A mantle plume origin for the Siberian traps: uplift and extension in the West Siberian Basin, Russia

The West Siberian Basin (WSB) records a detailed history of Permo-Triassic rifting, extension and volcanism, followed by Mesozoic and Cenozoic sedimentation in a thermally subsiding basin. Sedimentary deposits of Permian age are absent from much of the basin, suggesting that large areas of the nascent basin were elevated and exposed at that time. Industrial seismic and well log data from the basin have enabled extension and subsidence modelling of parts of the basin. Crustal extension (β) factors are calculated to be in excess of 1.6 in the northern part of the basin across the deep Urengoy graben. 1-D backstripping of the Triassic to Cenozoic sedimentary sequences in this region indicates a period of delayed subsidence during the early Mesozoic. The combination of elevation, rifting and volcanism is consistent with sublithospheric support, such as a hot mantle plume.

This interpretation accords with the geochemical data for basalts from the Siberian Traps and the West Siberian Basin, which are considered to be part of the same large igneous province. Whilst early suites from Noril'sk indicate moderate pressures of melting (mostly within the garnet stability field), later suites (and those from the West Siberian Basin) indicate shallow average depths of melting. The main region of magma production was therefore beneath the relatively thin (ca. 50–100 km) lithosphere of the basin, and not the craton on which the present-day exposure of the Traps occurs. The indicated uplift, widespread occurrence of basalts, and short duration of the volcanic province as a whole are entirely consistent with published models involving a mantle plume. The main argument against the plume model, namely lack of any associated uplift, appears to be untenable.     

亚洲3个大火成岩省(峨眉山、西伯利亚、德干)对比研究

峨眉山(～260 Ma)、西伯利亚(～250 Ma)和德干(～66 Ma)大陆溢流玄武岩是世界上3个重要的大火成岩省.大火成岩省至少具有4个通常被用于识别古地幔柱的标志:(1)先于岩浆作用的地表隆升;(2)与大陆裂谷化和裂解事件相伴;(3)与生物灭绝事件联系密切;(4)地幔柱源玄武岩的化学特征.虽然这3个大火成岩省都是来源于原始地幔柱,但是它们的地球化学特征有本质上的差异,反映其地幔柱曾与不同的上地幔库相互作用.(1)峨眉山和西伯利亚大陆溢流玄武岩的母岩浆,在上升过程中经受了与地球化学上和古老克拉通岩石圈地幔相同的上地幔库(EM1型幔源)的相互作用;(2)而德干大火成岩省没有受到地壳(或岩石圈)混染的原生玄武岩则显示地幔柱和EM2之间的Sr-Nd同位素变化.这种差异有可能制约了3个大火成岩省的成矿潜力.峨眉山和西伯利亚大火成岩省含有世界级岩浆矿床,而德干大火成岩省则不含矿.     

