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1.
From mica fission-track maps the serpentinized and weathered portions of four ultramafic rocks from oceanic ridge systems contained 0.5 to 2.4 ppm U compared to only 2.5 ppb in clinopyroxene, 0.6 ppb in chromite and less than 7.0 ppb in olivine. Orthopyroxene grains contained 0.4 ppb U which is three orders of magnitude lower than had previously been reported.Long thin tracks from (n, α) reactions with boron were recorded in cellulose nitrate plastic and were counted like fission tracks. The track density from boron was 2×104 times higher than that from uranium fission alone. Boron in serpentine was variable on a 50-μm scale attaining 155 ppm concentrations. Orthopyroxene grains, in contrast, had maximum concentrations of 0.8 ppm.Most of the uranium and boron in the rocks is believed to have been introduced during serpentinization. From known crystal-melt partitioning ratios the uranium and boron distributions are consistent with the ultramafic rocks being cumulates or residues from partial melting events. 相似文献
2.
Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt
The Western Mediterranean Jurassic ophiolites contain abundant volcanic rocks that resemble modern ocean floor basalts. In this paper we report analyses of rare earth elements (REE), transition elements (Ni, Co, Cr, V, Sc) and high field strength elements (Ti, P, Y, Zr, Ta, Hf) for metabasalts from representative ophiolite outcrops in the Eastern Alps, Corsica and the Northern Apennines (Liguria).The chemical characteristics of the metabasalts range from “normal” to “transitional” mid-ocean ridge basalt (MORB). Most chemical variation in the metabasalts from the different areas can be explained by low-pressure fractional crystallization, by differences in degree of partial melting, and by minor chemical heterogeneities of the source, but the “transitional” MORB characteristics of some metabasalts from Corsica (Balagne) might reflect formation from a source with different mineral and chemical composition. The estimated REE pattern of the source of the Liguria-type metabasalts corresponds to the pattern for certain Ligurian ultramafic rocks, which might therefore represent the residue from extraction of some ophiolitic lavas.While the Liguria-type ophiolitic metabasalts might represent products of a “normal”, for instance, oceanic ridge, the “transitional” metabasalts might be lavas erupted, during the early stages of opening of a small ocean basin, or along the continental margin of a larger ocean basin. 相似文献
3.
The silica content of basaltic rocks is an unreliable variable with which to distinguish ultramafic-mafic complexes developed at ocean ridges from those potentially formed beneath volcanic island arcs. Data from Appalachian ophiolites supports the view that silica metasomatism is responsible for the high silica content of supposed calc-alkaline basaltic rocks found in ophiolites such as Troodos, and that the high-silica (70 wt.%) leucocratic rocks associated with ophiolites are of tholeiitic rather than calc-alkaline parentage. The use of titanium as a discriminant of tectonic environment is also suspect because the titanium content of basalts associated with Appalachian ophiolites as well as those recently recovered from the Atlantic ocean floor ranges from values even lower than those typical of island arc tholeiites to values typical of abyssal tholeiites. However, the internal stratigraphy of ophiolites in both the Appalachian and Tethyan systems can only be explained on the basis of the postulate that ophiolites originate at oceanic spreading centres rather than beneath island arcs. 相似文献
4.
V. Dietrich R. Emmermann R. Oberhänsli H. Puchelt 《Earth and Planetary Science Letters》1978,39(1):127-144
This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc). 相似文献
5.
David C. Gerlach Hans G. AvéLallemant William P. Leeman 《Earth and Planetary Science Letters》1981,53(2):255-265
The Canyon Mountain ophiolite, Oregon, is exceptional in lacking sheeted dikes, basaltic pillow lavas, and sediments that are characteristic of many other ophiolites. Instead, the uppermost portion of the complex consists of a significant volume of plagiogranites, which, in addition to minor basalts, intrude a large section of keratophyres believed to be of volcanic origin. The trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite. It is suggested that the plagiogranites, basalts, and keratophyres comprise a sill complex. Both the plagiogranites and the keratophyres are similar, respectively, to low-K2O plutonic and extrusive rocks of island arcs. The mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation, related to magmatism and diapirism above a Benioff zone. Radiometric age dates of plagiogranites confine the minimum age of the complex to the Early Permian. The Canyon Mountain ophiolite may thus be correlative with other fragments of a Lower Permian arc terrane throughout northeastern Oregon which were chaotically mixed during renewed subduction in middle to late Triassic time. 相似文献
6.
