Mantle plume,large igneous province and continental breakup——Additionally discussing the Cenozoic and Mesozoic mantle plume problems in East China

Based on the former workers' study results such as numerical simulation of fluid mechanics, seismic tomography of the whole earth and igneous rocks, the basie characteristics of mantle plumes are summarized in detail, namely the mantle plume, from the D" layer near the core-mantle boundary (CMB) of 2900 km deep, is characterized by the shape of large head and thin narrow conduit, by the physical property of high temperature and low viscosity. The LIP (large igneous province) is the best exhibition when the mantle plume ascends to the surface. According to the basie characteristics of the mantle plumes and the LIP, as well as the temporal-spatial relationships between the mantle plume and Continental breakup, the detailed research on petrology, geochemistry, temporal-spatial distribution, tectonic background of the Cenozoic-Mesozoic igneous rocks and gravity anomaly distribution in East China has been done. As a result, the Mesozoic igneous rocks in Southeast China should not be regarded as an example of typical LIP related to mantle plumes, for their related characteristics are not consistent with those of the typical LIPs related to mantle plumes. The Cenozoic igneous rocks in Northeast China have no the typical characteristics of mantle plumes and hotspots, so the Cenozoic volcanism in Northeast China might have no the direct relationships with the activity of mantle plumes.

Plume-lithosphere interaction in the Comei Large Igneous Province: Evidence from two types of mafic dykes in Gyangze, south Tibet, China

Two suites of mafic dykes,T1193-A and T1194-A,outcrop in Gyangze area,southeast Tibet.They are in the area of Comei LIP and have indistinguishable field occurrences with two other dykes in Gyangze,T0902 dyke with 137.7±1.3 Ma zircon age and T0907 dyke with 142±1.4 Ma zircon age reported by Wang YY et al.(2016),indicating coeval formation time.Taking all the four diabase dykes into consideration,two different types,OIB-type and weak enriched-type,can be summarized.The“OIB-type”samples,including T1193-A and T0907 dykes,show OIB-like geochemical features and have initial Sr-Nd isotopic values similar with most mafic products in Comei Large Igneous Provinces(LIP),suggesting that they represent melts directly generated from the Kerguelen mantle plume.The“weak enriched-type”samples,including T1194-A and T0902 dykes,have REEs and trace element patterns showing withinplate affinity but have obvious Nb-Ta-Ti negative anomalies.They show uniform lowerεNd(t)values(−6‒−2)and higher 87Sr/86Sr(t)values(0.706‒0.709)independent of their MgO variation,indicating one enriched mantle source.Considering their closely spatial and temporal relationship with the widespread Comei LIP magmatic products in Tethyan Himalaya,these“weak enriched-type”samples are consistent with mixing of melts from mantle plume and the above ancient Tethyan Himalaya subcontinental lithospheric mantle(SCLM)in different proportions.These weak enriched mafic rocks in Comei LIP form one special rock group and most likely suggest large scale hot mantle plume-continental lithosphere interaction.This process may lead to strong modification of the Tethyan Himalaya lithosphere in the Early Cretaceous.

Alkali feldspar syenites with shoshonitic affinities from Chhotaudepur area: Implication for mantle metasomatism in the Deccan large igneous province

Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.

Crustal velocity structure in the Emeishan large igneous province and evidence of the Permian mantle plume activity

The Emeishan large igneous province(ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts were provided by geological and geochemical methods, but the geophysical investigation is very limited. In order to better understand the deep structure and features of ELIP, we have studied the crustal velocity structure using the data acquired from the Lijiang-Panzhihua-Qingzhen wide-angle seismic profile. This profile crosses the three sub-zones of the ELIP(the inner, intermediate, and outer zones), divided based on the differential erosion and uplift of the Maokou limestone. The results provided by the active source seismic experiment demonstrate:(1) The average depth of the crystalline basement along the profile is about 2 km.(2) The middle crust in the Inner Zone is characterized by high-velocity anomalies, with the average velocity of 6.2-6.6 km/s, which is about 0.1– 0.2 km/s higher than the normal one. The velocity of the lower crust in the inner zone is 6.9-7.2 km/s, higher than those observed in the intermediate and outer zones(6.7-7.0 km/s). Relatively low velocity anomalies appear in the upper, middle and lower crusts near the junction of the inner zone and intermediate zone, probably due to the effect of the Xiaojiang fault(XJF).(3) The average velocity of the crust is comparatively low on both sides of XJF, especially on the east side, and the average velocity of the consolidated continental crust is also low there. This may suggest that the XJF extends at least down to 40 km deep, even beyond through the crust.(4) The depth to the Moho discontinuity decrease gradually from 47-53 km in the inner zone, via 42-50 km in the intermediate zone to 38-42 km in the outer zone. In the inner zone, the Moho uplifts locally and the(consolidated) crust is characterized by high-velocity anomalies, which are likely related to intensive magma intrusion and underplating associated with melting of plume head. Overall the crustal velocity structure in the study area recorded the imprint left by the Permian Emeishan mantle plume.

