Relative contributions of crust and mantle to the generation of the Tianshan Carboniferous rift-related basic lavas, northwestern China

The Tianshan Carboniferous–Permian rift-related volcanism in northwestern China represents a newly recognized large igneous province extending over at least 1.5 × 10⁶ km². The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics, and are interpreted to result from a mantle plume head with component of ε_Nd(t) ≈ +5, ⁸⁷Sr/⁸⁶Sr(t) ≈ 0.704 and La/Nb ≈ 0.9. On the basis of petrogeochemical data, the Carboniferous basic lavas can be generally incorporated into low-Ti/Y (LT, Ti/Y < 500) magma type that can be further divided into three subtypes: LT1, LT2 and LT3. The chemical evolution of the LT1, LT2 (in central Tianshan) and LT3 (in western Tianshan and Jungar) lavas is controlled by an olivine (ol) + clinopyroxene (cpx) fractionation, but gabbroic fractionation accounts for the chemical variation of the LT3 lavas from eastern Tianshan. Elemental and isotopic data suggest that the chemical variation of Tianshan Carboniferous basic lavas cannot be explained by crystallization from a common parental magma.The Sr–Nd isotopic variation of the crustally contaminated LT3 lavas is related to the nature of lithosphere through which the plume-derived melts have erupted. The involvement of an older (Precambrian) lithosphere led the LT3 lavas in western Tianshan to have lower to negative ε_Nd(t) (−1.2 to +6.1) and variable ⁸⁷Sr/⁸⁶Sr(t) (0.7036–0.7061), whereas the LT3 lavas from eastern Tianshan and Jungar are characterized by high ε_Nd(t) (+4.2 to +9.7) and low ⁸⁷Sr/⁸⁶Sr(t) (0.7035–0.7044), that are related to the contamination of upper crust containing early Paleozoic and Devonian arc-basin volcanic rocks and/or to a pre-Carboniferous subduction enrichment of the lithospheric mantle source region. The observed geochemical variations in the Tianshan data are consistent with an AFC process.The Tianshan Carboniferous rift-related volcanic rocks display a spatial petrogeochemical variation in which predominantly uncontaminated LT1 and less-contaminated LT2 tholeiitic lavas erupted in central Tianshan rift and predominantly the strongly contaminated LT3 tholeiites erupted in the circumjacent regions of the central Tianshan rift. The LT1 and LT2 lavas were generated by a higher degree (10–30%) of partial melting in the garnet stability field of the mantle plume compared to the LT3 lavas. The lower degree (<10%) of partial melting in the spinel–garnet transition zone of the mantle plume, as is characteristic of the LT3 lavas, may be the result of a relatively lower geotherm.     

华南新元古代中期裂谷火山岩系：Rodinia超大陆裂谷化-裂解的地质纪录

华南新元古代中期（746-827Ma）双峰式（玄武岩-流纹岩）火山岩喷发于大陆板内裂谷环境。它们极有可能与导致Rodinia超大陆裂谷化-裂解的地幔柱（或超级地幔柱）活动有关。根据岩石地球化学数据,华南新元古代中期裂谷基性熔岩可以划分为高Ti／Y（HT,Ti／Y〉500）和低Ti／Y（LT,Ti／Y〈500）两个岩浆类型。HT熔岩又可进一步划分为HT1和HT2等两个亚类。HT1熔岩主要分部于华南中-西部裂谷盆地之中,总体上属于碱性玄武质岩浆系列;HT2和LT熔岩主要分布于华南中-东部裂谷盆地之中,总体上属于拉斑玄武质岩浆系列。元素和同位素数据表明,华南新元古代中期裂谷基性熔岩的化学变化不是由一个共同的母岩浆结晶分异作用所产生。华南中-西部地区裂谷基性熔岩的母岩浆经受了辉长岩质结晶分离作用,而华南中-东部地区裂谷基性熔岩的化学演化则是受控于单斜辉石（cpx）士橄榄石（01）结晶分离作用。各个双峰式火山岩系中,基性和酸性熔岩间为分异结晶关系。华南新元古代中期裂谷火山岩系极有可能是源于共同的地幔柱,该地幔柱组分的成分为：eNd（f）≈＋6,Mg#≈0．7,La／Nb≈0．7。华南新元古代中期裂谷基性熔岩存在空间上的地球化学变化：华南中一西部HT1熔岩的母岩浆,没有受到明显的大陆岩石圈混染,保存了鲜明的地幔柱信号;而大陆地壳或大陆岩石圈混染作用对于华南中-东部LT和HT2熔岩的形成则有着重要贡献。研究揭示,华南新元古代中期裂谷基性熔岩的母岩浆总体上产生于上涌地幔柱较深层位的石榴子石稳定区（深度：100～130km）。中-西部裂谷基性熔岩的母岩浆（碱性玄武质）产生于深度较大（～130km）、部分熔融程度较低（〈10％）的条件下,中-东部裂谷基性熔岩的母岩浆（拉斑玄武质）产生于深度稍浅（～100km）?     

