Elsevier

Gondwana Research

Volume 18, Issue 1, July 2010, Pages 60-81
Gondwana Research

The Troodos ophiolitic complex probably formed in a subduction initiation, slab edge setting

https://doi.org/10.1016/j.gr.2009.12.003Get rights and content

Abstract

Miyashiro (1973) famously initiated a debate on the tectonic setting of ophiolite complexes by proposing that ‘the Troodos ophiolitic complex was probably formed in an island arc’. This paper evaluates and updates Miyashiro's work by: (a) using the Mehegan–Robinson set of 137 fresh volcanic glass analyses to sidestep the controversy over the effect of alteration on major element classification diagrams; (b) using the volcanic glass database for Troodos analogues such as back-arc basins, slab edges, forearcs and subduction initiation terranes; (c) revising Miyashiro's classifications by including the boninitic series and subdividing the tholeiitic series into high- and medium-Fe series; and (d) extending Miyashiro's methodologies to new developments in the interpretation of major element data. We conclude that the Troodos Massif is made up of oceanic crust built from a high-Si8, moderate-Fe tholeiitic magma, overlain by boninites. Its low K8/H8 is consistent with near-trench crustal accretion and its Na8–Fe8 systematics indicate that TP = c.1400 °C for the Lower Lavas, consistent with rapid slab roll-back and/or sideways influx of hot mantle. Overall, the geochemical characteristics and geological setting support models in which the Troodos Massif formed by slab roll-back following subduction initiation, probably near a slab edge.

Introduction

Citation data demonstrate that Akiho Miyashiro's contribution to the interpretation of ophiolites rates as one of his more important, and certainly one of his most controversial, achievements. The controversy centred on the publication by Miyashiro (1973), which presented the hypothesis that the Troodos Massif (viewed at the time as the world's most complete and best-studied ophiolite complex: Fig. 1) ‘probably formed in an island arc’. This hypothesis ran counter to the consensus at the time, that the Troodos Massif and other ophiolites formed at mid-ocean ridges (e.g. Gass, 1968, Moores & Vine, 1971). It stimulated three replies (Hynes, 1975, Moores, 1975, Gass et al., 1975), with responses by Miyashiro, 1975a, Miyashiro, 1975b to the first two of these.

The essence of Miyashiro's argument was that the lavas of the Troodos Massif follow both tholeiitic (iron-enrichment) and calc-alkalic (non-iron-enrichment) trends and so are more similar to island arcs than mid-ocean ridges. In the most cited of his papers (at the time of writing), Miyashiro (1974) developed his methods of comparing tectonic environments on the basis of volcanic rock series, and it was these that he used in his 1973 paper. He also applied this methodology to ophiolites in general in a further paper (Miyashiro, 1975c), in which he discussed further the origin of the Troodos Massif. The essence of the counter-arguments was that the lavas of the Troodos Massif were too altered for Miyashiro's methods to be valid, and that the geology of the Troodos Massif, with its sheeted dyke complex and absence of any arc-derived component in its overlying sediments, precluded an origin in an island arc.

The ‘Troodos Debate’ is particularly interesting from a philosophy of science perspective, and is discussed or referenced in the majority of the papers in the volume edited by Dilek and Newcomb (2003) on ‘Ophiolite Concept and the Evolution of Geological Thought’. It demonstrates, in particular, how a scientific paradigm may be successfully challenged. In this case, the paradigm that the Troodos Massif and other ophiolites formed at mid-ocean ridges was replaced by a new paradigm that they could form above subduction zones. The present consensus, that the Troodos Massif formed at a supra-subduction zone spreading axis, implies that both sets of protagonists were partly correct: Hynes, Moores, Gass and others for proposing an origin at a spreading axis and Miyashiro for proposing a subduction-related setting. This outcome was not foreseeable at the time because of the lack of information on modern supra-subduction zone spreading axes in general and the subduction-initiation extensional regime in particular.