The Emeishan large igneous province:A synthesis

The late Permian Emeishan large igneous province （EL1P） covers -0.3× 10-6 kmL of the western margin of the Yangtze Block and Tibetan Plateau with displaced, correlative units in northern Vietnam （Song Da zone）. The ELIP is of particular interest because it contains numerous world-class base metal deposits and is contemporaneous with the late Capitanian （-260 Ma） mass extinction. The flood basalts are the signature feature of the ELIP but there are also ultramafic and silicic volcanic rocks and layered mafic- ultramafic and silicic plutonic rocks exposed. The EL1P is divided into three nearly concentric zones （i.e. inner, middle and outer） which correspond to progressively thicker crust from the inner to the outer zone. The eruptive age of the ELIP is constrained by geological, paleomagnetic and geochronological evidence to an interval of 〈3 Ma. The presence of picritic rocks and thick piles of flood basalts testifies to high temperature thermal regime however there is uncertainty as to whether these magmas were derived from the subcontinental lithospheric mantle or sub-lithospheric mantle （i.e. asthenosphere or mantle plume） sources or both. The range of Sr （Isr ≈ 0.7040-0.7132）, Nd （ENd（t） ≈ -14 tO ＋8）, Pb （206-pb/204-pb1 ≈ 17.9-20.6） and Os （Yos ≈ -5 to ＋11） isotope values of the ultramafic and mafic rocks does not permit a conclusive answer to ultimate source origin of the primitive rocks but it is clear that some rocks were affected by crustal contamination and the presence of near-depleted isotope compo- sitions suggests that there is a sub-lithospheric mantle component in the system. The silicic rocks are derived by basaltic magmas/rocks through fractional crystallization or partial melting, crustal melting or by interactions between mafic and crustal melts. The formation of the Fe-Ti-V oxide-ore deposits is probably due to a combination of fractional crystallization of Ti-rich basalt and fluxing of C02-rich fluids whereas the Ni-Cu-（PGE） deposits are related to crystallization and crustal contamination of mafic or ultramafic magmas with subsequent segregation of a sulphide-rich portion. The ELIP is considered to be a mantle plume-derived LIP however the primary evidence for such a model is less convincing （e.g. uplift and geochemistry） and is far more complicated than previously suggested but is likely to be derived from a relatively short-lived, plume-like upwelling of mantle-derived magmas. The emplacement of the ELIP may have adversely affected the short-term environmental conditions and contributed to the decline in biota durin~ the late Caoitanian.

Periodicities in the emplacement of large igneous provinces through the Phanerozoic:Relations to ocean chemistry and marine biodiversity evolution

Large igneous provinces （LIPs） are considered a relevant cause for mass extinctions of marine life throughout Earth＇s history. Their flood basalts and associated intrusions can cause significant release of SO4 and CO2 and consequently, cause major environmental disruptions. Here, we reconstruct the long-term periodic pattern of LIP emplacement and its impact on ocean chemistry and biodiversity from δ34Ssulfate of the last 520 Ma under particular consideration of the preservation limits of LIP records. A combination of cross-wavelet and other time-series analysis methods has been applied to quantify a potential chain of linkage between LIP emplacement periodicity, geochemical changes and the Phanerozoic marine genera record. We suggest a mantle plume cyclicity represented by LIP volumes （V） of V= （350-770） × 103km3sin（27πt/ 170 Ma）＋ （300-650）× 103 km3 sin（2πt/64.5 Ma ＋ 2.3） for t= time in Ma. A shift from the 64.5 Ma to a weaker -28-35 Ma LIP cyclicity during the Jurassic contributes together with probably independent changes in the marine sulfur cycle to less ocean anoxia, and a general stabilization of ocean chemistry and increasing marine biodiversity throughout the last -135 Ma. The LIP cycle pattern is coherent with marine biodiversity fluctuations corresponding to a reduction of marine biodiversity of -120 genera/Ma at 600 x 103 km3 LIP eruption volume. The 62-65 Ma LIP cycle pattern as well as excursion in -34Ssulfate and marine genera reduction suggest a not-vet identified found LIP event at - 440-450 Ma.

