Primary basalts and magma genesis

Three Eocene lavas from Skye, NW Scotland, have been subjected to anhydrous experimental studies within their melting ranges at pressures up to 30 kb. Two of these, an olivine-phyric magnesian alkali basalt and a near-aphyric Mg-poor transitional basalt, appear to show four-phase points on their liquidi at high pressures which are thought to have genetic significance. From experimental and mineralogical evidence, the magnesian basalt is postulated to be a primary magma, erupted without significant compositional change from its genesis by slight partial melting of a relatively Fe-rich spinel lherzolite upper mantle at about 60 km depth. The liquid seems to have had a reaction relationship with Ca-poor pyroxene (pigeonite) in the residual lherzolite. Partial crystallization of batches of this magma, delayed during its ascent at depths of about 40 km, is thought to have given rise to the Mg-poor basaltic liquids. The third lava studied experimentally, a sparsely olivine-phyric hawaiite, does not have olivine on the liquidus in any part of its anhydrous P-T diagram and therefore cannot have been derived under anhydrous conditions from olivine-saturated sources. The mineralogy and chemistry of the lavas are used to support an hypothesis that the hawaiites are products of partial crystallization of pockets of basalt magma at depths approximating to the crust/ mantle boundary beneath Skye, with rising to sufficient values to make the residual liquids comparatively rich in normative feldspar. Finally, the genesis of all other Skye Eocene lavas is reviewed in the light of the new experimental data.     

Primary basalts and magma genesis

A representative Quaternary clinopyroxene leucitite lava from the Alban Hills, Roman comagmatic province, central Italy, has been subjected to anhydrous thermal experiments within its melting range at pressures up to 45 kb. The lava contains 2.2% of leucite and 1.4% of diopside phenocrysts in a fine-grained groundmass, suggesting that these phases were crystallizing on the liquidus of the magma immediately prior to its eruption. This situation is reproduced experimentally at 14 kb and 1260 °C. As there is no evidence that the H₂O content of this lava was appreciable, it is concluded that the anhydrous experimental results give a valid indication that this leucitite equilibrated with its phenocrysts at approximately 50 km depth, about 25 km into the upper mantle, before final rapid uprise. Comparison of the bulk compositions of Alban Hills mafic leucitites with that of the eutectic in the synthetic system Diopside-Leucite as a function of pressure confirms the conclusion of the high-pressure experiments. In contrast, the dilc ratios of other Roman province mafic leucitites indicate that they equilibrated within the upper crust prior to eruption. Published Sr and O-isotope studies show unequivocally that, when the Alban Hills mafic leucitites and their phenocrysts equilibrated, the magmas contained a substantial fraction of melt from crustal rocks. These data are reconciled with the experimental demonstration that the magmas evolved entirely within the upper mantle by postulating that their crustal components were derived from partial fusion of Tyrrhenian ocean-floor sediments, subducted beneath Italy during the anticlockwise rotation of the Corsica-Sardinia lithospheric microplate. The Roman province volcanics show considerable chemical similarities with lavas from converging plate margins elsewhere, together with substantial differences from other occurrences of strongly-potassic rocks. It is concluded that this magma type may be polygenetic.     

Primary basalts and magma genesis

The Pliocene-Holocene lavas of the Snake River Plain, Idaho, U.S.A., have a bimodal composition range, consisting predominantly of basalts (olivine-tholeiites), with subordinate intercalated tholeiitic andesites but with very few analyses falling between these groups. The more-magnesian of the tholeiitic andesites contain more total Fe, alkalis, TiO₂ and P₂O₅ but less SiO₂ than the less-magnesian basalts. Derivation of the tholeiitic andesites from the basalts by low-pressure fractional crystallization or by major-element crustal contamination does not seem possible, although some minor-element exchange with ancient crust apparently has occurred. Two lavas, representative of the least-magnesian basalts and the most-magnesian tholeiitic andesites, respectively, have been subjected to anhydrous experimental studies within their melting ranges at pressures up to 35kb. Both appear to show four-phase points on their liquidi at about 8kb and these are thought to have genetic significance. Microprobe analyses of the interstitial glasses in partially-crystalline runs on the basalt between 8 and 12kb show that these reproduce all the characteristic features of the Snake River Plain most-magnesian tholeiitic andesites, notably their reduced Si-saturation. The compositions of the most Mg-rich Snake River Plain basalts are such that they may perhaps be primary magmas, produced by partial fusion of a relatively Fe-rich spinel-lherzolite upper mantle at 50 to 60km depth; a proposal which accords well with the geophysics of this currently-active region. Partial crystallization of batches of this magma, delayed during ascent within the crust at depths of about 30 km, is thought to have given rise to the tholeiitic andesites.     