The ‘Troodos Debate’ also highlighted a fissure between field-based and laboratory based geoscientists. Miyashiro did not consider it important or necessary to visit the Troodos Massif to reach his conclusions: the published analyses told him all he needed to know. Gass and others were similarly dismissive of the geochemistry: if the data did not support the obvious conclusion from field observations, then the geochemists must be the ones in the wrong. Thus the Troodos Debate provided proof, should it have been needed, that the best solutions to problems take into account all lines of evidence.

Twenty-five years on, there is consensus that subduction zones play an important role in the genesis of many, if not most, ophiolites and that Miyashiro was the first to demonstrate this by means of geochemical and petrological methods. The debate did, however, raise other important issues that have not been fully resolved. Was Miyashiro correct to claim that alteration was insufficient to affect his interpretation of volcanic rock series on Cyprus? Does the Troodos Massif exhibit both tholeiitic and calc-alkalic series and, if so, what does this mean for its origin? Do volcanic rock series have a role to play in the fingerprinting of ophiolites? If the Troodos Massif did form by spreading above a subduction zone with no actual arc volcanism, what was its precise setting of formation? The discovery, in the 1980s, of significant quantities of fresh volcanic glass (Robinson et al., 1983), means that this is one of the few ophiolites where all these questions can be answered. This paper briefly reiterates the relevant parts of the ‘Troodos Debate’ and then attempts a detailed examination of these questions, including an up-to-date reassessment of Miyashiro's concept of the application of volcanic rock series to ophiolite interpretation.

Section snippets

The Troodos Massif as an island arc: Miyashiro's arguments

Miyashiro (1973) based his interpretation of the Troodos Massif on his study of volcanic rock series, subsequently published in more detail by Miyashiro (1974). Essentially, he took the three main rock series (alkalic, calc-alkalic and tholeiitic), established methods for distinguishing between them and investigated their tectonic significance. He first used an alkali-silica diagram to separate alkalic from subalkalic. He then subdivided sub-alkalic into calc-alkalic and tholeiitic with three

Troodos glass data set

The principal data set used in this paper is that of Mehegan and Robinson (Robinson et al., 1983, Flower & Levine, 1987, Mehegan, 1988). It comprises a set of 137 glasses collected from eleven sections through the lava sequence on the east, west and south sides of the Troodos Massif (Fig. 1). Mehegan (1988) recorded the location of all the glasses with respect to the lava stratigraphy, which by consensus was subdivided into an Upper and a Lower Sequence. He assigned each glass into one of three

Troodos analogue data set

As is evident from the Troodos glass studies, and as demonstrated using other criteria, the Troodos Massif represents oceanic crust with an arc-like subduction signature and an associated boninite volcanic sequence. To interpret its setting, we take the approach here of comparing the Troodos glasses with glasses and fresh rocks from water-rich ridges in extant subduction settings. When Miyashiro compared the Troodos Massif with present-day settings, he was restricted to a limited number of

Re-evaluation of igneous rock series in the Troodos massif using volcanic glasses

When Miyashiro, 1973, Miyashiro, 1975c carried out his classic work on the use of igneous rock series to fingerprint ophiolites, only three series came under consideration: alkalic, calc-alkalic and tholeiitic. Of these, Miyashiro recognised that the Troodos lavas are all sub-alkalic and so focused on the tholeiitic-calc-alkalic distinction. However, the subsequent recognition of the importance of the boninitic series in many ophiolite complexes makes a big difference to the interpretation.

Tectonic interpretation based on Troodos glasses

This re-interpretation of the Troodos Massif as a combination of tholeiitic and boninitic lavas, specifically high-Si8 tholeiite lavas overlain by boninites, is similarly indicative of subduction, though not of normal arc or back-arc settings (e.g., Fig. 12). The settings that fit this scenario are (a) subduction initiation, (b) slab edges or (c) a combination of the two. We can now investigate these options further using other major element methodologies.

