The progress of the study of the sea-floor spreading has mutual relationship with that of the ophiolite. The scientists to study ophiolites should fully understand the present progress of the study of the sea-floor spreading, and vice versa. We obtained the concept of the sea-floor spreading in 1960's in terms of diverging plate boundaries and started to observe geological process at the spreading axis in 1970's. Swarth mapping of topography by multi narrow beam sonar and long range sidescan sonar, manned submersible observation, multichannel seismic reflection survey of the spreading axis, and detailed geochemical study of sampled rocks dramatically improved our understanding of the geological process along the spreading axis in early 1980's. The trend of the study of the sea-floor spreading is shifting toward the global understanding of its dynamic process in late 1980's.
DSDP/ODP Hole 504 B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, penetrating 1.55 km into a section that can be divided into four zones-Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Cyprus CY2A is located near Agrokipia deposits and cored into 90 Ma crust. The alteration is divided into four zones-Zone I: low-grade alteration; Zone II: silicified hydrothermal alteration; Zone III: propylitic alteration; Zone IV: lower greenschist facies alteration. Both drill cores were located at discharge zones in hydrothermal circulation systems. So the alteration characteristics of both cores were similar. Alteration temperatures of hydrothermally altered rocks were 200-350°C in Hole 504 B and 200-<300°C in CY2A. Boiling didn't occur in both cores. The hydrothermally altered rocks from both cores were formed under low seawater/rock ratio (1-5). The estimated isotopic and/or chemical compositions of hydrothermal solutions are within the range of those of endmember hydrothermal solutions from present seafloor hydrothermal fields. At least 2km3 of magma would be needed for the formation of 6 million tons of sulfide deposit if magma supplies heat energy to make hydrothermal solutions during the cooling from 1200°C to 350°C.
Magma reservoir models beneath ocean ridges are reviewed with reference to the spreading rates and the mode of fractional crystallization. Faster spreading rates (5cm/year) cause a large and long-lived magma chamber in which a thick plutonic sequence would be succesively produced away laterally from the chamber. Basalts extruded at fastspreading ridges underwent various degrees of low pressure fractionation causing conspicious chemical zonation in the chambers. On the other hand, small and transient magma chambers may be present beneath slow-spreading ridges. Comparatively less-evolved basalts, where petrological characteristics are governed mostly by the deep-seated processes such as degree of partial melting and polybaric fractionation, could appear at slow-spreading ridges because of absence of a large crustal magma chamber. Since sheeted dikes in ophiolites intruded vertically and the foliation of mantle tectonites were approximately horizontal in original, the configuration of the magma chamber interlayered between the tectonites and sheeted dike complex, if it existed, can be infered from the structure of a cumulate-gabbro sequence. The appearance of a thick plutonic sequence in the ophiolite where the crystallization order is olivine-plagioclase-clinopyroxene-orthopyroxene suggests that it was generated at a fast-spreading ridge. In such thick plutonic sequence, the fine-scale igneous layering (=time plane) obliquely across the major lithologic boundary of the plutonic sequence may reflect the horizontal chemical zonation of the magma chamber. On the other hand, the appearance of abundant whelritic cumulates crystallized at mantle deths and suffered high temperature deformation suggests that the ophiolite was generated at a slow-spreding ridge. Spreading rates of ancient ocean ridges at which the ophiolites were generated may be roughly estimated from the considerations on geochemical variation of the basaltic rocks. The occurrence of evolved basalts interpreted by extensive low pressure fractional crystallization suggests fast spreading rates, whereas the appearance of less evolved basalts showing complicated geochemical features such as different incompatible element ratios and crossing REE pattern suggests rather slow spreading rates. The studies on ophiolites would bring significant informations not only on the dynamic process beneath present-day ocean ridges but also on the tectonic framework and the evolution of ancient spreading center.
