Central European Cenozoic plume volcanism with OIB characteristics and indications of a lower mantle source

Average concentrations of incompatible elements and isotopes of radiogenic Sr, Nd and Pb in a 350 km long belt of Central European volcanics (CECV) resemble those in OIB in general and in OIB from the N Atlantic in particular. This similarity allows to infer comparable sources for CECV and OIB which might have been located in the lower mantle according to seismic tomography and chemistry, with the latter unlike a MORB source. The incompatible element contribution of lower mantle origin can be modelled from primitive mantle minus continental crust and upper mantle inventories. Alkali basaltic magmas from the modelled source are close in composition to CECV and OIB. The continental crust contains almost half of the silicate Earth's content of Rb, K, Pb, Ba, Th and U, which were mobilized to a major extent through dehydration of subducted ocean crust. Related losses from the lower mantle had to be replaced by deep subductions of oceanic lithosphere recognized from their isotopic imprint. From a balance based on Nd isotopes it can be concluded that average CECV contains 60% matter from residual primitive mantle and 40% from deeply subducted lithosphere (including some young upper mantle materials). Plume products from separate CECV regions developed, within 45 Ma, from rather depleted to more primitive isotopic signatures. Four periods of volcanism from Eocene to late Quaternary time are explained as four pulses of an almost stationary ultrafast plume uprise as modelled by Larsen and Yuen (1997). Magma production has increased from the first to the third pulse with the peak during Miocene time in the Vogelsberg region. The final pulse produced the Quaternary Eifel volcanoes. Tectonism from the Alpine orogen has probably triggered the synchronous volcanism of CECV, Massif Central etc. The European lithospheric plate has moved under the control of the opening Atlantic almost in an eastern direction with a velocity of 1 cm per year and has shifted extinct volcanoes off their source channels. Received: 10 July 1998 / Accepted: 10 February 1999     

Sources and geodynamics of the Late Cenozoic volcanism of Central Mongolia: Evidence from isotope-geochemical studies

In the Late Cenozoic, the volcanism of the South Khangai Volcanic Region (SKhVR) spanned the Khangai Range and its framing. Geochronological, petrochemical, geochemical, and isotope studies were performed for volcanic rocks of this region, which are represented by high-K basic and intermediate rocks of OIB affinity. Initial Sr, Nd, and Pb isotope ratios in the volcanic rocks of the SKhVR are close to those of the volcanic rocks of Pitcairn Island and form trends between PREMA, EMI, and EMII sources.     

The composition and formation of Miocene tholeiites in the Central European Cenozoic plume volcanism (CECV)

Tholeiites accompanying a majority of alkali basalts are restricted to the highly productive central part of the CECV plume activity in Vogelsberg and Hessian Depression. They mainly occur as quartz tholeiites which according to experiments of partial melting and material balances are products of olivine tholeiitic primary melts. The differentiation from olivine to quartz tholeiitic melts took place in lower crustal magma chambers where olivine tholeiitic melt intruded due to a density comparable with that of the country rocks. The fractionation due to separation of olivine and some clinopyroxene caused contamination of tholeiite magmas by tonalitic partial melts from the wall rocks of the magma chambers. The latter process is indicated by relatively high Rb, K and Pb and low Nb concentrations and by Nd, Sr and Pb isotopes. Contaminating crustal melts, which roughly attained a proportion of 10%, contained very low ¹⁴³Nd/¹⁴⁴Nd ratios from a Nd/Sm fractionation as old as 2.6 Ga. This is the first evidence from mafic rocks of this high age in the lower crust beneath Central Europe. Modelling with incompatible elements allows to recognize olivine tholeiites as products of about 1% partial melting of plume rocks consisting of 35% primitive and 65% depleted mantle materials. The production of tholeiites other than alkali basalts is restricted to the highest plume activity and the largest fraction of MORB type source rocks. Received: 10 December 1999 / Accepted: 23 June 2000     

