Magmatism in the Gregory Rift, East Africa: Evidence for Melt Generation by a Plume

The volume and composition of volcanic rocks associated withthe Gregory rift are interpreted in the light of inversionsperformed on the REE concentrations of the most magnesian basalts.When the estimated volume of salic rock ({small tilde}88 000km³) is converted into basalt ({small tilde}792 000 km³) thetotal volume of basaltic melt generated over the last 30 Myis at least 924 000 km³, corresponding to an average rate ofmelt production of {small tilde}0•03 km³/yr and an averagemelt thickness of between 7 and 26 km everywhere beneath therift. The mean compositions of the basaltic magmas erupted withinthe rift and on the rift flanks during the Upper Oligocene andMiocene, the Pliocene, and the Quaternary are taken to be representativeof the average compositions of melts produced by fractionalmelting in the asthenospheric mantle. When the REE concentrationsof the observed average compositions are inverted they suggestthat much of the melt was produced in the depth and temperaturerange of the transition from garnet to spinel peridotite. Fora mantle potential temperature of {small tilde}1500C the topof the melting region predicted from the inversions is at {smalltilde}70 km beneath the rift axis and {small tilde}80 km beneaththe rift flanks. Within the rift zone the predicted thicknessof melt increases from the Upper Oligocene and Miocene to thePliocene and is always greater than that predicted for the riftflanks, and the timeaveraged thickness of melt predicted is0/5 km. To generate the observed volume of melt the asthenosphericmantle must continually upwell through the melting region (extendingfrom 70 to 150 km) with a vertical velocity of between 40 and140 mm/yr. The results suggest that, volumetrically and compositionally,magmatic activity associated with the Gregory rift is quantitativelyconsistent with a model of a mantle plume upwelling beneaththinned continental lithosphere. Predictions made by such amodel are in broad agreement with geophysical observations. ^* Present address/reprint requests to: B.P. Exploration, 4/5 Long Walk, Stockley Park, Uxbridge UB11 1BP, UK     

Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana

New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and ⁴⁰Ar–³⁹Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.U–Pb is the only reliable method for dating crystallizationin these rocks; Rb–Sr is prone to hydrothermal resettingand Ar–Ar is additionally affected by initial excess ⁴⁰Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188–178 Ma), V2 (172–162Ma) and V3 (157–153 Ma). The first essentially coincideswith the Karoo–Ferrar mafic magmatism of South Africa,Antarctica and Tasmania. The silicic products of V1 are lower-crustalmelts that have incorporated upper-crustal material. The geochemistryof V2 and V3 ignimbrites is more characteristic of destructiveplate margins, but the presence of inherited zircon still pointsto a crustal source. The pattern of volcanism corresponds inspace and in time to migration away from the Karoo mantle plumetowards the proto-Pacific margin of Gondwana during riftingand break-up. The heat required to initiate bulk crustal fusionmay have been supplied by the spreading plume-head, but thinningof the crust during continental dispersion would also have facilitatedanatexis. KEY WORDS: Antarctic Peninsula; ignimbrites; Jurassic; Patagonia; U–Pb; zircon     

Carbonatite Magmatism and Plume Activity: Implications from the Nd, Pb and Sr Isotope Systematics of Oldoinyo Lengai

New Nd (0.51261–0.51268), Pb (²⁰⁶Pb/²⁰⁴Pb: 19.24–19.26),and Sr (0.70437–0.70446) isotopic compositions from tennatrocarbonatite lavas, collected in June 1993 from OldoinyoLengai, the only known active carbonatite volcano, are relativelyuniform, and are similar to data from the 1960 and 1988 flows.Three of the samples contain silicate spheroids, one of whichhas Nd and Sr isotopic ratios similar to host natrocarbonatite,consistent with an origin by liquid immiscibility or the mixingof melts with similar isotopic compositions. Pb isotope datafor two samples of trona are inconsistent with its involvementin the genesis of natrocarbonatite. New Pb isotope data fromsilicate volcanic and plutonic blocks (ijolite, nephelinite,phonolite, syenite) from Oldoinyo Lengai are highly variable(²⁰⁶Pb/²⁰⁴Pb, 17.75–19.34; ²⁰⁷Pb/²⁰⁴Pb, 15.41–15.67;²⁰⁸Pb/²⁰⁴Pb, 37.79–39.67), and define near-linear arraysin Pb-Pb diagrams. The isotopic data for the silicate rocksfrom Oldoinyo Lengai are best explained by invoking discretepartial melting events which generate undersaturated alkalinesilicate magmas with distinct isotopic ratios. Pb isotope ratiosfrom most ijolites and phonolites are predominantly lower andmore variable than from the natrocarbonatites, and are attributedto interaction between silicate melts involving HIMU and EMIsource components and an additional component, such as lower-crustalgranulites, DMM or PREMA (prevalent mantle). Variations in Nd,Pb and Sr isotope ratios from Oldoinyo Lengai, among the largestyet documented from a single volcano, are attributed to mantlesource heterogeneity involving mainly the mixing of HIMU andEMI mantle components. Based on the new isotopic data from OldoinyoLengai and data from other East African carbonatites, and mantlexenoliths, we propose a two-stage model in an attempt to explainthe isotope variations shown by carbonatites in this area. Themodel involves (I) the release of metasomatizing agents withHIMU-like signatures from upwelling mantle (‘plume’)source, which in turn metasomatize the sub-continental (old,isotopically enriched, EMI-like) lithosphere, and (2) variabledegrees and discrete partial melting of the resulting heterogeneous,metasomatized lithosphere. KEY WORDS: carbonatite; isotopes; Oldoinyo Lengai; mantle plumes ^*Telephone: (613) 788–2660, ext. 4419. Fax: (613) 788–4490. e-mail: kbell{at}ccs.carleton.ca     

