Composition and origin of serpentinites from the Mid-Atlantic Ridge near 24° and 30° North Latitude

Serpentinites occur in transverse fracture zones and adjacent areas in the Mid-Atlantic Ridge near 24° and 30° N. In two fracture zones, about 700 km distant from each other, serpentinites show practically the same trend and range of variation in chemical composition. Their CaO content ranges from 2.05 to 0.07% by weight. Serpentinites relatively high in CaO content contain pargasite, whereas those relatively low in CaO do not. Serpentinites relatively high in CaO are chemically similar to high-temperature peridotites which are widely believed to have been derived from the upper mantle. With a decrease in CaO, the Al₂O₃, TiO₂, K₂O and FeO contents and the Fe/Mg ratio tend to decrease, whereas the H₂O⁺ content tends to increase. This compositional variation is probably due partly to heterogeneity of uper mantle peridotite from which the serpentinites were derived, and partly to chemical migration during serpentinization. The interior of the Mid-Atlantic Ridge may be mainly made up of serpentinites. Alternatively, the Mid-Atlantic Ridge Serpentinites may have been formed by serpentinization of peridotites that were intruded into fracture zones from a great depth.Lamont-Doherty Geological Observatory Contribution No. 1354.     

Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24° and 30° North latitude

On cursory examination of hand specimens and thin sections, the abyssal tholeiite in a dredge haul may appear to be uniform in composition. Chemical analyses of a considerable number of fragments, however, have always revealed the existence of regular compositional variation in them. The MgO content decreases with increasing SiO₂. In abyssal tholeiites with relatively low Al₂O₃ contents, the SiO₂, total iron, Na₂O and P₂O₅ contents tend to increase and the MgO content tends to decrease with increasing iron/magnesia ratio, probably owing to crystallization differentiation.In a certain dredge haul, high-alumina abyssal tholeiites (with Al₂O₃ contents near or over 17%) occur in association with low-alumina abyssal tholeiites. The magma of high-alumina abyssal tholeiites would be generated from that of low-alumina abyssal tholeiites by differentiation at a depth around 30 km.In pillow lavas of abyssal tholeiite free from weathering and metamorphism, the chilled rim of the pillow usually has virtually the same chemical composition as the more crystalline core except for a decrease of K₂O content toward the rim. On the other hand, the weathered rim of pillow lavas shows marked compositional change. The Fe₂O₃/FeO ratio of unweathered abyssal tholeiite is in the range of 0.1 to 0.3. This ratio and the H₂O^– and H₂O⁺ contents increase with advancing weathering.Lamont-Doherty Geological Observatory Contribution No. 1339     

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.     

Compositional variation in pillow lavas from the Mid-Atlantic Ridge

Five fresh, two weathered, and two metamorphosed basalt pillows with a presently or originally glassy outermost layer, together with a weathered and a metamorphosed basalt fragment without an originally glassy layer, all from the Mid-Atlantic Ridge, were investigated in detail. Fresh pillows usually show only very slight compositional heterogeneity, except for the glassy outermost layer where the total iron and MgO contents are higher and the K₂O content and the $\text{Fe}{}_2\text{O}{}_3\text{FeO}$ ratio are lower than the interior. In particular, the K₂O content of the glassy outermost layer is only about a half of that of the core in some pillows. In weathered pillows the surface layer shows higher total iron, TiO₂, P₂O₅ and H₂O contents and a higher $\text{Fe}{}_2\text{O}{}_3\text{FeO}$ ratio and a much lower CaO content than the interior. Most of the compositional heterogeneity observed in metamorphosed pillows may be ascribed to chemical migration during weathering, though metamorphism also may cause considerable heterogeneity.     

Paired and unpaired metamorphic belts

Paired metamorphic belts occur in many parts of the circum-Pacific regions. A pair is composed of two contrasting belts running parallel: a high-pressure metamorphic belt which probably formed beneath a trench zone, and a low-pressure metamorphic belt which probably formed beneath a volcanic chain in the adjacent island arc or continental margin. The former and the latter belt are accompanied by basic-ultrabasic rocks and by granitic-andesitic-rhyolitic rocks, respectively.The rapid descent of a thick, cold oceanic plate along a convergent plate juncture should create an unusually low geothermal gradient to cause high-pressure metamorphism. If the rate of plate descent is relatively slow, or if the descending oceanic lithosphere is too thin or too hot, the resultant geothermal gradient will not be low enough to cause high-pressure, but may cause medium-pressure metamorphism. In this case, the contrast between the two associated metamorphic belts will be obscure. The heat transfer by the rise of magmas and mantle materials appears to be a necessary condition for the formation of low-pressure metamorphic belts.Presumably, paired and unpaired (single) metamorphic belts form by the same mechanism, and an unpaired belt represents paired belts in which the contrast between the two belts is obscure, or in which one of the two belts is undeveloped or lost.Progress in experimental petrology enables us to estimate the pressure and temperature during metamorphism, and to know the relations between the conditions of partial melting and the composition of the resultant magmas. This sets limits to our ideas about the relevant tectonic processes in plate junctures.     

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.     

Nature of alkalic volcanic rock series

The alkalic rocks are here regarded as a category in a classification of igneous rock series (rock associations) and not as a class in petrographic systematics. The alkalic series as a whole are characterized by higher Na₂O+K₂O content than the subalkalic series in the alkali vs. SiO₂ diagram.At least three different trends (types) of differentiation appear to exist in large-scale alkalic volcanic associations. One (here designated as the Kennedy trend) starts from weakly nepheline-normative basalt and shows increasing normative nepheline with advancing fractionation to reach a phonolitic composition. Another (here called the Coombs trend) starts from hypersthene-normative basalt and shows increasing normative hypersthene and then normative quartz with advancing fractionation to reach a comenditic composition. Besides these two trends, it seems that many alkalic associations exist which show a differentiation trend starting from nepheline-normative basaltic composition and leading to hypersthenenormative, and then to quartz-normative compositions (here designated as the straddle-B type).Alkalic rocks of these three trends are higher not only in Na₂O+K₂O but also in Rb, Ba, Sr and Zr than subalkalic rocks. The alkalic basalts as a whole are characterized by higher contents of such elements and not by any degree of silica undersaturation. It is widely believed that alkalic rocks are characterized by the presence of normative nepheline as well as by the absence of orthopyroxene and pigeonite. Indeed such a relationship holds for the Kennedy trend, but it is not always valid for other types of alkalic associations. Some alkalic rocks of the Coombs trend and straddle-B type have quartz (or other silica minerals) and orthopyroxene and pigeonite.     

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 SiO₂ > 52.5% and FeO*/MgO > 2.0 in contrast to mid-oceanic ridge volcanics which are almost entirely basic (SiO₂ < 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.