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1.
Pillow lavas in Bompoka island of the Andaman–Nicobar islands, forming a part of Sunda–Burmese forearc, are composed of plagioclase and clinopyroxene microphenocrysts in a fine-grained ferruginous groundmass along with glass. They are also characterized by several quench plagioclase and clinopyroxene morphologies. Zr/TiO2 versus Nb/Y relationship of these pillow lavas show that these are tholeiitic basalts in composition. These basalts have low MgO (5.19–6.12 wt%), Ni (84–118 ppm), and Cr (144–175 ppm) abundance and high FeO(T)/MgO (1.71–1.92) ratios, reflecting their fractionated nature. In Th/Yb versus Nb/Yb and Ti/Yb versus Nb/Yb binary diagrams, they show N-MORB affinity. However, La/Nb–Y and Ce/Nb–Th/Nb relationships along with a slight LREE depleted (LaN/YbN = 0.75–0.82) pattern and high Ba/Zr (0.28–0.40) ratios and LILE (K, Rb, Ba, Sr and Th) enrichment relative to N-MORB, suggest their back-arc basin basalt affinity. It is inferred that these pillow basalts have been derived from a metasomatised N-MORB-like mantle source in a trench-distal (wider) back-arc basin, probably near the leading edge of the Eurasian continent during Early to Late Cretaceous times, prior to the currently active Andaman–Java subduction system.  相似文献   

2.
《International Geology Review》2012,54(15):1865-1884
It is generally accepted that Neoproterozoic extension and dispersal of the supercontinent Rodinia was associated with mantle plume or superplume activities. However, plume-generated contemporaneous continental flood basalts (CFBs) have rarely been identified. In this study, we present geochronological and geochemical evidence for the basalts from the Liufangzui Formation of the Huashan Group in the Dahongshan region of east-central China. A representative sample yields a SHRIMP U–Pb zircon age of 824 ± 9 Ma, interpreted as the crystallization age of the rocks. Geochemically, these basalts belong to the subalkaline tholeiite series and display slight enrichments in light rare earth elements (LREE) and varying degrees of deficiency of high field strength elements (HFSE) such as Nb, Ta, and Ti. This pattern is very similar to that of CFBs from the Bikou Group and Tiechuanshan Formation in the northwestern Yangtze block in China and Siberia in Russia. The basaltic magmas underwent partially-fractional crystallization during ascent, but were not intensely influenced by crustal contamination. The characteristic element ratios and negative Hf isotopic analyses (?Hf(t) = ?6.6–2.6) in zircons indicate that the parental magmas of the basalts might have been derived from an enriched lithospheric mantle rather than from the depleted mantle such as normal mid-ocean ridge basalts (N-MORBs). The geochemical signatures and regional geological characteristics show that these basalts were formed along intraplate continental rifts rather than in island arcs or ocean basins. Considering the coeval basic volcanic rocks in South China, we propose that these Huashan Group basalts represent the remnants of plume-generated CFBs and have close spatiotemporal ties with a coeval basic igneous province in Australia. Our results support the Neoproterozoic location of the South China block adjacent to southeastern Australia in the reconstruction model of the supercontinent Rodinia.  相似文献   

3.
Flood basalt provinces may constitute some of the most catastrophic volcanic events in the Earth's history. A popular model to explain them involves adiabatic ascent of plumes of anomalously hot peridotite from a thermal boundary layer deep in the mantle, across the peridotite solidus. However, peridotitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relations of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)–oceanic basalt (GA1) compositional join. The hybrid basalt + peridotite compositions crystallised garnet lherzolite at subsolidus temperatures plus quenched ne-normative picritic liquids at temperatures just above the solidus, over the compositional range MPY90 to GA150MPY9050. The solidus temperature decreased slightly from ∼1500 °C for MPY90 to ∼1450 °C for GA150MPY9050. Compositions similar to GA130MPY9070 have 100% melting compressed into a melting interval which is approximately 50–60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few tens of percent basalt in peridotite, the lower solidus and compressed solidus–liquidus temperature interval may conspire to substantially enhance melt productivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable source for some flood volcanics. Evidence for this would include higher than normal Fe/Mg values in natural primary liquids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e. <Fo89–92). Modelling of fractionation trends in West Greenland picrites is presented to demonstrate that melts parental to the Greenland picrites were in equilibrium at mantle P–T conditions with olivine with Fo84–86, suggesting an Fe-enriched source compared with normal peridotite, and consistent with the presence of a basaltic component in the source. Received: 29 October 1999 / Accepted: 3 February 2000  相似文献   