Geochemical Characteristics of Jurassic Plume Magmatism in Ahlmannryggen Massif (Queen Maud Land,East Antarctica)

Doklady Earth Sciences - The Mesozoic dykes related to the distribution of Karoo plume on the territory of East Antarctica are studied. It is shown that magnesian high-Ti ferrobasalts are found in...     

Submarine Basalts of the Northern Kerguelen Plateau: Interaction Between the Kerguelen Plume and the Southeast Indian Ridge Revealed at ODP Site 1140

During Ocean Drilling Program Leg 183, basaltic cores were retrievedfrom the Northern Kerguelen Plateau (NKP) at Site 1140 on theextreme north of the plateau,     

Permian Mafic Magmatism and Related Gold Mineralization in the Bama District, SW China: A Result of Mantle Plume Activity

Please refer to the attachment(s) for more details     

Magmatism and Genesis of the Beiya Giant Gold-Polymetallic Deposit, SW China

Please refer to the attachment(s) for more details.     

Genesis of the Katugin rare-metal ore deposit: Magmatism versus metasomatism

Arguments in favor of magmatic or metasomatic genesis of the Katugin rare-metal ore deposit are discussed. The geological and mineralogical features of the deposit confirm its magmatic origin: (1) the shape of the ore-bearing massif and location of various types of granites (biotite, biotite–amphibole, amphibole, and amphibole–aegirine); (2) the geochemical properties of the massif rocks corresponding to A type granite (high alkali content (up to 12.3% Na₂O + K₂O), extremely high FeO/MgO ratio (f = 0.96–1.00), very high content of the most incoherent elements (Rb, Li, Y, Zr, Hf, Ta, Nb, Th, U, Zn, Ga, and REE) and F, and low concentrations of Ca, Mg, Al, P, Ba, and Sr); (3) Fe–F-rich rock-forming minerals; (4) no previously proposed metasomatic zoning and regular replacement of rock-forming minerals corresponding to infiltration fronts of metasomatism. The similar ages of the barren (2066 ± 6 Ma) and ore-bearing (2055 ± 7 Ma) granites along with the features of the ore mineralization speak in favor of the origin of the ore at the magmatic stage of the massif’s evolution. The nature of the ore occurrence and the relationships between the ore minerals support their crystallization from F-rich aluminosilicate melt and also under melt liquation into aluminosilicate and fluoride (and/or aluminofluoride) fractions.     

东昆仑祁漫塔格晚古生代末期幔源岩浆活动成因及地质意义:以鹰爪沟岩体为例

幔源岩浆活动的成因研究,对约束区域构造演化历史具有重要意义.东昆仑祁漫塔格鹰爪沟镁铁-超镁铁质层状岩体由橄榄辉长苏长岩、含长橄榄二辉岩及橄榄二辉岩组成.获得橄榄辉长苏长岩的LA-ICP-MS锆石U-Pb年龄为263±4 Ma,指示岩体形成于中二叠世晚期.岩石地球化学特征显示,鹰爪沟岩体具有低SiO₂、高MgO、FeO_t含量,富集LREE、LILE,亏损HREE以及Th、U、Nb、Ta、Ti等不相容元素等特征.全岩ε_Nd（t）=0.73~0.92,锆石ε_Hf（t）=8.33~13.50.综合区域地质背景资料,认为其形成于古特提斯洋俯冲作用下的活动大陆边缘裂谷,源自于受软流圈熔体和俯冲流体交代的岩石圈地幔.结合同期具有壳幔混源特征的花岗岩资料,认为东昆仑地区在中二叠世已有幔源岩浆底侵活动,主要形成于古特提斯洋俯冲体制下局部的伸展背景.      

