Preface

Ophiolites have continued to be an exciting and interesting topic of multidisciplinary research since their recognition as on-land exposures of fossil ocean crust in the early 1960s. Ideas on the formation of ophiolites in terms of the tectonic setting of their igneous accretion and the mechanisms of their incorporation into continental margins have varied over the years, undergoing a significant transformation since the 1972 Penrose definition of an ideal ophiolite suite. Systematic studies of ophiolites have helped us better understand the 3-D architecture of oceanic crust, while marine geological and geophysical investigations of in situ modern oceanic crust have provided us with better observations and ideas on the mode and nature of magmatic, tectonic, metamorphic, sedimentary, hydrothermal and biogenic processes that affect the evolution of oceanic crust in diverse tectonic settings. Thus, the synergistic research activities of the international ophiolite and the marine geology and geophysics communities have produced a wealth of new data and observations on oceanic lithosphere and ophiolites over the past 20 years toward the refinement of plate tectonic theory. Several major publications on ophiolites and oceanic crust have come out during this time (Gass et al. 1984; Peters et al. 1991; Parson et al. 1992; Prichard et al. 1993; Ishiwatari et al. 1994; Vissers & Nicolas 1995; Dilek et al. 2000; Dilek & Newcomb 2003; Dilek & Robinson 2003; Kusky 2004). The 32nd International Geological Congress (IGC-32) held in Florence, Italy, 20–28 August 2004, provided an excellent forum to discuss and evaluate the most recent observations, data and ideas on ophiolites and oceanic crust. Discussions were lively and involved active participation of both on-land and marine researchers in a total of six sessions focused on igneous petrogenesis of ophiolites, melt and fluid flow in the evolution of oceanic lithosphere, emplacement tectonics of ophiolites, ophiolites and suture zones of the Tethysides, record of oceanic rocks in Precambrian and Early Phanerozoic times, and ophiolites of the Circum-Pacific Orogenic Belts. It was clear from these discussions that the origin of ophiolites is far more complex than previously thought, and that the existing evidence points to the evolution of ophiolites in various tectonic environments along convergent and divergent plate boundaries. Perhaps the most telling information on ophiolite genesis has come recently from Tethyan ophiolites as well as from ophiolites and oceanic rocks in the accretionary-type Circum-Pacific Orogenic Belts. This thematic section of The Island Arc includes a collection of papers presented at the IGC-32 in order to disseminate the exciting new data and models on various aspects of ophiolites to the international community in a timely fashion, following the successful International Geological Congress in 2004. Tethyan ophiolites, occurring in nearly east–west-trending belts in the Alpine–Himalayan orogenic system and in the Caribbean region, display a record of rift-drift, sea floor spreading, and subduction zone tectonics that their respective ocean basins experienced during the Mesozoic (see Robertson 2002; Dilek & Flower 2003 for overview). These ophiolites show a progression in their igneous accretion and tectonic emplacement ages from west to east within the Alpine–Himalayan Orogenic Belt. This progression indicates that the latitudinal Mesozoic ocean basins within the Tethyan realm had a diachronous evolution through time. The timing and kinematics of the initial rifting of Pangea, the distribution and paleogeography of various ocean basins in which the Tethyan ophiolites formed, and the timing and mechanisms of the closure of these basins leading to widespread ophiolite emplacement are still a subject of debate; as such, there is great need for more systematic structural field studies, isotopic fingerprinting and geochronologic investigations of suitable ophiolite complexes and associated tectonic units in the Tethyan realm. The following six papers in this thematic issue examine the geology of several Tethyan ophiolites and present diverse data and new models on their evolution. Looking at the Tethyan ophiolites on a more global scale, Bortolotti and Principi explore the role of these ophiolites in better understanding the timing and mechanisms of the break-up of Pangea and development of the Tethyan basins throughout the Mesozoic. Widespread continental rifting within Pangea during the Permo-Triassic ultimately led to the separation of Africa, Southern Europe, and the North and South Americas later in the Mesozoic. These authors argue that the Tethyan Ocean developed in two distinct paleogeographic configurations. An Eastern Tethys may have evolved as a westward-propagating seaway during the Middle Late Triassic, cutting across the eastern equatorial portion of Pangea, whereas a Western Tethys likely developed along the northwestern edge of Gondwana during the Jurassic. Most of the ophiolites within the Eastern Mediterranean region and in the Himalayan orogenic belt are the products of the Eastern Tethys and include crustal and mantle rocks with mid-oceanic ridge basalt (MORB) and island arc tholeiite (IAT) affinities. This interpretation confirms earlier suggestions (Dilek & Flower 2003) that the Eastern Mediterranean