4.
Knickzones are common features along rivers on the basaltic plateaus of the Paraná Basin. According to current conceptual models, knickpoints are formed in massive basalts that have a high density of vertical joints. Vesicular–amygdaloidal basalts and those with horizontal joints tend to form reaches of low slope due to their lower resistance to erosion. However, field surveys revealed complexities in this general relationship. The research presented here sought to verify the controls on the genesis of knickzones in this type of geological environment. We studied a 61 km-long mixed bedrock–alluvial river. The longitudinal profile of the river was surveyed on a topographic map with 5 m contour intervals. Tectonic lineaments oriented transverse to the channel and longitudinal lineaments in which the river lies were identified from maps. A detailed field survey of the lithologic characteristics of the riverbed was also performed. The results show that knickzones may form in any litho-structural zone in the flood basalts. On the other hand, low slope zones are predominantly sculpted into vesicular–amygdaloidal basalts, which are less resistant to erosion. The fracture densities of vesicular–amygdaloidal basalts are similar in low slope zones and in knickzones (4.86 and 4.93 m/m2, respectively). This indicates that knickzones in this type of basalt are not caused by higher resistance to erosion. Approximately 60% of the 18 knickzones identified are associated with tectonic lineaments, irrespective of the structural characteristics of the basalts. Vesicular–amygdaloidal basalt and/or basalt with horizontal joints allow the fastest knickzone migration and aid in the formation of convexities. Knickpoints in these basalts do not migrate, but erosion in the pools advances downstream and breaks the bedrock steps, thus increasing the slope. Massive basalt with vertical joints causes slower migration, and its presence at convexities indicates local uplift. Convex segments are only formed upstream of faults.  相似文献   

5.
This study reports a geochemical investigation of two thick basalt sequences, exposed in the Bracco–Levanto ophiolite (northern Apennine, Italy) and in the Balagne ophiolite (central-northern Corsica, France). These ophiolites are considered to represent an oceanward and a continent-near paleogeographic domain of the Jurassic Liguria–Piedmont basin. Trace elements and Nd isotopic compositions were examined to obtain information about: (1) mantle source and melting process and (2) melt–rock reactions during basalt ascent. Whole-rock analyses revealed that the Balagne basalts are slightly enriched in LREE, Nb, and Ta with respect to the Bracco–Levanto counterparts. These variations are paralleled by clinopyroxene chemistry. In particular, clinopyroxene from the Balagne basalts has higher CeN/SmN (0.4–0.3 vs. 0.2) and ZrN/YN (0.9–0.6 vs. 0.4–0.3) than that from the Bracco–Levanto basalts. The basalts from the two ophiolites have homogeneous initial Nd isotopic compositions (initial εNd from +?8.8 to +?8.6), within typical depleted mantle values, thereby excluding an origin from a lithospheric mantle source. These data also reject the involvement of contaminant crustal material, as associated continent-derived clastic sediments and radiolarian cherts have a highly radiogenic Nd isotopic fingerprint (εNd at the time of basalt formation?=???5.5 and ??5.2, respectively). We propose that the Bracco–Levanto and the Balagne basalts formed by partial melts of a depleted mantle source, most likely containing a garnet-bearing enriched component. The decoupling between incompatible elements and Nd isotopic signature can be explained either by different degrees of partial melting of a similar asthenospheric source or by reaction of the ascending melts with a lower crustal crystal mush. Both hypotheses are reconcilable with the formation of these two basalt sequences in different domains of a nascent oceanic basin.  相似文献   