Siberian meimechites: origin and relation to flood basalts and kimberlites

Here we combine petrological-geochemical and thermomechanical modeling techniques to explain origin of primary magmas of both Maimecha–Kotui meimechites and the Gudchikhinskaya basalts of Norilsk region, which represent, respectively, the end and the beginning of flood magmatism in the Siberian Trap Province.We have analyzed the least altered samples of meimechites, their olivine phenocrysts, and melt inclusions in olivines, as well as samples of dunites and their olivines, from boreholes G-1 and G-3 within the Guli volcanoplutonic complex in the Maimecha–Kotui igneous province of the northern Siberian platform. The Mn/Fe and Ni/MgO ratios in olivines indicate a mantle peridotite source of meimechites. Meimechite parental magma that rose to shallow depths was rich in alkalis and highly magnesian (24 wt.% MgO), largely degassed, undersaturated by sulfide liquid and oxidized. At greater depths, it was, likely, high in CO₂ (6 wt.%) and H₂O (2 wt.%) and resulted from partial melting of initially highly depleted and later metasomatized harzburgite some 200 km below the surface. Trace-element abundances in primary meimechite magma suggest presence of garnet and K-clinopyroxene, in the mantle source and imply for genetic link to the sources of the early Siberian flood basalts (Gudchikhinskaya suite) and kimberlites. The analyzed dunite samples from the Guli complex have chemistry and mineralogy indicating their close relation to meimechites.We have also computed thermomechanical model of interaction of a hot mantle plume with the shield lithosphere of variable thickness, using realistic temperature- and stress-dependent visco-elasto-plastic rocks rheology and advanced finite element solution technique.Based on our experimental and modeling results we propose that a Permian–Triassic plume, with potential temperature of about 1650 °C transported a large amount of recycled ancient oceanic crust (up to 15%) as SiO₂-supersaturated carbonated eclogite. Low-degree partial melting of eclogite at depths of 250–300 km produced carbonate-silicate melt that metasomatized the lithospheric roots of the Siberian shield. Further rise of the plume under relatively attenuated lithosphere (Norilsk area) led to progressive melting of eclogite and formation of reaction pyroxenite, which then melted at depths of 130–180 km. Consequantly, a large volume of melt (Gudchikhinskaya suite) penetrated into the lithosphere and caused its destabilization and delamination. Delaminated lithosphere that included fragments of locally metasomatized depleted harzburgite subsided into the plume and was heated to the temperatures of the plume interior with subsequent generation of meimechite magma. Meimechites showed up at the surface only under thicker part of the lithosphere aside from major melting zone above because otherwise they were mixed up in more voluminous flood basalts. We further suggest that meimechites, uncontaminated Siberian flood basalts and kimberlites all shear the same source of strongly incompatible elements, the carbonated recycled oceanic crust carried up by hot mantle plume.     

Two melting regimes during Paleogene flood basalt generation in East Greenland: combined REE and PGE modelling

Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section “the Sortebre Profile” in central East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content (~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast, the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment related to fractional crystallisation also increased the S-capacity of these melts. An erratum to this article can be found at     

Plume dynamics beneath the African plate inferred from the geochemistry of the Tertiary basalts of southern Ethiopia

Southern Ethiopian flood basalts erupted in two episodes: the pre-rift Amaro and Gamo transitional tholeiites (45-35 million years) followed by the syn-extensional Getra-Kele alkali basalts (19-11 million years). These two volcanic episodes are distinct in both trace element and isotope ratios (Zr/Nb ratios in Amaro/Gamo lavas fall between 7 and 14, and 3-4.7 in the Getra-Kele lavas whereas ²⁰⁶Pb/²⁰⁴Pb ratios fall between 18-19 and 18.9-20, respectively). The distinctive chemistries of the two eruptive phases record the tapping of two distinct source regions: a mantle plume source for the Amaro/Gamo phase and an enriched continental mantle lithosphere source for the Getra-Kele phase. Isotope and trace element variations within the Amaro/Gamo lavas reflect polybaric fractional crystallisation initiated at high pressures accompanied by limited crustal contamination. We show that clinopyroxene removal at high (0.5 GPa) crustal pressures provides an explanation for the common occurrence of transitional tholeiites in Ethiopia relative to other, typically tholeiitic flood basalt provinces. The mantle plume signature inferred from the most primitive Amaro basalts is isotopically distinct from that contributing to melt generation in central Ethiopian and Afar. This, combined with Early Tertiary plate reconstructions and similarities with Kenyan basalts farther south, lends credence to derivation of these melts from the Kenyan plume rather than the Afar mantle plume. The break in magmatism between 35 and 19 Ma is consistent with the northward movement away from the Kenya plume predicted from plate tectonic reconstructions. In this model the Getra-Kele magmatism is a response to heating of carbonatitically metasomatised lithosphere by the Afar mantle plume beneath southern Ethiopia at this time.     