Geochemical data are presented for a suite of ultramafic and related rocks from the Dras ophiolitic melange of the Indus suture zone in the western Himalaya. Harzburgites from the suite have highly refractory chemistry. Lherzolites from the suite represent modified mantle material and are comparable to potential source rocks for MORB and to the lherzolites of the Bay of Islands ophiolites. Cumulus dunite, pyroxenite and gabbro units contain olivine, chromite, clinopyroxene and plagioclase as the major cumulus phases. Orthopyroxene is absent as a cumulus phase and in this respect the Dras ophiolite differs from the Marum, Betts Cove and Troodos ophiolites. However, the Dras cumulates are similar to the Vourinos and Bay of Islands cumulate sequence and are consistent with accumulation of low-pressure liquidus phases of mid-oceanic ridge-type magmas. Magmas parental to the Dras cumulate rocks contained high 100 Mg/(Mg + Fe2+) ratios of 77–79, high Ni, Cr and possibly Ca, low Ti and depleted LREE. Parent magmas were probably similar to those of normal MORB formed by two-stage (or dynamic) melting processes in the mantle. Peridotite fabrics suggest high-temperature plastic (mantle) deformation. Disruption, serpentinisation and melange formation were probably produced during emplacement in the Indus suture zone. 相似文献
7.
Abstract Ophiolites of the Mirdita–Subpelagonian zone form a nearly continuous belt in the Albanide–Hellenide orogen, including mid‐ocean ridge basalt (MORB) associations in the western Mirdita sector and supra‐subduction zone (SSZ) complexes, with prevalent island arc tholeiitic (IAT) and minor boninitic affinities in the eastern part of the belt (i.e. eastern Mirdita, Pindos, Vourinos). In addition, basalts with geochemical features intermediate between MORB and IAT (MORB/IAT) are found in the central Mirdita and in the Aspropotamos sequence (Pindos). These basalts alternate with pure MORB and are cut by boninitic dykes. The distinctive compositional characteristics of the mafic magmas parental to the different ophiolitic suites can be accounted for by partial melting of mantle sources progressively depleted by melt extractions. Partial melting processes (10–20%) of lherzolitic sources generated pure MORB, leaving clinopyroxene‐poor lherzolite as a residuum. Approximately 10% water‐assisted partial melting of this latter source, in an SSZ setting, may in turn generate basalts with MORB/IAT intermediate characteristics, whereas IAT basalts and boninites may have been derived from 10–20% and 30% partial melting, respectively, of the same source variably enriched by subduction‐derived fluids. In addition, boninites may also have been derived by comparatively lower degrees of hydrated partial melting of more refractory harzburgitic sources. A generalized petrologic model based on mass balance calculations between bulk rock and mineral compositions, indicate that most of the intrusives (from ultramafic cumulates to gabbronorites and plagiogranites), as well as sheeted dykes and volcanics (from basalts to rhyodacites) forming the bulk crustal section of the SSZ ophiolites, may be accounted for by shallow fractional crystallization from low‐Ti picritic parental magmas very similar in composition to IAT picrites from Pacific intraoceanic arcs. The most appropriate tectono‐magmatic model for the generation of the SSZ Tethyan ophiolites implies low velocity plate‐convergence of the intraoceanic subduction and generation of a nascent arc with IAT affinity and progressive slab roll‐back, mantle diapirism and extension from the arc axis to the forearc region, with generation of MORB/IAT intermediate basalts and boninitic magmas. 相似文献
8.