6.
The Emeishan flood basalts can be divided into high-Ti (HT) basalt (Ti/Y>500) and low-Ti (LT) basalt (Ti/Y<500). Sr, Nd isotopic characteristics of the lavas indicate that the LT- and the HT-type magmas originated from distinct mantle sources and parental magmas. The LT-type magma was derived from a shallower lithospheric mantle, whereas the HT-type magma was derived from a deeper mantle source that may be possibly a mantle plume. However, few studies on the Emeishan flood basalts involved their Pb isotopes, especially the Ertan basalts. In this paper, the authors investigated basalt samples from the Ertan area in terms of Pb isotopes, in order to constrain the source of the Emeishan flood basalts. The ratios of 206Pb/204Pb (18.31–18.41), 207Pb/204Pb (15.55–15.56) and 208Pb/204Pb (38.81–38.94) are significantly higher than those of the depleted mantle, just lying between EM I and EM II. This indicates that the Emeishan HT basalts (in the Ertan area) are the result of mixing of EMI end-member and EMII end-member.  相似文献   

7.
Enrichment in K2O in oceanic island basalts (OIB) is correlated with high SiO2, low CaO/Al2O3, and radiogenic isotopic signatures indicative of enriched mantle sources (EM1 and EM2). These are also chemical characteristics of the petit-spot lavas, which are highly enriched in K2O (3–4 wt%) compared to other primitive oceanic basalts. We present experimentally derived liquids with varying concentrations of K2O in equilibrium with a garnet lherzolite residue at 3 GPa to test the hypothesis that the major element characteristics of EM-type basalts are related to their enrichment in K2O. SiO2 is known to increase with K2O at pressures less than 3 GPa, but it was previously unknown if this effect was significant at the high pressures associated with partial melting at the base of the lithosphere. We find that at 3 GPa for each 1 wt% increase in the K2O content of a garnet lherzolite saturated melt, SiO2 increases by ~0.5 wt% and CaO decreases by ~0.5 wt%. MgO and $K_{D}^{{{\text{Fe}} - {\text{Mg}}}}$ K D Fe - Mg each decrease slightly with K2O concentration, as do Na2O and Cr2O3. The effect of K2O alone is not strong enough to account for the SiO2 and CaO signatures associated with high-K2O OIB. The SiO2, CaO, and K2O concentrations of experimentally derived partial melts presented here resemble those of petit-spot lavas, but the Al2O3 concentrations from the experimental melts are greater. Partitioning of K2O between peridotite and melt suggests that petit spots, previously considered to sample ambient asthenosphere, require a source more enriched in K2O than the MORB source.  相似文献   

8.
We have measured the Hf and Nd isotopic compositions of 38 basalts from the Ko’olau drill hole, Hawai’i. The basalts show limited variations in both 176Hf/177Hf and 143Nd/144Nd (ε Nd varies from +4.2 to +7.3 and ε Hf from +8.0 to +12.3). Their correlated variation has an R 2 of 0.86. The data form an array with a slope of 1.2 on an ε Hf–ε Nd isotope correlation diagram, while the slope of all Hawai’ian basalt data is 0.98. Both slopes are significantly shallower than that of the mantle array of 1.4 defined by ocean island basalts. Previous studies have shown that a shallow slope in Hf–Nd isotope space can be related to ancient pelagic sediments in the mantle source (Blichert-Toft et al. 1999; Salters and White 1998). However, the combined variations in Ko’olau basalts of Hf–Nd–Pb–Os isotopic compositions and trace element ratios, such as La/Nb, Th/La and Sr/Nd, are not consistent with the simple addition of a sediment component to the mantle. We instead propose that the shallow slope on the Hf–Nd isotope correlation diagram for Ko’olau shield stage basalts can be better explained if the enriched endmember contains either an ancient oceanic lithosphere component or the high-176Hf/177Hf component observed in the Salt Lake Crater (SLC) peridotite xenoliths (which also have a depleted lithosphere origin). Since Ko’olau basalts have high 187Os/188Os (0.135–0.160) and the SLC xenoliths have 187Os/188Os up to 0.13 (Lassiter et al. 2000) Os-isotopes are consistent with the latter being a component in the enriched Ko’olau source.
Vincent J. M. SaltersEmail:
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9.
The discovery of a layer of increased density in the liquid core at the boundary with the solid core gives grounds to suggest that solidification of the solid core occurs with an increase in total core volume and is accompanied by an increase in internal pressure in the core. This makes it possible to suggest a translational mechanism of energy transfer from the core to the Earth’s surface. It is suggested that the restoration of lithostatic equilibrium occurs via rising of a column of mantle material, uplift as result of elastic mantle deformation of the boundary of the transition layer at a depth of 420 km, and the formation of rising at the surface.  相似文献   