东非裂谷系统(EARS)地幔柱成因的新生代火山作用地球化学标志

本文应用岩浆岩地球化学图解对东非裂谷系统(EARS)火山岩的岩石地球化学数据进行处理,重点回顾和讨论埃塞俄比亚大裂谷(MER)、阿法(Afar)盆地及肯尼亚裂谷火山作用的构造环境和地幔柱成因.MER火山岩的岩石类型具双峰式火山岩套特征,以高Ti的大陆溢流玄武岩(CFB)-大陆洋岛玄武岩(OIB)-流纹岩系列为主,缺少中性岩,是来自地幔柱岩浆分异的结果,与板块俯冲作用无关.阿法(Afar)盆地和红海为CFB-MORB系列.肯尼亚裂谷(KR)及埃塞俄比亚大裂谷最南端图尔卡纳盆地只见以大陆OIB,缺乏流纹岩.EARS是一个主动型地幔柱,由地幔上涌冲击地壳底部而成,其火山岩以富集常量元素Ti、Fe和Mg,富集高场强元素Nb、Ta和相容元素V、Cr、Co和Ni为特征.绝大多数EARS火山岩的Nb/Zr>0.04和Ta/Hf比值>0.1,在地球化学-构造环境判别图上落在板内火山岩的范围内(大洋板内和地幔柱),集中在Nb/Zr>0.15和Ta/Hf比值>0.3范围内的火山岩样品可能为主动地幔柱成因.根据La/Nb、Ce/Pb和Ba/La比值,地幔柱成因的MER火山岩可分为受地壳混染地幔和未受地壳混染的原始地幔两种类型.La/Nb(≤1)、Ce/Pb(30~50)、Ba/Nb比值(>10)和La/Yb≤12是来源于未受地壳混染原始地幔的重要判别标志.地球化学证据(微量元素,Sr-Nd-Pb-He同位素)表明,MER-阿拉伯-也门地幔柱起源于深部核-幔边界的HIMU.MER-Afar的前裂谷和同裂谷火山岩(50-12 Ma)具有高的3He同位素标志(R/Ra比值>16.4)说明其非洲板块之下存在一个深藏的地幔岩浆源深度大于670 km,位于石榴子石-尖晶石橄榄岩过渡带(图6b、c、d).MER-Afar大火山岩省及其最南端图尔卡纳湖和肯尼亚地体(克拉通,改造的克拉通边缘和活动带)深部地壳底部之下的HIMU地幔可能是导致裂谷型的OIB和CFB异常发育的岩浆源区,而Afar洼地(吉布提)、红海和亚丁湾形成于后裂谷期(5~0 Ma)的MORB和亏损LREE玄武岩则归因于HIMU、富集地幔(EM1-EM2)与DM混合地幔源的熔融,其岩浆源区位地壳拉张减薄带之下的尖晶石橄榄岩区.     

Helium isotopes provide no evidence for deep mantle involvement in widespread Cenozoic volcanism across Central Asia

Small-volume alkali basaltic volcanism has occurred intermittently for the past + 30 My across a vast area of thick continental crust from southern Siberia, through Mongolia to northeast China. With a lack of evidence for Basin-and-Range-type crustal extension or rifting, models to explain the widely dispersed, yet long-lived, volcanism tend to favour involvement of one or more mantle plume(s). We examine the range of ³He/⁴He isotope values in olivine phenocrysts from basalts, and their entrained mantle xenoliths, from Hamar Daban in southern Siberia, and Hangai in central Mongolia, in order to examine whether upwelling lower mantle appears to be present beneath central Asia and thus test the validity of the plume model for this region. Our results show that the maximum ³He/⁴He value for the Siberian basalts is 8.12 ± 0.2R_a, and the maximum value for Mongolian basalts is 9.5 ± 0.5R_a. These values suggest that there is no significant contribution from a high ³He/⁴He primordial component that would strongly argue a lower mantle source. Overlap with commonly reported values for MORB leads us to propose that the source of the magmatism derives from the shallow asthenosphere. Alternative models to a deeply sourced mantle plume that may be able to explain the magmatism include: a shallow thermal anomaly confined to the upper mantle but either fed laterally or caused by thermal blanketing of the large Asian landmass; replacement or delamination of the lowermost lithosphere in response to tectonic stresses; or large-scale mantle disturbance or overturn caused by a protracted history of subduction beneath central Asia that ended regionally with the Jurassic closure of the Mongol-Okhotsk Ocean, but continues further afield with the present Indo-Asia collision.     

History of development of late Cenozoic volcanism in the Central Caucasus

Violent volcanism developed in the central part of the Caucasus during the last stage of the Alpine orogenic cycle. Three main epochs of volcanic development are here established: the first -late Miocene-early Pliocene; the second-late Pliocene; the third — Quaternary. These epochs of volcanic activity can be subdivided into a series of phases and subphases. The total volume of volcanic products is in the order of 2000 km³. The acidic volcanic rocks are mostly rhyolitic ignimbrites and have a volume larger than 800–820 km³.