Plate tectonics and geosynclines

Kay's (1951) classification of geosynclines, involving bulk sedimentary, volcanic and tectonic assemblages, is accommodated within the megaframework of oceanic expansion and contraction by lithospheric accretion and consumption. Apparently, entirely continental eugeosynclines do not exist; geosynclines occur in oceans with marginal continental shelves, continental rise, deep ocean basins, small ocean basins and island arcs. An orogen, resulting from crustal loss in trenches at Benioff zones, grows progressively away from the trench, either on the continental margin or as an island arc. The term, kinegeosyncline, is proposed for the contracting trough, trapped between continental margins and growing orogens. The arrival of a continental mass, with its continental margin sediments, at a trench results in collision and an orogen, which may suture continents together.     

Plate tectonics in Russia

After a short period of propagation of Wegener's continental drift ideas in Russia in the twenties and first half of the thirties of this century, there came a much longer period (1935–1960) of fixist reaction', when vertical tectonic movements were regarded as primary and most important and horizontal ones as secondary and subsidiary. In the sixties, interest in the manifestation of horizontal movements on strike-slip faults, overthrusts and nappe formation began to grow, and at the verge of sixties and seventies ideas of plate tectonics began to penetrate into Russia. But at the begining they met strong resistance mainly from the older generation of scientists. In spite of this, these ideas finally gained the support of the majority of Earth scientists in this country. Russian geologists and geophysicists made a significant contribution to the development of the plate tectonics theory as well as in its regional, global and theoretical aspects.     

花岗岩与大陆构造、岩浆热场与成矿

文中指出,花岗岩研究存在两个误区:(1)花岗岩没有自己独立的理论,花岗岩主要是效仿玄武岩的理论;(2)花岗岩的理论主要是应用板块构造的理论,而板块构造的理论并不适合大陆花岗岩。板块构造理论只能解决海洋里及海洋边缘的问题(如安第斯),不能解决大陆本身的问题。花岗岩在海洋里很少,绝大部分在大陆。花岗岩能否分离结晶是学术界争论的重大问题,本文从野外和镜下关系上、从理论上、方法学上和三段论法的逻辑学上进行了分析,指出花岗岩分离结晶作用是不可能的。混合作用是又一个争论的焦点,花岗岩是能够发生混合作用的,关键是笔者认为,花岗岩的混合作用主要发生在下地壳底部,而不是在花岗岩侵位和上升的过程中。目前,花岗岩研究领域里地球化学最受重视,花岗岩地球化学研究应当着力解决两个问题:(1)解决生产中提出的问题;(2)解决地球化学学科自身发展提出的问题。离开这两条,花岗岩地球化学研究即迷失了方向。笔者认为,花岗岩区分为大洋系列和大陆系列,位于海洋和海洋边缘受俯冲作用影响的为大洋系列;位于大陆内部的属于大陆系列。现存的一些花岗岩构造环境判别图仅适用于大洋系列的花岗岩,而不适用于大陆系列的花岗岩。大陆系列花岗岩的地球动力学意义不是构造环境或碰撞环境,而是地壳底部温度和压力状况。我们提出的花岗岩按照Sr-Yb的分类,适合于解释大陆花岗岩的地球动力学问题。花岗岩与成矿的关系是学术界争论的重大问题,争论的焦点主要集中在两个问题上:(1)岩浆是怎么形成的?岩浆源自哪里?(2)流体是怎么形成的?流体来自哪里,流体如何上升?为了探讨上述问题,文中主要讨论了岩浆热场理论,希望用这个理论去解释与花岗岩有关的各种成矿作用,包括与岩浆有关的各种变质和沉积热液成矿作用以及岩浆热场对煤和油气的影响等。在此基础上,提出了"与花岗岩有关的成矿组合"的概念。岩浆热场最重要的意义可能是解释了大规模岩浆活动为什么与大规模成矿作用有关的问题,指出大规模岩浆活动所造成的是1+12的效果。大数据技术是当今社会的热门话题,大数据的特点主要不在于数据的大,而在于思维的新,用新思维去处理数据才是大数据的特点。不强调"因果关系",重视"关联关系";不关注"为什么",只关心"是什么",是大数据思维的特点。其实这个特点可能恰恰最符合地质找矿的实际。板块构造引发了地球科学的一场变革,但是,它只限于地球科学理论上的变革。而大数据带来的这场变革主要不是体现在地球科学理论方面,而是地球科学研究方法和研究思路方面,它所带来的效益将是前所未有的。     