测定了日喀则地区蛇绿岩组合岩石的稀有气体同位素组成。白朗的玄武岩具有比较均一的He同位素组成 ,3He/ 4He平均值为 5 35 9Ra;变橄岩的He同位素比值范围为 1 10 4～ 3 384Ra,平均为 2 383Ra;从东部的仁布和大竹到西部的下鲁和白朗 ,辉绿岩的3He/ 4He由低变高。采于吉定的新鲜辉绿岩平均高达 31 5 7Ra,该值接近于在夏威夷发现的高比值。分步加热法结果显示高R值的He是在低温步释放的 ,根据这种高R值的He可以推测雅鲁藏布江蛇绿岩的成因环境有地幔柱的作用 ,该构造带很可能出现过裂谷洋盆或未能发育成熟的深大裂谷带 相似文献
9.
The Trap Series outcropping in the southeastern margin of the Main Ethiopian Rift has a total thickness of not less than 500–600 m. In this series, mainly composed of flood lavas and rare scoriaceous beds, basaltic rocks — which on a serial viewpoint are transitional basalts — are the best represented. Alkali-olivine basalts (with modal and normative nepheline) are interbedded within the lower and middle members of the series, while in the upper members andesine basalts and dark phonolitic tephrites are present. Among the transitional basalts, rocks of alkaline and of tholeiitic affinity are present as shown by petrographic and chemical analyses. On the basis of petrochemical and geovolcanological data, it is probable that these rocks and the phonolitic-tephrites originated from different primary magmas. 相似文献
10.
Jane D. Sills David Savage Janet V. Watson Brian F. Windley 《Earth and Planetary Science Letters》1982,58(3):345-360
Layered ultramafic-gabbro bodies occur widely in the Archaean of northwest Scotland. They were metamorphosed at granulite or high amphibolite facies and were tectonically thinned and broken up during deformation. They comprise repeated ultramafic-gabbro layers, locally with Ni-poor sulphide-rich tops, each rhythmic unit showing decreasing MgO, Ni and normative anorthite with stratigraphic height. Major, trace and rare earth element data are presented for the range of rock types. In ultramafic rocks, MgO varies from 22 to 37 wt.%, Ni from 1000 to 2500 ppm and TiO2 from 0.08 to 0.40 wt.%, while the MgO content of the gabbros ranges from 14 to 6 wt.%. The REE patterns are flat to LREE enriched with no significant Eu anomalies. In ultramafic rocks REE are from 4 to 10 times chondrite, and in the gabbros LREE range from 8 to 30 times chondrite and HREE from 6 to 15 times chondrite. Study of incompatible elements (Ti, Zr, Y) which are relatively immobile during metamorphism shows that neither garnet nor hornblende were involved in fractionation. Trace element modelling shows it is improbable that the ultramafic rocks represent primary MgO-rich liquids even though their incompatible element contents are quite high. The chemical trends are interpreted in terms of olivine and pyroxene settling from a tholeiitic high-Mg magma with 15–20 wt.% MgO derived by 30–40% partial melting of an undepleted mantle. The ultramafic rocks are the cumulates and the gabbros the derived liquids. 相似文献
11.
Basalts and a mafic dyke collected from the city of Panzhihua show characteristics of high-Ti and low-Ti Emeishan basalts respectively. The dyke yielded a SHRIMP zircon U–Pb mean age of 261 ± 5 Ma making it contemporaneous with the eruption of Emeishan basalts. The basalts have ISr ranging from 0.7059 to 0.7062 with εNd(T) ranging from ?1.1 to + 0.7 whereas the dyke has ISr ranging from 0.7056 to 0.7064 with εNd(T) ranging from + 0.3 to + 0.5. Trace element modeling shows that the two rock types can be generated by different degrees of partial melting from the same garnet-bearing source. Assimilation of crustal material is required in order to produce the depletion of some trace elements (e.g. Nb and Ta) of the dyke however crustal assimilation is not required to produce the basalts. Trace element modeling and isotopic data of the Emeishan basalts suggest that, in general, the high- and low-Ti basaltic rocks are likely derived from the same source and represent different degrees of partial melting with or without crustal assimilation. The location and geological relationships of the ‘high-Ti’ basalts indicate they erupted relatively early and within the central part of the ELIP, casting doubt on the previous spatial–compositional distribution of the Emeishan basalts. 相似文献
12.