Distribution of PGE in Permo-Triassic basalts of the Siberian Large Igneous Province

We have revealed the spatio-temporal regularities of distribution of platinum group elements (PGE) in basaltoids related to the activity of the Siberian mantle plume. As objects of study, we chose rift and flood basalts from the Norilsk district (sampled from the SD-9 borehole), flood basalts from the central part of the Tunguska syneclise (Lower Tunguska), Kuznetsk Basin traps, and subalkalic basalt from the Semeitau volcanoplutonic structure in eastern Kazakhstan. Based on the PGE patterns of basaltoids related to the activity of the Permo-Triassic Siberian plume, we have shown that the rocks that formed in the central part of the Siberian Large Igneous Province (LIP) at the early rift stage have low contents of PGE, whereas picrites and tholeiitic flood basalts have high contents. The rift (Semeitau structure) and flood (Kuznetsk Basin traps) basalts from the peripheral regions are characterized by extremely low PGE contents. The high PGE contents in magmas of the plume head are responsible for the high productivity of ultramafic-mafic trap magmatism. The elevated K contents in magmas and the high PGE contents in the mantle plume head are probably due to the ascent of deep-seated material from the core-lower-mantle boundary, as follows from the thermochemical model of the Siberian plume.     

Geochemistry of the Siberian Trap of the Noril'sk area,USSR, with implications for the relative contributions of crust and mantle to flood basalt magmatism

The sequence investigated of the Siberian Trap at Noril'sk, USSR, consists of at least 45 flows that have been divided into six lava suites. The lower three suites consist of alkalic to subalkalic basalts (the Ivakinsky suite), overlain by nonporphyritic basalts (the Syverminsky suite), and porphyritic and picritic basalts (the Gudchikhinsky suite). The upper three suites are tholeiitic. The uppermost 750 m of dominantly non-porphyritic basalt belong to the Mokulaevsky suite and are characterized by a nearly constant Mg number (0.54–0.56), SiO₂ (48.2–49.1 wt%), Ce (12–18 ppm), and Ce/Yb (5–8). The underlying 1100 m of dominantly porphyritic basalt belong to the Morongovsky and Nadezhdinsky suites. There is a continuous increase in SiO₂ (48.1–55.2 wt%), Ce (12–41 ppm), and Ce/Yb (5–18) from the top of the Mokulaevsky to the base of the Nadezhdinsky with little change in the Mg number (0.53–0.59). Mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid-ocean ridge basalts. The change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by Mokulaevsky magma. The Nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the Noril'sk-Talnakh mining camp. The Mokulaevsky and Morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the Noril'sk region; these magmas may have risen up out of a different conduit system.     

Petrology of the parental melts and mantle sources of Siberian trap magmatism

Based on the investigation of olivine phenocrysts and melt and spinel inclusions in them from the picrites of the Gudchikhinsky Formation and olivine phenocrysts and the whole-rock geochemistry from the Tuklonsky and Nadezhdinsky formations of the Noril’sk region, the compositions and conditions of formation and evolution of the parental melts and mantle sources of Siberian trap magmatism were evaluated. Olivine phenocrysts from the samples studied are enriched in Ni and depleted in Mn compared with olivines equilibrated with the products of peridotite melting, which suggests a considerable role of a nonperidotitic component (olivine-free pyroxenite) in their mantle source. The onset of Siberian trap magmatism (Gudchikhinsky Formation) was related to the melting of pyroxenite produced by the interaction of ancient recycled oceanic crust with mantle peridotite. During the subsequent evolution of the magmatic system (development of the Tuklonsky and Nadezhdinsky formations), the fraction of the pyroxenite component in the source region decreased rapidly (to 40 and 60%, respectively) owing to the entrainment of peridotite material into the melting zone. The formation of magmas was significantly affected by the contamination by continental crustal material. The primitive magmas of the Gudchikhinsky Formation crystallized under near-surface conditions at temperatures of 1250–1170°C and oxygen fugacities 2.5–3.0 orders of magnitude below the Ni-NiO buffer. Simultaneously, the magmas were contaminated by continental silicic rocks and evaporites. The parental magmas of the Gudchikhinsky rocks corresponded to tholeiitic picrites with 11–14 wt % MgO. They were strongly undersaturated in sulfur, contained less than 0.25 wt % water and carbon dioxide, and were chemically similar to the Hawaiian tholeiites. They were produced by melting of a pyroxenite source at depths of 130–180 km in a mantle plume with a potential temperature of 1500–1580°C. The presence of low melting temperature pyroxenite material in the source of Siberian trap magmas promoted the formation of considerable volumes of melt under the thick continental lithosphere, which could trigger its catastrophic collapse. The contribution of pyroxenite-derived melt to the magmas of the Siberian trap province was no less than 40–50%. This component, whose solid residue was free of sulfides and olivine, played a key role in the origin of high contents of Ni, Cu, and Pt-group elements and low sulfur contents in the parental trap magmas and prevented the early dispersion of these elements at the expense of sulfide melt fractionation. The high contents of Cl in the magmas resulted in considerable HCl emission into the atmosphere and could be responsible for the mass extinction at the Paleozoic-Mesozoic boundary.     

