The Troodos ophiolitic complex probably formed in a subduction initiation, slab edge setting
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)
- et al.
The H2O content of basalt glasses from southwest Pacific back-arc basins
Earth and Planetary Science Letters
(1993) - et al.
The geology, petrology, and petrogenesis of Saba Island, Lesser Antilles
Journal of Volcanology and Geothermal Research
(2001) - et al.
Structure and geochemistry of Tethyan ophiolites and their petrogenesis in subduction roll-back systems
Lithos
(2009) - et al.
Suprasubduction zone ophiolite formation alongthe periphety of Mesozoic Gondwana
Gondwana Research
(2007) - et al.
Island arc tholeiite to boninite melt evolution of the Cretaceous Kizildag (Turkey) ophiolite: model for multi-stage early arc-forearc magmatism in Tethyan subduction factories
Lithos
(2009) - et al.
Stable isotope compositions and water contents of boninite series volcanic rocks from Chichi-jimam Bonin Islands, Japan
Earth and Planetary Science Letters
(1987) Narrow subducting slabs and the origin of backarc basins
Tectonophysics
(1993)- et al.
Petrology and Geochemistry of the central North Fiji Basin spreading centre (Southwest Pacific) between 16ºS and 22ºS
Marine Geology
(1991) - et al.
Comments on "The Troodos ophiolitic complex was probably formed in an island arc", by A. Miyashiro and subsequent correspondence by A. Hynes and A Miyashiro
Earth and Planetary Science Letters
(1975) - et al.
Lithosphere tearing at STEP faults: response to edges of subduction zones
Earth and Planetary Science Letters
(2005)