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1.
Abstract   The Lower Sorachi Group of the Sorachi–Yezo Belt in central Hokkaido, Japan is a peculiar accretionary complex characterized by numerous occurrences of greenstones (metabasalts and diabases), which are mostly composed of aphyric basalts. Clinopyroxene-rich phenocryst assemblage in phyric basalts is different from olivine–plagioclase assemblage in mid-oceanic ridge basalts (MORB). The greenstones are geochemically uniform, and show a lower-Ti trend than MORB in an FeO*/MgO-TiO2 diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO2−10MnO−10P2O5 diagram. In a MORB-normalized spider diagram, the greenstones show a flat pattern from P to Y, which are lower than those of normal mid-oceanic ridge basalt (N-MORB). These indicate that the greenstones were derived by a higher degree of partial melting from a depleted mantle similar to a N-MORB source, and experienced olivine–clinopyroxene fractional crystallization. However, a positive spike of Nb in the spider diagram cannot be explained, and may be attributed to mantle heterogeneity. These characteristics are analogous to those of oceanic plateau basalts (OPB) such as in Ontong Java Plateau, Manihiki Plateau and Nauru Basin, suggesting that the greenstones in the Lower Sorachi Group are of oceanic plateau origin. The present study proposes new field divisions to distinguish OPB from MORB in the conventional FeO*/MgO–TiO2 and TiO2−10MnO−10P2O5 diagrams.  相似文献   

2.
Abstract The Jurassic Tamba accretionary complex is divided into two tectono‐stratigraphic suites (Type I and II nappe groups), which are further divided into six complexes (nappes) each of which is characterized by a rock sequence of Late Paleozoic greenstone/limestone, Permian to Jurassic chert and Jurassic terrigenous clastic rocks. The mode of occurrence of the greenstone is divided into two types. The major basal type occurs as a large coherent slab associated with Permian chert and limestone, constituting the basal part of each complex, and the minor mixed type occurs as fragmented allochthonous greenstone blocks and lenses mixed with chert, limestone and sandstone in the Jurassic mudstone matrix. Most of the basal greenstones have uniform geochemical characteristics, which indicate enriched‐mid‐oceanic ridge basalt (MORB) affinity. Their geochemical compositions are akin to the reported Permo‐Carboniferous and Triassic oceanic plateau basalts. Mixed greenstones are divided into two petrochemical types: (i) tholeiitic basalt with normal‐MORB affinity, which is predominant in the uppermost complex of the Type II suite (upper nappe group); and (ii) tholeiitic and alkalic basalts of oceanic island or seamount origin, which are common in all complexes of the Tamba Belt. Geochemical characteristics of the greenstones thus vary in accordance with their occurrences and the structural units to which they belong. This relationship reflects the difference in topographic relief and crustal thickness of the accreted oceanic edifices – the remnants of thick oceanic plateau crust tended to accrete to the continental margin as a large basal greenstone body, whereas thin normal oceanic crust with small seamounts or oceanic islands accreted as mixed greenstones because of their mechanical weakness. The Type II suite (upper nappe group) contains the basal and mixed greenstones, whereas the Type I suite (lower nappe group) includes only mixed greenstones. This distinction may reflect the temporal change of subducting edifices from a thick oceanic plateau to a thin normal oceanic crust, and suggests that the accretion of a large oceanic plateau may be responsible for building accretionary complexes with thick basal greenstones slabs.  相似文献   

3.
Abstract Greenstones, representing remnants of paleo-oceanic crust, occur in Permian and Jurassic accretionary complexes of the Inner Zone in the Southwestern Japan arc. The formation age of most of the greenstones is early Carboniferous, based on fossil ages for overlying limestones and Sm-Nd isotope ages of the greenstones themselves. The geochemistry of such greenstones is similar to those of present-day oceanic islands. Greenstones of the Permian accretionary complex (Akiyoshi belt) are alkalic and tholeiitic in composition. Some alkali basalts show peculiar features from an EM-1 mantle source, such as the Gough Island and Tristan da Chunha basalts in the South Atlantic. Greenstones of the Jurassic accretionary complex (Tamba belt) are also alkali and tholeiitic basalts with both basalt types in the northern part of the Tamba belt coming from strongly depleted characters similar to a mid-ocean ridge basalt source mantle. The variable geochemistry of the oceanic basalts is explained by hypothesis on existence of a Carboniferous mantle plume below the spreading ridge which divides the Farallon and Izanagi plates. The Akiyoshi belt seamounts and/or oceanic islands of the Farallon plate and Tamba belt seamounts and/or oceanic islands of the Izanagi plate formed simultaneously by the upwelling of the thermal plume. Some part of the Akiyoshi belt basalts originated locally from an EM-1 mantle source, while basalts from the northern parts of the Tamba belt have a normal-type mid-ocean ridge basalt (N-MORB) source component. Existence of an N-MORB signature is consistent with the presence of a spreading center in a Carboniferous 'Pacific Ocean' that caused separation of the Farallon and Izanagi plates. Disparity in accretion ages of the basaltic rocks in the Permian and Jurassic may have been caused by differences in the relative motion of the two plates.  相似文献   