Basic magma tectonics contrasted to folding of basic rocks

In the Precambrian gneisses of SE Norway, several generations of amphibolitised basic dykes/sills occur. While prekinematically emplaced dykes and sills show normal (but complex) fold structures, other metabasic intrusions have imposed a deformation on to the host rock. Apparent fold structures which have developed due to fabric control exerted by the host rocks, are recorded from late kinematic intrusions. Attention is drawn to the problem of distinguishing primary intrusive structures from simple and multiple fold-structures.

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Zusammenfassung In die präkambrischen Gneise Südost-Norwegens sind mehrere Generationen amphibolisierter basischer Gänge bzw. Sills eingedrungen. Während präkinematisch angelegte Gänge und Sills normale (aber komplexe) Faltenstrukturen zeigen, haben andere metabasische Intrusionen das Nebengestein beansprucht. Deutliche Faltenstrukturen, die sich in der Gefügekontrolle des Nebengesteins manifestieren, werden von spätkinematischen Intrusionen berichtet. Besondere Beachtung wird der Unterscheidung primärer Intrusivstrukturen von einfachen und multiplen Faltenstrukturen geschenkt.

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Résumé Dans le SE de la Norvège, les gneiss Précambriens contiennent plusieurs générations de dykes et sills basiques amphibolitisés. Les dykes et sills prétectoniques montrent des structures de plissement, normales quoique compliquées. Par contre, les autres intrusions basiques ont déformé les roches encaissantes. Localement, des intrusions tardi-tectoniques présentent des plissements apparents controlés et provoqués par la structure de la roche encaissante. L'attention est portée sur le problème de distinction entre des structures intrusives primaires et des structures de plissement, simples et multiples.

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Plate tectonics and volcanism in the gibraltar arc

Late Tertiary and Quaternary volcanism of southeastern Spain can be fitted in a platetectonics model, taking into account the post-Paleozoic evolution of the stable and semimobile Iberian areas and the new orogenic belts bordering the Mediterranean between Africa and the Iberian Peninsula.The occurrence and distribution of calc-alkaline and potassic volcanism suggest an oceanic crust sinking downwards from the Iberian plate. This active margin is causally related to the convergence and collision of Iberia and Africa during Late Cretaceous—Early Miocene time span.A pre-collision distensive phase is inferred from the stratigraphie and tectonic record between the Triassic and Late Cretaceous, while since the Late Miocene another distensive phase is related to the actualistic features.     

Plate tectonics and the undation model: A comparison

The modern geodynamic model known as ‘New Global Tectonics’ or ‘Plate Tectonics’, elaborated in the past twenty years, is the synthetic result of the great post-war advances achieved in marine geophysical methods, deep-sea drilling technology, worldwide monitoring of earthquakes and paleomagnetics. It represents a ‘mobilistic’ view of the earth based on continental drift and sea-floor spreading (as outward manifestation) and convection currents in the mantle (as interior driving motor). The predictions of this model appear to correspond fairly well with observations about the kinematic evolution of the lithosphere during its younger (or Phanerozoic) phase. Extrapolations to Precambrian times run into conflict, however, with our knowledge of the earlier global evolution and some major revisions of the model appear to be imperative, especially in the field of geochemistry.In comparison, the model of the ‘undation theory’, conceived since 1931, has grown into a synthesis between the (sometimes extreme) mobilistic and fixistic positions. Its driving force is supposed to be primarily provided by the liberation of free energy in the earth's core and its stepwise rise to the surface in an irreversibly outward transport. This general form of energy flow occurs through rheological and geochemical processes, which lead to accumulation of potential energy and its release expressed primarily by vertical deformations of the surface — called ‘undations’ —, of various dimensions.By means of the prognosis—diagnosis method of scientific verification, this view has been repeatedly checked in test-cases of global tectonics.A notable item of this paper is the checking of both models by a comparison of global tectonic features and the major anomalies of the earth's gravity field (as ascertained from the deviations observed in the orbits of man-made satellites).A plausible rheological interpretation of these geoidal anomalies through their correlation with the major tectonic features of the earth can be produced by the undation model, whereas (so far) the model of the new global tectonics has remained silent on this allimportant geodynamic issue.     