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Zusammenfassung Im Zentralteil des Großen Kaukasus tritt im Spätstadium des alpinen orogenen Zyklus ein starker Vulkanismus auf. Drei Hauptzeiten der vulkanischen Entwicklung werden verzeichnet: zunächst im späten Miozän bis frühen Pliozän; zweitens im späten Pliozän; drittens im Quartär. Die Zeiten der Vulkantätigkeit können in Phasen und Subphasen unterteilt werden. Das Gesamtvolumen der Vulkanprodukte beträgta etwa 2000 km³, wobei saure Gesteine, vorwiegend rhyolithische Ignimbrite, nicht weniger als 800–820 km³ umfassen.

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Résumé Dans la partie centrale du Grand Caucase, au stade postérieur du cycle orogénique se manifeste un volcanisme très fort. On distingue trois époques principales d'activité volcanique: 1re - miocène postérieur-pliocène antérieur;2me -pliocene postérieur; 3 me - quaternaire. Les époques de l'activité volcanique sont subdivisées en phases et sous-phases. Le volume commun des produits volcaniques est environ 2000 km³; les roches acides généralement sont les brêches de nuées ardents à liparite, dont le volume est non moins 800–820 km³.

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday     

Biotic effects of late Maastrichtian mantle plume volcanism: implications for impacts and mass extinctions

During the late Maastrichtian, DSDP Site 216 on Ninetyeast Ridge, Indian Ocean, passed over a mantle plume leading to volcanic eruptions, islands built to sea level, and catastrophic environmental conditions for planktic and benthic foraminifera. The biotic effects were severe, including dwarfing of all benthic and planktic species, a 90% reduction in species diversity, exclusion of all ecological specialists, near-absence of ecological generalists, and dominance of the disaster opportunist Guembelitria alternating with low O₂-tolerant species. These faunal characteristics are identical to those of the K–T boundary mass extinction, except that the fauna recovered after Site 216 passed beyond the influence of mantle plume volcanism about 500 kyr before the K–T boundary. Similar biotic effects have been observed in Madagascar, Israel, and Egypt. The direct correlation between mantle plume volcanism and biotic effects on Ninetyeast Ridge and the similarity to the K–T mass extinction, which is generally attributed to a large impact, reveal that impacts and volcanism can cause similar environmental catastrophes. This raises the inevitable question: Are mass extinctions caused by impacts or mantle plume volcanism? The unequivocal correlation between intense volcanism and high-stress assemblages necessitates a review of current impact and mass extinction theories.     

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge,Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ?_Nd (+8.0–+8.4) and low ⁸⁷Sr/⁸⁶Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (?_Nd=+7.5?+8.4; ⁸⁷Sr/⁸⁶Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ?_Nd (+4.6–+4.9) and (⁸⁷Sr/⁸⁶Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.     

Late Cenozoic volcanic province in Central and East Asia

The paper presents materials on the inner structure of the Late Cenozoic within-plate volcanic province in Central and East Asia, in which two subprovinces are distinguished: Central Asian and Far Eastern, which comprise a number of autonomously evolving volcanic areas. Some of the volcanic areas are proved to have evolved for a long time, starting in the Late Mesozoic. In spite of differences in their age and structural setting, the volcanic areas evolved according to similar scenarios in the Late Cenozoic. Magmatism in the province was related to a mantle source of the within-plate type. The magmatic associations are dominated by mafic alkaline high-K rocks. The rocks are geochemically close to basalts of the OIB type, and their isotopic composition corresponds to a combination of mantle sources of the PREMA, EMI, and EMII types at the predominance of PREMA. Geological, geochemical, and isotopic lines of evidence suggest that magmatism in the province was related to mantle plumes. This is consistent with geophysical data, which testify that the volcanic areas are underlain by upwellings of the asthenospheric mantle or plumes. Seismic tomography data indicate that the “stems” of the plumes can be traced down to the upper and lower mantle. The province is thought to have been produced when the eastern margin of the Asian plate overlapped one of the branches of the Pacific superplume at approximately 160 Ma. This branch of the superplume is pronounced in the modern mantle structure as a cluster of mantle plumes that control (according to seismic tomography data) the interaction zone of the Pacific and Asian lithospheric plates.     

Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo–Asian collision?