4.
An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ∼2.7 Ga Wawa greenstone belts. Large, positive initial ?Nd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ?Nd values in the MA (+0.14 to +1.68), Nb-enriched basalts and andesites (NEBA) (+1.11 to +2.05), and ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts and andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ?Nd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and ‘normal’ arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes and intra-oceanic arcs tend to have different Nd-isotopic compositions.  相似文献   

5.
The average chemical compositions of the continental crust and the oceanic crust (represented by MORB), normalized to primitive mantle values and plotted as functions of the apparent bulk partition coefficient of each element, form surprisingly simple, complementary concentration patterns. In the continental crust, the maximum concentrations are on the order of 50 to 100 times the primitive-mantle values, and these are attained by the most highly incompatible elements Cs, Rb, Ba, and Th. In the average oceanic crust, the maximum concentrations are only about 10 times the primitive mantle values, and they are attained by the moderately incompatible elements Na, Ti, Zr, Hf, Y and the intermediate to heavy REE.This relationship is explained by a simple, two-stage model of extracting first continental and then oceanic crust from the initially primitive mantle. This model reproduces the characteristic concentration maximum in MORB. It yields quantitative constraints about the effective aggregate melt fractions extracted during both stages. These amount to about 1.5% for the continental crust and about 8–10% for the oceanic crust.The comparatively low degrees of melting inferred for average MORB are consistent with the correlation of Na2O concentration with depth of extrusion [1], and with the normalized concentrations of Ca, Sc, and Al ( 3) in MORB, which are much lower than those of Zr, Hf, and the HREE ( 10). Ca, Al and Sc are compatible with clinopyroxene and are preferentially retained in the residual mantle by this mineral. This is possible only if the aggregate melt fraction is low enough for the clinopyroxene not to be consumed.A sequence of increasing compatibility of lithophile elements may be defined in two independent ways: (1) the order of decreasing normalized concentrations in the continental crust; or (2) by concentration correlations in oceanic basalts. The results are surprisingly similar except for Nb, Ta, and Pb, which yield inconsistent bulk partition coefficients as well as anomalous concentrations and standard deviations.The anomalies can be explained if Nb and Ta have relatively large partition coefficients during continental crust production and smaller coefficients during oceanic crust production. In contrast, Pb has a very small coefficient during continental crust production and a larger coefficient during oceanic crust production. This is the reason why these elements are useful in geochemical discrimination diagrams for distinguishing MORB and OIB on the one hand from island arc and most intracontinental volcanics on the other.The results are consistent with the crust-mantle differentiation model proposed previously [2]. Nb and Ta are preferentially retained and enriched in the residual mantle during formation of continental crust. After separation of the bulk of the continental crust, the residual portion of the mantle was rehomogenized, and the present-day internal heterogeneities between MORB and OIB sources were generated subsequently by processes involving only oceanic crust and mantle. During this second stage, Nb and Ta are highly incompatible, and their abundances are anomalously high in both OIB and MORB.The anomalous behavior of Pb causes the so-called “lead paradox”, namely the elevated U/Pb and Th/Pb ratios (inferred from Pb isotopes) in the present-day, depleted mantle, even though U and Th are more incompatible than Pb in oceanic basalts. This is explained if Pb is in fact more incompatible than U and Th during formation of the continental crust, and less incompatible than U and Th during formation of oceanic crust.  相似文献   