Origin of spinel peridotites of upper mantle derivation, especially that of ophiolitic peridotites, is discussed mainly on the basis of compositional relationship between olivine (Fo content) and chromian spinel (Cr/(Cr+Al) atomic ratio=Cr* ratio). Spinel peridotites are distributed in a relatively narrow band, the olivine-spinel mantle array, on the Fo-Cr* plane. The olivine-spinel mantle array (=OSMA) may by a trend for residual peridotites and have a fertile tip at Fo87, Cr*=0.08 and a refractory tip at Fo93, Cr*=0.95.Lherzolite is distributed in a fertile part of the OSMA (Cr*<0.6) and harzburgite, in a refractory part (Cr*>0.4). In a subsolidus stage, the Fo-Cr* relation in spinel peridotites is notaltered by temperature variation but is severely altered by a metasomatic process. Mantle peridotites from know tectonic settings are summarized as follows; lherzolite with Cr* of 0.6-0.1 (mostly 0.4-0.2) from the ocean floor, lherzolite with Cr*<0.4 (mostly around 0.1) from the oceanic hot spot, harzburgite-lherzolite with Cr* of 0.6-0.1 from the island arc or the marginal basin (Japan island arcs), and harzburgite-lherzolite with Cr* of 0.8-0.4 from the fore-arc area. Subcontinental upper mantle is mostly made up by lherzolite with Cr* less than 0.2. Olivine and chromian spinel are also early precipitating phases from primary or nearly primary magmas. Variation of the Cr* ratio of chromian spinel monitored by the Fo content of coexisting olivine makes a “fractionation line” on the Fo-Cr* plane. The cumulative peridotite, which always contains chromian spinel, is directly correlated with its parental magma on the Fo-Cr* plane. The residual peridotite for each magma suite could be estimated by extrapolating a fractionation line back to the OSMA as follows; lherzo lite with Cr*<0.6 (mostly 0.6 to 0.4) for MORB, lherzolite with Cr*<0.5 (mostly 0.5 to 0.2) for alkali basalts, harzburgite with Cr*>0.9 for boninites, harzburgite with Cr* of 0.9-0.7 for high-magnesia andesites or high-magnesia, high-silica arc tholeiites, harzburgitelherzolite with Cr*<0.7 for arc subalkalic basalts, harzburgite with Cr* of ca. 0.7 for intraplate tholeiites. The genetical consanguinity between residual peridotites and cumulate or volcanic rocks within an ophiolite complex could be examined in terms of the Fo-Cr* relationship.
Recent results of dredging and drilling of inner trench slopes have revealed that the abundant ophiolitic rocks (fore-arc ophiolite) crop out in the Izu-Ogasawara, Mariana, Yap, Tonga, and Middle America regions. These are commonly composed of serpentinized peridotite, gabbro, and volcanics of basic to intermediate composition. The fore-arc ophiolite of the Tonga Trench is in a striking contrast to those of the Izu-Ogasawara and Mariana Trenches. The former forms crude layering of unaltered peridotite, gabbro, and volcanic units in the ascending order, whereas the latter occurs as so-called serpentinite melange at the topographic highs of “diapiric seamount” and “horst block”. The difference may depend on the degree of fracturing of fore-arc regions by subductionrelated vertical movement and subduction of seamounts from oceanic side. Serpentinite diapirism for the “diapiric seamounts” in the Izu-Ogasawara and Mariana fore-arcs is strongly supported by the evidence of shearing of the dredge samples obtained from a fore-arc seamount, northeast of Tori-Shima. Each dredge sample is surrounded by several slickensided surfaces and is intensely sheared along its margin, suggesting brecciation at the diapiric stage. Milonitization under the amphibolite facies condition is recognized in an arc-derived rock of another fore-arc seamount in the Izu-Ogasawara Trench. This highly deformed rock has possibly been uplifted by diapirism from the deeper part of islandarc. Another possibility, however, is worth considerable that the rock, together with the other ophiolitic rocks, was primarily included near the surface of fore-arc region and has been only a little risen with low-density serpentinite along the faults where serpentinization of peridotite was promoted. To reveal the mechanism of diapir, petrological reconnaissance of both diapiric seamounts and surrounding areas will must be carried out in the Izu-Ogasawara and Mariana fore-arcs in the near future.