峨眉山大火成岩省：地幔柱活动的证据及其熔融条件

对苦橄岩中橄榄石斑晶及其中熔体包裹体的电子探针分析表明，峨眉山大火山岩省的原始岩浆具高镁（ MgO > 16％）特征。玄武岩的 REE反演计算揭示，参与峨眉山玄武岩岩浆作用的地幔具有异常高的潜能温度（ 1 550℃）。这些特征以及峨眉山玄武岩的大面积分布和一些熔岩所显示的类似于洋岛玄武岩 (OIB)的微量元素和 Sr- Nd同位素特征均为地幔热柱在能量和物质上参与峨眉山溢流玄武岩的形成提供了确凿证据。峨眉山两个主要岩类（高钛和低钛玄武岩）可能是不同地幔源区物质在不同条件下的熔融产物。低钛玄武岩形成于温度最高、岩石圈最薄的地幔柱轴部。地幔（ ISr≈ 0.705,ε Nd(t)≈＋ 2）熔融始于 140 km，并一直延续到较浅的深度（ 60 km,尖晶石稳定区 ),部分熔融程度为 16％，这类岩石可能代表了峨眉山玄武岩的主体。而高钛玄武岩的母岩浆的形成基本局限在石榴子石稳定区（ > 70 km），其源区特征为 : ISr≈ 0.704,ε Nd(t)≈＋ 5，可能代表了热柱边部或消亡期地幔小程度部分熔融（ 1.5％）的产物。     

碧口群火山岩岩石成因研究

新元古代(846~776Ma)碧口群火山岩喷发于大陆板内裂谷环境。该火山岩系以基性火山岩为主,酸性火山岩次之,中性火山岩少见。根据岩石地球化学数据,碧口群裂谷基性熔岩总体上属于低Ti/Y(<500)岩浆类型。元素和同位素数据表明,碧口群基性熔岩的化学变化不是由一个共同的母岩浆的结晶分异作用所产生。它们极有可能是源于地幔柱源(εNd(t)≈+3,87Sr/86Sr(t)≈0.704,La/Nb≈0.7)。地壳混染作用对于碧口群裂谷基性熔岩的形成有重要贡献。我们的研究揭示,碧口群火山岩存在空间上的岩石地球化学变化。东部红岩沟和辛田坝—黑木林地区的碧口群基性熔岩以拉斑玄武岩为主,产生于幔源石榴子石稳定区的高度部分熔融。相反,西部白杨—碧口地区的碧口群基性熔岩的母岩浆则是形成于幔源的尖晶石-石榴子石过渡带:碱性熔岩是产生于部分熔融程度较低的条件下,拉斑玄武质熔岩则是产生于部分熔融条件较高的条件下。它们经受了浅层位辉长岩质(cpx+plag±ol)分离作用,化学变异较大。     

Petrogenetic Evolution of the Bayan Gol Ophiolite— Geological Record of an Early Carboniferous "Red Sea Type"Ocean Basin in the Tianshan Mountains, Northwestern China

The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous “Red Sea type” ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcanosedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous tiffing and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2)younger (late Early Carboniferous, -324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).     

