Troodos Ophiolite Complex Research Articles

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

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

Comment on “the troodos ophiolitic complex was probably formed in an island arc”, by A. Miyashiro

The use of bulk chemical analyses from the Troodos complex to determine the chemical affinity of the ophiolites is unjustified since these rocks exhibit conspicuous alteration in thin section. Very similar rocks on mainland Greece can be shown, on mineralogical grounds, to have been mainly tholeiitic originally, with a subordinate alkaline trend. This is compatible with their origin at an oceanic ridge. Regional geological evidence provides strong support for this origin. However, bulk chemical analyses from them could be used, like those from Troodos, to argue against a mid-ocean ridge origin. This suggests that the use of bulk chemical analyses from these rocks is very unreliable. In the presence of other, more reliable data, inferences from bulk chemistry can be rejected. The Troodos ophiolites are very similar, in both petrology and geological position, to those in Greece and probably originated in the same way.

Origin of the troodos and other ophiolites: A reply to hynes

Geologic and tectonic evidence on the origin of the Troodos ophiolitic complex is ambiguous, being compatible with its creation in an island arc or a continent as well as in a mid-oceanic ridge. However, there is decisive petrologic and chemical evidence against its origin in a mid-oceanic ridge (and in a marginal sea). A large proportion (about half) of volcanic rocks in the sheeted complex and lower pillow lavas of Troodos have SiO 2 > 52.5% and FeO*/MgO > 2.0 in contrast to mid-oceanic ridge volcanics which are almost entirely basic (SiO 2 < 52.5%) with FeO*/MgO < 2.0. (FeO* means total iron as FeO.) A considerable proportion of volcanic rocks in Troodos belong to the calc-alkalic series. These facts indicate that the Troodos massif was formed probably in an island arc or a continental region. In order to rebut the above chemical evidence, Hynes resorts to an assumption of thorough compositional change of the Troodos volcanic rocks by metasomatism. However, this assumption is not plausible.

Origin of the troodos and other ophiolites: A reply to moores

Advocates of the hypothesis of mid-oceanic ridge origin for ophiolites endeavored to show similarities between ophiolites and mid-oceanic ridges as evidence for their hypothesis. Some of the similarities claimed to exist appear to be true. However, this does not prove their hypothesis, because analogous similarities exist between ophiolites and some island arcs as well. Such advocates claim that rocks of the Troodos ophiolitic complex were subjected to such intense metasomatic changes that my hypothesis of island-arc origin for Troodos based on the bulk chemical analyses of rocks is not justified. However, a detailed examination of compositional variation in Troodos volcanic rocks has revealed that, though some components (e.g., K) were strongly mobile during secondary processes, other components were not, and the presently observed compositional variation resulted mainly from crystallization differentiation. Their assumption of large-scale silica metasomatism for Troodos cannot explain the compositional features of rocks and is contradictory to the hypothesis of mid-oceanic ridge origin, because such metasomatism does not appear to take place in mid-oceanic ridges.

Oxygen and hydrogen isotope studies of serpentinization in the Troodos ophiolite complex, Cyprus

Oxygen and hydrogen isotopic compositions were measured on 12 serpentine and 2 actinolite samples from the Troodos ophiolite complex, Cyprus. The single analyzed antigorite (δD= −60, δ 18O= 7.1) is isotopically similar to all previously analyzed antigorites from ultramafic bodies. However, although their D/H ratios are relatively “normal” (δD= −70to−92), the δ 18O values of most of the Troodos lizardite-chrysotile serpentines (+12.6 to +14.1) are much higher than the 2.0–9.3‰ range typically found in such serpentines. Such high δ 18O values have previously been found only in the serpentine-like mineraloids termed “deweylites”, which apparently formed at Earth-surface temperatures, and in a single sample from Vourinos, Greece that is in contact with high- 18O limestone. The Troodos lizardite-chrysotile samples cannot have formed by reaction with heated ocean waters, but instead must have formed in contact with large amounts of some type of meteoric, metamorphic, or formation water, either (1) at very low temperatures in a near-surface environment, or (2) at about 100°C from waters that were abnormally enriched in 18O (δ 18O ≈ +4 to +8) . The latter possibility seems most plausible inasmuch as extensive evaporites were deposited throughout the Mediterranean Sea during the late Miocene, and this would have been accompanied by strong 18O enrichments of the local meteoric waters. Heated ocean waters, however, probably were responsible for the formation of the actinolitic amphiboles (δ 18O= 4.6 to 5.5; δD= −51to−46) from the gabbro and ultramafic zones in the Troodos complex. The amphiboles must have formed at considerably higher temperatures and at an earlier stage than the lizardite-chrysotile serpentinization.

Stratabound pyritic sulphide occurrences in an ophiolite rock assemblage near Tumut, New South Wales

Abstract Pyrite‐rich sulphide with minor chalcopyrite, sphalerite, marcasite, and pyrrho‐tite, occurs in low‐grade metabasaltic rocks near Tumut, New South Wales. The rocks are chemically akin to modern abyssal tholeiites and form part of a well‐developed ophiolite rock assemblage. Rock alteration and sulphide mineral assemblages are similar to those found in pyrite‐rich sulphide deposits in the Troodos ophiolite complex, Cyprus, and in other ophiolite complexes, although low‐grade regional metamorphism has caused textural and mineralogical changes. It is suggested that the Tumut occurrences formed from submarine volcanic exhalations, associated with abyssal theoliite volcanism; and that the ophiolite rock assemblage formed contemporaneously with a sea‐floor spreading episode. The ophiolite rock assemblage was later obducted, perhaps during continental collision.

The Troodos ophiolitic complex was probably formed in an island arc

The Troodos ophiolitic complex in Cyprus has been widely regarded as a fragment of oceanic crust that was created in a mid-oceanic ridge. However, about one-third of the analyzed rocks of the lower pillow lavas and sheeted complex in it follows a calc-alkalic trend. This strongly suggests that the massif was created as a basaltic volcano in an island arc with a relatively thin oceanic-type crust rather than in a mid-oceanic ridge. Other chemical features and the structure of the massif are also consistent with an island arc origin. Some other ophiolitic complexes also appear to have been formed in island arcs.