Yap Islands fringing southeastern margin of the Philippine Sea plate consist chiefly of metamorphic rocks which are not expected to exist on common oceanic islands. A model for the formation of such anomalous island arc is present by showing spontaneous down going slab at the paleo-Kyushu-Palau transform fault after change of the direction of the movement of the Pacific plate from NNW to WNW at 42Ma. Amphibolites, amphibolite-cataclasites and debris flow deposits consisting mostly of amphibolite were formed during the process of the ocean floor metamorphism of the West Philippine Basin followed by the activity of the fault movements of the transform fault and finally uplifting of the paleo Yap terrain. Island arc type volcanism took place after the deposition of channel fill sediments in the forearc area and ceased at about 10Ma by the collision of the Caroline Ridge with Yap Arc system.
Geochemical characteristics of volcanic rocks in an ophiolite body is a clue to the identification of tectonic setting for the formation of ophilites. Enricheminet of large ion lithophile elements relative to high field strength elements is well-known characteristics of supra-subduction zone rocks and grows through selective trasportation of the former elements with slab-derived H2O. High-magnesian andesites or boninites, undoutedly subduction zone rocks, have been found in many ophiolite complexes. Those characteristic magmas can be generated by partial melting of hydrous peridotite in near-trench mantle wedge under anomalously higher geothermal conditions associated with back-arc spreading.
Two possible examples of active back-arc ophiolite emplacement can be demonstrated through some multichannel reflection seismic profiles obtained from the Japan Sea. Lithospheric convergence has been suggested along the eastern margin of the Japan Basin. Especially, some interesting structures are seen around the Okushiri Ridge.(1) Subduction with back-thrusting type: Along the 44° line in latitude, the northern part of the Okushiri Ridge is detached from the oceanic crust of Japan Basin by two thrust zones having different polarities and ages. The main boundary between the Japan Basin and the Okushiri Ridge coincides with an thrust plane (approximately 2-3° dip) with the deformation of lower sedimentary unit (Middle Miocene to Pliocene). The boundary between the Okushiri Ridge and the Musashi Basin, however, is represented by active back-thrustings against the main thrust of counter boundary between the Japan Basin and the Okushiri Ridge. The formation of the back-thrustings may relate to the subduction of the Japan Basin.(2) Obduction type: In the middle to southern part of the Okushiri Ridge, the basement, oceanic crust layer H, is continuously traceable down to the Japan Basin. Active thrusts are observed in the boundary between the Okushiri Ridge and the Shiribeshi Trough, which dip to the west. The sediments of the Shiribeshi Trough are disturbed and faulted with listric reverse faults because of the obduction of oceanic crust, the Japan Basin, against to the island arc. And also some back-thrustings develop around the boundary between the Japan Basin and the Okushiri Ridge against to the obduction. It may relate to the degree of convergence of the main detachment thrust. This lithospheric convergence may initiate from Middle Miocene, it is just after the spreading of the Japan Basin under the compressive regime. These two styles of lithospheric detachment may be an indication of nascent back-arc closing. Back-thrustings are very important structure for the detachment of pre-ophiolite sequences. Moreover, the continuation of such back-arc closing should result in an ophiolite emplacement in a short time. It is suggested that an embryo of ophiolite complex, the Okushiri Ridge, is about to be emplaced on the Japanese arc.
The stress history of Semail ophiolite, northern Oman mountains, is presented. The axial orientation of regional stress field is obtained using conjugated ductile shear zone pairs and sheeted dikes. The results are; the early E-W extension changing into the later E-W compression at 100Ma. The conversion of stress field is best interpreted by the initiation of a subduction; this model is not inconsistent with the petrological and geochemical data (UMINO et al. 1989). The subducting plate, the African plate, dipped gently to the north, and generated an arc, the Semail Arc, above it. The Semail Arc was topographical highs trended N30 W. Assuming that the A1 orientation in the Arc is controlled by the relative motion of converging plate, the subduction should be sinistrally oblique. The plate convergence was completed at the final Arabia Continent-Semail Arc collision.
