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Ophiolites in Earth History: Introduction Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Ophiolites in Earth history: introduction YILDIRIM DILEK 1 & PAUL T. ROBINSON 2 1Department of Geology, Miami University, Oxford, 0H45056, USA (e-mail: [email protected]) 2Department of Earth Sciences, Dalhousie University, Halifax, N.S. B3H 3J5, Canada Ophiolites record significant evidence for tectonic ment; and regional occurrences of ophiolites and and magmatic processes from rift-drift through their geodynamic implications. accrefionary and collisional stages of continental margin evolution in various tectonic settings. Ophiolites, mantle plumes and orogeny Structural, petrological and geochemical features of ophiolites and associated rock units provide Ophiolite occurrences around the world are not a essential information on mantle flow field effects, random geological phenomenon. Ophiolites with including plume activities, collision-induced certain age groups in different orogenic belts aesthenospheric extrusion, crustal growth via mag- characterize distinct ophiolite pulses, which mark matism and tectonic accretion in subduction- times of enhanced ophiolite genesis and emplace- accretion cycles, changes in the structure and ment. Examining the geological record of motm- composition of the crust and mantle reservoirs tain-building episodes and related events, Dilek through time, and evolution of global geochemical shows that ophiolite pulses overlap significantly cycles and seawater compositions. Ophiolite stud- with the timing of major collisional events during ies over the years have played a major role in the assembly of supercontinents, their break-up better understanding of mid-ocean ridge and sub- and increased mantle plume activities that devel- duction zone processes, mantle dynanlics and oped extensive large igneous provinces (LIPs). heterogeneity, magma chamber processes, fluid These global events have been involved in the flow mechanisms and fluid-rock interactions in Wilson cycle evolution of ancient ocean basins oceanic lithosphere, the evolution of deep bio- that in turn contributed to ophiolite genesis in sphere, the role of plate tectonics and plume diverse tectonic settings. Suprasubduction zone tectonics in crustal evolution during the Precam- ophiolites represent anomalous oceanic crust gen- brian and the Phanerozoic, and mechanisms of eration in subduction rollback cycles during the continental growth in accretionary and coltisional closing stages of basins prior to terminal continen- mountain belts. Through multi-disciplinary inves- tal collisions. Accelerated LIP formation asso- tigations and comparative studies of ophiolites and ciated with superplume activities may have modern oceanic crust and using advanced instru- facilitated both the generation and tectonic empla- mentation and computational facilities, the inter- cement of ophiolites at global scales. These spatial national ophiolite community has gathered a and temporal relations suggest that ophiolite wealth of new data and syntheses from ophiolites pulses, mantle plume activities and orogenic around the world during the last 10 years. The events have been closely linked through complex purpose of this book is to present the most recent mantle dynamics in Earth history. data, observations and ideas on different aspects of 'ophiolite science' through case studies and to document the mode and nature of igneous, meta- Tethyan ophiolites in the Alpine- morphic, tectonic, sedimentological and/or bio- Himalayan orogenic system logical processes associated with the evolution of oceanic crust in different tectonic settings in Papers in this section present diverse data from Earth's history. It comprises 32 papers collected in Tethyan ophiolites and provide refined geody- six sections on temporal relations anaongst ophio- namic models for their evolution. Flower & Dilek lite genesis, mantle plume events and orogeny in examine the processes of arc-trench rollback and Earth history; Tethyan ophiolites in the Alpine- forearc accretion, and present an 'actualistic' Himalayan orogenic system; magmatic, meta- model for ophiolites based on recent observations morphic and tectonic processes in ophiolite gen- of forearc evolution in western Pacific and Medi- esis; hydrothermal and biogenic alteration of terranean marginal basins. Collision-induced man- oceanic crust; mechanisms of ophiolite emplace- tle flow and 'slab-pull' forces may result in rapid From: DILEK, Y. & ROBINSONP. T. (eds) 2003. Ophiolites in Earth History. Geological Society, London, Special Publications, 218, 1-8. 0305-8719/03/$15 9The Geological Society of London 2003. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 Y. DILEK & P. T. ROBINSON arc-trench rollback pulses and associated exten- the embryonic Piedmont-Ligurian Ocean. Perva- sional episodes (splitting of nascent volcanic pro- sive melt infiltration and melt-rock reaction pro- to-arcs), producing proto-ophiolites in arc-forearc duced gabbroic intrusions with a wide range of settings. These ophiolites commonly include high- compositions characteristic of the melting column temperature metamorphic soles, boninitic rocks, beneath mid-ocean ridges. These observations are juxtaposed refractory peridotites and high-tem- critical to better understand the effects of melt perature epidosites that are generally absent in percolation and impregnation in development of mid-ocean ridge, normal arc and back-arc basin plagioclase-enriched peridotites. The Ligurian environments. As subduction rollback continues, ophiolites clearly do not represent a typical Pen- arc-forearc complexes become increasingly het- rose-type, idealized oceanic crust. erogeneous, displaying significant internal age and Bazylev et al. present mineral and bulk-rock structural discrepancies, a common feature both in chemistry data from the Jurassic Brezovica ultra- the SW Pacific subduction zone environments and mafic massif (Serbia) in the Dinarides and show Tethyan ophiolites. When an arc-trench rollback that its petrogenetic evolution involved two dis- cycle is terminated by a collision, heterogeneous tinct magmatic stages. A suite of spinel harzbur- forearc lithosphere is accreted as ophiolites in the gites was produced during the first stage as a initial stages of the evolution of collisional oro- result of partial melting of the mantle and segre- genic belts. This model demonstrates the apparent gation of tholeiitic melts. Percolation of melt correspondence of subduction nucleation and through these spinel harzburgites and melt-rock mantle flow to plate collisions at regional and reaction produced dunites and refractory harzbur- global scales. gites during the second stage and generated high- In a companion paper, Dilek & Flower explore Ca boninitic melt. The authors conclude that the the application of the arc-trench rollback and second magmatic stage had to occur in a supra- forearc accretion model to Neo-Tethyan ophiolites, subduction zone setting. specifically to the Mirdita (Albania), Troodos Saccani et aL present new field and geochem- (Cyprus) and Semail (Oman) ophiolites. Neo- ical constraints from the Western Hellenides in Tethyan oceans evolved as east-west-oriented Greece, documenting that initial stages of sea- basins separated by discrete continental fragments, floor spreading and oceanic crust formation in the which were rifted off from the northern edge of Pindos basin probably occurred in the Mid- to Gondwana beginning in the Triassic. Triassic rift Late Triassic, earlier than previously thought. assemblages containing within-plate-type alkaline Pillow lavas from the Argolis Peninsula have basalt to transitional (T-MORB) and mid-ocean MORB trace element characteristics and are di- ridge basalt (MORB) are spatially associated with vided into T-MORB and normal MORB (N- ophiolites in the eastern Mediterranean region and MORB). These are the oldest unequivocally dated may represent the precursor of Late Triassic oceanic crust in the Hellenide sector of the Pindos oceanic crust, which was subsequently consumed Basin. Early Triassic rifting produced shoshonitic to produce the suprasubduction zone ophiolites. and calc-alkaline lavas derived from a mantle The three ophiolites examined here include a source that was previously contaminated by sub- basement of typical 'oceanic' lithosphere intruded duction components. Associated alkaline basalts and overlain by boninitic (ultra-refractory) to calc- were derived from ocean island basalt-type (OIB) alkaline series rocks that formed in a proto-arc- mantle source. Mixing of mantle sources produced forearc setting. This progression was a result of enriched MORB (E-MORB) and T-MORB, and upper plate extension and further melting of then N-MORB lavas were erupted in Mid(?)- to previously depleted asthenosphere that occurred in Late Triassic, suggesting that sea-floor spreading response to successive stages of slab rollback. had reached a steady state. The authors cite the This igneous evolution of the ophiolites involved Red Sea as a modern analogue with along-strike subduction initiation and one or more episodes of chemical variations for the Pindos Basin. proto-arc splitting before the termination of slab Sarkarinejad describes the internal structure of rollback cycles as a result of trench-continent the Cretaceous Neyriz ophiolite in southern Iran, collisions. and presents structural and microstructural obser- Miintener & Pieeardo examine the Lanzo and vations for the existence of a NW-trending palaeo-
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