10.
《International Geology Review》2012,54(14):1791-1805
Newly discovered basalts in the Dabure area (central Qiangtang block, northern Tibet) were subjected to laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating, geochemical analyses, and zircon Hf isotope analyses. Dating of magmatic zircons from three basalt samples indicates that the Dabure basalts formed during the late Ediacaran (~550 Ma). Xenocrystic zircons yield ages of 700–1150 Ma, providing evidence of the Cryogenian crust in the Tibet block. The Dabure basalts are alkaline, rich in Ti and Fe, and are strongly enriched in light rare earth elements without Eu anomalies. The basalts are geochemically similar to within-plate basalts but are relatively depleted in Nb and Ta. Although the analysed zircons show differences in their Hf isotope compositions, the geochemical data suggest that the Dabure basalts were derived from enriched mantle and that the source magmas were contaminated by the continental crust. The basalts may have erupted during rifting at ~550 Ma (from the dating of magmatic zircons), and may have been a product of the initial breakup of Gondwanaland.  相似文献   

11.
The lavas of the Zig-Zag Dal Formation of eastern North Greenland constitute a Mesoproterozoic tholeiitic flood basalt succession up to 1,350 m thick, extending >10,000 km2, and underlain by a sill complex. U–Pb dating on baddeleyite from one of the sills thought to be contemporaneous with the lava extrusion, gives an age of 1,382±2 Ma. The lavas, subdivided from oldest to youngest into Basal, Aphyric and Porphyritic units, are dominantly basaltic (>6 wt.% MgO), with more evolved lavas occurring within the Aphyric unit. The most magnesian lavas occur in the Basal unit and the basaltic lavas exhibit a generalised upward decrease in Mg number (MgO/(MgO + Fe2O3T)) through the succession. All of the lavas are regarded as products of variable degrees of olivine, augite and plagioclase fractionation and to be residual after generation of cumulates in the deep crust. The basaltic lavas display an up-section fall in the ratio of light to heavy rare-earth elements (LREE/HREE) but an up-section rise in Zr/Nb, Sc, Y and HREE. The older lavas (Basal and Aphyric units) are characterised by low Nd and Hf in contrast to higher values in the younger (Porphyritic unit) lavas. The Porphyritic Unit basalts are characterised by a notable enrichment in Fe and Ti. The Zig-Zag Dal succession is inferred to reflect an increase in melt fraction in the sub-lithospheric mantle, with melting commencing in garnet–lherzolite facies peridotites and subsequently involving spinel-facies mantle at increasingly shallow depths. Melting is deduced to have occurred beneath an attenuating continental lithosphere in conjunction with ascent of a mantle plume. Lithospheric contamination of primitive melts is inferred to have diminished with time with the Porphyritic unit basalts being products of essentially uncontaminated plume-source magmas. The high iron signature may reflect a relatively iron-rich plume source.  相似文献   

12.
The geochemistry of pillow basalts from the Chonos Metamorphic Complex (CMC) and the Eastern Andes Metamorphic Complex of Aysén (EAMC) indicates contrasting tectonic environments for these basic lavas. They have E-MORB and continental alkaline affinities, respectively. The MORB-like basalts are metamorphosed in the pumpellyite–actinolite metamorphic facies, with mineral associations indicative of relatively high P/T metamorphism. The continental alkali basalts exhibit pumpellyite–chlorite assemblages developed in a low to intermediate P/T regime. These contrasting eruptive and metamorphic settings agree with recently established age differences between the complexes, and invalidate direct correlation between them.  相似文献   