Modelling of thermochemical plumes and implications for the origin of the Siberian traps

Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P–T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t₁ between plume formation and eruption. These relationships are used to determine thermal power and time t₁ for the Tunguska syneclise and the Siberian traps as a whole.

The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts.     

大陆板内玄武岩数据挖掘:成分多样性及在判别图中的表现

通常认为,大陆溢流玄武岩(CFB)、裂谷玄武岩(CRB)、板内玄武岩(WPB)均产于板内构造环境,其地球化学特征与OIB类似,源于富集的下地幔,与地幔柱的活动有关。本文利用GEOROC数据库对全球CFB、CRB和WPB数据进行挖掘,发现上述三类玄武岩判别图投图几乎落入了全部的构造环境域,有些甚至主要落入MORB和IAB区,而不是落入WPB区。结果表明原先的玄武岩判别图的判别功能值得商榷,尤其对大陆玄武岩来说,许多判别图都存在问题。全体CFB、CRB和WPB的地球化学成分变化巨大,暗示其源区具有强烈的不均一性:部分CFB、CRB和WPB来自富集的地幔柱,仍然具有经典的OIB的特征;部分来自MORB的源区,与MORB的再循环作用有关;部分来自岛弧岩石圈之下的亏损地幔源区,以强烈亏损Nb-Ta为特征,类似岛弧玄武岩的地球化学特征。许多地区的大陆玄武岩可分为低钛和高钛两类,低钛玄武岩大多是亏损或强烈亏损的,而高钛玄武岩通常是富集型的。本文的研究表明,富集型大陆玄武岩可能来自富集的下地幔,而亏损的和强烈亏损的玄武岩可能来自具有MORB或岛弧特征的软流圈地幔。进一步指出,源区性质可能是大陆玄武岩多样性的主控因素,其次为部分熔融程度、熔融深度、结晶分离、陆壳混染以及AFC过程。     

Evidence for an enriched mantle source for jøtunite (orthopyroxene monzodiorite) associated with the St. Urbain anorthosite, Quebec

Mafic orthopyroxene monzodiorite (jøtunite) lithologies are exposed in the St. Urbain plutonic suite as a marginal facies to quartz mangerite and massif anorthosite intrusive bodies and as dikes within a variety of host rocks. High concentrations of Ti, Fe, P, K, Ba, Nb, La, Ce, Zn, Ga, Zr and Y characterize these rocks and are distinctive of many mafic lithologies associated with anorthosite massifs worldwide. Characteristically low concentrations of Ni and Cr, in conjuction with low Mg numbers, have been used by previous investigators as evidence for either partial melting of mafic granulitic lower crust or extensive fractional crystallization of a mantle-derived magma. In an attempt to distinguish between these competing models, we note that jøtunite display many features that bear a strong resemblance to continental tholeiitic flood basalts, including chemical signatures on normalized multi-element (‘spider’) diagrams. Ratios of incompatible trace elements and patterns on rare earth and ‘spider’ diagrams collectively indicate that the jøtunite rocks were derived from an enriched, rather than depleted, mantle source. Enrichment may have occured by subduction-derived fluids or by mixture with a plume component prior to partial melting so that isotopic and trace-element compositions are decoupled. Small amounts of partial melting of mafic granulite has been advanced as an alternative model; we show, however, that the experimental data on which this model is built are not applicable. Our preferred model begins with partial melting of a trace-element enriched mantle source that fractionates olivine at high to moderate pressures. Increasing concentrations of P (and Ti) eventually caused a contraction of the olivine stability field in favor of orthopyroxene. Fractional crystallization may yield the series of rocks from anorthosite, leuconorite, oxide-apatite gabbronorite, to jøtunite. Mafic magmas emplaced into continental crust are typically attributed to incipient rifting or mantle upwelling, which are features common to many models for the genesis of anorthosite and related rocks.     