Time series analysis of mantle cycles Part Ⅱ:The geologic record in zircons, large igneous provinces and mantle lithosphere

Igneous and detrital zircons have six major U/Pb isotopic age peaks in common(2700 Ma,1875 Ma.1045 Ma,625 Ma,265 Ma and 90 Ma).For igneous rocks,each age peak is comprised of subpeaks with distinct geographic distributions and a subpeak age range per age peak≤100 Myr.There are eight major LIP age peaks(found on≥10 crustal provinces)of which only four are in common to major detrital zircon age peaks(2715 Ma,1875 Ma,825 Ma,90 Ma).Of the whole-rock Re depletion ages,58%have correspo nding detrital zircon age peaks and 55%have corresponding LIP age peaks.Ten age pea ks are fou nd in common to igneous zircon,detrital zircon,LIP,and Re depletion age time series(3225 Ma,2875 Ma,2145 Ma,2085 Ma,1985 Ma,1785 Ma,1455 Ma,1175 Ma,825 Ma,and 90 Ma).and these are very robust peaks on a global scale as recorded in both crustal and mantle rocks.About 50%of the age peaks in each of these time series correspond to predicted peaks in a 94-Myr mantle cycle,including four of the ten peaks in common to all four time series(2875 Ma,1785 Ma,825 Ma and 90 Ma).Age peak widths and subpeak ranges per age peak suggest that mantle events responsible for age peaks are<100 Myr and many<50 Myr in duration.Age peak geographic distributions show three populations(≤1000 Ma,2500-1000 Ma,>2500 Ma),with the number of new provinces in which age peaks are represented decreasing with time within each population.The breaks between the populations(at 2.5 Ga and 1 Ga)fall near the onsets of two transitions in Earth history.The First Transition may represent a change from stagnant-lid tectonics into plate tectonics and the Second Transition,the onset of subduction of continental crust.The major factor controlling geographic distribution of age peaks is the changing locations of orogeny.Before^2 Ga,age subpeaks and peaks are housed in orogens within or around the edges of crustal provinces,mostly in accretionary orogens.but beginning at 1.9 Ga,collisional orogens become more important.The coincidence in duration between magmatic flare-ups in Phanerozoic arcs and duration of age subpeaks(10-30 Myr)is consiste nt with subpeaks representing periods of enhanced arcrelated magmatism.probably caused by increased subduction flux.The correlation of isotopic age peaks between time series supports a cause and effect relationship between mantle plume activity,continental magma production at convergent margins,and crustal deformation.Correlation of over half of the detrital zircon age peaks(and six of the nine major peaks)with Re depletion age peaks supports an interpretation of the zircon peaks as crustal growth rather than selective preservation peaks.

Crust-derived felsic magmatism in the Emeishan large igneous Province:New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block,China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province(ELIP),southwestern China.Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection,and crustal magmas have rarely been studied.Here we investigate a suite of mafic dykes and Ⅰ-type granites that yield zircon U-Pb emplacement ages of 259.9±1.2 Ma and 259.3±1.3 Ma,respectively.The εHf(t)values of zircon from the DZ mafic dyke are–0.3 to 9.4,and their corresponding TDM1 values are in the range of 919–523 Ma.The εHf(t)values of zircon from the DSC Ⅰ-type granite are between–1 and 3,with TDM1 values showing a range of 938–782 Ma.We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time.The δ18O values of zircon from the DSC Ⅰ-type granite ranges from 4.87‰to 7.5‰.The field,petrologic,geochemical and isotopic data from our study lead to the following salient findings.(i)The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar.(ii)The DZ mafic dyke and high-Ti basalts have the same source,i.e.,the Emeishan mantle plume.The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite,with low degree partial melting(<10%).(iii)The Hf-O isotope data suggest that the DSC Ⅰ-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material.(iv)The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume,which partially melted the overlying crust,generating the felsic magma.