青藏高原古新世一始新世早期（65-40Ma）火山岩——同碰撞火山作用的产物

印度-亚洲大陆的碰撞开始于65Ma左右,大约在45／40Ma完成,之后转入碰撞后阶段至今。碰撞过程（～65～40Ma）中,已消减的新特提斯大洋板片回转,不仅导致会聚速率提高,还诱使青藏岩石圈之下的对流软流圈上涌,并发生减压熔融,产生碰撞期（或同碰撞）火山作用。西藏中部和南部的古新世一始新世早期（～65～40Ma）火山岩即是此碰撞期（或同碰撞）火山作用的产物。该碰撞期（或同碰撞）火山岩系并非是单一的长英质中酸性火山岩,其成分变化很宽．从玄武质到流纹质均有发育。它们源于成分为ENd（t）≈＋3、87Sr／88Sr（f）≈0．705和La／Nb≈0．8的软流圈源。根据岩石地球化学数据,古新世一始新世早期基性熔岩可以划分为高Ti／Y（HT,Ti／Y≥500）和低Ti／Y（LT,Ti／Y〈500）两个岩浆类型。LT熔岩又可以进一步划分为LT1和LT2等两个亚类。HT和LT1熔岩为未遭受地壳混染的基性熔岩,以具有高Nb／La值（0．88～1．53）和原始地幔标准化分配曲线上缺乏Nb、Ta和Ti负异常为特征;而LT2熔岩却为受到了强烈地壳混染的基性熔岩．其Nb／La值很低（O．20～0．49）,Nb、Ta和Ti明显亏损。西藏中部拉嘎拉玄武岩和邦达错碱性玄武岩的化学演化受控于橄榄石（ol）＋单斜辉石（cpx）结晶分离作用;而西藏南部林子宗火山岩系的化学变异则是经受了辉长质结晶分离作用。元素和同位素数据表明．青藏高原古新世一始新世早期基性熔岩并不是单一母岩浆结晶分离的产物。遭受地壳混染的LT2熔岩的Sr—Nd同位素变化特点与其软流圈源熔体上升过程中所卷入的不同岩石圈组分有关。下地壳组分的卷入导致典中组、帕那组和拉嘎拉玄武岩的LT2熔岩具有低-负εNd（t）值（＋1．3～-3．9）和较低87St／86Sr（f）值（0．7046～-0．7065）;而达孜基性火山岩和年波组的LT2?     

云南宾川地区峨眉山玄武岩地球化学特征:岩石类型及随时间演化规律

分布于云南宾川地区、厚逾5000m的晚二叠世峨眉山玄武岩以拉斑玄武岩为主,少量碱性玄武岩。根据它们的岩相学和主量元素、微量元素特征,将其划分为两个地球化学类型:低钛玄武岩(LT)和高钛玄武岩(HT)。低钛玄武岩主要分布于岩石剖面的中下部,其主要地球化学标志为低Ti/Y(<500),高Mg^#(048～067),低的∑REE(<120μg/g)、(La/Yb)_N(<6)和(Sm/Yb)_N(<25)。高钛玄武岩分布于剖面的顶部,以高Ti/Y(>500),低Mg^#(039～053)、高的∑REE(>150)、(La/Yb)_N(>9)和(Sm/Yb)_N(>3)为特征。根据LT的分异指数和Th、U异常特征等,将其分为LT1和LT2两个亚类。其中LT1位于岩石剖面下部,表现为明显的Th和U正异常,高Mg^#(061～067)和低Nb/U比值等。LT2位于岩石剖面中部,具Rb、Ba正异常,无Th和U异常和较低的分异指数(Mg^#=048～054)。LT玄武岩可能是峨眉地幔热柱主活动期地幔柱头部熔融的产物,后经较弱的橄榄石+单斜辉石±斜长石结晶分异形成。剖面最底部的LT1玄武岩显示较强的“壳源”印记,可能与岩石圈地幔中富集组份的活化有关。这些壳源物质的参与程度自底部向上有降低的趋势。晚期HT玄武岩为地幔热柱消亡期的产物,其在地壳浅部经历强烈的以斜长石为主的结晶分异,壳源物质的混染不明显。     

A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic–ultramafic intrusions in the Emeishan Large Igneous Province, SW China