6.
The Bangong–Nujiang suture (BNS) between the Lhasa and Qiangtang terranes is an important boundary and its petrogenesis is controversial. Diabase from the accretionary prism in the southern Qiangtang terrane yields a zircon U–Pb age of 181.3 ± 1.4 Ma. All the diabases show tholeiitic basalt compositions, gentle enrichment patters of light rare earth elements (REE), variable enrichment in incompatible element concentrations (e.g. Th and Rb), and no anomaly in high field strength elements (e.g. Nb and Ta), similar to that of enriched mid‐ocean ridge basalt (E‐MORB). They have relatively homogeneous whole rock Nd (εNd(t) = 7.3–9.1) and zircon Hf–O isotopic compositions (εHf(t) = 14.8–16.1, and δ18O = 4.57–6.12‰), possibly indicating melting of the depleted mantle and no significant crustal contamination during the petrogenesis. The element variations suggest that the diabases were formed by plume–ridge interaction at a mid‐ocean ridge within the Bangong–Nujiang ocean.  相似文献   

7.
The Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area are divided into five lithological units, four of which contain greenstone bodies. The Lower Cretaceous Naizawa Complex consists of two lithologic units. The Basaltic Unit (B‐Unit) is a large‐scale tectonic slab of greenstone, consisting of depleted tholeiite similar to that of the Lower Sorachi Ophiolite (basal forearc basin ophiolite) in the Sorachi‐Yezo Belt. The Mixed Unit of Naizawa Complex (MN‐Unit) contains oceanic island‐type alkaline greenstones which occur as slab‐like bodies and faulted blocks with tectonically dismembered trench‐fill sediments. Repeated alternations of the two units in the Naizawa Complex may have been formed by the collision of seamounts with forearc ophiolitic body (Lower Sorachi Ophiolite) in the trench. The Upper Cretaceous Horobetsugawa Complex structurally underlies the Naizawa Complex in its original configuration, and it also contains greenstone bodies. Greenstones in the MH‐Unit occur as blocks and sedimentary clasts in a clastic matrix, and exhibit depleted tholeiite and oceanic‐island alkaline basalt/tholeiite chemistry. This unit is interpreted as submarine slide and debris flow deposits. Greenstones in the PT‐Unit occur at the base of several chert‐clastic successions. Most of the greenstones are severely sheared and show normal‐type mid‐ocean ridge basalt composition. The PT‐Unit greenstones are considered to have been derived from abyssal basement peeled off during accretion. The different accretion mechanism of the greenstones in the Naizawa and Horobetsugawa complexes reflects temporal changes in subduction zone conditions. Seamount accretion and tectonic erosion were dominant in the Early Cretaceous, due to highly oblique subduction of the old oceanic crust and minimal sediment supply. Whereas, thick sediments with minor mid‐ocean ridge basalt and olistostrome accreted in the Late Cretaceous, due to near‐orthogonal subduction of young oceanic crust with voluminous sediment supply.  相似文献   

8.
The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from 2.7― 5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ~(39)Ar/~(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.  相似文献   

9.
SEONG HEE  CHOI  JONG IK  LEE  CHUNG-HWA  PARK  JACQUES  MOUTTE 《Island Arc》2002,11(4):221-235
Abstract   Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history.  相似文献   

10.
Mesozoic and Cenozoic volcanic rocks are widely distributed in the circum-Pacific area of eastern China. These rocks have long been genetically linked to westward subduction of the paleo-Pacific oceanic plate to the eastern Asia continent[1,2]. Research in re-cent years[3―6] has attained conclusions that a simple paleo-Pacific subduction model does not work well in interpreting all the volcanic rocks in eastern China, although some of them could be attributed to circum-Pacific interaction …  相似文献   

11.
Abstract Mélange units containing greenstones are common throughout the Cretaceous-Miocene Shimanto Supergroup in the Ryukyu Is and southwest Japan. Most greenstones in the accretionary complex originated in oceanic spreading ridges and seamounts, and they formed far from the convergent margin. Some mélange-like units in the supergroup, however, contain greenstones that were extruded upon and intruded into unconsolidated fine-grained terrigenous clastic sediments. It is inferred that eruption of the in situ greenstones resulted from igneous activity in the trench area. Geochemical signatures indicate that the greenstone protoliths were similar to mafic lavas generated at spreading ridges. Fossil ages of the strata containing in situ greenstones become younger over a distance of 1300 km eastward from Amami-Oshima (Cenomanian-Turonian) in the Ryukyu Is to central Japan (Late Maestrichtian-earliest Paleocene), implying that a site of igneous activity in the trench area migrated eastward along the Ryukyu Is and southwest Japan margin. Plate reconstructions of the northwest Pacific Ocean suggest the presence of the Kula-Pacific ridge near Late Cretaceous to early Paleogene Japan. In this context, it is suggested that the greenstones formed in response to Kula-Pacific ridge-forearc collision.
Ancient ridge-forearc collisions are best recognized by the presence of mid-ocean ridge basalt (MORB) extruded on sediments inferred to have accumulated in the trench area. Diachronous occurrences of the strata associated with these MORB in an orogenic belt are useful for documenting the ridge collision through time.  相似文献   