The Indo–Asian continental collision is known to have had a great impact on crustal deformation in south-central Asia, but its effects on the sublithospheric mantle remain uncertain. Studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the Asian continent, similar to the observed lateral extrusion of Asian crustal blocks. Here we present supporting evidence from P-wave travel time seismic tomography and numerical modeling. The tomography shows continuous low-velocity asthenospheric mantle structures extending from the Tibetan plateau to eastern China, consistent with the notion of a collision-driven lateral mantle extrusion. Numerical simulations suggest that, at the presence of a low-viscosity asthenosphere, continued mass injection under the Indo–Asian collision zone over the past 50 My could have driven significant lateral extrusion of the asthenospheric mantle, leading to diffuse asthenospheric upwelling, rifting, and widespread Cenozoic volcanism in eastern China.     

Paleomagnetism of Late Paleozoic,Mesozoic, and Cenozoic rocks in Mongolia

Rock complexes in Mongolia experienced two remagnetization events. Almost all secondary remanence components of normal polarity were acquired apparently in the Cenozoic, after major deformation events, and those of reverse polarity were associated with intrusion of bimodal magmas during the Late Carboniferous–Permian reverse superchron. Active continental-margin sequences in some areas of Mongolia were folded prior to the Late Carboniferous–Permian magnetic event. The primary origin of magnetization in Late Paleozoic and Mesozoic rocks has been inferred to different degrees of reliability. According to paleolatitudes derived from most reliable paleomagnetic data, the analyzed rocks were located far north of the North China block throughout the Late Paleozoic and Early Mesozoic. Mongolia, as well as Siberia, moved from the south to the north in the Paleozoic, back from the north to the south between the latest Triassic and the latest Jurassic, and remained almost within the same latitudes in Cretaceous and Cenozoic time. These paleolatitudes show no statistical difference from those for the Siberian craton at least since the latest Permian (275–250 Ma). Older Mongolian complexes (with ages of 290, 316, and 330 Ma) likewise may have formed within the Siberian continent, which makes their paleomagnetic determinations applicable to calculate the polar wander path for Siberia. The paleolatitudes of Early Carboniferous sediments in Mongolia differ significantly from those of Siberia, either because of overprints from the reverse superchron or because they were deposited away from the Siberian margin.     

Petrology of the Cenozoic volcanism in the Upper Benue valley,northern Cameroon (Central Africa)

Thirty-one plugs of alkaline volcanic rocks of Cenozoic age (37 Ma in mean) occur in the Upper Benue valley, northern Cameroon (Central Africa). The complete alkaline series (alkaline basalts, hawaiites, mugearites, phonolites, trachytes and rhyolites) is represented. Basalts contain phenocrysts of olivine, Al-Ti-rich diopside, and Ti-magnetite, and hawaiites-abundant microphenocrysts of plagioclase. Mugearites have a trachytic texture and contain xenocrysts of K-feldspar, apatite, quartz and unstable biotite. Phonolites are peralkaline. Trachytes (peralkaline and non-peralkaline) and rhyolites are characterised by their sodic mineralogy with aegirine-augite, richterite, and arfvedsonite phenocrysts. There is a large compositional gap between basaltic and felsic lavas, except the mugearites. Despite this gap, major- and trace-element distributions are in favour of a co-magmatic origin for the basaltic and felsic lavas. The Upper Benue valley basalts are similar in their chemical and isotopic features to other basalts from both the continental and oceanic sectors of the Cameroon Line. The Upper Benue valley basaltic magmas (⁸⁷Sr/⁸⁶SrƸ.7035; k Nd=+3.9) originate from an infra-lithospheric reservoir. The Sr-Nd isotopic composition and high Sr contents of the mugearites suggest that they are related to mantle-derived magmas and that they result from the mixing, at shallow crustal levels, of a large fraction of trachytic magma with a minor amount of basaltic magma. Major-element modelling of the basalt-trachyte evolution (through hawaiite and mugearite compositions) does not support an evolution through fractional crystallization alone. The fluids have played a significant role in the felsic lavas genesis, as attested by the occurrence of F-rich minerals, calcite and analcite. An origin of the Upper Benue valley rhyolitic magmas by fractional crystallization of mantle-derived primitive magmas of basaltic composition, promoted or accompanied by volatile, halogen-rich fluid phases, may be the best hypothesis for the genesis of these lavas. These fluids also interact with the continental crust, resulting in the high Sr-isotope initial ratios (0.710) in the rhyolites, whereas the Nd isotopic composition has been less affected (k Nd=+0.4).