Geochemical Constraints for the Genesis of Post-collisional Magmatism and the Geodynamic Evolution of the Northern Taiwan Region

Taiwan is an active mountain belt created by the oblique collisionof the northern Luzon arc with Asia. Late Pliocene extensionalcollapse of the northern Taiwan mountain belt (NTMB) was accompaniedby magmatism that formed the Northern Taiwan Volcanic Zone (NTVZ;2·8–0·2 Ma). The geochemical characteristicsof the NTVZ magmas can thus provide constraints both for themantle source composition and the geodynamic processes operatingin the late orogenic stage of the region. The NTVZ volcanicrocks consist dominantly of calc-alkaline andesites and basalts,along with subordinate but heterogeneous lavas including low-K,shoshonitic and ultrapotassic magmas. From the NE to the SWin the NTVZ, the magmas show systematic compositional variationsfrom low-K to calc-alkaline and then shoshonitic. This spatialgeochemical variation, characterized by southwesterly increasein potassium and incompatible trace elements, appears to besubparallel to the southwestern part of the modern Ryukyu subductionsystem. Sr–Nd isotope ratios of the NTVZ volcanic rocks(⁸⁷Sr/⁸⁶Sr     

The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin

The incompatible element signatures of volcanic rocks formingJeju Island, located at the eastern margin of the Asian continent,are identical to those of typical intraplate magmas. The sourceof these volcanic rocks may be a mantle plume, located immediatelybehind the SW Japan arc. Jeju plume magmas can be divided intothree series, based on major and trace element abundances: high-aluminaalkalic, low-alumina alkalic, and sub-alkalic. Mass-balancecalculations indicate that the compositional variations withineach magma series are largely governed by fractional crystallizationof three chemically distinct parental magmas. The compositionsof primary magmas for these series, using inferred residualmantle olivine compositions, suggest that the low-alumina alkalicand sub-alkalic magmas are generated at the deepest and shallowestdepths by lowest and highest degrees of melting, respectively.These estimates, together with systematic differences in traceelement and isotopic compositions, indicate that the upper mantlebeneath Jeju Island is characterized by an increased degreeof metasomatism and a change in major metasomatic hydrous mineralsfrom amphibole to phlogopite with decreasing depth. The originalplume material, having rather depleted geochemical characteristics,entrained shallower metasomatized uppermost mantle material,and segregated least-enriched low-alumina alkalic, moderatelyenriched high-alumina alkalic, and highly enriched sub-alkalicmagmas, with decreasing depth. KEY WORDS: Jeju Island; magma genesis; mantle plume; subcontinental mantle     

Early Cretaceous Basaltic and Rhyolitic Magmatism in Southern Uruguay Associated with the Opening of the South Atlantic

The Early Cretaceous volcanic rocks of southern Uruguay comprisemafic and felsic volcanics. The position of these outcrops atthe southern edge of the Paraná–Etendeka continentalflood basalt province provides an opportunity to investigatepossible lateral variations in both mafic and more evolved rocktypes towards the margins of such an area of plume-related magmatism.The mafic lavas are divided into two compositionally distinctmagma types. The more voluminous Treinte Y Trés magmatype is similar to the low-Ti basalts of the Paraná floodbasalt province. The Santa Lucía magma type is a distinctand rare basalt type with ocean-island basalt type asthenosphericaffinities (high Nb/La, low ⁸⁷Sr/⁸⁶Sr_i). The felsic volcanicsare divided into two series, the Lavalleja Series and the AigüaSeries. The Lavalleja Series are chemically and isotopicallysimilar to the Paraná–Etendeka low-Ti rhyolites,and are considered to be related to the Treinte Y Tréslavas by extensive fractionation and crustal assimilation. TheAigüa Series have low ¹⁴³Nd/¹⁴⁴Nd_i and low ⁸⁷Sr/⁸⁶Sr_i andunlike the rhyolites of the Paraná, are interpreted asmelts of pre-existing mafic lower crust that subsequently underwentextreme fractionation. The differences observed in the felsicsuites may be linked to differences in the volumes of the associatedbasalts and the amounts of extension. KEY WORDS: South America; flood basalts; felsic volcanics; crustal melts; plume     

Evolution of the Kerguelen plume and its impact upon the continental and oceanic magmatism of East Antarctica