The Japanese ophiolites are divided into nappe type and melange type by their occurrences. The nappe type formed in a short period (a few 10Ma), and emplaced shortly after its igneous formation keeping its original igneous stratigraphy. They generally escaped from post-emplacement, high-pressure metamorphism. The melange type is a mixture of ophiolitic fragments of various ages (a few 100Ma) and lithologies, and most of them are affected by high-pressure metamorphism. The age of igneous formation of the Japanese ophiolites ranges from early Paleozoic to Cenzoic. Large nappe-type ophiolites formed in Ordovician, Permian, Jurassic, and Cretaceous periods, corresponding to the world-wide ophiolite pulses. The Paleozoic ophiolites are mainly distributed in Honshu, while the Mesozoic ophiolites are in Hokkaido. The Cenozoic melange-type ophiolites occur in front of the Izu arc, which collided against Japan in Miocene. The residual peridotite of Japanese ophiolites ranges from fertile lherzolite to highly depleted harzburgite. Mafic-ultramafic cumulates show diverse crystallization sequences including plagioclase, clinopyroxene, and orthopyroxene types. MORBs are dominant among the ophiolitic volcanics, while picrites and alkali basalts are also common among melangetype ophiolites. A few ophiolites bear calc-alkali rocks or hydrous mantle peridotite suggestive of island-arc origin. Pervasive pre-emplacement metamorphism of the cumulate rocks in the amphibolite and granulite facies as well as the absence of sheeted dike complexes are common features of the Japanese ophiolites.
Basaltic rocks in several accretionary complexes in Japan were chemically analyzed. Using minor element diagrams for discrimination of their tectonic settings, it is verified that there is definitely a good relationship between the tectonic settings and modes of occurrence. Basaltic rocks in the schistose and sheared metamorphic rocks are mostly ocean floor basalts (N- or T-MORBs) which are commonly associated with bedded chert or siliceous claystone. They might have been accreted by underplating of ocean floor materials at deeper levels in the accretionary prisms during oceanic plate subduction. On the other hand basaltic rocks isolately involved within not so highly metamorphosed matrices are mostly within plate basalts of hot-spot origin commonly capped with reef of pelagic limestone. Seamount materials might be accreted either by off-scraping at shallow depths of accretionary prisms or by collapsing at the trenches from seaward. One more particular occurence is that ophiolitic materials involved in the Mineoka tectonic belt are dominantly MORBs with minor hot-spot basalts in sheared serpentinite. Oceanic plate materials associated with seamount rocks were presumed to be obducted landward during specific tectonics around the TTT-type triple junction.