ophiolites are heterogeneous in age and composition. The fossil oceanic crust derived from the Western Tethys constitutes the ophiolitic occurrences with MORB affinities in the Alps, Northern Apennines (Italy) and Betics (Iberian Peninsula), and is mostly Middle–Late Jurassic in age. The opening of the Central Atlantic and the Caribbean Tethys during the Late Jurassic–Early Cretaceous was an artifact of major left-lateral wrench tectonics between Gondwana and Laurasia as the dismantling of Pangea progressed. Bortolotti and Principi conclude that both the break-up of Pangea and the development of the Tethyan ophiolites were time-progressive events younging from east (Middle–Late Triassic) to west (Late Jurassic–Early Cretaceous) over time. Current and future studies of rift-drift assemblages and the Tethyan and Caribbean ophiolites should be able to test this hypothesis. Beccaluva et al. discuss the magmatic evolution of the Jurassic ophiolites in the Albanide–Hellenide segment of the Alpine Orogenic Belt. They propose that the Mirdita ophiolites in the Albanides and the Pindos and Vourinos ophiolites in the western Hellenides were derived from the Pindos–Mirdita Basin, which had evolved between the Adrian and Pelagonian continental blocks. The ophiolites in the western part of the Albanide–Hellenide Orogenic Belt have typical MORB associations, whereas the ophiolites to the east have prevalent IAT and minor boninite affinities, with basaltic rocks in the central Mirdita region and in Pindos displaying geochemical features intermediate between MORB and IAT (MORB/IAT). Beccaluva et al. attribute these compositional differences to various degrees of partial melting of mantle sources, which were progressively depleted by successive melt extractions. They explain the MORB generation in the west through partial melting (10–20%) of lherzolitic mantle sources that might have left a clinopyroxene-poor lherzolite residuum. The intermediate MORB/IAT assemblages in central Mirdita were derived from approximately 10% partial melting of this residuum in a supra-subduction zone (SSZ) setting, whereas the IAT and boninitic units farther east were derived form 10–20% and 30% partial melting, respectively, of the same mantle source, which was variably enriched by subduction-derived fluids. The authors suggest that shallow fractional crystallization processes may have played a significant role in the chemical evolution of the SSZ magmas. The tectonic model in this study envisions a west-dipping (in the present coordinate system) subduction zone in the Pindos–Mirdita Basin whose rollback toward the Pelagonian continental block to the east enabled the tectonic extension and magmatic plumbing in the upper plate that generated the SSZ ophiolites in the Albanide–Hellenide Belt. Looking more closely at the geology of the Mirdita ophiolites in Albania, Bortolotti et al. discuss that the Jurassic oceanic crust preserved in these ophiolites have both MORB and IAT affinities, indicating a two-stage crustal growth for their igneous evolution. A slightly older MORB component of these ophiolites developed in a mid-ocean ridge environment, and was subsequently trapped in an SSZ setting above an east-dipping (in the present coordinate system) subduction zone. It is rather interesting that given similar geochemical observations and geological information, the geodynamic models on the Albanide–Hellenide ophiolites as presented in the papers by Beccaluva et al. and Bortolotti et al. differ significantly, in that the inferred subduction zone polarities are exactly the opposite. This has strong implications for both the igneous accretion and the tectonic emplacement processes of the Mirdita ophiolites. The westward-dipping subduction zone model of Beccaluva et al. implies that the ophiolites were first emplaced onto the Pelagonian platform as a result of its collision with the trench. The eastward-dipping subduction zone model of Bortolotti et al. depicts the Pelagonian platform as part of Adria throughout the Mesozoic and Cenozoic and emplaces the ophiolites westward onto Pelagonia–Adria as part of large nappe sheets. These authors suggest a single Tethyan seaway for the origin of all ophiolite occurrences (Western Hellenic, Vardar Zone, etc.) in the Balkan Peninsula. In this regard, the model by Bortolotti et al. deviates significantly from the existing interpretations of the Mesozoic evolution of this region (see, e.g., Smith 1993; Robertson & Shallo 2000; Stampfli & Borel 2004; Dilek et al. 2005), which envisage the Pelagonian block as an insular continental entity separating the Pindos and Vardar (and/or Maliac) Basins within the Tethyan realm during much of the Mesozoic. Relatively unmetamorphosed platform carbonates of the Pelagonian block with a continuous sedimentary record of Triassic through Upper Jurassic–Lower Cretaceous rocks are in support of this latter interpretation and are incompatible with the model of Bortolotti et al., which suggests significant unroofing of a deeply buried Pelagonian continental crust at the easternmost edge of Apulia starting in the Middle–Late Miocene. Saccani and Photiades document the lithologic and geochemical characteristics of sub- and supra-ophiolitic mélange units spatially associated with the Mirdita–Subpelagonian zone ophiolites in the Albanide–Hellenide Belt. Characterized