Subduction initiation, slab edge setting for the Troodos Massif

In the subduction initiation models based on the Eocene of the Western Pacific, old, cold Pacific oceanic lithosphere first sinks and rapidly rolls back, providing a setting for supra-subduction zone extension and sea-floor spreading; only then does true (slab-parallel) subduction dominate and arc volcanism begin (Stern & Bloomer, 1992, Hall et al., 2003). This model has been applied to the Troodos Massif and other Tethyan ophiolites, most recently in a series of papers by Dilek and coworkers

Conclusions

Miyashiro (1973) and related papers raised many questions about the role of subduction in ophiolite genesis, the use of volcanic rock series to determine tectonic setting, and the effect of alteration in interpreting paleovolcanic rocks. We identified some of the key questions at the start of this paper and here attempt to summarise our answers.

Acknowledgements

The database used in this paper was collected by the late Jim Mehegan, and this paper is dedicated to his memory as well as that of Akiho Miyashiro. In addition we are grateful to Charlie Langmuir and his co-workers for giving us access to their unpublished work on the Eastern Lau Spreading Centre. We have discussed this work with numerous colleagues, including (most recently) Chris MacLeod, Johan Lissenberg, Alastair Robertson, Yildirim Dilek, Bob Stern and Mark Reagan, and are most grateful

References (132)

  • R.F. Gribble et al.

    MORB mantle and subduction components interact to generate basalts in the southern Mariana Trough back-arc basin

    Geochimica et Cosmochimica Acta

    (1996)
  • C. Hall et al.

    Catastrophic initiation of subduction following forced convergence across fracture zones

    Earth and Planetary Science Letters

    (2003)
  • A. Hynes

    Comment on "The Troodos ophiolitic complex was probably formed in an island arc", by A. Miyashiro

    Earth and Planetary Science Letters

    (1975)
  • O. Ishizuka et al.

    Early stages in the evolution of Izu–Bonin arc volcanism: new age, chemical, and isotopic constraints

    Earth and Planetary Science Letters

    (2006)
  • G.A. Jenner et al.

    Composition of back-arc basin volcanics, Valu Fa Ridge, Lau Basin: evidence for a slab-derived component in their mantle source

    Journal of Volcanology and Geothermal Research

    (1987)
  • V.S. Kamenetsky et al.

    Phenocryst and melt inclusion chemistry of near-axis seamounts, Valu Fa Ridge, Lau Basin: insight into mantle wedge melting and the addition of subduction components

    Earth and Planetary Science Letters

    (1997)
  • P.T. Leat et al.

    Magma genesis and mantle flow at a subducting slab edge: the South Sandwich arc-basin system

    Earth and Planetary Science Letters

    (2004)
  • J. Malpas et al.

    The Ayia Vavara Formation of SW Cyprus — a product of complex collisional tectonics

    Tectonophysics

    (1992)
  • A. Miyashiro

    The Troodos ophiolitic complex was probably formed in an island arc

    Earth and Planetary Science Letters

    (1973)
  • A. Miyashiro

    Origin of the Troodos and other ophiolites; a reply to Hynes

    Earth and Planetary Science Letters

    (1975)
  • A. Miyashiro

    Origin of the Troodos and other ophiolites: a reply to Moores

    Earth and Planetary Science Letters

    (1975)
  • M. Monzier et al.

    High-Mg andesites from the southern termination of the New Hebrides island arc (SW Pacific)

    Journal of Volcanology and Geothermal Research

    (1993)
  • E.M. Moores

    Discussion of "Origin of Troodos and other ophiolites; a reply to Hynes", by Akiho Miyashiro

    Earth and Planetary Science Letters

    (1975)
  • J.A. Pearce

    Basalt geochemistry used to investigate past tectonic environments on Cyprus

    Tectonophysics

    (1975)
  • T. Plank et al.