Petrological–geochemical study showed that the alkaline-ultramafics of the Jetty Oasis (rift zone of the Lambert glacier, East Antarctica) are similar in the age (117–110 Ma) and geochemistry to the ultrapotassic alkali basalts of eastern India (Jharia and Raniganj intrusions). Alkaline magmatism in India and Antarctica is related to the activity of the Kerguelen plume, which significantly affected the evolution of the entire eastern Indian Ocean, in particular, determined geodynamic peculiarities of the ocean opening (existence of non-spreading blocks, fragments of the Gondwana lithosphere in oceanic areas) and geochemical characteristics of erupted tholeiitic magmas. Enriched magma sources related to the Kerguelen plume were formed by melting of ancient Gondwana-derived continental fragments, which experienced multiple transformations during its evolution up to the formation of metasomatized mantle under the impact of the Kerguelen plume on the Antarctic and India margins.     

The composite magnetic anomaly map of the East Antarctic

Airborne and marine magnetic observations in East Antarctica and adjacent seas of the Indian Ocean were compiled for a magnetic anomaly map of the Antarctic. For East Antarctica, over 260,000 line km of Russian reconnaissance magnetic data were used that had been collected since 1955 mainly at line spacings of about 5, 20 and 50 km. For the offshore areas, magnetic data from American, Australian, German, Japanese, and Russian marine expeditions were incorporated. Digitally recorded data and data digitized from published and unpublished maps and profiles were included in the compilation. Local grids of these data were developed and merged into a regional grid at an interval of 5 km. The prime product of this compilation is a shaded-relief map that shows the most complete and coherent perspective to date of the region's magnetic character. In combination with other types of data, the compilation provides new insight on the tectonic features and history of this largely inaccessible region of the world. It maps out approximately 4300 km of the Antarctic Continental Margin Magnetic Anomaly (ACMMA) related to Gondwana breakup, new cratons and mobile belts, and large submarine igneous provinces.     

Isotope-Geochemical Features and Genesis of Gases in the East Carpathian Region

The paper presents data on the composition of a gas phase of underground fluids in the East Carpathian region, including ³He/⁴He, ⁴⁰Ar/³⁶Ar, and ⁴He/²⁰Ne ratios. The argon isotope composition of these gases was used to estimate the fractions of atmospheric Aratm and radiogenic ⁴⁰Ar_rad formed in the rocks, N₂/Aratm ratio, and to reveal the admixture of nonatmogenic (“excess”) nitrogen in most samples. The CO₂ content in gases positively correlates with the fraction of mantle component in fluid helium. At the same time, the CO₂ content shows a negative correlation with the total helium (and light ³He enriching mantle derivatives), thus excluding the simultaneous influx of CO₂ and helium from a common mantle source in the fluids. A wide spectrum of ³He/⁴He = R in gases of the region spanning three orders of magnitude confirms the concept of mixing of the crustal and mantle components in the helium. However, even gases with similar R values show a wide scatter of He concentrations. This is mainly caused by the additional influx of other gases: CH₄ formed during OM transformation or CO₂ released during the thermal metamorphism of carbonate sequences. Correlation of the CH₄/³He ratio and the helium isotope composition in the Carpathian gases indicates the crustal origin of hydrocarbons, which formed economic gas pools in the Ciscarpathian Trough and the adjacent part of the Folded Carpathians. Lateral chemical and isotope variations revealed in the underground fluids are related to the tectonic zoning of the region. The helium isotope variations are also consistent with the geodynamic setting of the region (thinning of the crust and lithosphere towards the Pannonian Basin, growth of the background conductive heat flow and corresponding ascent of isotherms). In combination with geothermal data, they reflect specifics of the mantle heat-and-mass flow discharge.     

河流相储层隔夹层成因及其分布特征

储层隔夹层研究是揭示油藏非均质性的不可缺少的组成部分,特别是油田开发至高、特高含水期,储层隔夹层的类型、分布等对剩余油的分布具有重要的控制作用。河流相储层隔夹层从成因上可分为沉积作用形成的隔夹层和成岩作用形成的隔夹层两大类型。沉积作用形成的隔夹层是指在沉积过程中形成的粒度相对较细、孔隙度较低、渗透率较小的岩层或界面,河流相储层中如点砂坝侧积层、河道砂体细粒微相等。成岩作用形成的隔夹层是在成岩过程中因钙质胶结而形成的,此类隔夹层分布零星,横向预测难,其分布受到沉积微相及断层的控制。点砂坝是河流相储层的主体,点砂坝侧积层是河流相储层中一类重要的隔夹层,因此着重研究了河流相储层中点砂坝的沉积模式及构型要素,它们控制着点砂坝砂体中隔夹层的分布。