The fluid regime of high-temperature metamorphism during granitoid magma genesis

The strong negative correlation between the temperatures and melt H₂O contents of granitoid magmas implies that the crustal melting reactions that produced the magmas were strongly buffered, with T and aH₂O co-varying within a narrow band. This observation can only be explained if the partial melting reactions that created the magmas were either fluid-absent from the outset or evolved toward this condition as melting progressed. Since these melting reactions occur during upper amphibolite- to granulite-facies metamorphism, it is reasonable to conclude that metamorphic events responsible for the generation of granitoid magmas generally occur in the absence of excess pervasive fluid.     

Plate tectonics at Lamont: the first year-1966

During the period from the mid 1950s to the 1960s it seemed as if the community of earth scientists was slowly being dragged toward accepting continental drift. But resistance was often steadfast. Each new ideal or set of data produced a trickle of new converts, but also generated heated debate. The concept of sea-floor spreading and the corollary hypothesis that the process of spreading would generate a set of symmetrical magnetic anomalies was particularly disputatious. The discovery of the Eltanin 19 magnetic anomaly profile ended the dispute and the trickle became a flood.     

幔源岩浆氧化还原状态及对岩浆矿床成矿的制约

岩浆的氧化还原状态是控制许多基本地质过程的关键热动力学参数之一。估算玄武质岩浆和源区岩石氧化还原状态的常用方法主要包括多价元素的价态、多价元素的分配系数、共存矿物对的化学平衡和全岩化学比值。岩石学实验的深入和分析技术手段的快速发展使精确估算岩浆氧逸度成为可能。这有力地促进了对地幔源区成分、岩浆的部分熔融程度和熔融方式与分异演化历史,以及岩浆矿床的成因机制及成矿过程的研究。幔源岩浆的氧化还原状态复杂多变,不仅与构造背景有关,还与地幔深度(压力)、交代作用和部分熔融有着密切联系。而在岩浆到达浅部地壳后,结晶分异、岩浆去气和同化混染等过程也能不同程度地改变岩浆的氧逸度。因此,即使来自同一构造背景的幔源岩浆也呈现出明显的氧逸度不均一性。氧逸度的高低对源区部分熔融过程中金属元素的地球化学行为、岩浆的分异演化趋势、Fe-Ti-V氧化物饱和时间的早晚和S在岩浆中的溶解度具有明显的控制作用。因此,岩浆的氧逸度对钒钛磁铁矿矿床和汇聚板块边缘的岩浆硫化物矿床的成矿过程具有显著的影响。     

Early Precambrian Earth history: Plate and plume tectonics and extraterrestrial controls

The Hadean and Archean geologic history of the Earth is discussed in the context of available knowledge from different sources: space physics and comparative planetology; isotope geochronology; geology and petrology of Archean greenstone belts (GB) and tonalite-trondhjemite-granodiorite (TTG) complexes; and geodynamic modeling review to analyse plate-tectonic, plume activity, and impact processes. Correlation between the age peaks of terrestrial Hadean-Early Archean zircons and late heavy bombardment events on the Moon, as well as the Hf isotope composition of zircons indicating their mostly mafic sources, hint to an important role of impact processes in the Earth’s history between 4.4 and 3.8 Ga. The earliest continental crust (TTG complexes) formed at 4.2 Ga (Acasta gneisses), while its large-scale recycling left imprint in Hf isotope signatures after 3.75 Ga. The associations and geochemistry of rocks suggest that Archean greenstone belts formed in settings of rifting, ocean floor spreading, subduction, and plume magmatism generally similar to the present respective processes. The Archean history differed in the greater extent of rocks derived from mantle plumes (komatiites and basalts), boninites, and adakites as well as in shorter subduction cycles recorded in alternation of typical calc-alkaline andesite-dacite-rhyolite and adakite series that were generated in a hotter mantle with more turbulent convection and unsteady subduction. The Archean is interpreted as a transient period of small plate tectonics.     