Baddeleyite and zircon U-Pb ages of the ultramafic rocks in Chigu Tso area,Southeastern Tibet and their constraints on the timing of Comei Large Igneous Province

A suite of ultramafic and mafic rocks developed in the Chigu Tso area,eastern Tethyan Himalaya.Baddeleyite and zircon U-Pb ages acquired by SIMS and LA-ICP-MS from olivine pyroxenite rocks in the Chigu Tso area are 138.9±3.0 Ma and 139.0±1.9 Ma,respectively.These two Early Cretaceous ages are similar with the ages of the more abundant mafic rocks in the eastern Tethyan Himalaya,indicating that this suite of ultramafic and mafic rocks in the Chigu Tso area should be included in the outcrop area of the Comei Large Igneous Province(LIP).These ultramafic rocks provide significant evidence that the involvement of mantle plume/hot spot activities in the formation of the Comei LIP.Baddeleyite U-Pb dating by SIMS is one reliable and convenient method to constrain the formation time of ultramafic rocks.The dating results of baddeleyite and zircon from the olivine pyroxenite samples in this paper are consistent with each other within analytical uncertainties,suggesting that baddeleyite and zircon were both formed during the same magmatic process.The consistency of baddeleyite U-Pb ages in the Chigu Tso area with zircon U-Pb ages for a large number of Early Cretaceous mafic rocks in the eastern Tethyan Himalaya further support that zircon grains from such mafic rocks yielding Early Cretaceous ages are also magmatic in origin.

Contrasting oxidation states of low-Ti and high-Ti magmas control Ni-Cu sulfide and Fe-Ti oxide mineralization in Emeishan Large Igneous Province

Magmatic Ni-Cu-(PGE) sulfide and Fe-Ti oxide deposits in plume-related large igneous provinces(LIPs)are commonly related to low-Ti and high-Ti series magmas, respectively, but the major factors that control such a relationship of metallogenic types and magma compositions are unclear. Magma fOcontrols sulfur status and relative timing of Fe-Ti oxide saturation in mafic magmas, which may help clarify this issue. Taking the Emeishan LIP as a case, we calculated the magma fOof the high-Ti and low-Ti picrites based on the olivine-spinel oxygen barometer, and the partitioning of V in olivine. The obtained fOof the high-Ti series magma(FMQ + 1.1 to FMQ + 2.6) is higher than that of the low-Ti series magma(FMQ-0.5to FMQ + 0.5). The magma fOof the high-Ti and low-Ti picrites containing Fo > 90 olivine reveals that the mantle source of the high-Ti series is likely more oxidized than that of the low-Ti series. The results using the ’lambda REE’ approach show that the high-Ti series may have been derived from relatively oxidized mantle with garnet pyroxenite component. The S contents at sulfide saturation(SCSS) of the two series magmas were calculated based on liquid compositions obtained from the alpha Melts modeling, and the results show that the low-Ti series magma could easily attain the sulfide saturation as it has low fOwith S being dominantly as S. In contrast, the oxidized high-Ti series magma is difficult to attain the sulfide saturation, but could crystallize Fe-Ti oxides at magma MgO content of ~7.0 wt.%. Thus, contrasting magma fOof low-Ti and high-Ti series in plume-related LIPs may play an important role in producing two different styles of metallogeny.

Major,trace element and Sr-Nd isotope evidence for a sublithospheric mantle source for the Umkondo large igneous province