Based on published data, we reappraise the classification of high-Ti and low-Ti basalt from the Emeishan large igneous province (ELIP) and the correlations between basalts and mafic–ultramafic intrusions. Because of the lack of clear spatial and temporal variations of different types of basalts, we suggest that the basalts in the ELIP cannot be classified into high-Ti and low-Ti groups, by TiO₂ contents and/or Ti/Y ratios. The distinctive characteristics of these high-Ti and low-Ti lavas probably result largely from the different fractionating assemblages. Whether or not fractional crystallization of the Fe–Ti oxides occurred probably is the key factor that controls the Ti abundances and Ti/Y ratios in the residual melts, e.g., lavas, although the nature of the mantle sources, variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas. Therefore, neither Ti abundance nor Ti/Y ratios in basalts can reflect the nature of their mantle source. Moreover, the different types of mafic–ultramafic intrusions in the ELIP cannot simply be attributed to be genetically related special types of basalts, either high-Ti or low-Ti basalts. It is likely that they are merely the cumulus phases, i.e. chamber or conduit of the basaltic lavas. Hence, caution should be exercised in the use of high-Ti or low-Ti basalts as prospecting vectors for ore deposits in the region. Potential implications are proposed that both the Fe–V–Ti oxide and Cu–Ni–(PGE) sulfide mineralization in the ELIP intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes.     

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     

Experimental study of interaction between fluid-bearing basaltic melts and peridotite: A mantle-crustal source of trap magmas in the Norilsk area

The paper reports data on the geology and tectono-magmatic reactivation of the Norilsk area and on the stratigraphy and geochemistry of its volcanic sequence, with the discussion of the sources and genesis of the ore magmas and the scale of the ore-forming process. According to the geochemistry of the lavas and intrusive rocks (Ti concentration and the La/Sm and Gd/Yb ratios), two types of the parental magmas are recognized: high-Ti magmas of the OIB type (from bottom to top, suites iv, sv, and gd of phase 1) and low-Ti magmas (suites hk, tk, and nd of phase 2 and suites mr-mk of phase 3), which were derived from the lithospheric mantle. The magmatic differentiation of the parental low-Ti magma of the tk type into a magma of the nd type was associated with the derivation of an evolved magma of the nd type, which was depleted in ore elements, and an ore magma, which was a mixture of silicate and sulfide melts, protocrysts of silicate minerals, and chromite. Judging from their geochemical parameters, the intrusions of the lower Norilsk type were comagmatic with the lavas of the upper part of the nd suite, and the ore-bearing intrusions of the upper Norilsk type were comagmatic with the lavas of the mr-mk suites. When the ore-bearing intrusions were emplaced, their magmas entrained droplets of sulfide melt and protocrysts of olivine and chromite and brought them to the modern magmatic chamber. These protocrysts are xenogenic with respect to the magma that formed the intrusions. In certain instances (Talnakh and Kharaelakh intrusions), the moving magma entrained single portions of sulfide magma, which were emplaced as individual subphases. The experimental study of the peridotite-basalt-fluid system shows that mantle reservoirs with protoliths of subducted oceanic crustal material could serve as sources of relatively low-temperature (1250–1350°C) high-Ti magnesian magmas of the rifting stage from an olivine-free source.     

U-Pb geochronology of Paraná volcanics combined with trace element geochemistry of the zircon crystals and zircon Hf isotope data

The Paraná volcanic province is a window into mantle and crustal processes in the Cretaceous. The variability and complexity of this province can be determined through the study of minerals. An integrated study of zircon from Paraná lavas (one high-Ti basalt, one low-Ti andesite, one high-Ti rhyodacite and one high-Ti andesite sill) was achieved using backscattered electron imaging, sensitive high resolution ion microprobe (SHRIMP-IIe) for U-Pb geochronology, and laser ablation inductively coupled mass spectrometer (LA-ICPMS) to determine the Lu-Hf isotopes and the trace-element compositions. U-Pb-Hf and trace-element data indicate that zircon crystallized from the magma at approximately 134 Ma. This South American large igneous province originated from the mantle and was contaminated by crust either in the mantle or during ascent and crystallization of magma. Contaminant continental crust had Precambrian age. Trace elements point to a new compositional field for zircon, different from other types of provinces. Examination of volcanic zircon improves our understanding of age and contamination of the Paraná volcanics.     

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units,Greece: petrogenesis and geodynamic implications

ABSTRACT

Pillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.     