峨眉山大火成岩省中苦橄岩与其共生岩石的地球化学特征及其对源区的约束

本文对在峨眉山玄武岩省中新发现的苦橄质岩石及其共生玄武岩的地球化学特征进行了研究,结果表明,除苦橄质岩石外,与其共生的玄武质岩石均属高Ti玄武岩。其主要元素特征与大多数大陆溢流玄武岩省相似,表现为高Fe_8、(CaO/Al_2O_3)_8和低Na_8,指示其形成压力高;其稀土和微量元素配分曲线相似,表现为轻稀土富集、高场强元素(HFSE)相对亏损,并且不存在Nb、Ta的负异常,而存在P和K的相对亏损。一些反映源区特征的比值,如La/Ta、La/Sm、(La/Nb)_(PM)、(Th/Ta)_(PM),Ta/Hf,Nb/Zr等,变化范围小,均指示了其地幔柱成因,且上升过程中很少或没有受到岩石圈地幔或地壳物质的混染,是石榴子石二辉橄榄岩在>75km时经大约7％的部分熔融的产物。而地幔柱的轴部位置可能位于现今云南丽江县城一带。     

The petrogenesis of the Kangâmiut dyke swarm,W. Greenland

Previous studies have shown that the 2.04 Ga Kangâmiut dyke swarm of SW Greenland was injected into an active tectonic environment associated with the formation of the Nagssugtoqidian orogenic belt. Major and trace element modelling of the swarm shows that its chemical evolution was controlled by simple clinopyroxene–plagioclase fractionation. However, such trends — although typical of continental flood basalts and mafic dyke swarms — are at variance with their mineralogy and petrography, which show that locally hornblende is the dominant primary ferromagnesian mineral. Modelling of intradyke fractionation alone shows that hornblende could locally have been an important crystallising phase within several dykes. Normal basaltic fractionation must have occurred before dyke injection at the exposed crustal levels, where the influx of water into the dykes is believed to be responsible for the transition from clinopyroxene–plagioclase (tholeiitic) to hornblende–plagioclase±oxides (calc–alkaline) crystallisation. Overall geochemical trends are dominated by tholeiitic fractionation because (1) hornblende fractionation tended to buffer chemical composition; (2) the presence of water in the surrounding country rocks may have resulted in the advection of heat away from the dyke and consequently resulted in rapid crystallisation, particularly in thin dykes. There is no evidence from trace element data, and particularly Pb isotopic ratios, of any significant assimilation of country rocks occurring during clinopyroxene–plagioclase fractionation, although this does not preclude contamination of the mantle source prior to magma generation. It is likely that the incompatible element enrichment within the dykes resulted from subduction-related mantle metasomatism. The Kangâmiut dyke swarm was both a syn-tectonic and thermal event, which triggered it may be linked to passage of a slab window underneath the metasomatised region, or a mantle plume ascending under a subduction zone.     

Early Stage in the Evolution of the Paleoasian Ccean at the Western Margin of the Siberian Craton: Geochemical and Geochronological Evidence

The discovery of glaucophane relicts in the high-pressure tectonites of the Yenisei suture zone of the Yenisei Ridge suggests the manifestation of the “Chilean-type” convergent margin on the western Siberian Craton, which was controlled by subduction of oceanic crust beneath the continental margin. These rocks are restricted to the tectonic suture between the craton and the Isakovka ocean-island terrane and experienced two metamorphic stages. Petrogeochemical characteristics of the mafic tectonites indicate that their protoliths are N-MORB and E-MORB basalts. More primitive N-MORB basalts were formed at the initial spreading stages through melting of the upper depleted mantle. Higher Ti basalts were formed by melting of enriched mantle protolith at the later spreading stages. U–Pb zircon age of 701.6 ± 8.4 Ma of the metamorphosed analogues of normal basalts marks the initiation of oceanic crust in the region. Revealed sequence of spreading, subduction (640–620 Ma), and shear deformations (~600 Ma) records the early stages in the evolution of the Paleoasian ocean in its junction zone with the western margin of the Siberian craton: from formation of fragments of oceanic crust to the completion of accretionary–subduction events.     