as ‘block-in-matrix-type’, these mélange units consist mainly of Triassic transitional to alkaline within-plate basalts, Triassic normal (N) and enriched (E) MORB, Jurassic N-MORB, Jurassic MORB/IAT and Jurassic boninitic rocks. Although widespread in the SSZ-type ophiolites, basalts with IAT affinities have not been found in these mélanges, whereas volcanic rocks interpreted to have formed in forearc settings (i.e. boninites) are common. Saccani and Photiades suggest that incorporation of N-MORB material into subophiolitic mélanges was carried out mostly through tectonic processes during ophiolite obduction, whereas incorporation of forearc material (MORB/IAT rocks and boninites) into supra-ophiolitic mélanges was a result of sedimentary processes in an inner-arc or back-arc tectonic setting. The Cretaceous Oman ophiolite occurring in a 600-km-long and 150-km-wide thrust sheet in the southeast Arabian Peninsula represents a Tethyan oceanic lithosphere approximately 15 km thick with a Penrose-type layer-cake pseudostratigraphy. Nearly 8 km of this fossil oceanic lithosphere consists of a mantle sequence comprising dunitic and harzburgitic peridotites cut by pyroxenite dykes. Gently dipping foliations and associated lineations in these peridotites are at right angles to the average trend of the northwest-running sheeted dykes and represent high-temperature plastic flow structures within the upper mantle sequence (Nicolas et al. 1988). A flat-lying and gently undulating Moho forms the transition zone between the mantle rocks below and the crustal sequence above. Ultramafic cumulates and layered gabbros, which form the bottom units of the oceanic lower crust in the Oman ophiolite, played a major role in the development of magma chamber models during the early 1980s and 1990s (Thy & Dilek 2003; references therein). Kawamura et al. examine the origin of layering in cumulate gabbros in the Oman ophiolite by documenting the secular changes in magnetic susceptibility and its anisotropy within a section 800 m thick. They note that the magnetic susceptibility decreases as the number of magnetic grains is reduced from melanocratic to leucocratic layers, suggesting cyclic downward accumulation of olivine grains in a magma chamber. The preferred subhorizontal alignment of magnetite grains, which were derived from the alteration of olivine minerals, and the anisotropic fabric of plagioclase and clinopyroxene grains suggest simple shear deformation in lithospheric conditions, following the formation of layered gabbros through cumulus processes. Rollinson reviews the extant data and models on the chromitite occurrence in the mantle section of the Oman ophiolite and presents a new model for its genesis. He demonstrates that the Omani chromites range in composition from high Cr♯, boninite-like to low Cr♯, MORB-like end-members, reflecting their crystallization from a spectrum of different melt compositions. He proposes that, unlike in previous models invoking different tectonic environments for the origin of this wide variation in melt compositions, different melt/rock ratios were responsible for the evolution of successively more Cr-rich melts and hence for the crystallization of chromites with higher Cr♯ through the reaction of a basaltic melt with depleted harzburgitic mantle. If the variable chromite chemistry in the Oman mantle sequence is indeed a product of a compositionally evolved melt through processes of melt migration and melt–rock reaction, the volcanic products of these evolved melt(s) should be found in the extrusive sequence. Careful chemostratigraphic analyses of volcanic units in the Oman and other ophiolites could provide a means to test this model. It has been shown that the internal structure of in situ oceanic lithosphere is a result of the spatial and temporal relations among the spreading rate, magma supply, tectonic extension and thermal regime beneath the mid-ocean ridges and the spreading axes in back-arc basins (Macdonald 1982; Phipps Morgan et al. 1994; Dilek et al. 1998). These factors also affect the mode and nature of hydrothermal activities and associated processes along sea floor spreading centers (Humphris & Tivey 2000). Chemical reactions between the rocks and hydrothermal fluids control the isotopic signatures of hydrothermal fluids and are in turn influenced by spreading rates. Isotopic signatures of hydrothermal fluids at slow-spreading centers tend to be, for example, more rock-dominated than those at fast-spreading centers (Bach & Humphris 1999). Therefore, carefully documented hydrothermal alteration histories of well-preserved ophiolites may prove to be useful in deciphering the tectonic evolution of fossil oceanic crust (e.g. spreading rate, tectonic setting of igneous accretion, etc.). Fonneland et al. document the volcanic stratigraphy and alteration history of the Late Ordovician Solund-Stavjord ophiolite complex (SSOC) in the western Norwegian Caledonides and present a tectonic model for the evolution of this intermediate- to fast-spreading fossil oceanic crust. They demonstrate that the δ18O values of the whole-rock samples display a general depletion structurally downward in the ophiolite, and that the calculated minimum water/rock ratios show the largest variations in volcanic rocks and gabbros and generally the lowest values and range in the