Plate tectonics and cratonal geology in Northeast Africa (Egypt,Sudan)

The stratigraphical interpretation of the strata of Nubia allows for the first time — in connection with structural, petrological and sedimentological investigations — to reconstruct the geological development of this cratonal area. After cratonization during the PanAfrican event, extensional trends in WSW-ENE direction caused a structural relief, striking NNW-SSE. The collision between Gondwana and the northern continents during the Carboniferous resulted in the uplifting of large parts of the northeast African Plate and was accompanied by more or less East-West striking faults and magmatic intrusions. Erosion of Paleozoic sediments in middle and southern Egypt and reversal of the main drainage direction was the consequence. This caused deposition of Karroo-type strata in northern Sudan, including glacial deposits at the base along the Sudanes Egyptian border.The breaking apart of Pangea during Jurassic time led to a northward tilt of NE-Africa again and consequently, the main drainage system began to follow its original north-ward directions. New structural elements developed, partly following older trends. Differing resistence against the northward drift of northeast Africa including Arabia led to the separation of Arabia from Africa and to the formation of the Red Sea — Gulf of Suez — Gulf of Akaba-Graben system. All major structural events were accompanied by magmatic activity.

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Zusammenfassung Die Klärung der stratigraphischen Stellung der Sedimentserien Nubiens hat zusammen mit den Ergebnissen sedimentologischer, tektonischer und petrologischer Untersuchungen Einblicke in die Entwicklung dieses kratonalen Gebietes zur Folge, die bisher nicht möglich waren.Die bekannten globaltektonischen Gro\ereignisse haben danach seit dem frühen Paläozoikum auf dem Ostsahara Kraton die Verteilung von Abtragungs- und Ablagerungsgebieten entscheidend beeinflu\t und waren dort von bruchtektonischen Vorgängen und magmatischen Ereignissen begleitet.Danach bestand im Paläozoikum bis ins Karbon ein WSW-ENE gerichtetes Dehnungsrelief mit daraus resultierender Anordnung entsprechend NNW-SSE gerichteter Gro\schollen. Die Kollision Gondwanas mit den Nordkontinenten hatte im Karbon Aufwölbung von Teilen des Ostsahara Kratons entlang einer Ost-West Achse zur Folge, begleitet von Ost-West gerichteten Bruchsystemen und nachfolgend von der Intrusion intermediärer Magmatite.Eine Folge der Aufwölbung war die völlige Umkehr der Entwässerungssysteme und entsprechende Erosion vorher mit paläozoischen Sedimenten bedeckter Bereiche vor allem Mittel- und Südägyptens. Lokale Vergletscherung im späten Karbon und Karroo-ähnliche Sedimentationsbedingungen bis in den Unterjura waren im Nordsudan eine weitere Folge.Mit dem Auseinanderbrechen des Superkontinentes Pangea während des Jura stellte sich die bereits im Paläozoikum übliche Nordneigung der Ostsaharatafel wieder ein, neue bruchtektonische Elemente entwickelten sich, und es kam zu erneuter Umkehr der Haupt-Entwässerungsrichtung und damit zur Annäherung an die heutige Situation. Differenzierter Widerstand gegen die Nordrift der Ostsaharaplatte führte schlie\lich zur Abtrennung Arabiens und damit zur Entwicklung des Grabensystems Rotes Meer — Akaba — Suez.

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Résumé L'établissement de la stratigraphie de la série sédimentaire de Nubie et des régions voisines, joint aux données récentes sédimentologiques, tectoniques et pétrologiques, permet aujourd'hui de reconstituer l'histoire de ce domaine cratonique, chose qui était jusqu'ici impossible.Après la cratonisation du Sahara occidental à la fin du cycle panafricain, les événements géodynamiques (tectonique cassante et magmatisme) ont déterminé dans cette région la répartition des aires d'érosion et de sédimentation.Ainsi au PaléozoÏque s'installe une tendance à l'extension dans le sens WSW-ENE qui perdura jusqu'au Carbonifère avec pour conséquence un relief structural d'orientation NNW-SSE. La collision du Gondwana avec les continents septentrionaux au cours du Carbonifère a provoqué le soulèvement d'une grande partie de l'Est saharien, accompagné d'un système de cassures E-W et d'intrusions magmatiques.Ce soulèvement a eu pour conséquence une inversion complète du système de drainage et l'érosion des sédiments paléozoÏques en Egypte moyenne et méridionale. Une autre conséquence a été, dans le Nord du Soudan, l'établissement de glaciers locaux et le dépÔt de sédiments de type Karroo.La dislocation de la Pangée au Jurassique a été accompagnée d'un nouveau basculement vers le Nord du Sahara oriental, avec nouvelle inversion du système de drainage. Finalement, la résistance inégale rencontrée dans sa dérive vers le Nord par la plaque nord-est africaine amena la séparation de l'Arabie avec formation du système des grabens Mer Rouge — Akaba — Suez.