The Mesoproterozoic(1.11 Ga)Umkondo large igneous province(LIP)in southern Africa and Antarctica was emplaced in<5 Myr and is dominated by low-Ti tholeiitic doleritic-gabbroic sills.It is of particular interest because it is the least studied LIP in southern Africa with both sublithospheric and lithospheric mantle sources proposed and it coincides with the early assembly of Rodinia,so it has importance in understanding the nature of magmatism and tectonics in and around the Kalahari craton during the Mesoproterozoic.In this study,we compiled a large database of existing(~750)and new(~100)major and trace element data for the Umkondo province,as well as 42 new Sr-Nd isotopic measurements,to provide constraints on its magma sources and geochemical evolution.Major element compositional variations in the low-Ti tholeiites are explained by low-pressure(1 kbar)three-phase fractional crystallisation(olivine,clinopyroxene and plagioclase)of a parent magma with~10 wt.%MgO in oxidising conditions(QFM+1).Inverse models show that the low-Ti tholeiitic magmas were derived as residual melts after the crystallization of 12%-33%olivine from primary komatiitic-basaltic magmas(up to~20 wt.%MgO)in equilibrium with mantle olivine(Fo90).Low Sm/Yb and TiO2/Yb-Nb/Yb indicate that the primary magmas were derived by 2%-20%shallow(40-50 km)partial melting of spinel lherzolite.High Sm/Yb is restricted to dyke swarms and may imply limited magma production from deeper(up to~70 km)garnet lherzolite-like sources.The low-Ti tholeiites of the Umkondo province are enriched in large ion lithophile elements(Rb-Sr-Cs-K)and depleted in high-field strength elements(Zr-Hf-Nb-Ta),indicating the involvement of crustal material and/or the subcontinental lithospheric mantle.This is supported by covariations in Th/Nb,Nb/Yb,Nb/La and Ce/Sm with generally negativeΔNb.Sr-Nd isotopes lend support to the notion that the Umkondo magmas were derived from depleted and/or enriched sublithospheric mantle sources and subsequently contaminated by enriched lithospheric material during emplacement(initial(at 1.11 Ga)^(87)Sr/^(86)Sr between 0.704820 and 0.737464 andεNd between-8.9 and+5.3).The Vredefort sills are significant as they display the most depleted Sr-Nd isotopic signatures(average initial ^(87)Sr/^(86)Sr of 0.705342 and averageεNd of 0.4)and are the least contaminated magma suite in the Umkondo province.Because of(i)the large volume of low-Ti magmas,(ii)evidence of a primary hot and MgO-rich(komatiitic)magma,and(iii)the short duration of magmatism,we suggest that the Umkondo province was formed by plume-induced melting of the sublithospheric mantle beneath the Kalahari craton in an extensional setting.This contrasts with previous suggestions that the heat source developed in response to the“thermal insulation”of the mantle beneath a thickened Kalahari craton in the absence of a mantle plume.There is further evidence from the elevated Zn/Fe that the sublithospheric mantle was lithologically heterogeneous and consisted of mixed peridotite and pyroxenite domains.There is a general lack of ultramafic cumulates in the low-Ti magma suite that may imply there was deeper ponding and storage of the primary magmas that fractionated large quantities of ultramafic rocks.There is also a paucity of high-Ti rocks in the Umkondo province that may reflect limited direct melting of the lithospheric mantle or that they are simply not as well-preserved in this province compared to the Karoo province.The similar trace element and Sr-Nd isotopic compositions of the Umkondo sills in southern Africa with the Borgmassivet sills in Antarctica support the concept that the Kalahari craton and Grunehogna terrane were adjoined at 1.11 Ga.The timing of the Umkondo province indicates there was localised lithospheric extension and upwelling asthenospheric mantle during a time of dominantly compressional tectonics on Earth at the end of the‘boring billion’.

Mantle Source Components and Magmatic Evolution for the Comei Large Igneous Province:Evidence from the Early Cretaceous Niangzhong Mafic Magmatism in Tethyan Himalaya

The Niangzhong diabase dikes,dated at 138.1±0.4 Ma,are located within the outcrop area of the Comei large igneous province(LIP).These diabase samples can be divided into two groups:samples in Group 1 show varying MgO(1.50 wt.%-10.25 wt.%)and TiO_(2)(0.85 wt.%-4.63 wt.%)contents,and enriched initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7056-0.7112,ε_(Nd)(t)=-0.3-+3.8),with OIB-like REEs and trace elements patterns,resulting from low degree melting of garnet-bearing lherzolite mantle sources;in contrast,samples in Group 2 show limited MgO(4.14 wt.%-7.75 wt.%)and TiO_(2)(0.98 wt.%-1.69 wt.%)contents,and depleted initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7075-0.7112,ε_(Nd)(t)=+5.5-+6.2),with N-MORB-like REEs and trace elements patterns,resulting from relatively high degree melting of spinel-bearing lherzolite mantle source.Combined with the published representative data about Comei LIP,we summarize that the source components for Comei LIP products include OIB end-member,enriched OIB end-member,and N-MORB end-member,respectively.Melts modeling suggests that magmas in the Comei LIP evolve in a relatively high oxygen fugacity condition,which influenced their fractionation sequences and led to systematic changes of TiO_(2)contents,Ti/Y and Ti/Ti*ratios.From the spatial and temporal distribution of above three end-member samples,deep process of Kerguelen plume during the Comei LIP formation can be interpreted as the interaction among the Kerguelen plume,the overlying lithospheric mantle,and the upwelling asthenosphere.The magmatism of Comei LIP began at~140 Ma and then lasted and peaked at~132 Ma with the progressively lithospheric thinning of eastern Gondwana upon the impact of Kerguelen plume.

