Geology of the Semail ophiolite, northern Oman Mountains, is reviewed. The ophiolite is one of the lragest in the world with an extension of 500 km along the Oman Mountains, which is a part of a series of nappes abducted onto the Arabian Continent during the closure of the Tetheys Sea at the end of Cretaceous. It preserves almost complete sections up to 14 km thick which are expected for the upper part of oceanic lithosphere formed at medium to fast spreading ridges : pillow and sheet flows with little intervening volcaniclastics, well developed sheeted dike swarm, large continuous plutonic bodies, and harzburgitic tectonite with foliations and lineations indicative of diapiritic upwelling, in desending order. These are superposed and modified by subsequent magmatic and tectonic episodes during the obduction, such as later lavas and plutonic bodies with island-arc affinities, deep erosion down to the sheeted dike swarm, steep low-T sheare zones in the mantle peridotite, and milonitic deformation at the base of the nappes. Although the Semail ophiolite possesses many structural and volcanological similarities to the mid-ocean ridges, depleted but less primitive lava geochemistry cannot be reconciled by the origin as such, and still needs further research not only on ophiolites but also on the ocean floors.
Ophiolites of primary igneous origin have been more or less modified by later metamorphic processes, and there are several types of ophiolite metamorphism such as (1) the low-pressure type as recognized in ophiolites of the Mediterranean, Alps, Oman, Newfoundland, Chile, Taiwan, Borneo, California Coast Range, Hidaka Western Zone, and Horokanai, (2) the high-pressure type as recognized in dismembered ophiolites of Franciscan, New Caledonia, Alps, and Sanbagawa, (3) the medium-pressure type as recognized in Yakuno ophiolite, and (4) the polymetamorphosed (from low-pressure to high-pressure) type as recognized in ophiolites of Kurosegawa. Of these, the first, low-pressure type has the nature broadly similar to that of metamorphic rocks dredged or drilled from ocean-floor; both types of metamorphism are characterized by (a) the facies series includes the zeolite, prehnite-actinolite (partly, prehnite-pumpellyite), greenschist, amphibolite, and granulite facies, but the top sedimentary rocks of the ophiolite sequence exhibit no metamorphic feature, (b) the metamorphic grade increases down the ophiolite sequence, and the order of about 100°C/km is inferred for the geothermal gradient, (c) the igneous structure and/or texture are commonly preserved in the low-to mediumgrade rocks, and (d) the element migration mainly due to hydrothermal circulation of hot brine occurs generally in the low-grade rocks. It follows that there is a similar genetic environment between the low-pressure type of ophiolite metamorphism and ocean-floor metamor-phism. Therefore, the comparative study of both metamorphism may provide an excellent opportunity for understanding the origin of ophiolites, and it is proposed that the oceanic or backarc spreading ridge may be the most plausible tectonic setting for this type of metamorphism.
Petrological constitution of the oceanic lithosphere is discussed in comparison with the origin and derivation of ophiolitic suites. Recent ODP results for Hess Deep, equatorial Pacific, indicate the petrological similarity of deep-seated oceanic rocks of a fast spreading ridge system with those of some ophiolites, such as Samail ophiolite, Oman. Mantle peridotite from Hess Deep is harzburgite with chromian spinel of Cr# from 0.5 to 0.6. Dunite is common around gabbro-troctolite intrusions. Chromian spinel is sometimes concentrated within dunite and troctolite. Interaction of high-pressure MORB with harzburgite may be more pervasive in the fast spreading ridge than in the slow spreading one. Lithospheric slice as an ophiolite complex has a much more complicated history than the present-day oceanic lithosphere. Some petrological discrepancy, therefore, is expected between ophiolite and oceanic lithosphere. Arc-related rocks in the ophiolite may have been formed at a relatively later stage of arc-like setting which is inevitable for the oceanic lithosphere to be obducted onto continental margins. Highly refractory rocks with Cr-rich spinel (Cr# > 0.7), which are commonly found in ophiolites and have not been found in the present-day ocean floors, were formed at arc-related settings or could be rarely present in the oceanic setting. It is noteworthy that all of the constituent rocks of ophiolites, and even those from ocean floor, were not always formed at the same tectonic setting.
The location of the magnetized rocks of the oceanic crust that are responsible for sea-floor spreading magnetic anomalies has been a long-standing problem in geophysics. The recognition of these anomalies was a keystone in the development of the theory of plate tectonics. Our present concept of oceanic crustal magnetization is much more complex than the original, uniformly magnetized model of Vine-Matthwes-Morley. Magnetic inversion studies indicated that the upper oceanic extrusive layer (layer 2A, 0.5 km thick) was the only magnetic layer and that it was not necessary to postulate any contribution from deeper parts of oceanic crust. Direct measurements of the magnetic properties from the sea floor, however, have shown that (i) the magnetization of layer 2A is insufficient to give the required size of observed magnetic anomalies and (ii) some contribution from lower intrusive rocks is necessary. Recent ODP studies reported high magnetization intensities in the gabbroic rocks and the peridotites. The source of the lineated magnetic anomalies must reside in most of the oceanic crust and in the upper portion of upper mantle.
Modern seismological methods applied to studies of oceanic crustal structures are revealing detailed two-dimensional, sometimes three-dimensional, models. These new models are being constructed in areas where their geology and tectonics are much better defined than before. We are in an era in which researchers are vigourously testing new ideas on how oceanic crusts form, evolve and eventually recycle into mantle or accrete onto land. This requires synthesis of seismological, petrological and geological evidences, including ophiolites. This paper aims to show how seismological models of oceanic crusts are constructed, how seismological parameters are dependent on other parameters, and what is the present view of an oceanic crust.