河南信阳地区龟山岩组新元古代变质玄武岩的地球化学特征、Sr-Nd同位素组成及地质意义

对信阳地区商丹断裂带南侧龟山岩组新元古代变质玄武岩进行了岩石学、地球化学及Sr-Nd同位素研究,分析结果显示该套玄武岩为亚碱性拉斑玄武系列,分为低Ti及高Ti两种类型：低Ti型较富Mg,不相容元素富集程度及稀土分馏程度较低,具有E-MORB的微量元素地球化学特征,Sr-Nd同位素组成相对富集,可能来自地幔柱引发的岩石圈地幔的部分熔融,并受到一定程度的地壳混染;高Ti型较富Fe,强烈富集不相容元素,具有OIB的地球化学特征,Sr-Nd同位素组成较为亏损,可能来自地幔柱的部分熔融,并较少受到地壳物质的影响。综合构造判别显示该套玄武岩可能为地幔柱伸展背景下的岩浆活动产物,可能为区域上沿商丹断裂带分布的中—新元古代局部伸展背景岩浆活动产物的组成部分。     

Petrogenesis of the Flood-Basalt Sequence at Noril'sk,North Central Siberia

The 3500-m-thick sequence of volcanic rocks at Noril'sk, formed during a brief interval (～1 m.y.) at the Permian/Triassic time boundary (～251 Ma), represents the earliest part of the ～6500-m-thick sequence presently ascribed to the Siberian flood-basalt province. It is composed of picritic and basaltic lavas of both low-Ti and high-Ti parentage. Extensive geological, geochemical, and isotopic study of the lava sequence and related intrusions allows detailed reconstruction of its petrogenesis. Various crustal-related processes-fractionation, crustal contamination, sulfide separation, and magma mixing-participated in the formation of the lavas. The geochemical and isotopic characteristics indicative of these processes, as well as mantle-related signatures of lava compositions, are discussed. Based on these characteristics, detailed interpretations of lava genesis and evolution throughout the Noril'sk sequence are presented. Eight varieties of lavas are recognized to be primitive, similar in composition to primary mantle melts; they varied from low-Mg basalts to olivine tholeiites or picrites, with normal tholeiites predominating.

The primitive lavas are subdivided into four groups (magma types) on the basis of trace-element ratios (principally, Gd/Yb, Th/U, La/Yb, Ta/La, Ti/Sc, and V/Yb) and isotopic data. Three of the groups include both basaltic and picritic primitive lavas (with low-Mg basalts present in one of them), whereas the fourth group is represented exclusively by tholeiites. Distinctions among the groups cannot be related to degree of melting, and isotopic data indicate that none of the magma types could have formed by mixing or contamination of other types. Apparently, only differences in source composition and/or depth of melting can explain the magmatic diversity.

This multitude of primitive magma types may be explained by melting in different layers of the upper mantle, which is complexly layered beneath Siberia to depths of 270 km. Moreover, no clear boundary between lithosphere and asthenosphere is evident in the deep seismic profile. A large-scale event is necessary to account for melting in different parts of the upper mantle and formation of the great volume of the Siberian flood basalts in ～1 m.y. Extension, caused by ascent of a mantle plume, would provide a reasonable explanation, but no plume-related uplift is documented in north-central Siberia prior to, or during, eruption of the volcanic sequence.     

Petrogenesis and geodynamic implications of the Late Carboniferous felsic volcanics in the Bogda belt,Chinese Northern Tianshan

An integrated study on petrology and geochemistry has been carried out on the Late Carboniferous I-type felsic volcanics of the Liushugou Formation in the Bogda belt to constrain the late Paleozoic tectonic evolution of the Bogda belt. The felsic volcanics were dated to be 315 to 319 Ma and are composed of trachy-andesite–trachyte ignimbrites and rhyolite lavas. They are in conformable contact with high-Al basalt. The eruption of the felsic volcanics and high-Al basalt is not bimodal volcanism, but is related to bimodal magma (basaltic and rhyolitic magmas). MELTS modeling and comparison with previous basaltic melting experiments indicate that the felsic volcanics are likely produced by partial melting of hydrated mafic crust rather than fractional crystallization of high-Al basalt. It is also supported by relatively large amounts of felsic volcanics to high-Al basalts and remarkably different incompatible element ratios (e.g., Th/Zr, Nb/Zr and U/Zr) of the rocks. The Bogda felsic volcanics have positive ε_Nd(t) values (6.2–7.4), low Pb isotopes and low zircon saturation temperatures, consistent with a derivation from a juvenile crust in an arc setting. The intermediate ignimbrites display melting–mingling textures and abundant feldspar aggregates and have various δEu ratios, indicating that magma mingling and feldspar fractionation processes may have played an important role in the genesis of the ignimbrites. In contrast, the Early Permian felsic rocks in this region are of post-collisional A-type. We therefore propose that the Bogda belt was an island arc in the Late Carboniferous and then switched to a post-collisional setting in the Early Permian due to the arc–arc collision at the end of the Late Carboniferous.     