    The chemical composition of subducting sediment and its consequences for the crust and mantle

    Chemical Geology

    (1998)
  • K.D. Putirka et al.

    Ambient and excess mantle temperatures, olivine thermometry, and active v passive upwelling

    Chemical Geology

    (2007)
  • M. Rautenschlein et al.

    Isotopic and trace element composition of volcanic glasses from the Akaki Canyon, Cyprus: implications for the origin of the Troodos ophiolite

    Earth and Planetary Science Letters

    (1985)
  • C.J. Robinson et al.

    Low degree melting under the Southwest Indian Ridge: the roles of mantle temperature, conductive cooling and wet melting

    Earth and Planetary Science Letters

    (2001)
  • A.H.F. Robertson

    Overview of the genesis and emplacement of Mesozoic ophiolites in the eastern Mediterranean Tethyan region

    Lithos

    (2002)
  • G. Rosenbaum et al.

    Relative motions of Africa, Iberia and Europe during Alpine orogeny

    Tectonophysics

    (2002)
  • E. Abbate et al.

    The rhythm of Phanerozoic ophiolites

    Ofioliti

    (1985)
  • Acland, A.S., 1996. Magma genesis in the Northern Lau basin, S.W. Pacific. PhD thesis, University of Durham, Durham,...
  • W. Alvarez

    Geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics

    Journal of Geophysical Research

    (1982)
  • R.J. Arculus

    Use and abuse of the terms calcalkaline and calcalkalic

    Journal of Petrology

    (2003)
  • R.J. Arculus et al.

    Igneous stratigraphy and major element geochemistry of Holes 786A and 786B

    Proceedings ODP Scientific Results

    (1992)
  • P.D. Asimow et al.

    The importance of water to oceanic mantle melting regimes

    Nature

    (2003)
  • U. Bednarz et al.

    Petrological and chemical evolution of the northeastern Troodos Extrusive Series, Cyprus

    Journal of Petrology

    (1994)
  • A. Bézos et al.

    Origins of chemical diversity of back-arc basins: a segment-scale study of the Eastern Lau Spreading Center

    Journal of Geophysical Research

    (2009)
  • S.H. Bloomer et al.

    Gabbroic and ultramafic rocks from the Mariana Trench: an island arc ophiolite

    American Geophysical Union Geophysical Monograph Series

    (1983)
  • S.H. Bloomer et al.

    Petrology and geochemistry of boninitic series volcanic rocks from the Mariana Trench

    Contributions to Mineralogy and Petrology

    (1987)
  • S.H. Bloomer et al.

    Tectonic origins

    Nature

    (1990)
  • W.E. Cameron et al.

    Boninites, komatiites and ophiolitic basalts

    Nature

    (1979)
  • J.S. Collier et al.

    Seismic mapping of a magma chamber beneath the Valu Fa Ridge, Lau Basin

    Journal of Geophysical Research.

    (1992)
  • L.A. Coogan et al.

    Hidden melting signatures recorded in the Troodos ophiolite plutonic suite: evidence for widespread generation of depleted melts and intra-crustal melt aggregation

    Contributions to Mineralogy and Petrology

    (2003)
  • A.J. Crawford et al.

    Classification, petrogenesis and tectonic setting of boninites

  • L.V. Danyushevsky et al.

    H2O abundance in depleted to moderately enriched mid-ocean ridge basalts; Part I: incompatible behaviour, implications for mantle storage, and origin of regional variations

    Journal of Petrology

    (2000)
  • Y. Dilek et al.

    Arc-trench roll-back and forearc accretion: 2. A model template for ophiolites in Albania, Cyprus, and Oman

    Geological Society of London Special Publication

    (2003)
  • Y. Dilek et al.

    Regional tectonics of the Eastern Mediterranean ophiolites

  • R.A. Duncan et al.

    The genesis of refractory melts in the formation of oceanic crust

    Contributions to Mineralogy and Petrology

    (1987)
  • Cited by (0)

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