Estimating the Parental Magma Composition and Temperature of the Xiaohaizi Cumulate-Bearing Ultramafic Rock: Implication for Magma Evolution of the Tarim Large Igneous Province, Northwestern China

The ultramafic dikes in the Tarim large igneous province（Tarim LIP）, exposed in the Xiaohaizi area in the northwestern Tarim Basin of northwestern China, have porphyritic textures, and the olivine and clinopyroxene are as the major phenocryst phases. The groundmass therein consists of clinopyroxene, plagioclase and Fe-Ti oxides, with the cryptocrystalline texture. The olivine phenocrysts in one typical ultramafic dike have Fo（Mg/（Mg＋Fe）） numbers ranging from 73 to 85, which are not in equilibrium with the olivine（Mg# of 89） from the host rock crystalized. Combined with microscope observation, both the olivine and clinopyroxene phenocrysts as well as some Fe-Ti oxides in the ultramafic rock are accounted as cumulates. The liquid（parental magma） composition of SiO2 of 45.00 wt.%–48.82 wt.%, MgO of 9.93 wt.%– 18.56 wt.%, FeO of 5.85 wt.%–14.17 wt.%, CaO of 7.54 wt.%–11.52 wt.%, Al2O3 of 8.70 wt.%–11.62 wt.% and TiO2 of 0.00 wt.%–3.43 wt.% in the Xiaohaizi ultramafic rock was estimated by mass balance, and the results show a reasonable liquid proportion in the cumulate-bearing ultramafic dike（ca. 45%–60% in the whole rock）. The estimated parental magma composition corresponds to a melting temperature of 1 300–1 550 oC, which is equal or higher than those of a normal asthenosphere mantle, supporting the involvement of a mantle plume. Combined with other previous studies, an evolution model for the formation processes of the Xiaohaizi ultramafic dike of the Tarim LIP is proposed.

Iceland Plume and its Magmatic Manifestations:LIP-Dornr?schen in the North Atlantic

Plume-lithosphere interactions are key in the coupling of deep Earth and surface processes,impacting deformation and evolution of sedimentary basins and continental topography at different spatial scales(Cloetingh et al.,2022,2023).The North Atlantic region is a prime example of the interaction between plate tectonic movements and thermal instabilities in the Earth's mantle.The opening of the Labrador Sea/Baffin Bay and the North Atlantic,the widespread volcanism and the localized uplift of the topography in Greenland and the North Atlantic are traditionally attributed to the thermal effect of the Iceland mantle plume.

沙湾地区玄武岩对峨眉山地幔柱岩浆过程的响应

四川省乐山市沙湾地区附近出露大面积的二叠纪玄武岩,是峨眉山大火成岩省的重要组成部分。研究区岩石学及地球化学研究表明,其岩性主要为致密块状辉石玄武岩、斜斑玄武岩和粗面玄武岩;主量元素显示玄武岩具有高钛拉斑系列的成分特点,稀土元素表现出玄武岩呈轻稀土元素富集的右倾模式,轻、重稀土元素分馏程度较高;微量元素相对富集Th、U等大离子亲石元素及高场强元素,强烈亏损Sr和P等与流体相关的微量元素;与云南宾川上仓附近晚期喷发的高钛玄武岩及洋岛玄武岩(OIB)特征相似,暗示峨眉山周边的玄武岩形成于地幔柱诱发下的板内拉张环境,原始岩浆源于交代富集地幔源区的石榴石二辉橄榄岩的低程度部分熔融,具有与再循环洋壳有关的辉石岩相似的成分特点,且在上升过程中发生斜长石的分离结晶作用及微弱的地壳混染。本次玄武岩对比工作揭示,位于大火成岩省中带的沙湾地区玄武岩与内带的宾川上仓晚阶段喷发产物具有同源性和等时性的特点,暗示该玄武岩浆属于大火成岩省活动晚期地幔柱边缘部分熔融的产物。