Origin of Late Palaeoproterozoic Great Vindhyan basin of North Indian shield: Geochemical evidence from mafic volcanic rocks

On the western and southern margins of the sickle shaped Vindhyan basin of north Indian shield, there are basal Vindhyan mafic volcanic rocks referred to as Khairmalia volcanics and Jungel volcanics respectively. These volcanics vary in composition from low-Ti tholeiite to high-Ti alkali basalt showing close affinity with continental flood basalts (CFB) and ocean island basalts (OIB) respectively. The parental magmas of Khairmalia and Jungel alkali basalts were formed by different degrees of partial melting of a garnet lherzolite. The magma of Khairmalia tholeiites was generated by a relatively higher degrees of partial melting of a garnet + spinel lherzolite. The geochemical data coupled with available geological and geophysical data favour a rift type origin of this basin which evolved as a peripheral basin showing many similarities with Paleogene Himalayan foreland basin. The existing radiometric age data suggest that the origin of Vindhyan basin is linked with Aravalli–Satpura orogeny. At about 1800–1600 Ma collision occurred along the Aravalli-Delhi fold belt (ADFB) and Central Indian Tectonic Zone (CITZ) with west and south subduction respectively. During this process the subducting lithosphere suffered extensional deformation on its convex side and some pre-existing large faults in the already thin leading edge of subducted plate also reactivated and tapped magma generated by decompressional melting of the subcontinental mantle. The simultaneous processes such as flexural subsidence, reactivation of pre-existing faults, heating, thermal cooling and contraction during volcanism, resulted in the formation of curvilinear warp parallel to the emerging mountain front. The Lower Vindhyan volcano–sedimentary succession was deformed and exposed to erosion before the deposition of Upper Vindhyan rocks. The orogenic forces were active intermittently throughout the Vindhyan sedimentation.     

南秦岭中段西乡群火山岩岩石成因

南秦岭中段新元古代中期（730-845Ma）西乡群（自下而上包括孙家河组、大石沟组和白勉峡组）火山岩喷发于大陆板内裂谷环境。它们极有可能与导致Rodinia超大陆裂谷化裂解的地幔柱活动有关。根据岩石地球化学数据．南秦岭中段新元古代中期西乡群裂谷基性熔岩总体上属于低Ti／Y（LT,Ti／Y〈500）岩浆类型。LT熔岩又可进一步划分为LT1和LT2等2个亚类。LT1熔岩以高Nb／La（0．87～0．98）、低Thw／NbN（≈1）、缺乏Nb—Ta和Ti的亏损、具有“大隆起”式微量元素原始地幔标准化分配型式、（^87SrSr^86Sr）（t）=0．703869、εNd（t）=4．83为特征,属于拉斑玄武质岩浆系列;LT2熔岩以低Nb／La（〈0．75）、高ThN／NbN（〉1．4）、Nb—Ta和Ti亏损明显和Sr—Nd同位索比值变化较大为特征。元素和同位素数据表明,西乡群裂谷火山岩的化学变化不是由一个共同的母岩浆结晶分异作用所产生。孙家河组、大石沟组和自勉峡组中TiO2含量大于1．09％的火山岩的母岩浆经受了辉长岩质结晶分离作用。而白勉峡组中TiO2含量小于0．69％的基性熔岩的化学演化则是受控于单斜辉石（cpx）±橄榄石（ol）结晶分离作用。西乡群火山岩系中,基性、中性和酸性熔岩间为分异结晶关系。南秦岭中段新元古代中期西乡群裂谷火山岩系极有可能是源于共同的地幔柱,该地幔柱组分的成分为;εNd（t）≈＋5,^87Sr／^86Sr（t）≈0．704,La／Nb≈0．7。南秦岭中段新元古代中期西乡群裂谷基性熔岩存在空间上的地球化学变化：LT1熔岩的母岩浆,没有受到明显的大陆岩石圈混染,保存了鲜明的地幔柱信号;而大陆地壳或大陆岩石圈混染作用对于LT2熔岩的形成则有着重要贡献。研究揭示,南秦岭中段新元古代中期西乡群裂谷基性熔岩的母岩浆总体上产生于上涌地幔柱上部层位（地幔柱头）3GPa?     