Characteristics of bottom topography and gravity anomalies in mid-oceanic ridges and their implications for tectonics and sub-bottom structures have been studied since the 1960's. Recently global gravity data derived from satellite altimetry have become available. Combina-tion of these global gravity data and results of global seismic studies has revealed that the difference in the pattern of gravity anomalies between slow-spreading and fast-spreading ridges is derived from the difference in the distribution of melt layers in the whole upper mantle beneath them : the former is characterised by a deeper melt layer of limited size, while the latter is characterised by a widespread melt layer in which minimum S-wave velocity anomaly is located shallower than 100 km in depth.
A compilation of 17 sulfide-bearing, sea-floor hydrothermal sites in western Pacific area was made to compare them with volcanogenic massive sulfide deposits (VMSDs) which were likely to have formed in analogous setting of ancient arc-backarc systems. Critical examination of these modes of occurrence shows that all the hydrothermal activity is limited at or above magmatic centers which occur either on backarc spreading center, in backarc rift, or on volcanic front. About half of the known localities (8 sites) are related to bimodal volcanism and are considered to be the analog of Kuroko type deposit. The other half (9 sites) are accompanied with andesitic to basaltic volcanism and are categolized into Besshi-and Cyprus-type deposits. This coincides well with the statistics that 56% of the known VMSDs belong to Kuroko type (Rona, 1988). Therefore, the present-day sea-floor mineralization well represents ancient mineralization of VMSDs now found in strata of arc-backarc affiliation. The close spatial relationship between volcanism and hydrothermal activity suggests that both present-day and ancient deposits are of volcanogenic origin. Several lines of evidence such as isotope ratio of constituent elements (Ishibashi and Urabe, 1995) also support this conclusion. The ultimate source of these volatile elements such as oxygen, hydrogen, carbon, sulfur, nitrogen, and probably chlorine is believed to be dehydration of sediments which were subducted with oceanic plate beneath the arc-backarc systems.
The heat and mass flux from hydrothermal activity along mid-ocean ridge crests has a relationship with the formation of oceanic crust and plays a significant role on material cycle in the ocean. It is estimated that 1.7×1020 J/yr of heat energy is transported by hydrothermal activity along mid-ocean ridges. The heat flux from one high temperature black smoker is estimated to be 250 MW and the energy amitted from the vent field with several tens of smokers in the area of several to several tens km2 may be less than 10, 000 MW. On the other hand, the sporadic megaplume is believed to require 2-3 orders of magnitude larger heat flux than a normal plume. Large amount of heat energy (2.0×1018 J) would be needed for the formation of three million tons of sulfide deposit. The sulfide deposits are distributed every two kilometers on average. These observations suggest that large-scale hydrothermal activity is more episodic than steady-state. Fluid inclusions in the altered gabbro and plagiogranite in the oceanic crust and ophiolites have large salinity and temperature variations, which is interpreted as resulting from phase separation occurring in hydrothermal or magmatic fluids within the transition zone between the hydrothermal system and the magma chamber. The chemical flux by submarine hydro-thermal activity tends to be overestimated. It is important to estimate real net chemical flux from hot springs to the ocean.
Some modes of occurrences of ophiolitic rocks from plate subduction boundaries are simply reviewed and the present problems are summarized. The occurrences indicate that most of the basaltic rocks in accretionary complexes are not of MORB origin but of hot spot origin; while those form non-accretionary type landward slopes of trenchs are mixtures of island arc, MORB and hot spot origins, although the first is dominant. Future more detailed data which make good relations between the tectonic settings and occurrences may provide some implications on the ophiolite problems.
Serpentine seamount was reviewed with recent seven Shinkai 6, 500 dives at Izu-Bonin and Mariana forearcs. The seamounts consist mostly of serpentine flows with huge xenoliths of peridotite, high-pressure type metamorphic rocks such as jadeite-quartz assemblage and aragonite, glaucophane schist as well as MORB like basaltic rocks and chert. Active seamount has the following three characteristic features ; 1) serpentine flow, 2) carbonate chimney, and 3) huge peridotite xenolith in the flow. Serpentine flow shows a lobate structure with several meters width and 2-3 m high, sometimes flows down along the steep cliff. Drilling results indicate that flow unit shows the sedimentary structure similar to the interior structure of the mud diapir. Carbonate chimneys exist as an isolated tree and more than 20 chimneys stand make a line in some cases. Active carbonate chimney has yellowish green bacterial mats on the surface but dead ones are covered with thin black manganese coating. The fate of carbonate chimneys is fell down and dissolves into the sea water when the depth exceeds more than 4, 000 m, becomes a member of sediments when buried and sometimes makes a vein. In the metamorphic belts onland such as Sanbagawa, Kamuikotan metamorphic terrains and Mineoka belt there are serpentinite bodies of both alpine and stratiform types. The former type was thought to be of solid intrusion along fault. These serpentinite bodies yield white sediments as a vein, layer of sediments as well as rodingites. Rodingites always crop out with serpentinites in the Alps and other orogenic belts. Serpentinite bodies have highly sheared and brecciated parts and most geologists thought those parts to be the results of strong shearing when solid intrusion. However, if we look at the body carefully, sedimentary structure and white veins, and sediments will be frequently observed and the body is closely accompanied by rodingites. We strongly insist to think that these white materials are the products of the carbonate chimneys and sedimentary structures are also the product of serpentine flow at the forearc serpentine seamount and emplace into the metamorphic terrains during the secondary processes. During the emplacement, rodingite and magnesite ore would be formed by metasomatism with serpentinites. Most of the serpentinites in the orogenic belt must be of serpentine seamount origin such as in Izu-Bonin, Mariana, Tonga and Middle America Trenches where the crust are thin compared with those of the ordinary island arcs and continental margins.