12.
Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike silicic liquids produced from partial melting of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet pyroxenite, MIX1G, at 2.0-2.5 GPa show that some pyroxenite partial melts are strongly silica-undersaturated [M.M. Hirschmann et al., Geology 31 (2003) 481-484]. These low-pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO2 (<48 wt%), low Al2O3 (<12 wt%) and high CaO (>12 wt%) at moderate MgO (12-16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIX1G or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825°C, respectively, which are less than 50°C cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725°C, indicating a narrow melting interval (∼100°C). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550°C, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the pyroxenite-derived partial melts.  相似文献   

13.
New inductively coupled plasma mass spectrometry (ICP-MS) trace element data are presented on a suite of arc lavas from the northern Mariana and southern Bonin island arcs. The samples were dredged from seamounts in the Central Island Province (CIP), the Northern Seamount Province (NSP) and the Volcano Arc (VA), and they range in composition from low-K tholeiites to shoshonites. Previous studies on these samples concluded that the primary compositional control was two-component mixing between a fluid-metasomatized mid-ocean ridge basalt (MORB) source and an enriched, ocean island basalt (OIB)-like, mantle component, with subducted sediment material playing a secondary role. However, the new trace element data suggest that the compositional variations along the Mariana arc can be better explained by the addition of spatially varying subduction components to a spatially varying mantle source. The data suggest that the subduction component in the CIP and VA is dominated by aqueous fluids derived from altered oceanic crust and a pelagic sediment component, while the subduction component in the NSP is dominated by more silicic fluids derived from volcanogenic sediments as well as from pelagic sediment and altered oceanic crust. The mantle wedge in the CIP and VA is depleted relative to a normal mid-ocean ridge basalt source by loss of a small melt fraction, while the mantle wedge in the NSP is enriched either by possible gain of a small melt fraction or addition of a sediment-derived melt. Because the subduction of seamounts controls the arc and back-arc geometries, so the concomitant variation between subducted material and mantle composition may be no coincidence. The high field strength element (HFSE) data indicate a high degree of melting (∼ 25–30%) throughout the arc, ∼ 10% of which may be attributed to decompression and ∼ 20% to fluid addition.  相似文献   

14.
New rare earth element (REE) data for Archaean basalts and spinifex-textured peridotites (STP) show a range of La/Sm ratios (chondrite-normalized) from 0.36 to 3.5, with the bulk of the data in the range 0.7–1.3. This supports the hypothesis, based on Sr isotope initial ratios, that the Archaean mantle was chemically heterogeneous. We suggest that the bulk mantle source for Archaean basaltic magmas was close to an undepleted earth material. An average chemical composition of the Archaean mantle is estimated using chemical regularities observed in Archaean STP and high-magnesian basalts. TiO2 and MgO data show an inverse correlation which intersects the MgO axis at about 50% MgO (Fo92). TiO2 abundance in the mantle source is measured on this plot by assigning anMgO= 38% for the mantle. Concentrations of other elements are also estimated and these data are then used to obtain a composition for the bulk earth. We suggest an earth model with about 1.35 times ordinary chondrite abundances of refractory lithophile elements and about 0.2 times carbonaceous type 1 chondrite abundances of moderately volatile elements (such as Na, Rb, K, Mn). P shows severe depletion in the model earth relative to carbonaceous chondrites, a feature either due to volatilization or core formation (preferred). Our data support the hypothesis of Ringwood that the source material for the earth is a carbonaceous chondrite-like material.The generation of mid-ocean ridge basalts (MORB) is examined in the light of the model earth composition and Al2O3/TiO2, CaO/TiO2 ratios. It is suggested that for primitive basalts, these values can be used to predict the residual phases in their source. Comparison of chemical characteristics of inferred sources for 2.7-b.y. Archaean basalts and modern “normal” MORB indicates that the MORB source is severely depleted in highly incompatible elements such as Cs, Ba, Rb, U, Th, K, La and Nb, but has comparable abundances of less incompatible elements such as Ti, Zr, Y, Yb. The cause of the depletion in the MORB source is examined in terms of crust formation and extraction of silica-undersaturated melts. The latter seems to be a more likely explanation, since the degree of enrichment of highly incompatible elements in the crust only accounts for up to 40% of their abundances in the bulk earth and cannot match the depletion pattern in normal MORB. A large volume of material, less depleted than the source for normal MORB must therefore exist in the mantle and can serve as the source for the ocean island basalts and “normal” MORB.Three different mantle evolution models are examined and each suggests that the mantle is stratified with respect to abundances of incompatible trace elements. We suggest that no satisfactory model is available to fully explain the spectrum of geochemical and geophysical data. In particular the Pb and Sr isotope data on oceanic basalts, the depletion patterns of MORB and the balance between lithophile abundances in the crust and mantle, are important geochemical constraints to mantle models. Further modelling of the mantle evolution will be dependent on firmer information on the role of subduction, mantle convection pattern, and basalt production through geologic time together with a better understanding of the nature of Archaean crustal genesis.  相似文献   