北祁连山奥陶纪弧后盆地火山岩浆成因

本文对北祁连山早古生代弧后盆地熔岩的岩石地球化学研究结果加以报道。样品的分布将南部弧后盆地拉伸最早阶段发育的岛弧裂谷化区和北部的弧后海底扩张区联系起来。熔岩的岩相学和地球化学特点反映了拉伸方式的改变,北部是典型的弧后盆地基性熔岩,向南则逐渐向岛弧熔岩过渡。海底扩张区以玻质(现已脱玻化)、少斑基性熔岩为特征,长英质熔岩和斑状基性熔岩产于南部岛弧裂谷化区。成熟岛弧部分(Y<20×10-6,TiO2<0.60%,Th/Yb>0.60)和弧后扩张区(Y>20×10-6,TiO2>1.0%,Th/Yb<0.60)在地球化学上相互有别。从由海底扩张形成的弧后盆地基性熔岩,向南经过逐渐与岛弧岩石相似的熔岩,直至裂谷区最南部的岛弧熔岩,它们的地球化学成分显示逐渐的变化。这种变化反映了弧后盆地形成过程中弧后盆地之下地幔对流方式和熔体产生作用的改变:从初始岛弧裂谷之下由消减板片俯冲引起的地幔下沉,转变为弧后海底扩张带之下的地幔上隆。早期岛弧裂谷阶段,裂谷轴捕获了岛弧岩浆流,从而使得喷出的熔岩在成分上与岛弧熔岩无法区分;随着弧后拉张继续,弧后盆地变宽,岛弧岩浆流逐渐离开裂谷轴,最终产生一个似洋中脊的减压熔融系统———弧后盆地岩浆系统。     

Geochemistry and Petrogenesis of the Proterozoic Bandal Mafic Rocks,Himachal Pradesh,NW Himalaya

The Proterozoic Bandal mafic rocks, exposed in Kullu-Rampur window, Lesser Himalaya, Himachal Pradesh, indicate two distinct (high-Ti and low-Ti) magma types. The high-Ti basalts are characterised by high-TiO₂ (> 2 wt%), Ti/Y, Ti/Zr, TiO₂/K₂O and low Rb/Sr ratios. They are enriched in high field strength (HFS) elements (Nb, Zr, Ti) relative to low field strength (LFS) incompatible elements (K, Rb). The low-Ti basalts are charactersied by low TiO₂ (< 2 wt%), Ti/Y, Ti/Zr and high Rb/Sr and Rb/Ba ratios. Quartz-normative composition, continental tholeiite characteristics with Nb/La less than 1 are some of the common factors of the two groups of the Bandal mafic rocks. The trace element concentrations and their ratios of the two groups of the basalts indicate that they have been derived from the asthenosphere at different depths, low-Ti at shallow and high-Ti at deeper levels. Some of the chemical features like low Mg #, Cr, Ni, high incompatible element concentrations (especially Ba), light rare earth element (LREE) enriched patterns point towards assimilation and fractional crystallisation (AFC) process which may have played a significant role in the generation of these basalts.Furthermore, the Bandal mafic rocks, apart from field settings, are geochemically similar to other Proterozoic mafic bodies like the Rampur volcanics, Mandi-Darla volcanics, Garhwal volcanics and Bhimtal-Bhowlai volcanics of the Lesser Himalaya. This widespread Proterozoic continental tholeiitic magmatism over an area of 170,000 km² in the Lesser Himalaya provides an evidence of plume activity in the region.     

新疆托里蛇绿岩中两种不同玄武岩的地球化学特征及成因

托里蛇绿岩中分布着高Ti玄武岩与低Ti玄武岩,在地球化学特征上存在明显的差异;表明二者之间不存在过渡关系,有着不同的形成条件。它们分别代表岛弧拉张不同阶段之产物,本文讨论了玄武岩的地球化学特征,从而揭示了蛇绿岩形成于弧后盆地的微扩张环境。