Meta-igneous mafic and ultramafic rocks, which constitute about 60% of the granulitic xenoliths enclosed in the Neogene alkali basalts of the Bournac pipe (French Massif Central) have well preserved magmatic trends of element variations. The meta-igneous suite was probably derived from at least two different parental magmas and it may be a part of a gabbroic complex which resembles mafic bodies associated with anorthosites. The xenoliths are also very similar to many other granulitic xenoliths and to meta-igneous mafic granulitic massifs. This indicates that the gabbroic intrusions may be widespread in the lower crust and the close association of gabbroic rocks with meta-sedimentary granulites suggests a model for the composition of the lower continental crust. 相似文献
13.
The lead content of five whole-rock peridotite inclusions (four lherzolites and one harzburgite) in alkali basalt ranges from 82 to 570 ppb (parts per billion). Approximately 30–60 ppb of this amount can be accounted for by analyzed major silicate minerals (olivine ≤ 10 ppb; enstatite 5–28 ppb; chrome diopside ～400 ppb). Through a series of acid leaching experiments, the remainder of the lead is shown to be quite labile and to reside in either glassy or microcrystalline veinlets or accessory mineral phases, such as apatite and mica. The lead isotopic composition of the peridotites (206Pb/204Pb= 18.01–18.90;207Pb/204Pb= 15.52–15.61;208Pb/204Pb= 37.80–38.86) lies within the range of values defined by many modern volcanic rocks and, in particular, is essentially coextensive with the abyssal tholeiite field. In all but one instance, isotopic differences were found between the peridotite and its host alkali basalt. Two of the peridotites clearly demonstrated internal isotopic heterogeneity between leachable and residual fractions that could not simply be due to contamination by the host basalt. However, there is no evidence that these ultramafic rocks form some layer in the mantle with isotopic characteristics fundamentally different from those of the magma sources of volcanic rocks. 相似文献
14.
Abstract Spinel lherzolite is a minor component of the deep-seated xenolith suite in the Oki-Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; < 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo86 ) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U-shaped chondrite-normalized rare-earth element (REE) patterns. The abundance of Fe-rich ultramafic and mafic cumulate xenoliths in Oki-Dogo alkali basalts suggests that the later formation of those Fe-rich cumulates from alkaline magma was the cause of Fe- and light REE (LREE)-enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South-west Japan arc, which are non-metasomatized in terms of major-element chemistry (e.g. Fo > 89), and are rarely associated with Fe-rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE-enrichment in mantle rocks has been more prominent and prevalent than Fe and other major-element enrichment during the metasomatism. 相似文献
15.