塔里木大火成岩省方解霞黄煌岩的成因及其对大陆板内深部碳循环的启示

作为典型深部地幔来源的岩石,碱性超镁铁质煌斑岩的源区特征和岩浆演化为揭示再循环壳源物质对地幔的改造及深部碳-水-硫等循环提供重要信息。本次研究通过对塔里木大火成岩省瓦吉里塔格方解霞黄煌岩岩石学、矿物学和全岩地球化学研究,揭示方解霞黄煌岩的岩石成因及其相关的大陆板内深部碳循环过程。瓦吉里塔格方解霞黄煌岩显示出碱性(Na_(2)O+K_(2)O=1.12%~2.18%)和超镁铁质特征(SiO_(2)=28.67%~31.20%;Mg^(#)=69~73),同时富集Fe_(2)O_(3)^(T)(13.57%~15.32%)、TiO_(2)(1.89%~2.11%)和地幔相容元素(Ni、Cr)。在微量元素上,样品富集轻稀土元素、大离子亲石元素但亏损高场强元素,具有强烈的K、P、Ti、Nb-Ta、Zr-Hf负异常,类似火山弧玄武岩的特征。(^(87)Sr/^(86)Sr)i变化于0.703765~0.703914之间,εNd(t)值变化于4.48~4.75之间,(^(206)Pb/^(204)Pb)i值为18.4563~18.7527,均与FOZO地幔端元特征相似。全岩微量元素和同位素组成表明,瓦吉里塔格方解霞黄煌岩可能是交代富集的岩石圈地幔和地幔柱共同作用的结果。瓦吉里塔格方解霞黄煌岩中斑晶橄榄石具有高的100×Mn/Fe(1.02~3.29)和10000×Zn/Fe值(10.6~23.0),但其Mn/Zn值为11.1~16.4,暗示源区中含有辉石岩组分,而不是均一的橄榄岩地幔。这些斑晶橄榄石中Sc与Ni呈正相关关系,但Co与Ni之间没有明显的相关性,进一步表明源区中的辉石岩组分可能是俯冲洋壳加入的结果。这些岩石富碳酸盐,结合橄榄石中Ca和Ti同时富集趋势,表明其地幔源区存在碳酸盐交代作用。据此,推测受交代的含PIC型脉体的岩石圈地幔在碳酸盐化地幔柱作用下发生低程度部分熔融产生方解霞黄煌岩岩浆。本次研究为板内岩浆作用相关的深部碳循环过程研究提供了借鉴和指示意义。

峨眉山地幔柱主要研究进展及四川盆地二叠纪成盆动力学机制

近年来峨眉山大火成岩省及其地幔柱成因研究取得了重要进展,但关于地幔柱在四川盆地的作用范围和二叠纪成盆动力学机制等方面存在认识分歧。基于大量文献调研并结合近期研究,采用深部地幔活动控制表层系统演变的研究思路,系统梳理峨眉山大火成岩省及其深部地质特征。地幔柱是诱发峨眉山玄武岩大规模喷发并形成大火成岩省的主导因素,但地幔柱仅对四川盆地部分地区具有直接影响,具体表现为地幔柱上涌引起地壳隆升并导致中二叠统茅口组顶部出现地层差异剥蚀和对盆地中南部二叠纪沉积的控制作用,而盆地中北部中晚二叠世—早三叠世拉张槽群和“凹凸相间”沉积-构造格局的形成则主要受控于南秦岭洋岩石圈板块的拉张。