15.
We present the first data on bulk‐rock major and trace element compositions for a suite of eclogite‐ and blueschist‐facies rocks from the Bantimala Complex, Indonesia, with the aim of better constraining the protolith origins and nature of the subducted crust. The eclogites can be classified into two groups: glaucophane‐rich eclogite and glaucophane‐free eclogite, whereas the blueschists are divided into albite–epidote glaucophanite and quartz–glaucophane schists. SiO2 contents of the eclogites are 43.3–49.6 wt%, with Na2O + K2O contents 3.7–4.7 wt%. The blueschists show a wider range of compositions, with SiO2 = 40.7–63.8 wt% and Na2O + K2O = 2.7–4.5 wt%. Trace element data suggest that the eclogite protoliths include both enriched and normal mid‐oceanic ridge basalt (E‐MORB and N‐MORB) and also gabbroic cumulates. The blueschists show more variation in protoliths, which include N‐MORB, Oceanic Island Basalt (OIB) and Island Arc Basalt (IAB). Plots of element concentrations against the immobile Zr show considerable mobility of large ion lithophiles but not of high field‐strength elements during high‐pressure metamorphism, and indicate that the high SiO2 content of some blueschists is probably due to metasomatism by a LILE‐rich siliceous aqueous fluid. Strong correlations between K, Rb, Ba and Cs suggests that enrichment of these elements occurred by a single process. All the protoliths were subducted, metamorphosed to blueschist/eclogite‐facies and subsequently exhumed. It is noteworthy that the samples deduced to have come from thicker‐crust environments (OIB, IAB) were subducted to shallower depths (blueschist‐facies) than MORB‐derived samples, all except one of which reached eclogite‐facies conditions. The geochemical data of this study demonstrate the variety of ocean floor types that were subducted under the southeast margin of Sundaland in the late Jurassic period.  相似文献   

16.
Abstract The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)‐ and transitional (T)‐types ([La/Yb]N = 0.77–11.2) of mid‐oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40–1.20) are as low as those of back‐arc basin basalts (BABB). Cr‐spinels in the metavolcanic rocks have Cr? of 40–73 and an Fe3+? of 9–24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back‐arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5‐km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine–plagioclase crystallization sequence, different from the commonly observed olivine–clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb]N = 0.18–0.55) so that it has an REE pattern that mimics normal (N)‐type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO2 contents (0.2–1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off‐ridge environment during sea floor spreading of the back‐arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N‐ to E‐MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back‐arc basin.  相似文献   

17.
Abstract During the Hakuho‐Maru KH03‐3 cruise and the Tansei‐Maru KT04‐28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest–southeast elongated, rectangular massif, 60 km × 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on‐land ophiolite outcrops. Volcanic rocks included mid‐oceanic ridge basalt (MORB)‐like tholeiitic basalt and dolerite, calc‐alkaline basalt and andesite, boninite, high‐Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB‐type and island arc‐type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Cr? 50–80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra‐subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast–southwest‐oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu–Ogasawara (Bonin)–Mariana forearc, the Hahajima Seamount may be better interpreted as a fault‐bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (<2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave‐eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body.  相似文献   