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Geochemistry of granitic rocks from the Hercynian Sardinia-Corsica batholith: Implication for magma genesis

The Sardinia-Corsica batholith was structured in the late stage of the Hercynian orogenesis. The granitoids intrude mainly metamorphic complexes grading from zeolite up to amphibolite facies. The batholith is heterogeneous consisting of complexes with different affinity, chemical composition, age and degree of deformation. The present paper reports major-and trace-element data for selected samples coming only from Sardinian outcrops.

The rocks range from gabbro-diorite to tonalite, monzogranite and leucogranite. The two latter lithologies are the most abundant, gabbrodiorites and tonalites occurring in minor amounts and mainly in northern-central Sardinia. Over 75% of the granitoids contain microgranular enclaves of magmatic origin. The age of the rocks falls in the interval between 307 and 281 Ma. Sr isotope initial ratios are high, ranging between 0.7083 and 0.7107.

REE, Rb, Sr, Ba, Zr, Th, Ta, Hf, Co and Sc abundances were determined on selected samples. All elements follow three types of trends vs. CaO, which is used as differentiation index. Two trends show positive and negative correlations while the third one shows a bell-shaped pattern. LREE have different degrees of enrichment (La = 20−120× ch) and HREE show variable fractionation with prevailing (Tb/Yb)_n<1. The two peraluminous samples have very different geochemical characteristics.

From the geochemical point of view all the rocks coming from the Sardinian segment of the batholith display a typical calc-alkaline chemical character showing the imprint of both “normal and mature” continental arc geodynamic environments.

Geochemical trends suggest some petrogenetic constraints. The complete sequence of differentiation can be neither the product of crystal/liquid fractionation processes starting from a single basic parent magma nor the product of an AFC process. On the contrary, a two-stage model can be proposed. In the first stage a mafic melt of subcrustal origin interacted with monzogranitic magmas derived from 25–35% degree of melting of a crustal biotite amphibolitic source. Such a mixing process acted together with a crystal/liquid fractionation process to give tonalites and granodiorites. In the second stage lesser degrees of melting of the same crustal source could give the late-stage leucogranitic masses.

A possible scenario, able to take into account field and geochemical data, can be suggested for the genesis of this suite and we propose it as a working model for future investigations.     

Geochemistry of the Grenville Dyke Swarm: role of plume-source mantle in magma genesis

Major and trace element data of samples from the approximately 590 Ma Grenville dyke swarm, in the southeastern parts of the Canadian Shield, were used to evaluate the extent of crustal contamination of the dyke magma and to model its probable mantle source. The dyke rocks are mostly saturated, quartz tholeiites. A few samples represent transitional basalts. Their overall chemistries are similar to those of continental flood basalts. Their compositions resemble those of liquids initially produced in the garnet peridotite mantle zone and then subjected to polybaric fractionation. However, fractionation models fail to mass-balance Ti, Fe, K, and P. Geochemical and isotopic criteria indicate that crustal contamination was minimal. The incompatible elements Zr, Y, K, Rb, Nb, V, Ta, Hf, and Th show hyperbolic elemental ratio-ratio distributions indicative of mixing between two end-members. One end-member has elemental ratios similar to those of a depleted N-MORB source and the second of a P-MORB i.e. ertile or plume MORB source. The more enriched dyke compositions can be successfully reproduced by mixing models in the ratio of 6:1 between the most depleted MORB-like dyke magma and a liquid composition similar to metasomatic glass veinlets coexisting with mica peridotite mantle xenoliths, and subsequent crystal fractionation. The Grenville dykes are probably a synrift swarm that was emplaced along a rift arm related to an Iapetan rrr triple junction that is thought to have formed over a mantle plume. The chemistry of the dyke swarm is consistent with its derivation from a plume head, in that the minor enriched source can be identified with plume-source mantle and the major depleted source with entrained mantle. The magma probably originated in the upper cooler parts of the plume head.