sheeted dyke complex. This hydrothermal alteration history of the SSOC is similar to that of the 5.9-my-old, intermediate-spreading oceanic crust of the Costa Rica rift, although there are some geochemical differences between the two as a result of the evolution of the SSOC in a back-arc setting rather than in a mid-ocean ridge environment (as in the Costa Rica Rift). Ophiolites in accretionary-type orogenic belts differ from Tethyan-type ophiolites both in terms of their internal architecture and in terms of their emplacement processes. They are commonly polygenetic, developed on and across a deformed, heterogeneous oceanic basement and may include fully developed island arc sequences and/or seamount fragments (Dilek 2003). Structurally, they overlie subduction–accretion complexes and have been incorporated into active continental margins through progressive underthrusting of oceanic material and ridge–trench interactions. The prolonged history of their tectonic evolution is therefore quite revealing, not only for the ophiolites themselves, but also for the geological history of the oceanic plates on which they were developed. The four papers in the last section of the thematic issue present new data and observations from several Pacific Rim ophiolites and oceanic remnants. Ueda and Miyashita describe the stratigraphy and petrology of the Oku-Niikappu complex (ONC), a remnant arc fragment in a Cretaceous accretionary complex in central Hokkaido, Japan, and propose a new tectonic model for the evolution of this subduction–accretion system in the Mesozoic Pacific northwest. The inferred remnant ensimatic arc origin of the ONC implies that a back-arc basin may have developed facing the Eurasian active margin during the Middle Cretaceous, rather than a single, large oceanic plate, as previous models have suggested. The collapse of this back-arc basin resulted in tectonic accretion of the remnant arc fragments into the Eurasian continental margin, which in turn affected the magmatic, metamorphic and tectonic evolution of the continental upper plate. Mori and Ogawa discuss the complex tectonic evolution of the polygenetic, Cretaceous–Miocene Mineoka Ophiolite Belt in the Boso Peninsula, central Japan. The igneous evolution of this ophiolite has been discussed in previous papers by Ogawa and his coworkers (Ogawa 1983; Hirano et al. 2003; Ogawa & Takahashi 2004). Mori and Ogawa show in this paper that the Mineoka ophiolite underwent several episodes of deformation during and after its emplacement into the Japanese continental margin. Uplift and exhumation of high-grade metamorphosed rocks and ‘knockers’ within the ophiolite belt were a result of transpressional tectonics associated with right-lateral strike-slip faulting in the trench–trench–trench-type Boso triple junction area. The Mineoka ophiolite was subsequently placed in a forearc setting as this triple junction evolved, and it was trapped by the Neogene accretionary prism to the south. Fujioka et al. describe the morphology and structure of the Hahajima Seamount at the junction between the and on the of a of magnetic and and from the Hahajima Seamount include and this of suggests different sources of for their Fujioka et al. a simple seamount interpretation of the Hahajima Seamount and suggest that it may represent a block of a forearc to some forearc ophiolites. et al. document the and deformation history of the ophiolite in on their This ophiolite is approximately km southeast of the triple junction and consists of a Penrose-type ophiolite pseudostratigraphy. The ophiolitic units to record two distinct episodes of deformation and associated An earlier is inferred to have resulted from the collision of the with the whereas the later is inferred to have resulted from the emplacement of the ophiolite into the South forearc setting following the ridge–trench This during emplacement is likely to have of the forearc basement tectonic which in turn and of the volcanic and units in the Thus, the crustal evolution of the ophiolite to have involved different magmatic episodes in mid-ocean ridge and forearc (SSZ) tectonic This is significantly different from the model of et al. which suggests that SSZ characteristics of volcanic rocks in the ophiolite may have resulted from of the mantle beneath the Southern by material derived from the subduction zone. These models and interpretations on the likely sources and mechanisms of subduction of ophiolitic and on the genesis of heterogeneous of extrusive sequences in ophiolites have strong implications for SSZ evolution of oceanic crust and need to be with future that this thematic issue and research on ophiolites and associated continental margin rocks. It is clear that on-land of fossil oceanic lithosphere provide information the evolution of oceanic plates and continental margins in and that they significant for the chemical of the mantle and its evolution through time. studies on ophiolites, on Precambrian ophiolites and should new information on oceanic crust continental margin changes in the and the role of oceanic lithosphere in continental growth through time. to the to this thematic issue for their time and to the of the international community for their timely and which the of the and to the for his and support throughout the of this thematic and a and helped us this thematic issue in a timely