中国东部新生代玄武岩记录古太平洋俯冲带壳幔相互作用

大陆玄武岩通常具有与洋岛玄武岩相似的地球化学成分,其中含有显著的壳源组分.对于洋岛玄武岩来说,虽然其中的壳源组分归咎于深俯冲大洋板片的再循环,但是对板片俯冲过程中的壳幔相互作用缺乏研究.对于大陆玄武岩来说,由于其形成与特定大洋板片在大陆边缘之下的俯冲有关,可以用来确定古大洋板片俯冲的地壳物质再循环.本文总结了我们对中国东部新生代玄武岩所进行的一系列地球化学研究,结果记录了古太平洋板片俯冲析出流体对地幔楔的化学交代作用.这些大陆玄武岩普遍具有与洋岛玄武岩类似的地球化学成分,在微量元素组成上表现为富集LILE和LREE、亏损HREE,但是不亏损HFSE的分布特点,在放射成因同位素组成上表现为亏损至弱富集的Sr-Nd同位素组成.在排除地壳混染效应之后,这些玄武岩的地球化学特征可以由其地幔源区中壳源组分的性质来解释.俯冲大洋地壳部分熔融产生的熔体提供了地幔源区中的壳源组分,其中包括洋壳镁铁质火成岩、海底沉积物和大陆下地壳三种组分.华北和华南新生代大陆玄武岩在Pb同位素组成上存在显著差异,反映它们地幔源区中的壳源组分有所区别.中国东部新生代玄武岩的地幔源区是古太平洋板片于中生代俯冲至亚欧大陆东部之下时,在>200 km的俯冲带深度发生壳幔相互作用的产物.在新生代期间,随着俯冲太平洋板片的回卷引起的中国东部大陆岩石圈拉张和软流圈地幔上涌,那些交代成因的地幔源区发生部分熔融,形成了现今所见的新生代玄武岩.      

Geochemistry and origin of metabasites from the granulite-gneiss complex of the Angara-Kan block,southwestern Siberian craton

In this paper, we present data on major and trace elements in highly metamorphosed mafic rocks from the granulite-gneiss complex of the Angara-Kan block (southwestern Siberian craton), identify igneous protoliths of the metabasites, and assess the mobility of elements during metamorphism. Two types of rocks with different geologic relations and compositions were recognized. Garnet-bearing two-pyroxene granulites (Cpx + Pl + Grt + Opx) occur as sheet- and boudin-like bodies, which were folded and deformed with their host paragneisses. Dikes, which in most cases underwent only brittle deformation, are composed of metabasites characterized by the assemblage Cpx + Hbl + Pl + Grt. The major element compositions of igneous protoliths for the mafic granulites and metabasite dykes correspond to variously differentiated basaltic magmas. The protoliths of the metabasites are depleted in K₂O, LILE, Zr, Nb, and LREE and were derived from a depleted mantle source. The major and trace element compositions of the dike metabasites are similar to those of low-K tholeiitic basalts of oceanic island arcs. Continental intraplate basalts derived from an enriched mantle source are possible igneous protoliths for the mafic granulites enriched in Ba, LREE, Nb, Ta, Zr, and Hf. It is assumed that lower Rb, Th, and U contents in the mafic granulites compared with continental flood basalts, high K/Rb and La/Th, and moderate Th/U ratios reflect the loss of Rb, Th and U during granulite-facies metamorphism.     