18.
Emilio  Saccani  Adonis  Photiades 《Island Arc》2005,14(4):494-516
Abstract Ophiolitic mélanges associated with ophiolitic sequences are wide spread in the Mirdita–Subpelagonian zone (Albanide–Hellenide Orogenic Belt) and consist of tectonosedimentary ‘block‐in‐matrix‐type’ mélanges. Volcanic and subvolcanic basaltic rocks included in the main mélange units are studied in this paper with the aim of assessing their chemistry and petrogenesis, as well as their original tectonic setting of formation. Basaltic rocks incorporated in these mélanges include (i) Triassic transitional to alkaline within‐plate basalts (WPB); (ii) Triassic normal (N‐MORB) and enriched (E‐MORB) mid‐oceanic ridge basalts; (iii) Jurassic N‐MORB; (iv) Jurassic basalts with geochemical characteristics intermediate between MORB and island arc tholeiites (MORB/IAT); and (v) Jurassic boninitic rocks. These rocks record different igneous activities, which are related to the geodynamic and mantle evolution through time in the Mirdita–Subpelagonian sector of the Tethys. Mélange units formed mainly through sedimentary processes are characterized by the prevalence of materials derived from the supra‐subduction zone (SSZ) environments, whereas in mélange units where tectonic processes prevail, oceanic materials predominate. In contrast, no compositional distinction between structurally similar mélange units is observed, suggesting that they may be regarded as a unique mélange belt extending from the Hellenides to the Albanides, whose formation was largely dominated by the mechanisms of incorporation of the different materials. Most of the basaltic rocks surfacing in the MOR and SSZ Albanide–Hellenide ophiolites are incorporated in mélanges. However, basalts with island arc tholeiitic affinity, although they are volumetrically the most abundant ophiolitic rock types, have not been found in mélanges so far. This implies that the rocks forming the main part of the intraoceanic arc do not seem to have contributed to the mélange formation, whereas rocks presumably formed in the forearc region are largely represented in sedimentary‐dominated mélanges. In addition, Triassic E‐MORB, N‐MORB and WPB included in many mélanges are not presently found in the ophiolitic sequences. Nonetheless, they testify to the existence throughout the Albanide–Hellenide Belt of an oceanic basin since the Middle Triassic.  相似文献   

19.
We report new trace element data for an extensive suite of quench basalt glasses dredged from the southern Mid-Atlantic Ridge (MAR) between 40°S and 52.5°S. Ratios between highly incompatible trace elements are strongly correlated and indicate a systematic distribution of incompatible element enriched mid-ocean ridge basalt (MORB) (E-type: Zr/Nb=5.9-19, Y/Nb=0.9-8.4, (La/Sm)n=1.0-2.9) and incompatible element depleted MORB (N-type: Zr/Nb=30-69, Y/Nb=11-29, (La/Sm)n=0.48-0.79) along this section of the southern MAR. A notable feature of N-type MORB from the region is the higher than usual Ba/Nb (4-9), La/Nb (1.2-2.4) and primitive mantle normalised K/Nb ratios (>1). Ba/Nb ratios in E-type MORB samples from 47.5 to 49°S are especially elevated (>10). The occurrence and geographic distribution of E-type MORB along this section of the southern MAR can be correlated with the ridge-centred Shona and off-axis Discovery mantle plumes. In conjunction with published isotope data for a subset of the same sample suite [Douglass et al., J. Geophys. Res. 104 (1999) 2941], a model is developed whereby prior to the breakup of Gondwana and the opening of the South Atlantic Ocean, the underlying asthenospheric mantle was locally contaminated by fluids/melts rising from the major Mesozoic subduction zone along the south-southwest boundary of Gondwana, leaving a subduction zone geochemical imprint (elevated (K/Nb)n and 87Sr/86Sr ratios, decreased 143Nd/144Nd ratios). Subsequent impingement of three major mantle plume heads (Tristan/Gough, Discovery, Shona) resulted in heating and thermal erosion of the lowermost subcontinental lithosphere and dispersal into the convecting asthenospheric mantle. With the opening of the ocean basin, continued plume upwelling led to plume-ridge interactions and mixing between geochemically enriched mantle derived from the Shona and Discovery mantle plumes, material derived from delamination of the subcontinental lithosphere, and mildly subduction zone contaminated depleted asthenospheric mantle.  相似文献   

20.
The basaltic rocks from the central and southern islands of the New Hebrides-Aneityum, Tanna, Erromango, Efate, Emae, Tongoa and Epi, have geochemical features typical of island arc volcanics. They are enriched in LILE and depleted in Zr, Hf, Nb and Ta compared to N-type MORB. The rocks were derived from a similar upper mantle source as N-type MORB but with a higher degree of partial melting. In addition their source was enriched in LILE (K, Rb, Sr, Ba and LREE) probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab. The basalts from Futuna island which is located farther from the trench, display characteristics typical of calc-alkaline rocks. The Futuna basalts were generated from a different LILE-enriched upper mantle source. It seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere.  相似文献   

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