13.
Quaternary basaltic volcanoes are distributed in the northern part of the Sanandaj–Sirjan Zone (N-SSZ). Those in the Ghorveh area of the N-SSZ are characterized by low SiO2, high alkalis, and LILE + LREE enrichment. They also have high Mg numbers (Mg# = 65–70) and high contents of Cr (>300 ppm), Ni (>177 ppm), and TiO2 (>1.5 wt.%), suggesting that they crystallized directly from primary magma. The basalts are classified as high-Nb basalts (HNB), with Nb concentrations greater than 20 ppm. Their 87Sr/86Sr values range from 0.7049 to 0.7053 and their ?0Nd values lie between –0.2 and 1.1. The small negative values of ?0Nd indicate involvement of continental material in the evolution of the source magma in the area. Based on these new chemical and isotopic data and their relationship to the Plio-Quaternary volcanic adakites in northern Ghorveh, we propose that the partial fusion of metasomatized mantle associated with adakitic magma was responsible for generation of the HNB rocks following late Miocene collision of the Arabian and Iranian plates. Rollback of Neotethyan oceanic spreading and mantle plume activity caused a thinning of the northern SSZ lithosphere; furthermore, the S wave tomography model beneath the N-SSZ supports this hypothesized lithospheric thinning. The HNB rocks have close spatial proximity and temporal association with adakites, which were formed by the subduction of young (<25 Ma) oceanic crust. Our discussion clarifies the role of the oceanic slab in the post-collision generation of the HNB basalts in this area. Our data confirm the relationship of the HNB rocks to the subduction zone instead of to the oceanic island basalt (OIB) type magma in extensional zones.  相似文献   

14.
At the southern part of the East Pacific Rise (EPR), between 6°S and 30°S a survey on volcanic and hydrothermal activity was performed and samples were obtained by means of TV-controlled grab.This paper deals with altered and mineralized basalt sampled between 7°S and 23°S from five sites in a hydrothermal field.These basalts of tholeitic composition are vitreous to holocrystaline. They have suffered pervasive alteration during which rock-forming minerals (pyroxene, plagioclase) have been replaced by kaolinite, chlorite and smectite. As a consequence, strong depletion of Ca, Cr, Ni, Mg, Sr and Al took place, accompanied by an enrichment of Fe, Cu, Co, Mo, Zn, and Pb. The ore mineral assemblage is rather uniform and consists of pyrite, marcasite, wurtzite, sphalerite, chalcopyrite, covellite and goethite.The igneous and hydrothermal activities can be subdivided into different stages: lithification, high-temperature alteration (<400 C), medium to low temperature alteration and two substages of ore formation (250°C – 150°C and <150°C, respectively).This active present-day ore deposition is interpreted in terms of a peripheral zone of a volcanic-hosted ore mineralisation. It may be compared with fossil ophiolite-hosted massive sulfides that formed throughout the Alpine (e.g. Arabia, Cyprus) as well as Caledonian orogeny (e.g. Scandinavia).
Zusammenfassung Im Südteil des East Pacific Rise (EPR) zwischen 6°S und 30°S wurde eine Übersichtsbeprobung in einem Gebiet starker vulkanischer und hydrothermaler Aktivität durchgeführt. Diese Studie befaßt sich mit alterierten und mineralisierten Basalten die in einem Hyrothermalfeld, an 5 Positionen zwischen 7°S und 23°S genommen wurden. Die tholeiitischen Basalte sind z.T. hyalin. z.T. holokristallin strukturiert. Im Verlauf der sehr starken Alteration wurden die Minerale Pyroxen und Plagioklas in Kaolinit, Chlorit und Smektit ungewandelt. Dieser Verdrängungsprozeß wurde von einer starken Abreicherung an Ca, Cr, Ni, Mg, Sr, Al und einer Anreicherung an Fe, Cu, Co, Mo, Zn und Pb begleitet. Die Vererzung zeigt eine einfache Mineralvergesellschaftung mit Pyrit, Markasit, Wurtzit, Sphalerit, Chalkopyrit, Covellin und Geothit.Die magmatische und hydrothermale Aktivität läßt sich in verschiedene Stadien untergliedern: Basaltentstehung, Hochtemperturalteration (< 400°), Mittelbis Tieftemperaturalteration und Vererzung mit 2 Substadien (250°C–150°C, < 150°C).Diese rezenten Vererzungen lassen sich interpretieren als die Randzone einer vulkanitgebundenen Erzmineralisation. Sie läßt sich mit fossilen Vertretern ophiolit-gebundener massiver Sulfiderze, wie sie im Verlauf den alpidischen (z.B. Arab.-Halbinsel, Zypern) und kaledonischen Orogenese (z.B. Norwegen) entstanden sind, vergleichen.
Résumé Une étude des activités volcanique et hydrothermale a été effectuée dans la partie sud de l'East Pacific Rise, entre 6° et 30° de latitude sud. Des échantillons y ont été prélevés au moyen d'un engin dirigé par TV.La présente note se rapporte aux basaltes altérés et minéralisés récoltés en cinq points d'un champ hydrothermal, entre 7°S et 23°S.Ces basaltes, de composition tholéiitique sont tantôt vitreux, tantôt holocristallins. Ils ont été le siège d'une forte altération hydrothermale au cours de laquelle les minéraux de la roche (pyroxène, plagioclase) ont été remplacés par de la kaolinite, de la chlorite et de la smectite. Il en est résulté un appauvrissement marqué en Ca, Cr, Ni, Mg, Sr et Al, accompagné d'un enrichissement en Fe, Cu, Co, Mo, Zn et Pb. La minéralisation présente une association simple à pyrite, marcassite, wurtzite, sphalérite, covelline et goethite.On peut distinguer plusieurs stades d'activité magmatique et hydrothermale: formation du basalte, altération de haute température (<400°C), altération de moyenne à basse température avec deux stades de minéralisation (250-150°C; <150°C). Ces dépôts de la nature actuelle peuvent s'interpréter comme la zone périphérique d'une aire de minéralisation volcanogène. On peut les comparer à des gisements fossiles de sulfures massifs liés à des ophiolites, comme il s'en est formé au cours des orogenèses alpine (p. ex.: péninsule arabique, Chypre) et calédonienne (p. ex.: Norvège).
»East Pacific Rise (EPR)« 6° 30° . , 7° 23° . , . , , . , Cr, Ni, Mg, Sr, Al Fe, u, , Mo, Zn Pb. , , , , , . : , ( 400°), (250°–150°, <150°) . , , . , , (.: , ) (.: ).
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15.
International Journal of Earth Sciences - This study presents new whole-rock elemental and isotopic data for the basalts from the Zhaotong area, located in the intermediate zone of the...  相似文献   