The Oligocene alkaline basalts of Toveireh area (southwest of Jandaq, Central Iran) exhibit northwest–southeast to west–east exposure in northwest of the central‐east Iranian microcontinent (CEIM). These basalts are composed of olivine (Fo70–90), clinopyroxene (diopside, augite), plagioclase (labradorite), spinel, and titanomagnetite as primary minerals and serpentine and zeolite as secondary ones. They are enriched in alkalis, TiO2 and light rare earth elements (La/Yb = 9.64–12.68) and are characterized by enrichment in large ion lithophile elements (Cs, Rb, Ba) and high field strength elements (Nb, Ta). The geochemical features of the rocks suggest that the Toveireh alkaline basalts are derived from a moderate degree partial melting (10–20%) of a previously enriched garnet lherzolite of asthenospheric mantle. Subduction of the CEIM confining oceanic crust from the Triassic to Eocene is the reason of mantle enrichment. The studied basalts contain mafic‐ultramafic and aluminous granulitic xenoliths. The rock‐forming minerals of the mafic‐ultramafic xenoliths are Cr‐free/poor spinel, olivine, Al‐rich pyroxene, and feldspar. The aluminous granulitic xenoliths consist of an assemblage of hercynitic spinel + plagioclase (andesine–labradorite) ± corundum ± sillimanite. They show interstitial texture, which is consistent with granulite facies. They are enriched in high field strength elements (Ti, Nb and Ta), light rare earth elements (La/Yb = 37–193) and exhibit a positive Eu anomaly. These granulitic xenoliths may be Al‐saturated but Si‐undersaturated feldspar bearing restitic materials of the lower crust. The Oligocene Toveireh basaltic magma passed and entrained these xenoliths from the lower crust to the surface. 相似文献
16.
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
17.
Field, petrographic and geochemical criteria identify the Eocambrian, 500 km2, mafic-ultramafic complex of Jabal al Wask as an ophiolite possibly formed in a back-arc environment. It is one of several ultramafic bodies that lie in NW-SE zones across western Arabia and northeast Africa. The basement between these zones is mainly granitic and seemingly developed by the cratonization of island arcs. It is suggested that the ultramafic zones are sutures between these arcs and that the ophiolites represent oceanic lithosphere remnants of back-arc seas.As palaeomagnetic evidence precludes any extensive movement of individual arcs, a palaeogeographic situation analogous to that of present-day southwest Pacific is envisaged. 相似文献
18.
In the Paleozoic basalts of Nova Scotia (Canada) metamorphosed to a greenschist facies grade, U and Th are closely associated with immobile elements (e.g. Zr and Nb). The coherence of these elements with K, typical of igneous rocks is, however, absent, U and Th are apparently not affected by greenschist facies metamorphism as are alkali and alkali earth elements and their variation thus reflects primary magmatic processes. It seems that during the early stages of metamorphism, U and Th were retained together with several other incompatible elements (Zr, Nb, La and Ce) in stable secondary phases or alternatively these elements may be held in primary accessory minerals such as zircon and apatite. 相似文献
19.
Akiho Miyashiro 《Earth and Planetary Science Letters》1975,25(2):227-235
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. 相似文献
20.
Cretaceous volcanic rocks (SCV) are widely developed in the northern part of the Sanandaj-Sirjan Zone, northwest Iran. Based on the mineralogy, texture and geochemical composition these rocks are divided in two main groups, the first and main one situated in the central part of the study area and the second one in the northeast. The former is dominantly basalts, andesitic basalts, and andesites and the latter comprises andesite, trachy-andesite to acidic variants, with porphyritic to microlithic porphyry and vitrophyric textures. Beside the differences between these two groups, the chemical compositions all of these rocks show a calc-alkaline affinity and enrichment in LIL elements (Rb, Ba, Th, U, and Pb) and depletion in Nb, Ti, and Zr, as evident in spider diagrams normalized to primitive mantle. The rocks are particularly enriched in Rb and depleted in Nb and Ti, as well as displaying high Rb/Sr and Rb/Ba ratios and low ratios of incompatible elements such as Nb/U (<10; range, 0.6–9), Th/U (<2), and Ba/Rb (<20). The significant U enrichment relative to neighbouring Nb and Th in the mantle-normalized variation diagram is mainly a result of source enrichment by slab-derived fluids. Significantly lower Nb/U ratios are observed in arc volcanics. These low values are generally ascribed to the strong capacity of LILE and the inability to transfer significant amounts of HFSE via slab-derived hydrous fluid. The results of geochemical modelling suggest a mantle lithospheric source that was metasomatized by fluids derived from a Neo-Tethyan subducted slab during the Middle to Late Cretaceous in the northern part Sanandaj-Sirjan Zone. 相似文献