塔里木大火成岩省碱性岩浆活动与稀有金属成矿作用

地幔柱活动导致大火成岩省的形成,并伴随大规模岩浆作用和成矿事件。碱性岩型稀有金属矿床是地幔柱成矿作用中非常重要的一类,但目前对该类矿床形成过程中稀有金属元素超常富集机制仍缺少系统性研究。本文搜集了塔里木大火成岩省两大构造域(塔里木克拉通及边缘地区和中亚造山带)碱性岩浆岩的年代学和岩石地球化学数据,分析其岩浆活动和成矿作用的基本特点,探讨地幔柱与不同类型大陆岩石圈相互作用导致稀有金属元素富集的差异性。塔里木克拉通及边缘地区碱性岩浆岩主要为碱性正长岩‒花岗岩和碳酸岩,形成于291~268 Ma,峰期年龄为~278 Ma,主要属于塔里木大火成岩省最晚阶段的岩浆活动;其中碱性正长岩‒花岗岩富碱、Th、REE、Nb、Zr,高Ga/Al、Nb/Ta值,多数显示典型A1型花岗岩的地球化学特征。中亚造山带碱性岩浆岩为碱性正长岩‒花岗岩,形成于325~255 Ma,大部分与塔里木大火成岩省岩浆活动时代一致,其碱、Th、REE、Nb、Zr含量和Ga/Al、Nb/Ta值明显低于塔里木克拉通及边缘地区的碱性正长岩‒花岗岩,并且多数显示典型A2型花岗岩的地球化学特征。塔里木克拉通及边缘地区的碱性正长岩‒花岗岩和碳酸岩赋存Nb-Zr-REE矿床,而中亚造山带的碱性正长岩‒花岗岩则赋存Li-Be-Ta-Nb-W-Sn-Mo矿床。地幔柱和再循环洋壳物质加入地幔源区导致塔里木克拉通及边缘地区的碱性岩浆岩富集Nb、Zr、REE;中亚造山带碱性岩浆岩的源区因近期俯冲交代影响的岩石圈地幔和新生地壳物质的参与而显著富集Li、Be、W、Sn、Mo等流体活动元素。塔里木克拉通及边缘地区碱性正长岩‒花岗岩贫硅、钠质、过碱性的特征有利于熔体中Nb、Zr、REE的富集;而中亚造山带碱性正长岩‒花岗岩富硅、钾质、过铝质的特征有利于Ta、Li、W、Sn的富集。塔里木克拉通和中亚造山带具有不同的地幔柱‒岩石圈相互作用,其碱性岩浆岩的岩浆源区不同,并经历了不同的岩浆演化过程,导致塔里木大火成岩省两大构造域具有不同的稀有金属成矿作用。

峨眉山大火成岩省西部苦橄岩及其共生玄武岩的地球化学:地幔柱头部熔融的证据

丽江地区的苦橄岩位于峨眉山大火成岩省的西部，其与辉斑玄武岩、无斑玄武岩和玄武质火山碎屑岩共生。苦橄岩中的斑晶主要为富镁橄榄石，其F0含量最高达91．6％，CaO含量最高达0．42％，其内含有少量玻璃包裹体，指示了橄榄石是在熔体中结晶形成的。苦橄岩中的铬尖晶石具有高的Cr#值（73-75）。计算的初始岩浆的MgO含量大约为22wt％，初始熔融的温度为1630-1680℃。研究结果表明，玄武质岩石是苦橄质岩浆通过橄榄石和单斜辉石分离结晶形成的。苦橄岩和玄武岩的Nd-Sr-Pb同位素比值差别不大，只落在一个很小的范围内（如εNd（t）=-1．3 to＋4．0）。高的εNd（t）值以及抗蚀变不相容元素的原始地幔标准化图解与洋岛玄武岩相似，并且其重稀土元素特征指示了源区有石榴子石的残余，而且是低部分熔融的产物。同位素比值与抗蚀变不相容元素比值（如Nb／La）的相关性表明，岩浆形成过程中有少量的大陆地壳物质或者相对低εNd（t）组分的大陆岩石圈地幔的混染。因此，总体上，苦橄岩的地球化学特征的研究结果支持了峨眉山大火成岩省是地幔柱头部熔融的成因模型。

峨眉山玄武岩的地幔热柱成因

根据峨眉山玄武岩的岩石组合、岩相学特征将峨眉火成岩省分为盐源 -丽江岩区、攀西岩区、贵州高原岩区和松潘 -甘孜岩区。通过对研究区二叠世的区域地质背景和古地理环境的分析 ,对峨眉山玄武岩喷发与地幔热柱的关系及其火山喷发的大地构造背景进行了进一步系统归纳和总结。根据地层学关系大致确定峨眉山玄武岩的主喷发期是阳新世 (中二叠 )晚期乐平世 (晚二叠 )早期 ,时限大致为 2 5 9Ma～ 2 5 7Ma。峨眉山玄武岩微量元素地幔标准化曲线特征与 OIB基本一致 ,反映出其成因与地幔热柱活动有密不可分的关系。