units of a typical Penrose-type, complete ophiolite sequence, whereas the Garbenschiefer amphibolites unit is composed dominantly of volcaniclastic and volcanic rocks that are com-monly found in immature island arcs.

Wawa Greenstone BeltsThe 2.7 Ga Wawa greenstone belt of the

Superior Province in Canada consists of Al-undepleted and Al-depleted komatiites and Mg- and Fe-tholeiites (Polat et al., 1998, 1999). Compositionally, these mafic volcanic and plutonic rocks are comparable to Phanerozoic ocean plateau basalts that subsequently were tectonically imbricated with primitive arc ba-salts (Polat et al., 1998, 1999).

Jormua ComplexThe 1.95 Ga (Peltonen et al., 1996) mafi c to

ultramafi c rocks of the Jormua Complex (JC) occur in the central part of an early Protero-zoic (2.31.92 Ga) metasedimentary sequence that is surrounded by Archean basement rocks in northeastern Finland (Kontinen, 1987; Peltonen et al., 1996). The Jormua Complex includes pillow lavas and volcanic breccias, a sheeted dike complex, mafi c cumulates, and upper-mantle peridotites, and it is tectonically disrupted into several blocks. The thickness of the Jormua Complex varies, and in places the lava sequence rests directly upon the upper-mantle rocks, typical of the Ligurian ophiolites in the Apennines. The crustal architecture of the

Jormua Complex is reminiscent of that seen in slow-spreading oceanic crust and in continental margin ophiolites (Peltonen et al., 1996, 2003).

SummaryIn the Bowen diagrams (MgO-TiO2), the

younger Garbenschiefer amphibolites of the Isua supracrustal belt plot exclusively in the fi eld of subduction-related ophiolites, whereas the un-differentiated amphibolites plot both in the subduction-related and subduction-unrelated fi elds (Fig. 9A). The Wawa and Jormua meta-basalts plot predominantly in the fi elds of plume and continental margin types of the subduction-unrelated ophiolites, respectively (Figs. 9B and 9C). In the multi-element diagrams, the undif-ferentiated amphibolites of Isua plot within the fi eld of subduction-related ophiolites and display their characteristic features, such as positive Pb anomalies, negative Nb and Ta anomalies, and strong enrichment of Ba and Th. On the other hand, the Garbenschiefer amphibolites show strong depletion of the middle (M) REEs, a typi-cal feature of boninites (Fig. 10A). The Wawa and Jormua metabasalts plot within the fi eld defi ned by the subduction-unrelated ophiolites and display the same features of fl at to moder-ately enriched patterns as the incompatibility of the ele ments increase (Figs. 10B and 10C). In the Ti-V discrimination diagram, the Isua data plot in two distinct fi elds, with the Garben-schiefer amphibolites exclusively in the boninite fi eld (Ti/V < 10), whereas the undifferentiated amphibolites have Ti/V ratios of 2030 (Fig. 11A) in the mixed MORB and island-arc fi elds (Shervais, 1982). The volcanic and dike rocks of the Wawa and Jormua sequences, on the other hand, plot entirely within the plume and conti-nental margin types, respectively, of subduction-unrelated ophiolites (Figs. 11B and 11C). In the Nb/Yb-Th/Yb discrimination diagram, all the Isua data plot in the subduction-related fi eld (Fig. 12A), whereas the Wawa and Jormua data plot in the subduction-unrelated fi eld (Fig2. 12B and 12C). The Wawa data defi ne a large spread between N-MORB and oceanic-island basalt (though mostly between N-MORB and E-MORB), while the Jormua data cluster tightly around E-MORB (Figs. 12B and 12C).

The geochemical character of the metavol-canic and intrusive rocks of the three selected Precambrian greenstone belts indicates that they originated in different tectonic environments. Thus, compared with the geochemical evolu-tion of Phanerozoic ophiolites, the Paleoarchean Isua rocks most likely represent a supra-subduction-zone forearc basin subtype ophio-lite, as suggested by Furnes et al. (2009). The Neo archean Wawa greenstone belt, on the other hand, is more akin to the structural and geo-




chemical character of plume-type ophiolites, in agreement with the interpretations of Polat et al. (1999). The early Proterozoic Jormua Complex resembles, both structurally and geochemically, continental margintype ophiolites, consistent with the interpretations of Peltonen et al. (2003).

CONCLUSIONS

Ophiolites are diverse in their internal struc-ture, geochemical makeup, and emplacement mechanisms, and they form in different tec-

tonic environments during the Wilson cycle evolution of ancient ocean basins from rift-drift and seafl oor spreading stages to subduction ini-tiation and closure phases. Mafi c-ultramafi c to felsic rock assemblages that originally formed in different tectonic settings may eventually be-come nested in collision zones, forming distinct ophiolite complexes with signifi cant diversity in their structural architecture, geochemical fi ngerprints, and emplacement mechanisms. Differences in the magmatic and structural archi tecture of ophiolites result from their prox-

imity to plumes or trenches, rates and geometry of spreading, mantle temperatures and fertility, and the availability of fl uids in the tectonic set-ting of formation during their primary igneous evolution. Ophiolites are broadly su

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Ophiolite Genesis and Global Tectonics -Dilek & Furnes 2011