塔里木西南缘棋盘河乡玄武岩锆石U-Pb年代学和地球化学研究

通过对塔西南棋盘河乡尤勒巴斯地区玄武岩进行了LA-ICP-MS 锆石 U-Pb同位素研究。获得锆石²⁰⁶ Pb/²³⁸ U同位素年龄为298.3±2.8 Ma(MSWD=2.6), 代表玄武岩的结晶年龄。本次研究的玄武岩具有高的Ti、Nb: Ti/Y为513.86~577.35、Nb含量为28×10^-6~35.7×10^-6、La/Nb为5.06~6.25以及低的Zr/Nb比值(10~10.86), 表明该玄武岩的形成与富集岩石圈地幔有关。而相对低的Nb/U(近30)和Ce/Pb(近15)比值, 指示研究区玄武岩来自大陆岩石圈或受一定程度的地壳混染。尤勒巴斯地区玄武岩具有高TiO₂和P₂O₅, 富集轻稀土和Rb、Ba, 指示具有地幔柱的地球化学成分特征。基于塔里木地区大规模的火山岩喷发以及富集不相容元素的地球化学特征和岩相古地理特征支持, 塔西南玄武岩可能是由地幔柱火山作用, 或由于地幔柱的供热和上升导致富集的岩石圈地幔部分熔融而形成。     

The last lavas erupted during the main phase of the Siberian flood volcanic province: results from experimental petrology

The final lavas of the Siberian flood basalts are a ∼1,000 m thick section of meimechites, high-alkali, high-titanium, hydrous lavas that contrast sharply with the tholeiites that precede them. This paper presents a phase equilibrium study indicating that a candidate primary meimechite magma with 1 wt% water originated at ∼5.5 GPa and 1,700°C, both hotter and shallower than other estimates for melting beneath continental lithosphere. The experiments also suggest that a higher volatile content was involved in meimechite source genesis. Both the absence of orthopyroxene in any experiment and the close field association with carbonatites suggest that the meimechite source region may have been metasomatized with a CO₂-rich fluid. A small additional quantity of CO₂ and water would move magma origination to ∼1,550–1,600°C.     

Geochemistry of the Wrangellia Flood Basalt Province: Implications for the Role of Continental and Oceanic Lithosphere in Flood Basalt Genesis

The Wrangellia terrane of North America contains a large volumeof Middle to Late Triassic oceanic flood basalts which wereemplaced on top of a preexisting island arc. Nd-, Sr-, and Pb-isotopiccompositions reflect derivation from a plume source with _Nd(T)+6 to + 7, ⁸⁷Sr/⁸⁶Sr_i0•7034, and ²⁰⁶Pb/²⁰⁴Pb_i19•0.Major and trace element compositions suggest the Wrangelliaflood basalts (WFB) formed through relatively small degreesof partial melting at greater depths than estimated for otheroceanic plateaux such as Ontong Java. It appears that the WFBdid not form in a rifting environment, and that preexistingarc lithosphere limited the ascent and decompression meltingof the source plume. Rocks from the preexisting arc are stronglydepleted in high field strength elements (HFSEs) relative tolarge ion lithophile elements (LILEs), but the WFB are not.Assimilation of arc lithospheric mantle or crust was thereforegenerally minor. However, some contamination by arc componentsis evident, particularly in basalts erupted in the early stagesof volcanism. Minor isotopic shifts, to lower _Nd(T) and ²⁰⁶Pb/²⁰⁴Pb_iand higher ⁸⁷Sr/⁸⁶Sr_i, are accompanied by shifts in trace elementratios towards more arclike signatures, e.g. low Nb/Th and Nb/La.Arc contamination is greatest in the most evolved basalts, indicatingthat assimilation was coupled with fractional crystallization.A comparison of the WFB with other continental and oceanic floodbasalts reveals that continental flood basalts generally formthrough smaller degrees of melting than oceanic flood basaltsand that the contribution of material from the crust and litho-sphericmantle is significantly greater. KEY WORDS: oceanic flood basalts; Wrangellia terrane; petrogenesis; Sr-Nd-Pb isotopes ^*Corroponding author     

峨眉山大火成岩省岩石成因与空间差异性研究——基于全区高Ti玄武岩地球化学数据分析与模拟

高Ti玄武岩成因是峨眉山大火成岩省(ELIP)研究的热点问题.由于高Ti玄武岩地球化学特征在空间上存在差异,其岩石成因尚未达成共识.本文系统收集了峨眉山大火成岩省中高Ti玄武岩地球化学数据以及锆石ID-TIMS U-Pb测年结果,并进行统一处理分析与模拟.研究结果显示,峨眉山大火成岩省形成于约259～258 Ma,高T...