16.
Fan  Jian-Jun  Li  Cai  Liu  Jin-Heng  Wang  Ming  Liu  Yi-Ming  Xie  Chao-Ming 《International Journal of Earth Sciences》2018,107(5):1755-1775
International Journal of Earth Sciences - In this paper, we present new major and trace element chemical data for the basalts and phonolites of the Nare ocean island fragment (NaOI), as well as...  相似文献   

17.
Many studies have examined the Japan Sea basalts recovered during Ocean Drilling Program (ODP) Leg127/128. Of these, the 40Ar–39Ar dating undertaken is important in constraining the timing of the formation of the Japan Sea; however, the implications of their results do not appear to be fully appreciated by the geological community. In this paper, I reassess the 40Ar–39Ar age data of the basalts with reference to Nd–Sr isotopic data. The 40Ar–39Ar dating was performed on basalts somewhat enriched in large-ion lithophile elements and recovered from ODP Sites 794, 795 and the lower part of 797, yielding the plateau ages of 21.2–17.7 Ma. These basalts show the Nd–Sr isotopic signature of a moderately depleted mantle source (εNd: 0.6–6.9). In contrast, the basalts from the upper part of Site 797 have yet to be dated due to their low K content, although their Nd isotopic compositions are similar to that of MORB (εNd: 8.4–10.4). By analogy to the secular Nd–Sr isotopic trends reported for Sikhote-Alin and northeast Japan, the age of the upper basalts at Site 797 may be inferred to be younger than the lower basalts, probably around 16 Ma. The Nd–Sr isotopic compositions of the Japan Sea basalts have been interpreted in terms of eastward asthenospheric flow, as have the lavas of the Sikhote-Alin and northeastern Japan. The timing of volcanic activity in the Japan Sea region (i.e., from 21.2 to 14.86 Ma) is consistent with the timing of rotational crustal movements inferred from paleomagnetic studies of the Japanese Islands (i.e., 14.8–4.2 Ma for southwest Japan and 16.5–14.4 Ma for northeast Japan).  相似文献   