A brief review of the study on ophiolite is given. 165 years have passed already since a first use of the term “ophiolite” by BRONGNIART (1813), but still have not yet obtained a broadly satisfying solution on its origin and emplacement. However, the rapidly increased data set during the last 15 years on both on-land ophiolite and oceanfloors clearly indicate the strong constraints on its origin and emplacement. The period during 1813-1927 was a time of description of ophiolite. BRONGNIART (1827) classified ophiolite into a group of igneous rocks, since then began a debate whether ophiolitic peridotite is igneous or the other in origin. SUESS (1909) had noticed that ophiolites appear characteristically in orogenic belts. It was STEINMANN (1927) who had first recognized a close association of peridotite, gabbro, diabase-spilite, and radiolarian chert suggesting a deep sea origin of ophiolite. The significance of his finding has never been looked back until the revolutional period of plate tectonics in the late' 60s. The second period of 1927-1949 was the time of debate on igeneous origin. BOWEN and his coworkers insisted igneous origin based on experimental petrology for the ultramafic rocks in general. But if so, an abnormally high temperature ca. 1, 900°C was necessary to explain the occurrence of dunite. BENSON (1926) pointed out that if BOWEN'S idea is true, the country rocks of ophiolite must be subjected a high-temperature contact metamorphism, but not in the field. HESS (1939) has given a new idea of serpentinite magma to solve the problem, but its possibility had completely been disproved by the experiment of MgO-SiO2-H2O by BOWEN and TUTTLE (1949). The third period (1949-1959) began by a break-through idea of DE ROEVER (1957), who speculated that ophiolitic peridotite is a piece of mantle material, which was brought into an orogen by a tectonic process. The fourth period (1959-1973) started by BRUNN (1959) who compared ophiolite with the rocks in the Mid-Atlantic Ridge. This period (1959-1973) was the time of plate tectonics. During the early' 60s the ocean-floor spreading theory was proposed by HESS and DIETZ, and both thought that the layer 3 is composed of serpentinite oreclogite. The year 1969 was a memorial year, when both MOORES and DAVIES distinguished cumulate peridotite from the underlying residue tectonite, the latter of which is a refractory mantle after the formation of oceanic crust by partial fusion of mantle peridotite. The best example of ophiolite was the Troodos massif in Cyprus, where the extensive-scale of parallel dike swarm develops indicating ocean-floor spreading. Thereafter an ophiolite boom has come out, and flood of papers appeared to regard ophiolite to be of mid-oceanic ridge in origin. However, several geologists have doubted mid-oceanic ridge origin by the facts of much thinner crust, more silicic volcanic composition, and frequent occurrence of phenocrystic augite in ophiolites. MIYASHIRO (1973) solved such problems, and concluded that Troodos was formed in an island-arc setting. This paper was very shocking for geologists who wanted to establish the basic framework of orogeny by plate tectonics in those days, but epoch-making on the study of ophiolite, and corresponding to the time, when the method of study has changed to be modernized and more interdisciplinary.
The criteria for the classification of present shelf sediments are summarized, and the classification of sedimentary environments and the facies of their sediments in a stormdominated shelf are presented. The present shelf sediments are classified by the seven criteria as follows.(1) Are the sediments modern or relict? and when were the sediments supplied? (2) Are the sediments palimpsest or not? (3) By which kind of physical processes were they deposited? (e. g. density currents, flood, tidal currents, geostrophic currents, oceanic currents, tsumanis, etc.)(4) What type of sedimentary environment? (e. g. foreshore, shoreface, inner shelf, outershelf, etc.)(5) What is the grain size? (6) What is the natur e of the constituents? (e. g. authigenic, biogenic, volcanic, clastic, etc.)(7) What is the sedimentary structure or bedform of the sediments? These classification criteria of the present shelf sediments are the key to the interpretation of the environment of ancient sedimentary rocks. The storm-dominated and siliciclastic shelf sediments are divided into nearshor e (foreshore and shoreface), inner shelf and outer shelf facies. Foreshore is the zone of beach and the facies is characterized by well-sorted sand and seaward dipping, low-angle wedge-shaped cross- or parallel-bedding. Upper shoreface is the zone in which longshore bars and troughs are recognized within water depth of less than about 6m, and the facies consists of coarse to pebbly sand with cosets of high -angle tabular or trough cross-bedding of 10 to 100cm thickness. Lower shoreface is the zone which is between 6 to about 20m of water depth, and the facies is comprised of well-sorted fine to very fine sand with amalgamated hummocky cross- or parallel-stratification. Inner shelf is the zone which ranges from 20 to about 70m of water depth, and the facies is made up of interbedded sand and mud. Also, hummocky cross-stratification is recognized in the sand beds deposited in a zone less than about 50m of water depth. Outer shelf is the zone below about 70m of water depth, and the facies is composed of bioturbated mud. The boundary between shoreface and innershelf corresponds to the mean fairweather wave base and the boundary between inner shelf and outer shelf to the mean storm wave base.