18.
The mechanism and process of lithospheric thinning beneath the North China Craton (NCC) are still debated. A key criterion in distinguishing among the proposed mechanisms is whether associated continental basalts were derived from the thinning lithospheric mantle or upwelling asthenosphere. Herein, we investigate the possible mechanisms of lithospheric thinning based on a systematic Re–Os isotopic study of Mesozoic to Cenozoic basalts from the NCC. Our whole-rock Re–Os isotopic results indicate that the Mesozoic basalts generally have high Re and Os concentrations that vary widely from 97.2 to 839.4 ppt and 74.4 to 519.6 ppt, respectively. They have high initial 187Os/188Os ratios ranging from 0.1513 to 0.3805, with corresponding variable γOs(t) values (+20 to +202). In contrast, the Re–Os concentrations and radiogenic Os isotope compositions of the Cenozoic basalts are typically lower than those of the Mesozoic basalts. The lowest initial 187Os/188Os ratios of the Cenozoic basalts are 0.1465 and 0.1479, with corresponding γOs(t) values of +15 and +16, which are within the range of ocean island basalts. These new Re–Os isotopic results, combined with the findings of previous studies, indicate that the Mesozoic basalts were a hybrid product of the melting of pyroxenite and peridotite in ancient lithospheric mantle beneath the NCC. The Cenozoic basalts were derived mainly from upwelling asthenosphere mixed with small amounts of lithospheric materials. The marked differences in geochemistry between the Mesozoic and Cenozoic basalts suggest a greatly reduced involvement of lithospheric mantle as the magma source from the Mesozoic to the Cenozoic. The subsequent lithospheric thinning of the NCC and replacement by upwelling asthenospheric mantle resulted in a change to asthenosphere-derived Cenozoic basalts.  相似文献   

19.
《International Geology Review》2012,54(11):1350-1362
ABSTRACT

Recent studies show that crustal carbonates recycled into the mantle can be traced using Mg isotopes of basalts. However, the species of recycled carbonates are poorly constrained. Carbonates have lower δ26Mg values and higher 87Sr/86Sr ratios relative to the mantle, but different carbonate species display different mixing curves with the mantle in the Mg-Sr isotopic diagram because of differences in their Sr and Mg contents. Thus a combined study of Mg-Sr isotopes can constrain the species of deeply recycled carbonates. Here, we present newly determined 87Sr/86Sr ratios of the <110 Ma basalts from Eastern China, and together with published Mg isotopic data we evaluate the species of recycled carbonates in the mantle and discuss their implication. The <110 Ma basalts display low δ26Mg values of ?0.60 to ?0.30‰ and relatively low initial 87Sr/86Sr ratios of 0.70328 to 0.70537, suggesting that their mantle source was hybridized by recycled carbonates with a light Mg isotopic composition which had more significant effects on Mg than Sr isotope ratios. Mg-Sr isotopic data indicate that the recycled carbonates consist of magnesite and aragonite, but the possibility of calcite and dolomite cannot be eliminated. Based on the carbonated peridotite solidus, the equilibrium line between dolomite and magnesite + aragonite, as well as the mantle adiabat, the initial melting depth of the carbonated mantle, the source region of the studied basalts, was constrained at ~300–360 km. Thus, the subducted depth of the west Pacific slab underlying the carbonated mantle and supplying recycled carbonates should be greater than ~300–360 km, consistent with the seismic tomography result that the west Pacific slab now stagnates in the mantle transition zone.  相似文献   

20.
Summary Fresh back-arc basin lavas were recovered during five dives of the submersible Mir during the 1990 cruise of the research vessel Akademik Mstislav Keldysh to the Lau Basin. Three dives were conducted on the central spreading center of the King's Triple Junction (KTJ) in the northeastern part of the Lau Basin east of Niuafo'ou Island at approximately 15°S. The lavas from the KTJ can be divided into types I and II based on their similarities to N-MORB and the BABB magma type ofSinton andFryer (1987) respectively. One dive each was made on the Central Lau Spreading Center (CLSC) at 18°S and the Eastern Lau Spreading Center (ELSC) at 19°S. Lavas sampled on the CLSC were associated with active hydrothermal sulphide chimneys occurring at the base of a collapsed caldera structure on the central volcanic axial high. Sampled lavas from both the CLSC and ELSC are all of type I geochemistry.The results of the Keldysh 90 cruise are integrated with previous work to evaluate the geochemical characteristics of the Lau Basin crust as a whole and geochemical zonation models for back-arc basin development. An important part of this review of Lau Basin basalt geochemistry is the recognition of boninites and rocks of boninitic affinity which occur at off-ridge locations throughout the Lau Basin. These are suggested to erupt during the initial stages of development of new spreading ridges associated with episodes of ridge jump or the propagation of back-arc spreading ridges into arc crust.Hf-Th-Ta systematics of the Lau Basin lavas are used to identify unmodified mantle source compositions and possible subduction-related enriched components. Unmodified mantle source compositions range from D-, N-, and E-MORB to OIB. All Lau Basin lavas show some evidence of enrichment by a H2O ± LILE ± LREE-enriched component, a slab derived hydrous fluid. The slab-derived fluid is not homogeneous in composition perhaps reflecting the presence or absence of a subducted sediment component.The sampled lavas from the KTJ confirm the uniqueness of the BABB magma type ofSinton andFryer (1987).
Zusammenfassung Dieser Artikel berichtet über erste Ergebnisse einer Forschungsexpedition in das Lau Basin, ausgeführt mit dem Forschungsschiff Akademiker Mstislav Keldysh und dem Mini-U-Boot Mir. Drei Tauchfahrten führten zur zentralen Achse der King's triple junction (KTJ) im nordöstlichen Teil des Lau Basins östlich der Insel Niuafo'ou (15°S). Die dort beprobten Laven werden auf der Basis ihrer chemischen Zusammensetzung eingeteilt in die Typen I und II, entsprechend der Klassifikation vonSinton andFryer (1987) in N-MORB und BABB. Eine vierte Tauchfahrt zum zentralen Lau Spreading Center (CLSC, 18°N) erbrachte Fragmente von Sulphidschloten eines aktiven Black Smoker innerhalb einer kollabierten Calderastructur. Eine fünfte Tauchfahrt zum östlichen Lau Spreading Center (ELSC, 19°N) erbrachte Laven vom Typus I (entsprechend N-MORB).Unter Berücksichtigung früherer Arbeiten gestatten die Ergebnisse der Keldysh 90 Expedition eine geochemische Charakterisierung des Lau Basins, sowie eine Bewertung bestehender Zonierungsmodelle fur die Entstehung und Evolution von Backarc Basins. Entlang der gesamten Achse im Lau Basin, aber abseits des eigentlichen aktiven Spreading Centers, werden Boninite und Laven boninitischer Affinität beobachtet. Diese Boninite können als integraler Teil des Frühstadiums eines Backarc Spreading Centers gedeutet werden, und entstehen möglicherweise dort, wo Spreading Centers in die Kruste eines Inselbogens vordringen oder dann, wenn es zu einer abrupten Positionsänderung der Achse kommt.Die Hf-Th-Ta Konzentrationen und Verhältnisse in Lau Basin Laven gestatten die Identifikation mehrerer Mantel-Endglieder (entsprechend D-MORB, N-MORB, E-MORB, und OIB), sowie einer an H2O und LILE-LREE angereicherten Komponente, die mit Subduktionstätigkeit und anschließender Mantelmetasomatose in Zusammenhang gebracht wird. Die angereicherte Komponente war vermutlich inhomogen aufgrund variabler Anteile subduzierter Sedimente. Die Laven des Lau Basins bestätigen die vonSinton andFryer (1987) vorgeschlagene geochemische Sonderstellung von BABB Magmentypen.

With 8 Figures  相似文献   

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